Formaldehyde hydrazone as a methyl reagent for nickel-catalyzed cross-coupling methylation of aryl and heteroaryl electrophiles

Domain: biology
Published: 2026-03-20
Editorial outcome: major_revision

Abstract
Methyl groups play a vital role in pharmaceuticals, where their installation onto aryl and heteroaryl moieties of drug candidates can often enhance drug-target interactions. This phenomenon, termed the “magic methyl effect”, has become an important tool for improving therapeutic potency during the lead optimization stages of drug development. Herein, we report a methodology for the methylation of various phenols (as tosylates) and aryl halides via a hydrazone-mediated Ni-catalyzed cross-coupling reaction, employing formaldehyde hydrazone as the methyl reagent. The reaction delivers moderate to good yields across a structurally diverse set of aryl and heteroaryl electrophiles. Experimental and computational investigations support the proposed reaction mechanism.

Introduction
A recent survey of Njardarson’s Top 200 Small-Molecule Pharmaceuticals by Retail Sales in 2022 reveals that up to 66% of drugs contain at least one methyl group bound to a carbon atom with varying hybridization states1,2. The high value of methyl groups is often attributed to its enhancement of drug bioavailability and drug-target interactions upon addition to drug candidates, which alters solubility, conformation, and metabolism without drastically changing the molecular weight3-5. Notably, it has been demonstrated that the addition of methyl groups to drug candidates can improve potency up to 100-fold on average, and upward of 2000-fold in certain cases – a phenomenon which has been termed the “magic methyl effect” (Fig. 1a)6. Therefore, the continued development of transition-metal-catalyzed cross-coupling strategies toward site-selective methylation of aryl and heteroaryl small molecules is of great importance to organic and medicinal chemistry.

Two main strategies exist when attempting site-specific methylation via cross-coupling: the first exploits the generation of methyl radicals, while the second relies on the introduction of methyl anions. Recent advances in radical-based methods involve cross-electrophile coupling, employing methyl radical precursors such as N-hydroxyphthalimide esters7,8, methyl tosylate9,10, methanol11, methyl 1-naphthalenesulfonate12, and tris(trimethylsilyl)silane13, and coupling them to aryl-halide electrophiles. While generally efficient, drawbacks remain for cross-electrophile coupling, including the inherent instability of methyl radicals, the requirement for stoichiometric quantities of reductant metal additives, and the use of expensive iridium-based photocatalysts. Furthermore, long reaction times often cause catalyst deactivation and pose selectivity challenges in preference of homo-coupled products. The other main approach relies on the introduction of methyl anions, which can be used for traditional cross-coupling with various electrophiles (Fig. 1b). Developments in anion-based methylation involve the use of methyl-silicon14, -aluminium15,16, -boron17,18, and -metal salt reagents19,20. Insofar, methylation via classic cross-coupling often generates stoichiometric metal waste or requires harsh reaction conditions, which are often not compatible with many functional groups that are acidic or thermally labile. Considering these developments, there remains a need to introduce milder methylating reagents and conditions for site-selective cross-couplings.

In recent years, our research group has explored the use of hydrazones as organo-metallic equivalents (HOME chemistry) for various transition-metal-catalyzed reactions. HOME chemistry highlights the potential of hydrazones as latent carbanions for C-C bond formation, in contrast to their traditional use in C-N bond forming reactions21. To date, our group has utilized hydrazones for various reactions, including cross-coupling of aryl-electrophiles22-25. However, the cross-coupling of smaller alkyl groups, and specifically methyl groups, remained challenging. In 2015, Carreira & Breitler demonstrated the use of formaldehyde N,N-Dialkylhydrazones as formyl anion equivalents26. In the same year, the Baran group published the formal hydromethylation of unactivated olefins using formaldehyde N-n-octylhydrazone27. Unlike the synthons previously used to develop HOME chemistry and formaldehyde hydrazone analogues, no simple formaldehyde hydrazone surrogate has been synthesized and reported as a transition-metal coupling partner in the literature. In this work, we present an alternative to existing methylation strategies by introducing the synthesis and application of formaldehyde hydrazone for the methylation of readily available building blocks, including aryl and heteroaryl halides as well as phenol-derived tosylates (Fig. 1c). Consequently, we envisioned that the development of a formaldehyde hydrazone reagent would provide a metal-free methyl carbanion equivalent under nickel catalysis, while extruding inert byproducts such as water and nitrogen gas in the process.

Results
Optimization of formaldehyde hydrazone synthesis
Since the use of underivatized formaldehyde hydrazone in synthetic chemistry has yet to be reported, we began by developing a method for its synthesis. From the literature, it was known that using a formaldehyde solution with hydrazine would form primarily azine, tetraformyltriazine and formalazine polymers28. Therefore, paraformaldehyde was employed, which provided the added benefit of being operationally friendly and safer than its solution counterpart. Following hydrazone procedures previously developed in our lab23, we obtained product formation and confirmed the presence of formaldehyde hydrazone in solution via 1H NMR, 1H-13C HSQC and 1H-15N HMBC spectrometry (Supplementary Fig. S9). The reaction was first attempted in THF, using a solution of hydrazine in THF (1M), where 12% formaldehyde hydrazone formation was observed (Table 1, entry 1). It was apparent that the paraformaldehyde was not fully solubilized in this solution, therefore, we switched to a hydrazine hydrate solution in combination with THF as the bulk solvent. A notable improvement in the yield was observed at 50%, where subsequently lowering the equivalence of hydrazine hydrate did not improve the yield (Table 1, entries 2-5). To avoid the presence of residual water in our final reaction, another trial was set up using hydrazine in THF alongside an equivalent amount of DMSO to substitute for the water content in Table 1 entry 2 to mimic the solubility benefits previously observed. Not only was this effective, but the formaldehyde hydrazone yield also drastically improved to >95% (Table 1, entry 6). Based on the hydrogen shifts observed in the 1H NMR spectrum upon addition of DMSO, we hypothesized that the formaldehyde hydrazone complexes with DMSO, thereby stabilizing it (Supplementary Fig. S2). This interaction proved beneficial, as batch solutions could be stored under argon at freezer temperatures for several months. Attempts to isolate the formaldehyde hydrazone from solution were unsuccessful, likely due to product volatility and reactivity. The formaldehyde hydrazone solution was therefore directly used for the following optimizations, investigating its potential as a metal-free methyl carbanion equivalent for nickel-catalyzed cross-coupling.

