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Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

29319
10.1038/s41467-022-29319-z
Article
Kinetic resolution of cyclic benzylic azides enabled by site- and enantioselective C(sp3)–H oxidation
Ye Pengbo
Feng Aili
Wang Lin
Cao Min
Zhu Rongxiu
http://orcid.org/0000-0002-0839-373X
Liu Lei kimberlyibarra@example.com

grid.27255.37 0000 0004 1761 1174 School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100 China
25 3 2022
25 3 2022
2022
13 162126 9 2021
21 2 2022
© The Author(s) 2022
https://creativecommons.org/licenses/by/4.0/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
Catalytic nonenzymatic kinetic resolution (KR) of racemates remains one of the most powerful tools to prepare enantiopure compounds, which dominantly relies on the manipulation of reactive functional groups. Moreover, catalytic KR of organic azides represents a formidable challenge due to the small size and instability of the azido group. Here, an effective KR of cyclic benzylic azides through site- and enantioselective C(sp3)–H oxidation is described. The manganese catalyzed oxidative KR reaction exhibits good functional group tolerance, and is applicable to a range of tetrahydroquinoline- and indoline-based organic azides with excellent site- and enantio-discrimination. Computational studies elucidate that the effective chiral recognition is derived from hydrogen bonding interaction between substrate and catalyst.

Catalytic kinetic resolution of racemates is among the best ways to prepare enantiopure compounds, but this process is difficult with organic azides given the small size of the azido group. Here, the authors show kinetic resolution of cyclic benzylic azides through site- and enantioselective C(sp3)–H oxidation.

Subject terms

Asymmetric catalysis
Asymmetric synthesis
Synthetic chemistry methodology
https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 92156008 22161142016 21971148 Liu Lei issue-copyright-statement© The Author(s) 2022
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pmcIntroduction

Catalytic nonenzymatic kinetic resolution (KR) of racemates is one of the most powerful and practical tools to prepare valuable enantiopure targets, especially in cases where other methods are not possible or provide insufficient enantiocontrol1–5. Chiral organic azides are versatile synthetic precursors for a range of nitrogen-containing molecules and have found dramatically expanded utility in medicine, biology, and material science6–15. However, catalytic KR to provide optically pure azides has remained elusive, principally due to two essential features of the azido moiety: (1) the instability hampering the design of new reactivity with excellent chemoselectivity; (2) the small size hampering the achievement of effective chiral recognition16–21. Existing isolated examples always focused on manipulating the azido moiety through azide–alkyne cycloaddition (AAC)22–26 (Fig. 1A) or extra reactive functional groups preinstalled in substrates (Fig. 1B)27,28, which typically suffer from the use of excess azide substrates, poor chiral recognition, and narrow substrate scope. Developing an effective KR of organic azides relying on the reactivity of C(sp3)–H bonds would be highly desired29.Fig. 1 Overview of KR methods to prepare chiral organic azides.

A KR of benzylic azides through azide–alkyne cycloaddition. B KR of allylic azides through asymmetric dihydroxylation of alkene. C KR of cyclic benzylic azides through asymmetric C(sp3)−H oxidation.

Nonenzymatic site- and enantioselective oxidation of ubiquitous C(sp3)−H bonds with a general scope and predictable selectivity represents a paradigm shift in the standard logic of organic synthesis30,31. However, such research topic has remained a formidable challenge, and current studies typically suffer from moderate enantioselectivity, low substrate conversion, and narrow substrate scope32–44. In particular, catalytic KR through C(sp3)–H oxidation dominantly focused on secondary alcohols45–52 and amines53,54 due to their high and well-known oxidized reactivity together with the presence of a strong interaction site with catalyst for efficient chiral recognition55,56. To our knowledge, selective oxidation of C(sp3)−H bond adjacent to azido moiety remains elusive. Moreover, organic azide lacks such an effective interaction site to direct substrate to an ideal location in the transition state. Therefore, chiral recognition of chemically similar C(sp3)−H bonds adjacent to azido group of two enantiomers would be difficult to accomplish.

