Asymmetric biomimetic transamination of α-keto phosphonates enabled by chiral pyridoxamines and synergistic solvent

Domain: chemistry
Published: 2026-02-14
Editorial outcome: major_revision

Abstract
Chiral α-aminophosphonate derivatives are recognized as valuable compounds in both medicine and organic chemistry. Biomimetic transamination of α-keto phosphonates represents one of the most efficient strategies for accessing these pharmaceutically relevant molecules. However, establishing a general amine-transfer platform that directly delivers N-unprotected chiral α-aminophosphonates remains challenging. We report an asymmetric transamination of α-keto phosphonates catalyzed by a chiral pyridoxamine catalyst, which efficiently affords a diverse range of chiral α-aminophosphonates with up to 86% yield and 98% ee. This methodology provides a straightforward approach to biologically active α-aminophosphonic acid derivatives. Mechanistic studies, supported by DFT calculations, reveal that trifluoroethanol plays a critical role in determining the enantioselectivity and stabilizing the transition states of the rate-limiting step.

Introduction
The synthesis of chiral primary amines from ketones is a highly valued transformation, given the significance of the amine products and the ready availability and low cost of the ketone substrates (Fig. 1a)1–3. Synthetic chemists have utilized the versatile reactivity of imines, such as reduction4,5, nucleophilic addition6–11, and 1,3-hydrogen shift12–21, to access chiral primary amines after deprotection. In nature, transamination of carbonyl compounds, enabled by transaminases via two 1,3-proton shift processes, represents an attractive strategy, as it directly yields N-unprotected chiral amines22–24. This approach avoids the condensation, protection, and deprotection steps required in conventional chemical routes. Thus, enzymatic transamination has been extensively studied since the 1950s25–28. Today, the enzymatic synthesis of chiral amines catalyzed by transaminases, particularly ω-transaminases (ω-TAs), has been industrialized and successfully applied in the production of various pharmaceuticals29,30. Although enzymatic transamination has emerged as a powerful tool for asymmetric synthesis of chiral amines, biomimetic transamination has also attracted considerable interest as a promising approach for chiral amine synthesis12–21,31–38. By emulating the structural and mechanistic features of transaminase enzymes, biomimetic catalysts not only retain key aspects of enzymatic efficiency but also benefit from the flexibility and tunability of small-molecule systems, enabling mild and enantioselective synthesis of diverse amines39–41. Our group has employed chiral pyridoxamines (1), biomimetic catalysts that mimic the structure and function of vitamin B6-dependent transaminases42, to achieve the asymmetric transamination of α-keto acids43,44 and α-keto amides45. This platform creates opportunities to develop new transformations by leveraging the unique structural and electronic features of these biomimetic pyridoxamine catalysts.

Chiral α‑aminophosphonates stand as an important class of organophosphorus compounds with broad relevance in medicinal and synthetic chemistry46–49. Their unique structural and electronic properties enable diverse biological activities, including antibacterial fosfomycin derivative Alafosfalin (hFPPs)50, the inhibitor of the human farnesyl pyrophosphate synthase51, imidazoline I2 receptors (I-IRs) ligand in Alzheimer’s disease therapy52, and the inhibitor of matrix metalloproteinases (MMPs)53 (Fig. 1b). Given their pharmacological potential, structural versatility, and synthetic adaptability, a variety of synthetic strategies have been developed for the preparation of chiral α‑aminophosphonates54–56. Among these, one of the most classical and widely utilized methods is the hydrophosphonylation57,58, represented by the Kabachnik–Fields reaction59–61 and Pudovik reaction62–65. In addition, chiral α‑aminophosphonates also can be efficiently accessed via asymmetric hydrogenation66,67 or nucleophilic addition68,69 of iminophosphonates (Fig. 1c, above). In recent years, significant efforts have been directed toward the development of organocatalyzed enantioselective 1,3-proton shift of Schiff bases70,71, a transformation that mimics the mechanism of biological half-transamination (Fig. 1c, below)15,42,72. In 2015, Albrecht and coworkers demonstrated this approach using a cupreine-derived catalyst to facilitate the 1,3-proton shift of in situ generated α-iminophosphonates (from α-ketophosphonates and benzylamines), yielding chiral α-aminophosphonates with excellent enantioselectivity70. More recently, Deng’s group reported a related strategy employing a betaine-derived quaternary ammonium salt catalyst, which exhibited remarkable efficiency and broad substrate compatibility (Fig. 1c, box)71. Despite being the most efficient approach to chiral α-aminophosphonates, enzymatic transamination has rarely been applied to non-proteinogenic α-keto phosphonates, likely due to the inherent substrate specificity of enzymes. Biomimetic asymmetric transamination offers a promising alternative for synthesizing unnatural chiral α-aminophosphonates. However, a general and efficient method for directly accessing N-unprotected derivatives remains scarce65. Herein, we report a chiral pyridoxamine-catalyzed biomimetic transamination of α-keto phosphonates that affords structurally diverse chiral α-aminophosphonates in up to 86% yield and 98% ee (Fig. 1d). Mechanistic studies reveal that both the catalyst structure and the solvent microenvironment play decisive roles in controlling the stereoselectivity.

Results
Optimization of reaction conditions
The transamination of α-ketophosphonate 2a was initially conducted with diphenylglycine (4a) as the nitrogen donor and chiral pyridoxamine 1a as the catalyst at 20 °C (Table 1). The effects of the solvent, ester group, additive, amino donor, and catalyst structure were systematically investigated. Solvent screening revealed that alcohols generally provided higher enantioselectivities, with trifluoroethanol (TFE) proving optimal (entries 1–6, for more details see Supplementary Table S1). Evaluation of the ester group of α-ketophosphonates suggests that both methyl ester and isopropyl ester are suitable for this reaction (Supplementary Table S1, entries 7–10 and 31). Various additives were examined, such as Brønsted acids (Table 1, entries 7–12) and buffer systems (e.g., NaOAc/AcOH, Supplementary Table S1, entries 26 and 27). Among the additives tested, HCOOH markedly accelerated the transamination, affording the product in 83% yield with 97% ee (Table 1, entry 12). In contrast, stronger acids such as CF3COOH (Table 1, entry 7) or bases like NaOAc (Table 1, entry 9) led to significant decreases in both yield and enantioselectivity. These results suggest that this biomimetic transamination operates optimally under neutral to weakly acidic conditions, which is consistent with the behavior of biological transamination systems. Several alternative amino donors, including N,N-dimethylglycine (4b), methylphenylglycine (4c), phenylalanine (4d), and benzylamine (4e), all afforded <5% yield under the optimized conditions (Supplementary Table S1, entries 32–35). These donors likely fail to promote the essential pyridoxal-to-pyridoxamine conversion required for the catalytic cycle44,45, whereas diphenylglycine readily undergoes decarboxylation to efficiently convert the pyridoxal to the pyridoxamine catalyst. Catalyst structure has a vital impact on the reactivity and enantioselectivity of this transamination (Table 1, entries 13–18). Catalyst 1a-e bearing an amine side chain all perform well for the transamination in terms of enantioselectivity (Table 1, entries 13–16), implying that the OH group in pyridoxamine 1d likely doesn’t participate in the catalysis via hydrogen bonding (Table 1, entry 15). Catalyst 1e, which has a primary amine side chain, showed slightly reduced reactivity (Table 1, entry 16). In contrast, catalysts 1f and 1g, featuring amide side chains, exhibited a dramatic decrease in both yield and enantiomeric excess (Table 1, entries 17 and 18). These results highlight that the basicity of the amine side chain is crucial for efficient transamination, thereby rendering a secondary amine indispensable for catalysis. Although α-ketophosphonates can act as acylating agents due to the good leaving ability of the phosphonate group, no such side products were observed under our optimized conditions, likely because the acidic environment effectively suppresses this pathway.