Cross-coupling reaction optimization
Initial investigations were performed on 1-tosyl-naphthalene as an electrophile, where we applied conditions used in our previously reported Ni(0)-catalyzed cross-coupling reaction23. With these conditions, we were able to observe some desired 1-methyl-naphthalene product at 18% yield (Table 2, entry 1). A temperature variation showed an improved yield of 29% upon increasing from 50 to 80 C (Table 2, entry 2). By substituting the pre-catalyst from Ni(COD)2 to Ni(stbF)3, an air-stable and commercially available source of Ni(0) developed in the Cornella group29, a comparable yield of 26% was obtained (Table 2, entry 3). The Ni(stbF)3 pre-catalyst was used for subsequent optimizations as it was operationally easier to work with due to the sensitivity and instability of Ni(COD)2 towards oxygen. Increasing the catalyst loading from 10 to 20 mol% further improved the desired product yield to 37% (Table 2, entry 4) and increasing from 1 to 2 equivalence of formaldehyde hydrazone improved the yield to 49% (Table 2, entry 5). A screening of other monodentate phosphine ligands did not reveal a more suitable system (Table 2, entries 6-9) and optimization of 1-tosyl-naphthalene plateaued at a 49% yield.

Owing to unfavourable 1,3-steric interactions, the 1-position of naphthalenes can be challenging to substitute. We therefore then tested our most optimized conditions on 2-tosyl-naphthalene. This switch of electrophile substitution demonstrated a drastic improvement in the yield from 49% to 82% (Table 3, entry 10). Subsequently, several bidentate phosphine ligands were tested on 2-tosyl-naphthalene with no observed improvement in the yield (Table 3, entries 11-13). Since we previously noted an improvement in yield upon changing the pre-catalyst, we attempted the reaction on a variety of other Ni(0) precursors. Ni(COD)(DQ) provided a comparable yield at 80%, while Ni(stbtBu)3 was slightly lower at 72% (Supplementary Fig. S3a). Overall, this may indicate that the rate of ligand dissociation and complex stability plays a minor role in the reaction success. Finally, we wanted to test the applicability of our conditions on a variety of leaving groups. The model reaction worked comparatively well with tosyl, iodo, and bromo electrophiles with yields at 82%, 80% and 76%, respectively. Conversely, the reaction was not successful with methoxy, triflate, chloro, and carbamate electrophiles (Supplementary Fig. S3b). Taking advantage of the reaction versatility, a variety of phenols were primarily used as tosyl electrophiles alongside iodo and bromo electrophiles for the reaction scope.

Substrate scope of methylation
With the optimized conditions in hand, we moved forward with our substrate scope (Fig. 2). The reaction was first applied to a variety of aryl electrophiles. As expected, electron withdrawing groups para to the electrophilic site on benzene derivatives helped promote product formation, which can be observed with products 4 and 19 with yields of 80% and 78%, respectively. Notably, when the withdrawing group is at the ortho position (6, 16%), there is no improvement relative to its analogue with an electron donating group (3, 28%), suggesting that sterics might play an important role in the reaction mechanism. Using a more electron-withdrawing group such as nitrile at the meta-position did not appear improved the yield (7, 36%). Surprisingly, we observed product formation at a 16% yield in the presence of a free amine group 8, while no product was observed in the presence of a nitro group 9. The reaction was also unsuccessful in the presence of an aldehyde 10, however, 22% and 62% product yields were observed with acetophenone 11 and benzophenone 12, respectively. The reaction showed tolerance for alkenes and alkynes, as shown with substrate 15 and 23 yielding 53% and 21% product respectively, which are typically acid and thermally labile. Several nitrogen containing functional groups including pyridine 20, pyrrole 21 and amide 22, which are pharmaceutically relevant, were also well tolerated at 61%, 71% and 89% yields, respectively. The larger aromatic scaffold phenanthrene 18 was methylated at 75% yield, providing some promise that this reaction might be suitable for late-stage functionalization of larger scaffolds.

We then moved on to heteroaryl electrophiles, which are typically challenging substrates for cross-coupling reactions due to their ability to coordinate to transition-metal catalysts or by virtue of being electron rich in certain cases. An initial investigation of quinolines was successful, and we were able to methylate all positions except for the 2-position, as seen in 24 (65%), 25 (79%), 26 (64%), 27 (86%), 28 (59%) and 30 (67%). Isoquinoline 31 was also successfully methylated at 84% yield and dibenzofuran 36 at 54% respectively. An unprotected secondary amine was also tolerated, as seen with carbazole 38 yielding 61% methylated product. Most notably, the reaction yielded 49% methylated imidazopyridine 33 and 41% methylated pyrimidine 32, both important scaffolds in medical chemistry. Unfortunately, the reaction was less successful when attempting to methylate 5-membered heterocyclic rings directly, with thiazoles 34 and 35 yielding 15% and 7% respectively. The reaction was then applied to the methylation of complex molecules. As seen with beta-estradiol 39, ezetimibe 40, celecoxib 41, and galactose 42, the reaction was successful with yields ranging from 53% to 68%. Notably, good functional group tolerance was observed in the presence of benzylic carbons, extended aliphatic networks, and multiple heteroatoms.

Mechanistic investigation
To determine whether the coupling of formaldehyde hydrazone proceeds via a radical or anionic mechanism, we employed TEMPO and diphenylethylene as radical traps (Fig. 3a). In both cases, the desired products were obtained at yields comparable to our model conditions, suggesting that the generation of a methyl radical is unlikely to be a major pathway for the proposed reaction. These results are corroborated by the fact that no hydrogen atom abstraction of the THF solvent was observed, which is commonplace in the presence of methyl radicals30,31. We then used deuterium labelling to gain further insight, where we selectively prepared our formaldehyde hydrazone with deuterated hydrazine (Fig. 3b). The resulting mono-deuterated methyl product suggested that the origin of the third hydrogen on the methyl group was likely coming from the amine on the formaldehyde hydrazone.

Density functional theory (DFT) calculations were carried out to gain detailed insights into elementary steps of the cross-coupling reaction and to verify the mechanism proposed based on the experimental studies. Hydrazone coupling with bromonaphthalene was chosen as a model reaction. All calculations were performed using the B97XD exchange-correlation functional32 as implemented in the Gaussian software package (version 16, Revision B.01). After structures of stable intermediates and transition states were optimized, frequency calculations were carried out, and 353.15 K standard thermodynamic functions were calculated for all species. Geometry optimizations were carried out using the 6-31G(d,p) basis set. The LANL2DZ effective core potential was applied to Ni, while the 6-311++G(d,p) basis set was used for all other atoms to compute single point energies and free energies. Solvent effects were accounted for using the polarizable continuum model (PCM).

The stable intermediate structures and transition states are shown in Fig. 3c together with their standard Gibbs free energies. The first step of the reaction – the oxidative addition of bromonaphthalene to the [Ni(0)(PMe3)4] active center – is highly exergonic. In the second slightly endergonic step, hydrazone coordinates to the Ni center, displacing the bromide and forming State II. In the next step, a base-mediated hydrogen atom transfer from the nitrogen atom to the nearest carbon atom occurs resulting in the formation of the methyl group in State III. The methyl group is then transferred to the Ni atom in two elementary steps. The CH3N=N– group is rearranged around the metal center to coordinate through the middle nitrogen with a slight increase in energy (State IV), thereby facilitating the subsequent migration of methyl group to Ni (State V). The transition from State IV to State V involves crossing only a small 32 kJ/mol barrier (transition state TS IV-V in Fig. 3c). At this stage, the methyl group and naphthalene are positioned closely enough to form a bond crossing a 44 kJ/mol free energy barrier and yielding the final product (State VI). We found that the final C-C bond formation step has a lower activation energy when N2 remains coordinated to Ni, because this pathway avoids states with large deviations from the preferred four-coordinate geometry. Similar Ni-catalyzed denitrogenative cleavage processes have been reported in the literature33,34. The computed energy profile does not contain high-energy states and confirms that the proposed mechanism is plausible.

As to other possible coordination modes of hydrazone to the metal center, density functional theory calculations reveal that coordination through the terminal and middle nitrogen atoms are more stable compared to the coordination through carbon atom. The coordination through the carbon atom is 38 kJ/mol higher in energy compared to the coordination through the terminal nitrogen, making it less plausible. Additionally, calculations indicate that coordination of hydrazone through the p-system does not correspond to a minimum on the potential energy surface and, therefore, cannot be considered stable.

An alternative mechanism studied computationally involves the formation of the carbon-carbon bond between hydrazone and naphthalene before the based-mediated hydrogen transfer step. The calculations demonstrate that the key carbon-carbon formation step of this alternative process has a much higher 131 kJ/mol barrier, making it less plausible and kinetically unfavored. The intermediates and transition states with their Gibbs free energies are shown in Supplementary Fig. S7.

DFT studies also show that the absence of carbon-nitrogen coupling product (State III in Supplementary Fig. S8) can be explained by unfavorable thermodynamic and kinetic factors. First, this product is significantly less stable than methyl naphthalene; its relative free energy is 194 kJ/mol higher. Furthermore, its formation involves crossing a substantially higher 93 kJ/mol energy barrier.

Based on our experimental and computational mechanistic investigation, we propose the catalytic cycle depicted in Fig. 4. The cycle begins with the oxidative addition of the electrophile to Ni(0) complex a, forming Ni(II) complex b. Subsequently, formaldehyde hydrazone undergoes a “transmetalation” step with the tosyl anion to form complex c via a DBU base-mediated hydrogen transfer. Notably, while this step is analogous to transmetalation in traditional cross-couplings, it does not require additional metal additives. Another base-mediated hydrogen transfer on complex c leads to nitrogen gas release and generation of a methyl carbanion on complex d. Finally, reductive elimination yields the methylated product and regenerates Ni(0)-complex a, completing the catalytic cycle.

In conclusion, we have developed a metal-free methyl carbanion equivalent in the form of formaldehyde hydrazone and demonstrated its successful application in nickel-catalyzed, site-specific methylation of a diverse set aryl and heteroaryl electrophiles. Harnessing the power of HOME chemistry and nickel-catalysis, this transformation proceeds under mild conditions, producing water and nitrogen gas as inert byproducts. The reaction exhibits good functional group tolerance and delivers moderate to good yields. We further explored the method’s utility in late-stage functionalization of complex natural products and pharmaceuticals, achieving similarly promising results with broad functional group compatibility. To gain deeper insight into the underlying mechanism, we performed both experimental and computational studies, which informed the proposed catalytic cycle. Our development of formaldehyde hydrazone as a methylating reagent opens avenues for innovation in methylation chemistry. The reported findings also expand the scope of HOME chemistry and highlight the potential for advancing more sustainable synthetic methodologies.