Substrate scope study
Under the optimized reaction conditions, substrate scope of the transamination was investigated (Fig. 2). Benzyl and phenethyl substituted α-ketophosphonates all proceeded the aminotransfer in the presence of pyridoxamine catalyst 1a to produce α-aminophosphonates 3b-d in excellent enantioselectivity. Substituents on arylethyl were then examined (for 3e-n). Both electron-donating groups (for 3e, 3f, 3g, and 3n) and electron-deficient groups (for 3h-m) generally gave moderate to high reactivity with 92-97% ee. Notably, in contrast with methyl phosphonate, isopropyl phosphonate produced the corresponding chiral α-amino phosphonates with significantly enhanced yields (3f vs 3g and 3h vs 3i). In addition, naphthylethyl (3q) and thiophenethyl (3r) α-aminophosphonates were respectively obtained in 96% ee with good to high yields. Aliphatic chain substituted α-keto phosphonates underwent the transamination smoothly, delivering a wide range of aliphatic α-aminophosphonates 3t-af in 66-86% yields and 85-98% ee. Various functional groups, such as hetero-atom (as in 3u and 3ac), ester (as in 3v and 3z), double bond (as in 3aa), triple bond (as in 3ab) and alicyclic substituent (as in 3ad) were all well tolerated.
The biomimetic transamination could be also applied to α-keto phosphonates containing complicated moieties. For example, citronellal- and deoxycholic acid-derived α-aminophosphonate 3ae and 3af were, respectively, obtained in good yields with high diastereomeric ratios by this transamination. Biologically active molecule (androgen receptor antagonist)73-tethered α-aminophosphonate 3ag was delivered with 68% yield and 90% ee. More importantly, aromatic α-keto phosphonate, a challenging substrate for transamination, was able to produce the desired product 3ah, albeit with moderate yield and enantioselectivty. Among these products, several chiral aminophosphonates corresponding to natural amino acids, such as 3b (Phe), 3c (Tyr), 3t (Ala), 3u (Met), and 3v (Glu), were successfully obtained, demonstrating the method’s broad applicability to the synthesis of chiral aminophosphonic acid analogs of proteinogenic amino acids. Unfortunately, Ile- and Val-derived α-keto phosphonates are not effective substrates for the asymmetric transamination, likely due to the steric effect of the β-substituents. The biomimetic reaction completes two half-transamination steps, thereby mimicking the full catalytic cycle of biological transamination22–24, and directly furnishes a broad range of chiral N-unprotected α-aminophosphonates without requiring any additional hydrolytic manipulation. Taken together, this reaction establishes an alternative platform to the previously reported enantioselective 1,3-proton-shift reactions of Schiff bases70,71, further expanding the accessible methodological scope for constructing chiral α-aminophosphonates.

Synthetic applications of α-aminophosphonates
The synthetic utility of the transamination was tentatively demonstrated (Fig. 3). The scaled-up reaction was investigated using the (S)-1a catalyzed transamination of 3.0 mmol of α-ketophosphonate 2a, affording (S)-α-aminophosphonate 3a in 80% yield and 97% ee, which are also identical to those obtained in the reaction in a small scale (83% yield and 97% ee), demonstrating good scalability for this process. Hydrolysis of (S)-3a gave chiral improving-aminophosphoric acid 5 in a 96% yield (Fig. 3a). The transamination can also be applied to efficient synthesis of bioactive molecules, such as MMPs inhibitor 8 (Fig. 3b). Transamination of 6-fluorohexanoylphosphonate (2ai) under biomimetic standard conditions afforded chiral α-aminophosphonate 3ai in 71% yield with 96% ee. Subsequent protection of the NH2 group using sulfonyl chloride 6 yielded compound 7 in 75% yield while maintaining 95% ee. Hydrolysis of this intermediate delivered the MMPs inhibitor 8. In contrast, the previous synthetic route started from 6-tetrahydropyranyl-protected 1,6-hexanediol (9) and required seven steps, including a chiral resolution, to furnish the corresponding N-protected α-amino phosphonate (8) in a mere 4% overall yield53 (Fig. 3b). In addition, this transamination also can be employed to the rapid synthesis of the enantiomeric analog (12) of proteasome inhibitor Ixazomib—a drug approved for the treatment of multiple myeloma (MM)74. α-Aminophosphonate 3aj, which was obtained from 2aj with 60% yield and 92% ee via catalyst (S)-1a, was condensed with (2,5-dichlorobenzoyl)glycine (10) followed by hydrolysis with TMSBr to produce chiral dipeptide phosphoric acid 12 with 92% ee (Fig. 3c).

Mechanistic study
The reaction follows a typical pyridoxamine catalyzed transamination mechanism42,72,75, which mimics the two half reactions of enzymatic transamination43. The first half-reaction (from α-ketophosphonate 2 to α-amino phosphonate 3) involves 1,3-proton shift of Schiff base 13 formed between α-ketophosphonate 2 and chiral pyridoxamine (R)-1a, which is promoted by the intramolecular amine side chain of catalyst43,44. The second half-reaction transfers the amine group of diphenyl glycine (4a) to internal imine 16, regenerating chiral pyridoxamine (R)-1a via the isomerization between Schiff bases 17 and 19 (Fig. 4). Although the internal ammonium species, such as intermediate 14 or 18, could in principle act as a proton donor for the 1,3-proton shift, in a reaction system containing multiple proton sources (HCOOH, CF3CH2OH and H2O), proton transfer mediated by external proton donors or via a proton-shuttle pathway also represents a plausible mechanism.