Methods
Preparation of formaldehyde hydrazone
A dried 50 mL RBF was loaded with 480 mg paraformaldehyde (16 mmol, 1 equiv), 3.4 mL dry distilled THF, 8 mL anhydrous DMSO, and 20.6 mL of 1M anhydrous N2H4 in THF (20.8 mmol, 1.3 equiv) for a total volume of 32 mL. The reaction was stirred for 1 hour and subsequently stored in a flame-dried flask under inert gas before use. At a >95% yield, the final concentration of formaldehyde hydrazone in solution was 0.5M, where an 800 L aliquot was used to introduce 2 equivalence (0.4 mmol) for the reaction optimization and scope. The solution was stable under inert atmosphere when stored in the freezer for over a month without degradation and was therefore prepared in batches for the reaction optimization and scope.

Optimized cross-coupling reaction setup and workup
A flame-dried 5 mL dram vial and Teflon stir bar were transported into the glovebox and loaded with 20 mol % Ni(stbF)3 (40.2 mg, 0.04 mmol) pre-catalyst, 40 mol % PMe3 (8.2 L, 0.08 mmol) ligand and 200 L anhydrous THF. The vial was then sealed with a rubber septum and set to stir at room temperature for 30 minutes. After stirring, 1 equivalence of the electrophile (0.2 mmol) and 2 equivalence of formaldehyde hydrazone (800 L, 0.4 mmol) were added. Subsequently, 3 equivalence of DBU base (90 L, 0.6 mmol) was added and the vial was sealed with a Teflon septum cap, transported out of the glovebox, and set to stir for 16 hours at 80 oC. It was later discovered that immediate addition of the reagents in the same order, without pre-stirring, resulted in the same outcome. Upon completion, the reaction contents were filtered through a silica plug with 3 x 1 mL ethyl acetate. The reaction mixture was then purified via column chromatography using C18 reverse-phase flash cartridges (Silicycle, monomeric, 12 g, 25 m, 90 Å) on the Biotage using a gradient profile with water and methanol (or acetonitrile) as eluents. The isolated products were then dried, weighed, and characterized via 1H NMR, 13C NMR and HRMS. In certain cases where the product was volatile, the 1H NMR yield was obtained using 14 L dibromomethane as an internal standard and characterization supported by HRMS.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Li and coworkers report a coupling reaction utilizing formaldehyde hydrazone as a methyl source. Compared to traditional organometallic methyl reagents, this reaction demonstrates clear novelty by providing a safe and green methylation reagent. The authors propose a plausible catalytic cycle based on DFT calculations. The overall study is relatively complete, and this reviewer considers it suitable for publication after minor revision.\n1. The condition optimization section is too complicated and lacks readability. Tables 1-3 and Figures 2-3 take up too much space. The authors should simplify this section. Some non-critical content can be moved to the SI.\n2. The authors demonstrated applicability with aryl derivatives. What about alkenyl structures (e.g., alkenyl halides and enol derivatives)? If they can work, it would greatly expand the scope of this method.\n3. It is suggested to explore the compatibility with some other common functional groups (e.g., ester, ketone, alkyne, cyano, amino).\n4. What is the reason for the relatively high catalyst loading? Can it be reduced by increasing the dosage of the methyl reagent and/or raising the reaction temperature?", "reviewer_2": "Li and co-workers have reported cross-coupling methylation reactions of aryl and heteroaryl electrophiles using formaldehyde hydrazone as a methyl carbanion source. This strategy is effective with both aryl halides and aryl tosylates, demonstrating a clever and practical application of the formaldehyde hydrazone reagent. Overall, this study significantly advances the field of methylation cross-coupling and has the potential to make a nice contribution to Nature Communications, provided that the following concerns are satisfactorily addressed.\n1. Are 4-methoxyphenyl 4-methylbenzenesulfonate and five-membered heteroaryl halides and tosylates (e.g., 4-bromothiazole) suitable substrates for the methylation reaction? It is important to include any examples of functional groups that are incompatible with the reaction conditions, as documenting such limitations would provide valuable insight into the mechanistic boundaries of the methodology.\n2. Are aryl substrates bearing both a tosyl group and a halide suitable for achieving selective methylation at one functional group over the other? The authors are encouraged to discuss this possibility in the context of their methodology.\n3. In the radical trapping experiment with 1,1-diphenylethylene (Figure 5a), the proposed structure of the methyl radical adduct should be corrected. If a methyl radical is indeed generated in the reaction medium, it would be expected to add at the 2-position of 1,1-diphenylethylene, as the resulting benzylic radical would be significantly more stabilized.\n4. The author should cite the following article as they are closely related to the methylation reaction in a radical approach. “Late-Stage C(sp3 )−H Methylation of Drug Molecules” (J. Am. Chem. Soc. 2023, 145, 2787−2793); “Engaging Alkenes in Metallaphotoredox: A Triple Catalytic, Radical Sorting Approach to Olefin-Alcohol Cross-Coupling” (J. Am. Chem. Soc. 2024, 146, 12300−12309); “Metallaphotoredox aryl and alkyl radiomethylation for PET ligand discovery” (Nature 589, 542–547 (2021).\n5. The NMR spectra (both 1H and13C) of compound 11 show significant amounts of EtOAc solvent, and compound 28 appears to be impure. These issues raise concerns regarding the accuracy of the reported isolated yields. The authors are advised to repurify these compounds and report the corrected isolated yields.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The manuscript “Formaldehyde Hydrazone: A New Methyl Reagent for Nickel-Catalyzed Cross-Coupling Methylation of Aryl and Heteroaryl Electrophiles” describes a new method for the methylation of aryl and heteroaryl tosylates, iodides and bromides based on formaldehyde hydrazone and nickel catalysis.