Density functional theory (DFT) calculations were performed to elucidate the detailed mechanism of transamination of α-ketophosphonate (2a) to chiral α-aminophosphonate (3a) and to investigate the origin of chiral induction. The potential energy surface (PES) for the step of the condensation between the 2a and (R)-1a is illustrated in Fig. 5. The reaction initiates with the nucleophilic attack of the amine group of pyridoxamine (R)-1a onto α-ketophosphonate 2a. This initial step proceeds via transition state TS1 (ΔG‡ = 15.2 kcal/mol) to generate intermediate M1. Assisted by a formic acid-mediated hydrogen-bonding network, a proton is transferred from N1 to O1 through TS2 to form intermediate M2. Subsequently, M2 undergoes a ligand exchange wherein a formic acid molecule dissociates as a formate ion enters, coordinating with the phenolic hydroxyl group to generate M3. This is followed by an intermolecular proton transfer from the phenolic hydroxyl group to the formate species via TS3. This stage generates intermediate M4, which undergoes C1–O1 bond cleavage and the elimination of a water molecule via TS4 to afford intermediate M5. A subsequent proton transfer from N2 to the formate ion proceeds via TS5, generating intermediate M6 (Fig. 5b). The entry of a TFE molecule into the system facilitates hydrogen bonding interaction with the side chain N2–H group, leading to the formation of intermediate M7. Subsequently, the benzylic C2–H undergoes deprotonation by the side-chain amine through TS6 (ΔG‡ = 14.4 kcal/mol) to produce intermediate M8. A stereoselective protonation step occurs via TS7R (ΔG‡ = 19.7 kcal/mol), furnishing the chiral intermediate M9R and ultimately the (R)-product. In contrast, the competing transition state TS7S is energetically unfavorable, characterized by a significantly higher barrier (ΔG‡ = 25.1 kcal/mol). Geometric analysis reveals that the N–H···O and O–H···O hydrogen bonds are markedly shorter in TS7R than in TS7S. These shorter distances indicate that the hydrogen-bonding network provides superior stabilization in TS7R, thereby lowering its activation barrier relative to TS7S. These findings clearly indicate a strong kinetic preference for the R-configurational pathway. Further transformation, including intramolecular cyclization and the subsequent release of α-aminophosphonate 3a, are discussed in the Supplementary Information (Fig. S2). Overall, the computational results identify the protonation of M8 via TS7R is the rate-determining and stereo-determining step for the initial half-transamination.
To further probe the critical role of TFE in the asymmetric 1,3-proton shift process, a series of control experiments were conducted with different alcohol solvents (Fig. 6). Gradual removal of fluorine substituents from the alcohols (from TFE to difluoroethanol and monofluoroethanol) led to a progressive decrease in enantioselectivity (entries 1–3), while ethylene glycol and ethanol afforded substantially lower ee values (entries 4 and 5). These results indicate that the electron-withdrawing effect at the β position of alcohol solvent is crucial for stereochemical induction during the 1,3-proton-shift step. To further rationalize this solvent effect, transition-state calculations were also performed using ethanol as an alternative hydrogen-bonding solvent for TS7S and TS7R. The calculated difference in Gibbs free energy of activation in ethanol is smaller than that in TFE, capturing the experimentally observed reduction in enantioselectivity (see Supplementary Fig. S3). Moreover, in the TS7R transition states in TFE and EtOH (Fig. 6 box), more acidic solvent forms stronger hydrogen-bonding interaction with formate, stabilizing this species and enhancing its proton-donating ability, which in turn improves both enantioselectivity and catalytic activity. In contrast, solvents with weaker acidity display diminished stereocontrol and catalytic efficiency (entries 2–5). Overall, TFE plays a decisive role in enabling both high activity and enantioselectivity, which is critical for the success of this biomimetic transamination.

Methods
General procedure for the asymmetric transamination
To a 5 mL vial equipped with a magnetic stirrer bar was added 2 (0.10 mmol), (R)-1a (0.0026 g, 0.005 mmol), diphenylglycine (4a) (0.0454 g, 0.2 mmol), trifluoroethanol (TFE, 0.51 mL), H2O (0.09 mL) and HCOOH (0.0161 g, 0.35 mmol) under N2 atmosphere. Upon stirring at 20 ˚C for 72 h, the reaction mixture was evaporated to remove most of the solvent and submitted to flash column chromatography on silica gel (DCM:MeOH:saturated NH3 solution in ethanol = 50:1:1) to give product 3. For some specific substrates, the reaction conditions were slightly changed, the detailed information of which can be found in the Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Liu, Xiao, Zhao, and coworkers reported chiral pyridoxamine-catalyzed asymmetric transamination of alpha-keto\nphosphonates to give unprotected alpha-aminophosphonates. While related molecules have been stereoselectively\nsynthesized via hydrophosphonylation and 1,3-proton shift reactions, this work provides N-unprotected alpha-\naminophosphonates directly, without protecting-group manipulations, in high yields and enantioselectivity. Furthermore, the\nsubstrate scope and transformation, as well as mechanistic investigations, are demonstrated. However, the following issues\nshould be clarified before further consideration.\n(1) In the introduction, the following paper should be cited after the following sentence in the manuscript: “However, a\ngeneral and efficient method for directly accessing N-unprotected derivatives remains scarce”.\nOrganocatalytic Direct Enantioselective Hydrophosphonylation of N-Unsubstituted Ketimines for the Synthesis of α-\nAminophosphonates ACS Catal. 2023, 13, 3158-3163. DOI: 10.1021/acscatal.2c05953\n(2) Throughout the manuscript, the catalyst should be drawn with appropriate salt composition (with I- and HCl or 2HCl) as\nshown in the SI pages 5 to 8.\n(3) For the synthesis of alpha-keto phosphonates 2 as starting materials shown in the SI, their preparative yields from\naldehydes should be reported for each substrate to compare the synthetic efficiency of the present method with that of the\nprevious ones. As most of the alpha-keto phosphonates 2 are unknown in the literature, either the two-step or each step\nyield of these molecules should be described in the SI.\n(4) The proposed mechanism shown in Fig. 4 should be revised as follows. (i) The catalyst structure should be 1a, but the N-\nmethylpyridinium salt structure of the bottom side of the catalyst is mistakenly drawn as a simple pyridinium salt. (ii) From 13\nto 14, the authors From 14 to 15, the authors proposed that the proton comes from an outer protic source, but the internal\nammonium salt could also act as the proton source. (iii) The curvy arrow on 15 should be redrawn because the arrow does\nnot appropriately represent the flow of electrons. (iv) From 15 to 16, H+ would not be necessary to make 16 and 3.\n(5) For the DFT calculation shown in Fig.5 and Figure S2, the following issues should be clarified. (i) The B3LYP functional\nused for the calculation would not be best suited for reaction energy evaluation, and the additional single-point calculation\nwith other functionals, such as M06-2X, is recommended. (ii) For the stereoselectivity-determining step, the hydrolysis of 15\nto give 3 and 16 should also be considered because the transition state energies after 1,3-proton shift, such as TS8R and\nTS9R, are higher than that of TS6R. (iii) The overall process from 1a to 16 and 3 is endergonic (∆G = 12.4 kcal/mol), and the\nauthors should demonstrate whether the net overall catalytic cycle would be exergonic (∆G < 0); otherwise, the calculated\nreaction energies should be reconsidered. (iv) For the 1,3-proton shift step from M4 to M7, the consideration of H3O+ as the\nproton source is difficult because the reaction proceeds even in the absence of carboxylic acid additives, as shown in Table\n1 and Table S1, and ammonium salts with pKa ~ 9 would not generate H3O+ with pKa = 0 in water. It would be better to\nconsider carboxylic acids, such as diphenylglycine and formic acid, and their carboxylate anions as alternative proton-\ntransfer agents for the proton-transfer step and other processes, which would give much lower energies than that of TS5\nshown in the proposed mechanism.\n(6) To support the control experiments in Fig. 6, a comparison of DFT calculation energies using these alternative solvents,\nespecially ethanol, should be performed. If the proposed reaction mechanism is correct, ethanol would give higher energy\nfor TS5, and the energy difference between TS6R and TS6S should be reduced.", "reviewer_2": "Tittle: Asymmetric Biomimetic Transamination of α-Keto Phosphonates Enabled by 1 Chiral Pyridoxamines and\nSynergistic\nSolvent.\nJournal: Nature Communications\nAccepted: 20/10/2025.\nKey results\nThe manuscript by Liu, Xiao, Zhao and co-workers describes the asymmetric synthesis of biorelevant α-aminophosphonates\nby a transamination reaction of α-ketophosphonates with diphenylglycine as the nitrogen source, using chiral pyridoxamines\nas biologically inspired catalysts. Enzymatic transamination of α-keto acids is the most important process to access optically\nactive α-amino acids in biological systems. Mimicking this fundamental process on the corresponding phosphorylated\nanalogues (α-keto phosphonates) and using pyridoxamines as biomimetic transaminases, the authors report an efficient\nprotocol for the synthesis of enantioenriched α-aminophosphonates, interesting isosteres of amino acids with a plethora of\nbiological activities derived from their ability of blocking enzymes related to the metabolism of peptides. In addition they\nreport some synthetic applications of the α-aminophosphonate substrates in the preparation of some bioactive molecules.\nFinally, they propose a plausible mechanism, providing control experiments and DFT studies that support their proposal and\nare able to explain the role of trifluoroethanol additive in cooperative catalysis\nValidity, data and methodology\nThe conclusions are reasonably supported by the data and the experimental procedures and characterization of the\ncompounds are carefully detailed. The precedents are well documented. However, I find hard to understand why the right\nnomenclature is used in the first part of the manuscript, such as “α-aminophosphonate” or “α-ketophosphonate”, and then the\nsame substrates are named as “phosphates”. This seems to be due to a cooperative manuscript writing process combined\nwith poor proofreading. According to IUPAC, a phosphate is a salt or ester derived from phosphoric acid (H3PO4), while\nphosphonate is a phosphonic (or phosphorous) acid derivative (H3PO3).\nThe scope presented is wide for aliphatic α-ketophosphonate substrates, but just one example of an aromatic ketone is\npresented (with a modest 50% yield) and only two different phosphonate substituents (Me and iPr) have been placed at the\nα-ketophosphonate substrates, showing similar results. I am disappointed that benzyl or phenyl phosphonates have not\nbeen tested since the use of such substituents at the phosphonate tetrahedral group often leads to surprising results which\nare not comparable to those of a planar carboxylate group. Moreover, dibenzylphosphonates can be easily deprotected by\nhydrogenolysis. It is true that a scope of 30 substrates is reported, but their structural differences are not substantial in order\nto demonstrate the general application of the reported procedure. Anyway, besides the limited number of examples reported\nusing different phosphonate esters and α-aryl α ketophosphonates, I think that the authors have demonstrated the value of\ntheir methodology and they provide a nice contribution to the field of the enantioselective synthesis α aminophosphonates\nand transamination reactions.\nSignificance and novelty\nRegarding the novelty of the procedure for the synthesis of the substrates, the most relevant methods for the preparation of\noptically active α-aminophosphonates are the enantioselective addition of phosphorus reagents to pre-formed (Pudovic) or\nin situ-formed (Kabachnick-Fields) imines, as well as the asymmetric hydrogenation or nucleophilic addition reactions with\nphosphorated imines or enamines (see: Molecules 2023, 28, 6150. https://doi.org/10.3390/molecules28166150 and other\nreview articles referenced therein). In this concern, the reported methodology does not imply any advantage in terms of yield\nor enantioselectivity compared with those classical methods. This includes Albreich’s and Deng’s methodology (Chem.\nCommun. 2015, 51, 3981-3984, https://doi.org/10.1039/C4CC09477H. J Am Chem Soc. 2024, 146, 16706-16713,\nhttps://doi.org/10.1021/jacs.4c04129) which, in addition, make use of cheap benzylamine as nitrogen source and release\nbenzaldehyde instead of carcinogen benzophenone.\nConcerning the relevance of the synthetic protocol, the asymmetric transamination of α-keto acids was reported by the same\nauthors in 2015 using chiral pyridoxal derived catalysts, leading to α-amino acids in moderate to good yields and\nenantioselectivities ranging from about 60 to 80% (Org. Lett. 2015, 17, 5784–5787,\nhttps://doi.org/10.1021/acs.orglett.5b02895). The use of diphenylglycine as the optimal source of nitrogen was determined\nalong this research. One year later they developed axially chiral pyridoxamines for the same transformation with improved\nresults in terms of reactivity and enantioselectivity (J. Am. Chem. Soc. 2016, 138, 10730–10733,\nhttps://doi.org/10.1021/jacs.6b03930). An elegant example of the utility of their protocol is the direct synthesis of peptides\nthrough the asymmetric biomimetic transamination of α-keto amides reported more recently by the same authors (Nat.\nCommun. 2021, 12, 5174, https://doi.org/10.1038/s41467-021-25449-y). The reaction reported in this manuscript is the\ncontinuation of the authors on the applications of pyridoxamine catalysts in transamination reactions of ketones, in this case\nusing α-keto-phosphonates. This is the same reaction reported in the previous three articles using different starting\nmaterials. One addition to those previous researches is the inclusion of a DFT study but, in fact, the mechanism depicted in\nthis manuscript is a simple modified copy of that proposed in JACS 2016.\nFeedback\nIt should be noted also the difficulties related to the generation of α-iminophosphonates by direct condensation of amines\nand α-ketophosphonates as proposed for the generation of intermediate 13 (from 1 and 2) of the depicted mechanism, since\nthe intermediate species are well known to undergo faster elimination of the phosphorus moiety than water (See: Sekine, M.;\nKume, A.; Hata, T. Tetrahedron 1981, 22, 3617-3620, https://doi.org/10.1016/S0040-4039(01)81975-4. Sekine, M.; Satoh,\nM.; Yamagata, H.; Hata, T. J. Org. Chem. 1980, 45, 4162-4167, https://doi.org/10.1021/jo01309a019). According to what is\nreported in these papers and my own experience, acylphosphonates (α-ketophosphonates) react fast with primary amines\n(especially aliphatic, including benzylic) to afford the amide substrates upon loss of the phosphonate moiety instead of\nwater. That makes acylphosphonates ideal acylating agents of amines and alcohols, avoiding the presence of acid sources.