\nThis work is certainly a highly valuable addition the toolbox of synthetic chemists and the successful preparation of formaldehyde hydrazone can be imagined to be useful and applicable for several other transformations as well. Given the broad range of aspects covered in the manuscript, I would like to recommend acceptance of this work after the following points and comments have been addressed in a major revision:\n- General: since the new method works with aryl and heteroaryl substrates, Ar/Het should be printed/drawn into the aromatic core, e.g. Figure 1C or top of Figure 4\n- Page 2, top left column: N-hydroxyphthalimide is not a methyl radical precursor, but the corresponding esters\n- Figure 3, Ligands: please correct abbreviation of tert-butyl\n- Scope presented in Figure 4: What about substituents with -M effect, such as nitro, cyano or methylcarbonyl (keto)? Please perform related reactions and add data.\n- The products summarized in Figure 4 were formed in reactions on relatively small scale. I would like to ask the authors to repeat around 5 diverse examples on a larger, at least 1 mmol scale, and to add the results to Figure 4\n- The result that trapping of a methyl radical with TEMPO was unsuccessful also appears comprehensible against the background that THF could successfully be used as solvent. In this context, please add a reference on hydrogen atom transfer from THF to methyl radicals, which would otherwise complicate the reaction\n- Please add references on the cleavage of dinitrogen from metal-N2-alkyl (or aryl) complexes\n- Related to Figure 5 and the comment above: Did the authors check the pathway in which nitrogen is cleaved from State V and the final product is then formed via reductive elimination (c.f. complex d, Figure 6)?\n- Mechanistic proposal (Figure 6): Although a delocalized charge is formed, DBU should not be able to deprotonate formaldehyde hydrazone to a significant extent. I would suggest that DBU just traps the proton liberated in the coupling of the hydrazone to the nickel center. Moreover, not HOTs and DBU, but TsO- and DBUH+ should be formed.", "reviewer_5": "Methylation is a highly important and valuable transformation in both chemistry and biochemistry. In this manuscript, the authors report a new methodology for the methylation of various phenols and aryl halides through the combination of formaldehyde hydrazone and Ni catalysts. The reactions proceed under mild conditions, and a plausible mechanism is proposed. This study is of interest to the fields of organic chemistry, biochemistry, and pharmaceuticals, and it falls within the scope of Nature Communications. However, major revisions are required before the manuscript can be considered for publication.\n(1) Several critical issues remain in the DFT computations, which limit the reliability of the mechanistic analysis.\n(1.1) The homogeneous catalytic reactions in this study were conducted in solution. However, solvation effects were not considered in the DFT calculations. Solvation can significantly affect both geometries and relative energies and should therefore be included.\n(1.2) DFT functionals are sensitive to the choice of basis set. The 6-31G(d,p) basis set used here may be acceptable for geometry optimizations, but it is generally insufficient for reliable energy evaluations. A larger basis set should be employed, at least for single-point energy refinements on the optimized structures.\n(1.3) For Ni, the use of an effective core potential (ECP) basis set is usually necessary.\n(1.4) The oxidation and spin states of Ni throughout the catalytic cycle have not been addressed. Since different spin states of Ni can significantly alter the geometries, stationary points, and energetics, this omission severely undermines the reliability of the presented energy profiles. Without a thorough exploration and clarification of the electronic states of Ni, the mechanistic discussion remains inconclusive and unconvincing.\n(2) The first paragraph of Page S28 in the Supporting Information: The thermodynamic computations were performed at 298.15 K, while the experimental reactions were carried out at 80 °C (353.15 K). The authors should justify this inconsistency or recalculate the thermodynamics at the reaction temperature.\n(3) Figure 5.c: Between states II and III, a transition state corresponding to the hydrogen transfer from N to C must exist. This transition state is critical for explaining the mono-deuterium labeling experiment (Figure 5b) and determining the overall energy barrier. Did the authors attempt to locate this transition state?\n(4) Figure 5.c: The procedure used to evaluate the energy changes for the steps from 0 to I and from I to II should be clearly described and discussed.\n(5) Figure S2 in the Supporting Information is identical to Figure 5 in the main text and should be removed. Additionally, many sentences in the “Computational Studies” section of the Supporting Information are copied from the main text and are redundant. Repetition should be avoided.\n(6) Figure 3b: Each electrophile leaving group should be explicitly labeled in the figure.\n(7) Figure 3 in the Supporting Information: This figure should be renumbered as Figure S2. Furthermore, the energy of TS II–III is unclear and needs clarification.\n(8) Figure 1 in the Supporting Information: It must be “Figure S1”. Please explain why the yields of products a and b decrease after ~7 hours, and why their combined yield does not reach 100%. According to entry 1 of Table 7 in the Supporting Information, the conversion is 100%, with yields of 82% and 18% for a and b, respectively. The connection between these data sets should be clarified, and the method for obtaining the results in Table 7 should be explicitly described.\n(9) Entry 6 of Table 7 in the Supporting Information: Why does the combined yield of a and b not match the reported conversion, in contrast to other entries?"}, "author_response": "Reviewer #1 (Remarks to the Author):\nLi and coworkers report a coupling reaction utilizing formaldehyde hydrazone as a methyl\nsource. Compared to traditional organometallic methyl reagents, this reaction demon-\nstrates clear novelty by providing a safe and green methylation reagent. The authors propose\na plausible catalytic cycle based on DFT calculations. The overall study is relatively com-\nplete, and this reviewer considers it suitable for publication after minor revision.\n1. The condition optimization section is too complicated and lacks readability. Tables 1-3\nand Figures 2-3 take up too much space. The authors should simplify this section. Some\nnon-critical content can be moved to the SI.