\nConsidering this possibility, the proposed amine-carbonyl condensation between 1a and 2 leading to imine 13 would be in\ncompetition with the formation of the amide derived from the elimination of the phosphonate (or phosphite) unit as depicted\nin the attached scheme. That would explain the decreased yields obtained in some cases.\nThe yield of α-aminophosphonate substrates in the transamination reaction ranges from 50 to 80% (Fig 2), but the authors do\nnot provide an explanation to those yields. Are there side products formed? Do the substrates undergo degradation during\nthe purification?, etc. It should be easy to detect the side products expected by the elimination of the phosphonate moiety,\nespecially the phosphorus-containing species.\nSummary and decision\nStrengths:\n• The protocol for the synthesis of α-aminophosphonates described is very valuable and implies an elegant example of the\napplications of the asymmetric pyridoxamine-catalyzed transamination reaction.\n• The scope is wide, but not too general, since all the examples are structurally related.\n• Some synthetic applications leading to biologically relevant molecules are reported.\n• A DFT study is performed, providing support to cooperative catalysis with TFE.\nWeaknesses:\n• The scope is wide, but not too general, since all the examples are structurally related.\n• Side products are not considered, neither identified.\n• DFT study does not consider the possibility of the elimination of the phosphonate moiety\n• Three examples of the asymmetric transamination of α-keto acid derivatives have been reported in the last years by the\nsame authors\n• The reported methodology does not imply any advantage in terms of yield or enantioselectivity compared with classical\nmethods. There are two similar methodologies among those\nAccording to my comments, although there is not any doubt of the quality of the research and the high value of the synthetic\nprotocol, considering the previous articles reported by Albreich, Deng and the authors of this manuscript in the same field, I\nbelieve that the manuscript lacks the significance and novelty required for publication in a top journal in Multidisciplinary\nSciences and therefore I am sorry to inform that I do not support its publication Nature Communications.", "reviewer_3": "Chiral α-aminophosphonates hold significant value in medicinal and organic chemistry. Traditional chemical syntheses of\nchiral α-aminophosphonates rely on methods such as hydrophosphonylation, asymmetric hydrogenation, and nucleophilic\naddition. In contrast, asymmetric transamination of carbonyl compounds, enabled by transaminases, offers an attractive\nroute to N-unprotected chiral α-aminophosphonates. In this manuscript, Zhao and co-workers describe an asymmetric\ntransamination of α-ketophosphonates catalyzed by a chiral pyridoxamine derivative. This transformation delivers a diverse\narray of chiral α-aminophosphonates in up to 86% yield and 98% ee. The authors also present mechanistic insights,\nincluding a notable synergistic solvent effect. I recommend acceptance of this work for Nature Communications on the basis\nof two key strengths:\n(1)It introduces the first general asymmetric transamination of α-ketophosphonates to afford N-unprotected chiral α-\naminophosphonates.\n(2)The method exhibits broad substrate scope, high enantioselectivity, and mild reaction conditions, rendering it practical for\nresearchers in adjacent fields.\nThe following points should be addressed during revision:\n(1)How do alternative amino donors perform in this reaction? Diphenylglycine, for instance, suffers from poor atom economy,\nand this aspect warrants discussion in the revised manuscript.\n(2)What is the scalability of this reaction?\n(3)Aminophosphonic acids serve as tetrahedral (bio)isosteric analogs of the corresponding planar amino acids. In this\nmanuscript, for instance, compounds 3s and 3ag are the P-analogs of alanine and leucine, respectively. To further highlight\nthe method's utility, the authors could synthesize additional chiral aminophosphonic acid analogs derived from the 20\nstandard amino acids, such as those mimicking Trp, Tyr, Phe, Met, Ile, or Val. Such examples would further enhance the\nwork's appeal and applicability.", "reviewer_4": "This paper reports a biomimetic transamination reaction based on a chiral pyridoxamine catalyst for the efficient and highly\nenantioselective synthesis of chiral α-aminophosphonates. The research is systematic and in-depth, demonstrating\nsignificant synthetic value and mechanistic importance. The experimental design is reasonable, the substrate scope is\nbroad, and the mechanistic study, which combines experimental and theoretical calculations, is highly convincing. DFT\ncalculations elucidate the rate-determining step and the origin of stereoselectivity, clarifying the crucial role of TFE solvent in\nstabilizing key transition states. It is recommended for acceptance after minor revision, pending the authors' response to the\npoints above and appropriate modifications.\n1)It is suggested to align the entries in Table 1 with the order of description in the main text, or to supplement the explanation\nof the logical sequence of the optimization path.\n2)The screening results for some conditions (e.g., types of additives, catalyst structures) are not sufficiently explained in\nterms of the underlying chemical principles; adding brief discussions is recommended.\n3)The structures of some substrates (e.g., \"3ab\", \"3ac\") are not fully shown in Figure 2. It is suggested to add the structural\nformulas or provide a clear table correlating substrate numbers with structures.\n4)The text refers to Fig. 4 and Fig. 5, but these were not provided in the submitted text. Ensure all mechanism figures are\ncompletely presented in the main text, with key intermediates and transition states labeled.\n5)It is recommended to add comparisons with existing methods (e.g., the work of Albrecht, Deng) regarding substrate scope,\nenantioselectivity, and operational simplicity to highlight the advantages of the present method.\n6)For example, \"3.8 kcal/mol above that of TS6R\" should clearly indicate whether it refers to the ΔΔG‡ value. Using\nstandardized expressions like \"ΔΔG‡ = 3.8 kcal/mol\" consistently is advised.\n7)The language is generally fluent, but there are minor grammatical and spelling errors: For instance, \"biomimetic\" is\nsometimes misspelled as \"biornimetic\" (line 33). A thorough proofreading of the entire text is recommended. Some sentence\nstructures are complex; consider splitting them appropriately to improve readability.\n8)The text refers to \"Fig. 1b\", \"Fig. 1c\", etc., but the figure captions do not clearly mark the sub-figure labels. It is suggested to\nexplicitly label sub-figures as (a), (b), (c), (d), etc., within the figures.\n9)Some reference numbers are superscripted, while others are not. Consistent formatting throughout the text is\nrecommended."}, "author_response": "POINT-TO-POINT RESPONSE\nResponses to Reviewer #1: Great thanks for the helpful comments and suggestions\n(1) In the introduction, the following paper should be cited after the following sentence\nin the manuscript: “However, a general and efficient method for directly accessing N-\nunprotected derivatives remains scarce”.Organocatalytic Direct Enantioselective\nHydrophosphonylation of N-Unsubstituted Ketimines for the Synthesis of α-\nAminophosphonates ACS Catal. 