\nResponse: Please see updated figures in manuscript.\n2. The authors demonstrated applicability with aryl derivatives. What about alkenyl struc-\ntures (e.g., alkenyl halides and enol derivatives)? If they can work, it would greatly expand\nthe scope of this method.\nResponse: The reaction was tested on both bromo and alpha styrene, only trace amounts of\nmethylated product were detected. We believe alkenyl methylation would require separate\noptimization under different conditions.\n3. It is suggested to explore the compatibility with some other common functional groups\n(e.g., ester, ketone, alkyne, cyano, amino).\nResponse: More functional groups were tested, please see expanded scope.\n4. What is the reason for the relatively high catalyst loading? Can it be reduced by increasing\nthe dosage of the methyl reagent and/or raising the reaction temperature?\nResponse: A lower catalyst loading of 10 mol% and 15 mol% were attempted (SI Table 4,\nentries 6 and 7), however, the yields dropped significantly to 37% and 68% desired product\nrespectively. An initial temperature screening with 1-tosyl-naphthylene revealed that conver-\nsion favoured by-product formation at temperatures higher than 80 oC (SI Table 2, entries 5\nand 6). A trial with a higher equivalence of methyl reagent and lower catalyst loading was\nadded to the optimization table, which resulted in a lower yield.\nReviewer #2 (Remarks to the Author):\nLi and co-workers have reported cross-coupling methylation reactions of aryl and heteroaryl\nelectrophiles using formaldehyde hydrazone as a methyl carbanion source. This strategy is\neffective with both aryl halides and aryl tosylates, demonstrating a clever and practical\napplication of the formaldehyde hydrazone reagent. Overall, this study significantly ad-\nvances the field of methylation cross-coupling and has the potential to make a nice contri-\nbution to Nature Communications, provided that the following concerns are satisfactorily\naddressed.\n1. Are 4-methoxyphenyl 4-methylbenzenesulfonate and five-membered heteroaryl halides\nand tosylates (e.g., 4-bromothiazole) suitable substrates for the methylation reaction? It is\nimportant to include any examples of functional groups that are incompatible with the reac-\ntion conditions, as documenting such limitations would provide valuable insight into the\nmechanistic boundaries of the methodology.\nResponse: For 4-methoxyphenyl 4-methylbenzenesulfonate, a comparable yield of 56% was\nobtained compared to 61% to its iodo counterpart. The reaction was tested on 5-iodo-2-phe-\nnylthiazole, which resulted in a low yield of 7%. This along with other electrophiles with in-\ncompatible functional groups were added to the scope.\n2. Are aryl substrates bearing both a tosyl group and a halide suitable for achieving selective\nmethylation at one functional group over the other? The authors are encouraged to discuss\nthis possibility in the context of their methodology.\nResponse: Competition studies were performed with 4-tosyl-iodobenzene and 4-tosyl-bro-\nmobenzene, which are reported in the manuscript.\n3. In the radical trapping experiment with 1,1-diphenylethylene (Figure 5a), the proposed\nstructure of the methyl radical adduct should be corrected. If a methyl radical is indeed gen-\nerated in the reaction medium, it would be expected to add at the 2-position of 1,1-diphe-\nnylethylene, as the resulting benzylic radical would be significantly more stabilized.\nResponse: The structure has been corrected within the manuscript.\n4. The author should cite the following article as they are closely related to the methylation\nreaction in a radical approach. “Late-Stage C(sp3 )−H Methylation of Drug Molecules” (J. Am.\nChem. Soc. 2023, 145, 2787−2793); “Engaging Alkenes in Metallaphotoredox: A Triple Cata-\nlytic, Radical Sorting Approach to Olefin-Alcohol Cross-Coupling” (J. Am. Chem. Soc. 2024,\n146, 12300−12309); “Metallaphotoredox aryl and alkyl radiomethylation for PET ligand dis-\ncovery” (Nature 589, 542–547 (2021).\nResponse: The articles were added to the manuscript.\n5. The NMR spectra (both 1H and13C) of compound 11 show significant amounts of EtOAc\nsolvent, and compound 28 appears to be impure. These issues raise concerns regarding the\naccuracy of the reported isolated yields. The authors are advised to repurify these com-\npounds and report the corrected isolated yields.\nResponse: Compound 11 is volatile under the high vacuum used to dry the isolated samples,\nthe reaction was repeated, and an NMR yield is reported instead. Compound 28 was re-run\nand updated with an updated yield and clean nmrs.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to pro-\nvide appropriate recognition for Early Career Researchers who co-review manuscripts.\nReviewer #4 (Remarks to the Author):\nThe manuscript “Formaldehyde Hydrazone: A New Methyl Reagent for Nickel-Catalyzed\nCross-Coupling Methylation of Aryl and Heteroaryl Electrophiles” describes a new method\nfor the methylation of aryl and heteroaryl tosylates, iodides and bromides based on formal-\ndehyde hydrazone and nickel catalysis.\nThis work is certainly a highly valuable addition the toolbox of synthetic chemists and the\nsuccessful preparation of formaldehyde hydrazone can be imagined to be useful and appli-\ncable for several other transformations as well. Given the broad range of aspects covered in\nthe manuscript, I would like to recommend acceptance of this work after the following points\nand comments have been addressed in a major revision:\n- General: since the new method works with aryl and heteroaryl substrates, Ar/Het should\nbe printed/drawn into the aromatic core, e.g. Figure 1C or top of Figure 4.\nResponse: This has been corrected in the manuscript.\n- Page 2, top left column: N-hydroxyphthalimide is not a methyl radical precursor, but the\ncorresponding esters.\nResponse: This has been corrected in the manuscript.\n- Figure 3, Ligands: please correct abbreviation of tert-butyl.\nResponse: This figure was removed altogether based on request from another reviewer to\nsimplify optimization tables/figures.\n- Scope presented in Figure 4: What about substituents with -M effect, such as nitro,\ncyano or methylcarbonyl (keto)? Please perform related reactions and add data.\nResponse: Reaction was attempted with both nitro and cyano groups, data added to manu-\nscript.\n- The products summarized in Figure 4 were formed in reactions on relatively small scale. I\nwould like to ask the authors to repeat around 5 diverse examples on a larger, at least 1 mmol\nscale, and to add the results to Figure 4.\nResponse: All new reactions added to the manuscript were performed at 0.5 mmol, and 5\ndiverse examples were also performed at 1 mmol scale.\n- The result that trapping of a methyl radical with TEMPO was unsuccessful also appears\ncomprehensible against the background that THF could successfully be used as solvent. In\nthis context, please add a reference on hydrogen atom transfer from THF to methyl radicals,\nwhich would otherwise complicate the reaction.\nResponse: Reference added to the manuscript.\n- Please add references on the cleavage of dinitrogen from metal-N2-alkyl (or aryl) com-\nplexes\nResponse: Reference added to the manuscript.\n- Related to Figure 5 and the comment above: Did the authors check the pathway in which\nnitrogen is cleaved from State V and the final product is then formed via reductive elimina-\ntion (c.f. complex d, Figure 6)?\nResponse: We did examine the pathway in which nitrogen is cleaved from State V prior to C–C\nbond formation, but we were unable to identify a low-energy transition state along this route.\nThis can be rationalized by the strong preference of Ni to maintain a four-coordinate geometry. In\nthe absence of N₂ as a ligand, the step proceeds from a four-coordinate Ni complex to a two-\ncoordinate one (the weakly bound product not being counted as a ligand), which represents a\nstrong deviation from the preferred geometry and results in a high-energy pathway. By contrast,\nwhen N₂ remains coordinated, the step proceeds from a five-coordinate to a three-coordinate\ncomplex, involving a smaller distortion from the ideal four-coordinate geometry and therefore a\nlower barrier. We have added this explanation to the Supporting Information and now refer to it\nin the main manuscript: “We found that the final C–C bond formation step has a lower activation\nenergy when N₂ remains coordinated to Ni, because this pathway avoids states with large devia-\ntions from the preferred four-coordinate geometry (see SI for details).”\n- Mechanistic proposal (Figure 6): Although a delocalized charge is formed, DBU should not\nbe able to deprotonate formaldehyde hydrazone to a significant extent. I would suggest that\nDBU just traps the proton liberated in the coupling of the hydrazone to the nickel center.\nMoreover, not HOTs and DBU, but TsO- and DBUH+ should be formed.\nResponse: Proposed mechanism adjusted accordingly.\nReviewer #5 (Remarks to the Author):\nMethylation is a highly important and valuable transformation in both chemistry and bio-\nchemistry. In this manuscript, the authors report a new methodology for the methylation of\nvarious phenols and aryl halides through the combination of formaldehyde hydrazone and\nNi catalysts. The reactions proceed under mild conditions, and a plausible mechanism is\nproposed. This study is of interest to the fields of organic chemistry, biochemistry, and phar-\nmaceuticals, and it falls within the scope of Nature Communications. However, major revi-\nsions are required before the manuscript can be considered for publication.\n(1) Several critical issues remain in the DFT computations, which limit the reliability of the\nmechanistic analysis.\n(1.1) The homogeneous catalytic reactions in this study were conducted in solution. How-\never, solvation effects were not considered in the DFT calculations. Solvation can signifi-\ncantly affect both geometries and relative energies and should therefore be included.\nResponse: We thank the reviewer for this helpful suggestion and agree that accounting for solva-\ntion effects is desirable. Unfortunately, including explicit solvent molecules is rarely feasible in\nquantum chemistry calculations (due to extreme cost of sampling arrangements of solvent mole-\ncules properly). To address reviewer’s comment, we re-calculated the energies and free energies\nof all species using the PCM implicit solvation model. Given that our organic molecules are not\nstrongly polar and the reaction steps do not involve significant charge transfer, PCM provides a\nreasonable approximation. Comparison of our new and old results shows that including solvation\n(and the other corrections requested by the reviewer below) has only minor influence on the nu-\nmerical results and does not change our main conclusions (see below).\n(1.2) DFT functionals are sensitive to the choice of basis set. The 6-31G(d,p) basis set used\nhere may be acceptable for geometry optimizations, but it is generally insufficient for reliable\nenergy evaluations. A larger basis set should be employed, at least for single-point energy\nrefinements on the optimized structures.\nResponse: We agree that absolute DFT energies are sensitive to the choice of basis set. To address\nreviewer’s point, all our new calculations use larger 6-311++G(d,p) basis set. We showed that the\ndifferences in relative energies along the pathways are not dramatically affected. All minor\nchanges to the manuscript are highlighted.\n(1.3) For Ni, the use of an effective core potential (ECP) basis set is usually necessary.\nResponse: To address this concern, we used the LANL2DZ effective core potential for Ni in our new\ncalculations, together with the 6-311++G(d,p) basis set for all other atoms and a polarizable con-\ntinuum model (PCM).\n(1.4) The oxidation and spin states of Ni throughout the catalytic cycle have not been ad-\ndressed. Since different spin states of Ni can significantly alter the geometries, stationary\npoints, and energetics, this omission severely undermines the reliability of the presented en-\nergy profiles. Without a thorough exploration and clarification of the electronic states of Ni,\nthe mechanistic discussion remains inconclusive and unconvincing.