2023, 13, 3158-3163. DOI: 10.1021/acscatal.2c05953\nResponse: We thank the reviewer for his/her kind suggestion. Actually, this reference\nhas been cited as the ref. 65 in the original text. We also now cited this paper in the\nsuggested location in the revised main text.\n(2) Throughout the manuscript, the catalyst should be drawn with appropriate salt\ncomposition (with I- and HCl or 2HCl) as shown in the SI pages 5 to 8.\nResponse: According to the reviewer’s suggestion, we have redrawn the catalysts in\ntheir salt forms, except in the mechanism-related figures (Figures 4–6), where the\niodide and chloride counterions of catalyst 1a were omitted to clearly illustrate the\nstructural features of the reaction intermediates. We have also added a note in the\ncorresponding figure captions to indicate that the counterions are intentionally omitted.\n(3) For the synthesis of alpha-keto phosphonates 2 as starting materials shown in the\nSI, their preparative yields from aldehydes should be reported for each substrate to\ncompare the synthetic efficiency of the present method with that of the previous ones.\nAs most of the alpha-keto phosphonates 2 are unknown in the literature, either the two-\nstep or each step yield of these molecules should be described in the SI.\nResponse: According to the reviewer’s suggestion, we have included the total yields\n(two steps) for the synthesis of all -keto phosphonates 2 in the Supplementary\nInformation.\n(4) The proposed mechanism shown in Fig. 4 should be revised as follows.\n(i) The catalyst structure should be 1a, but the N-methylpyridinium salt structure of\nthe bottom side of the catalyst is mistakenly drawn as a simple pyridinium salt.\nResponse: We thank the reviewer for pointing out the error. We have corrected this\nmistake in Fig. 4.\n(ii) From 14 to 15, the authors proposed that the proton comes from an outer protic\nsource, but the internal ammonium salt could also act as the proton source.\nResponse: We thank the reviewer for this insightful comment. Indeed, the internal\nammonium species generated in the catalytic cycle could also serve as a viable proton\nsource for the conversion of 14 to 15. Given the presence of multiple proton sources\n(HCOOH, CF₃CH₂OH and H₂O), DFT calculations support a formate-assisted proton-\nshuttle mechanism for the 1,3-proton shift (Figure 5b in the main text ). A totally\ninternal proton-donation pathway via the amine side chain is likely disfavored due to\ngeometric and energetic constraints, although it cannot be fully excluded. This step\ntherefore most plausibly involves a cooperative proton transfer from both internal and\nexternal sources. A brief clarification has been added to the revised manuscript to reflect\nthis mechanistic possibility.\n(iii)The curvy arrow on 15 should be redrawn because the arrow does not\nappropriately represent the flow of electrons.\nResponse: We thank the reviewer for pointing out this issue. The hydrolysis of\ncompound 15 proceeds through multiple elementary steps, in which the carboxylate\nanion functions as a proton-transfer agent. The detailed process is illustrated in Fig. S2\nof the Supplementary Information. Accordingly, the arrow in the main text has been\nremoved for clarity.\n(iv) From 15 to 16, H+ would not be necessary to make 16 and 3.\nResponse: We thank the reviewer for pointing out this error and agree with the reviewer\nthat the 15 → 16 conversion does not necessarily require an external proton, as\nintramolecular proton transfer from the ammonium species can furnish the intermediate\n16. We now have deleted the external H+ of the conversion of 15 → 16. The Fig. 4 has\nbeen updated in main text.\n(5)For the DFT calculation shown in Fig.5 and Figure S2, the following issues should\nbe clarified.\n(i) The B3LYP functional used for the calculation would not be best suited for reaction\nenergy evaluation, and the additional single-point calculation with other functionals,\nsuch as M06-2X, is recommended.\nResponse: We thank the reviewer for this insightful suggestion. Following the\nrecommendation, we have performed additional optimization calculations and single-\npoint energy calculations using the M06-2X functional on the B3LYP-optimized\ngeometries with formic acid as a proton-transfer agent (see Scheme 1 below). The\nupdated energy profiles have been incorporated into the revised manuscript and\nSupplementary Information\nScheme 1. Updated DFT calculation for transamination of α-keto phosphonate.\n(ii) For the stereoselectivity-determining step, the hydrolysis of 15 to give 3 and 16\nshould also be considered because the transition state energies after 1,3-proton shift,\nsuch as TS8R and TS9R, are higher than that of TS6R.\nResponse: We thank the reviewer for the constructive suggestion regarding the\nhydrolysis of 15. As suggested, we have re-calculated the hydrolysis pathway (leading\nto 3 and 16) using formic acid and its carboxylate anion as updated proton-transfer\nagents.The refined calculations indicate that the energy barrier for this hydrolysis step\nis indeed lower than that of the 1,3-proton shift (the corresponding transition state\nlabeled as TS7R in the revised manuscript). Since hydrolysis is more kinetically\nfeasible than previously estimated and the transition state TS7R has the highest energy,\nthe 1,3-proton shift constitutes the stereoselectivity-determining step. We have updated\nthe energy profile in Fig. 5 and the related discussion in the revised manuscript to\nincorporate these new findings.\n(iii) The overall process from 1a to 16 and 3 is endergonic (∆G = 12.4 kcal/mol), and\nthe authors should demonstrate whether the net overall catalytic cycle would be\nexergonic (∆G < 0); otherwise, the calculated reaction energies should be reconsidered.\nResponse:Following the suggestion to reconsider proton-transfer agents, we updated\nthe proton-transfer agents to a more realistic model involving formic acid and its\ncorresponding carboxylate anion. The net overall catalytic cycle from 1a to 3 and 16 is\nnow exergonic (ΔG = -21.9 kcal/mol), providing the necessary thermodynamic driving\nforce (Scheme 1). The updated energy profiles have been incorporated into the revised\nmanuscript (Fig. 5) and Supplementary Information (Fig. S2) .\n(iv) For the 1,3-proton shift step from M4 to M7, the consideration of H O+ as the\n3\nproton source is difficult because the reaction proceeds even in the absence of\ncarboxylic acid additives, as shown in Table 1 and Table S1, and ammonium salts with\npKa~ 9 would not generate H O+ with pKa = 0 in water. It would be better to consider\n3\ncarboxylic acids, such as diphenylglycine and formic acid, and their carboxylate anions\nas alternative proton-transfer agents for the proton-transfer step and other processes,\nwhich would give much lower energies than that of TS5 shown in the proposed\nmechanism.\nResponse:Thank the reviewer for the insightful suggestion on using diphenylglycine\nor formic acid as alternative proton-transfer agents. Considering the extremely low\nsolubility of diphenylglycine under the experimental conditions, we have re-calculated\nthe energy profile for the transamination by using formic acid and its corresponding\ncarboxylate anion as the proton-transfer agents. As shown in the updated Energy Profile\n(Scheme 1), the use of a formic acid/carboxylate shuttle significantly lowers the barrier\nfor deprotonation step (now labeled as TS6 in the revised manuscript) and the\nhydrolysis step compared to our initial model. As a result, the entire catalytic cycle\nbecomes thermodynamically exergonic. This model provides a much more accurate\nreflection of the experimental environment where the reaction proceeds efficiently. The\nFig.5 of manuscript has been updated and the corresponding discussion has also been\nincluded in the main text.\n(6) To support the control experiments in Fig. 6, a comparison of DFT calculation\nenergies using these alternative solvents, especially ethanol, should be performed. If\nthe proposed reaction mechanism is correct, ethanol would give higher energy for TS5,\nand the energy difference between TS6R and TS6S should be reduced.\nResponse: We thank the reviewer for this constructive suggestion. We agree that\ncomparing DFT-calculated energies in different solvents can further support the role of\nsolvent-assisted hydrogen bonding in asymmetric transamination. Calculations were\nperformed using ethanol as an alternative hydrogen-bonding solvent. The resulting\ndifference in Gibbs free energy of activation (ΔΔG‡) between TS6S and TS6R (now\nlabeled as TS7S and TS7R in the revised manuscript) is smaller than that obtained in\ntrifluoroethanol (See below, Scheme 2), which is consistent with the experimentally\nobserved decrease in enantioselectivity. Moreover, more acidic solvent (like TFE)\nforms stronger hydrogen-bonding interaction with formate, stabilizing this species and\nenhancing its proton-donating ability, which in turn improves both enantioselectivity\nand catalytic activity. In contrast, solvents with weaker acidity display diminished\nstereocontrol and catalytic efficiency (Fig. 6 in main text). The corresponding\ndiscussion and data have been added to the revised manuscript and Supplementary\nInformation.\nScheme 2. DFT calculation for TS7R and TS7S in TFE and EtOH solvents.\nResponses to Reviewer #2: Great thanks for the helpful comments and suggestions\n(1) However, I find hard to understand why the right nomenclature is used in the first\npart of the manuscript, such as “α-aminophosphonate” or “α-ketophosphonate”, and\nthen the same substrates are named as “phosphates”.\nResponse: We thank the reviewer for pointing out the errors. We have now adopted the\ncorrect nomenclature consistently throughout the revised main text and the\nSupplementary Information.\n(2) I am disappointed that benzyl or phenyl phosphonates have not been tested since\nthe use of such substituents at the phosphonate tetrahedral group often leads to\nsurprising results which are not comparable to those of a planar carboxylate group.\nResponse: We have examined different phosphonate substituents, such as Me, Et, iPr,\nnBu, as shown in Supplementary Table S1 (entries 7-10) of the original Supplementary\nInformation. According to the review’s suggestion, we attempted to synthesize the\nbenzyl- or phenyl-substituted phosphonates. However, the phenyl α-ketophosphonate\ncould not be fully purified, likely due to the instability of the OPh group. We therefore\nevaluated the benzyl α-ketophosphonate under the standard reaction conditions\n(Supplementary Table S1, entry 31), which afforded the corresponding product in 68%\nyield and 96% ee.\n(3) The yield of α-aminophosphonate substrates in the transamination reaction ranges\nfrom 50 to 80% (Fig 2), but the authors do not provide an explanation to those yields.\nAre there side products formed? Do the substrates undergo degradation during the\npurification?, etc. It should be easy to detect the side products expected by the\nelimination of the phosphonate moiety, especially the phosphorus-containing species.\nResponse: We thank the reviewer for raising this important point. In our biomimetic\ntransamination system, the reaction operates under mildly acidic conditions, where the\nNH group of pyridoxamine catalyst, diphenylglycine, and α-aminophosphonate are all\n2\nprotonated, resulting in a decrease in nucleophilicity. The acidic conditions likely\nrender phosphonate elimination highly unfavorable, preventing the formation of the\namide byproducts typically observed under neutral or basic conditions. To confirm the\nabsence of this side reaction in the current system, we analyzed the crude reaction\nmixtures by LC–MS and ³¹P NMR. No amide species resulting from phosphonate loss\nwere detected. For substrates that give lower isolated yields, the major issue is low\nconversion rather than the undesired phosphonate elimination. The corresponding\ndiscussion have been included in the revised manuscript.\n(4) The reported methodology does not imply any advantage in terms of yield or\nenantioselectivity compared with classical methods. There are two similar\nmethodologies among those.\nResponse:With regard to the reviewer’s concern about the advantages of the present\nwork over the related methodologies reported by Albrecht and Deng, this biomimetic\nreaction completes two half-transamination steps, thereby mimicking the full catalytic\ncycle of biological transamination, and directly furnishes a broad range of chiral N-\nunprotected α-aminophosphonates without requiring any additional hydrolytic\nmanipulation. This reaction establishes an alternative platform to the previously\nreported enantioselective 1,3-proton-shift reactions of Schiff bases, further expanding the\naccessible methodological scope for constructing chiral α-aminophosphonates. The\ndiscussion has been added to the end of the “Substrate Scope Study” section in the\nrevised manuscript.\nResponses to Reviewer #3: Great thanks for the helpful comments and suggestions\n(1) How do alternative amino donors perform in this reaction? Diphenylglycine,\nfor instance, suffers from poor atom economy, and this aspect warrants discussion in\nthe revised manuscript.\nResponse: We thank the reviewer for the kind advice. Regarding the performance of\nalternative amino donors, we conducted additional control experiments using several\nstructurally diverse candidates, including N,N-dimethylglycine (4b), phenylalanine\n(4c), benzylamine (4d) and methylphenylglycine (4e) (Supplementary Information,\nTable S1, entries 32-35). However, all these amino donors afforded less than 5% yield\nunder the optimized reaction conditions. This uniformly low reactivity highlights a key\nmechanistic requirement of the present biomimetic transamination system: unlike\ndiphenylglycine, these alternative donors are unable to efficiently convert the pyridoxal\ncatalyst 16 to the catalytically active pyridoxamine form 1a. The results have been\nincluded in SI and the corresponding discussion has been enclosed in the “Optimization\nof Reaction Conditions” section of the revised text.\n(2) What is the scalability of this reaction?\nResponse: According to the review’s advice, we conducted a 3.0 mmol-scale\ntransamination of α-ketophosphonate 2a, producing 0.78 g (S)-α-aminophosphonate 3a\nwith80% yield and 97% ee. The yield and enantioselectivity are also identical to those\nobtained in the reaction in a 0.2 mmol scale (83% yield and 97% ee), demonstrating\ngood scalability for this process. The result and discussion have been added in the main\ntext as Figure 3a.