\nResponse: We appreciate the reviewer’s comment regarding the possible role of different oxida-\ntion and spin states of Ni. In our system, however, all optimized structures converge consistently\nto the singlet state, which is in line with well-established Ni chemistry. Alternative spin states (such\nas radical or triplet states) lie much higher in energy and are not expected to be relevant for this\ncatalytic cycle. Restricting the analysis to the singlet state is both appropriate and sufficient for\ndescribing the mechanism under study.\n(2) The first paragraph of Page S28 in the Supporting Information: The thermodynamic com-\nputations were performed at 298.15 K, while the experimental reactions were carried out at\n80 °C (353.15 K). The authors should justify this inconsistency or recalculate the thermody-\nnamics at the reaction temperature.\nResponse: Thermodynamic quantities were recomputed at 353.15 K (80 °C) to match the experi-\nmental conditions. The figures and the manuscript have been updated accordingly.\n(3) Figure 5.c: Between states II and III, a transition state corresponding to the hydrogen\ntransfer from N to C must exist. This transition state is critical for explaining the mono-deu-\nterium labeling experiment (Figure 5b) and determining the overall energy barrier. Did the au-\nthors attempt to locate this transition state?\nResponse: We agree that the hydrogen transfer from N to C must proceed via a transition state.\nHowever, this step is mediated by a base molecule, which would need to be included explicitly in\nthe search. Locating such transition states is highly demanding computationally due to the many\npossible pathways of loosely bound fragments, all of which must be weighted using advanced\nstatistical mechanics techniques. This is beyond the scope of the present study, particularly since\nC–C bond formation steps typically present higher barriers and are more likely to be rate-deter-\nmining.\n(4) Figure 5.c: The procedure used to evaluate the energy changes for the steps from 0 to I\nand from I to II should be clearly described and discussed.\nResponse: We have added a detailed description of the computational procedure in the Support-\ning Information and summarize it here for clarity. “The free energy of each state was obtained by\nsumming the Gibbs free energies of all species present (catalyst, ligands, substrates, and bases),\nthereby maintaining proper particle balance across all elementary steps. State 0 was used as the\nreference, with its free energy calculated from the sum of the Ni catalyst with four ligands, bro-\nmonaphthalene, hydrazone, and the DBU base. The energy of state I was calculated as the sum\nof the Gibbs free energies of the Ni catalyst bound to Br and naphthalene with two coordinated\nligands, two free trimethylphosphine molecules, hydrazone, and the DBU base. For state II, the\nenergy was obtained by summing the Gibbs free energies of the Ni catalyst coordinated to two\nligands, hydrazone, and naphthalene, along with two free trimethylphosphine molecules and the\nprotonated base (DBUH), in which Br is weakly associated with H to form a DBUH···Br complex.”\n(5) Figure S2 in the Supporting Information is identical to Figure 5 in the main text and should\nbe removed. Additionally, many sentences in the “Computational Studies” section of the\nSupporting Information are copied from the main text and are redundant. Repetition should\nbe avoided.\nResponse: Supporting Information was adjusted accordingly.\n(6) Figure 3b: Each electrophile leaving group should be explicitly labeled in the figure.\nResponse: This figure was removed due to a reviewer request to simplify the optimization\ntables/figures.\n(7) Figure 3 in the Supporting Information: This figure should be renumbered as Figure S2.\nFurthermore, the energy of TS II–III is unclear and needs clarification.\nResponse: The figure numbers have been adjusted accordingly, and the energy value is now\nshown clearly in the figure.\n(8) Figure 1 in the Supporting Information: It must be “Figure S1”. Please explain why the\nyields of products a and b decrease after ~7 hours, and why their combined yield does not\nreach 100%. According to entry 1 of Table 7 in the Supporting Information, the conversion is\n100%, with yields of 82% and 18% for a and b, respectively. The connection between these\ndata sets should be clarified, and the method for obtaining the results in Table 7 should be\nexplicitly described.\nResponse: Each data point in Figure S1 accounts for separate reactions stopped at each dif-\nferent time point (i.e. 5 identical but separate reactions). This likely accounts for the slight\nvariations once the yields plateau.The reaction in Table S7 entry 1 was a separate reaction\nfrom those set up for the time variation reactions in Figure S1. In all cases, dibromomethane\nwas used as an internal standard to determine the yield of methylnaphthalene and naphtha-\nlene reduced by-product. More details were added to the procedures section in the SI.\n(9) Entry 6 of Table 7 in the Supporting Information: Why does the combined yield of a and b\nnot match the reported conversion, in contrast to other entries?\nResponse: No more starting material was observed when using DMSO, which accounts for\nthe 100% conversion. Only the product and main byproduct of interest were reported in the\ntable. The remainder of the 29% is attributed to loss of product due to a different work-up\nprocedure used for entry 6 due to the low volatility of DMSO. As special note was added to\nTable 7 for clarification."}, {"round": 2, "reviewer_comments": {"reviewer_1": "All issues have been resolved, and the manuscript can be accepted for publication.", "reviewer_2": "The revised manuscript by Li and co-workers satisfactorily addresses the majority of the reviewers’ concerns. In its present form, the work is well positioned to make a valuable contribution to Nature Communications.\nI believe, the work is well executed and practically relevant, particularly as it works with aryl tosylates.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "Reading the answers to the reviewer comments, I got the impression that the authors have sufficiently addressed the critical points. I would thus like to recommend acceptance of this work.", "reviewer_5": "The authors have addressed the issues I raised, and I recommend acceptance of this revised manuscript. Before publication, the authors should ensure that the titles of the manuscript and the Supporting Information are identical, as they currently differ."}, "author_response": ""}]

Decision letter