\n(3) Aminophosphonic acids serve as tetrahedral (bio)isosteric analogs of the\ncorresponding planar amino acids. In this manuscript, for instance, compounds 3s and\n3ag are the Panalogs of alanine and leucine, respectively. To further highlight the\nmethod's utility, the authors could synthesize additional chiral aminophosphonic acid\nanalogs derived from the 20 standard amino acids, such as those mimicking Trp, Tyr,\nPhe, Met, Ile, or Val. Such examples would further enhance the work's appeal and\napplicability.\nResponse: We thank the reviewer for the good suggestion. we attempted to synthesize\nthe corresponding α-keto phosphonates derived from the natural amino acids Trp, Tyr,\nMet, Ile, Val, and Glu. Under the standard conditions, Tyr-, Met, and Glu- derived α-\nketo phosphonates were smoothly transaminated into the corresponding chiral α-\naminophosphonates. Together with 3b (Phe) and 3t (Ala) from the previous studies,\nfive chiral aminophosphonic acid analogues of natural amino acids are now synthesized\nby the transamination.\nHowever, Ile- and Val-derived α-keto phosphonates could not afford the desired\nproducts, likely due to the steric effect of the β-substituents. And the Trp-derived α-keto\nphosphonate couldn’t be prepared from the corresponding 2-(1H-indol-3-\nyl)acetaldehyde, probably because it is not stable enough to serve as a viable synthetic\nprecursor. The results and the corresponding discussion have also been incorporated\ninto the revised text.\nResponses to Reviewer #4: Great thanks for the helpful comments and suggestions\n(1) It is suggested to align the entries in Table 1 with the order of description in the\nmain text, or to supplement the explanation of the logical sequence of the optimization\npath.\nResponse: We thank the reviewer for the kind suggestions. We have aligned the entries\nin Table 1 with the order of the description in the text.\n(2) The screening results for some conditions (e.g., types of additives, catalyst\nstructures) are not sufficiently explained in terms of the underlying chemical principles;\nadding brief discussions is recommended.\nResponse: According to the reviewer’s advice, we have added additional discussions\nregarding the effects of additives and catalyst structures. The additive studies have\nsuggested that this biomimetic transamination operates optimally under neutral to\nweakly acidic conditions, which is consistent with the behavior of biological\ntransamination systems. By analyzing the impact of the side chain of the pyridoxal\ncatalysts, we found that the basicity of the amine side chain is crucial for efficient\ntransamination, thereby rendering a secondary amine indispensable for catalysis. These\ndiscussions have been incorporated into the “Optimization of Reaction Conditions”\nsection of the revised manuscript.\n(3) The structures of some substrates (e.g., \"3ab\", \"3ac\") are not fully shown in Figure\n2. It is suggested to add the structural formulas or provide a clear table correlating\nsubstrate numbers with structures.\nResponse: Regarding the issues of incomplete or missing figures in the submitted text\nnoted by the reviewer, we have uploaded an additional PDF file to ensure that all the\nfigures are displayed in full.\n(4) The text refers to Fig. 4 and Fig. 5, but these were not provided in the submitted\ntext. Ensure all mechanism figures are completely presented in the main text, with key\nintermediates and transition states labeled.\nResponse: Regarding the issues of incomplete or missing figures in the submitted text\nnoted by the reviewer, we have uploaded an additional PDF file to ensure that all the\nfigures are displayed in full.\n(5) It is recommended to add comparisons with existing methods (e.g., the work of\nAlbrecht, Deng) regarding substrate scope, enantioselectivity, and operational\nsimplicity to highlight the advantages of the present method.\nResponse: We thank the reviewer for the kind suggestion. This biomimetic reaction\ncompletes two half-transamination steps, thereby mimicking the full catalytic cycle of\nbiological transamination, and directly furnishes a broad range of chiral N-unprotected\nα-aminophosphonates without requiring any additional hydrolytic manipulation. Taken\ntogether, this reaction establishes an alternative platform to the previously reported\nenantioselective 1,3-proton-shift reactions of Schiff bases, further expanding the\naccessible methodological scope for constructing chiral α-aminophosphonates. The\ndiscussion has been added to the end of the “Substrate Scope Study” section in the\nrevised manuscript.\n(6) For example, \"3.8 kcal/mol above that of TS6R\" should clearly indicate whether it\nrefers to the ΔΔG‡ value. Using standardized expressions like \"ΔΔG‡ = 3.8 kcal/mol\"\nconsistently is advised.\nResponse: We thank the reviewer for this helpful reminder. Standardized expressions\nof ΔΔG‡ have now been used consistently throughout the revised manuscript and the\nSupplementary Information.\n(7) The language is generally fluent, but there are minor grammatical and spelling\nerrors: For instance, \"biomimetic\" is sometimes misspelled as \"biornimetic\" (line 33).\nA thorough proofreading of the entire text is recommended. Some sentence structures\nare complex; consider splitting them appropriately to improve readability.\nResponse: We appreciate the reviewer’s suggestion. All identified spelling and\ngrammatical errors have been corrected, the entire manuscript has been thoroughly\nproofread, and several complex sentences have also been revised or split to ensure\nclarity and enhance overall readability.\n(8) The text refers to \"Fig. 1b\", \"Fig. 1c\", etc., but the figure captions do not clearly\nmark the sub-figure labels. It is suggested to explicitly label sub-figures as (a), (b), (c),\n(d), within the figures.\nResponse: Regarding the issues of missing the sub-figure labels of Fig. 1 noted by the\nreviewer, we have uploaded an additional PDF file to ensure that all the sub-figure\nlabels are displayed in full.\n(9) Some reference numbers are superscripted, while others are not. Consistent\nformatting throughout the text is recommended.\nResponse: We appreciate the reviewer’s kind reminding. All references have now been\nformatted consistently."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors revised their manuscript in response to the reviewers' comments and addressed the concerns. DFT calculations\nnow support the mechanistic proposal, and the importance of carboxylic acid as a proton source is emphasized. Although\nthe atom economy and catalyst loading should be improved in the future, this work directly provides unprotected\nenantioenriched aminophosphonates, a distinguishing feature compared to precedents. Thus, this reviewer recommends\npublication of this work in Nature Communications.", "reviewer_2": "The authors have made a strong effort in improving the quality of the manuscript and have answered to all the queries\ncoherently. Although I still have some minor concerns with the proposed amine-carbonyl condensation leading to imine 13,\nin competition with the formation of the amide and the elimination of the phosphonate, I think that the authors have made the\nopportune additional experiments and modifications and provide convenient explanations. For these reasons, despite my\ninitial rejection decision due to a perceived lack of significance, and especially in light of the very positive comments from\nthe other referees, I support the publication of the manuscript in Nature Communications.", "reviewer_3": "The authors have well addressed all my previous concerns and can be accepted now.", "reviewer_4": "The authors have revised carefully, and the manuscript can be acceptable for publication in Nature Communications."}, "author_response": ""}]

Decision letter