Herein, we report the KR of organic azides through site- and enantioselective C(sp3)–H bond oxidation (Fig. 1C). First, given the significance of benzo-fused nitrogen-containing heterocycles in modern pharmacology, we choose a range of racemic benzylic azides bearing such skeletons as substrates. Second, we select the readily modifiable salen as the basal ligand to search for suitable base-metal catalyst. Third, varying the protecting group on the nitrogen moiety might also provide an opportunity to tune the chiral recognition. Based on these considerations, a range (36 examples) of cyclic benzylic azides participate in oxidative KR with good to excellent selectivity factors (s up to 95).

Results and discussion

Reaction condition optimization

The oxidative KR of tetrahydroquinoline (THQ) based organic azide rac-1a was selected as the model reaction for optimization (Table 1). In the presence of PhIO as the oxidant, no reaction was observed for either chiral Fe(salen) C1 or Co(salen) C2 (entry 1). Chiral Mn(salen) C3 exhibited good oxidation catalysis reactivity, though poor chiral recognition was obtained (entry 2). Oxidation proceeded with excellent site selectivity at the C4–H bond adjacent to azido moiety over C2–H bond α to amide motif, affording azine 2a as oxidized product. Mn(salen) C4 having cyclohexanediamine skeleton provided better results than C3 with 1,2-diphenyl-1,2-ethanediamine (entry 3). Careful examination of the substituent effects on the basal salen ligand revealed C12 with 2,4-difluorophenyl moieties at C3(3') sites to be optimal (entries 4–10). Other oxidants such as H2O2 and NaClO afforded inferior oxidation reactivity (entry 11). Addition of PhIO as eight equal portions in 30 min intervals was beneficial for achieving an extremely high level of chiral discrimination, and (S)-1a was isolated in 48% yield with 98% ee (s = 91, entries 12 and 13).Table 1 Reaction condition optimizationa.

	
Entry	Catalyst	Conv. (%)b	ee (%)c	sd	
1	C1 or C2	<5	n.d.	n.d.	
2	C3	60	25	1.7	
3	C4	57	30	2.1	
4	C5	56	36	2.5	
5	C6	60	39	2.4	
6	C7	55	45	3.3	
7	C8	50	34	2.8	
8	C9	49	50	5.1	
9	C10	52	70	9.5	
10	C11	49	79	25	
11	C12	50	86	37	
12e	C12	<20	n.d.	n.d.	
13f	C12	52	94	50	
14g	C12	52	98	91	
	
n.d. not determined.

aReaction condition: to rac-1a (0.1 mmol) and catalyst (5 mol%) in EtOAc (1.0 ml) at rt was added PhIO (0.08 mmol) as two portions in 2 h intervals, unless otherwise noted.

bConversion was calculated from yield of recovered 1a.

cDetermined by chiral HPLC analysis.

dSelectivity (s) values were calculated through the equation s = ln[(1 − C)(1 − ee)]/ln[(1 − C)(1 + ee)], where C is the conversion.

eNaClO or 30% aqueous H2O2 as oxidant.

fPhIO was added as four portions in 1 h intervals over 3 h.

gPhIO was added as eight portions in 0.5 h intervals over 3.5 h.

Substrate scope

The scope of oxidative KR of THQ-based organic azides was explored (Fig. 2). In general, both electron-rich and -deficient THQ skeletons were well tolerated, as demonstrated by effective access to optically pure 1a-1m with good to excellent selectivity factors (s = 32–91). Resolution efficiency was not impaired for reaction on a 0.5 mmol scale. Common functional groups, including halide, acetate, silyl ether, benzoate, and triflate, were tolerated for further manipulation. Racemic THQ-based organic azides bearing two stereocenters were also suitable components with good enantio-discrimination. Oxidative KR of cis-2,4-disubstituted rac-1n proceeded, furnishing (2R,4S)-1n in 47% yield with 92% ee (The absolute configuration of recovered 1n was determined by X-ray diffraction analysis. See the Supporting Information for details). The reaction was not sensitive to the steric hindrance of C2-substituents, as demonstrated by access to respective enantiopure 1o-1u with good selectivity factors (s = 20–35).Fig. 2 Kinetic resolution of THQ-based organic azides.

Conditions: rac-1 (0.1 mmol), PhIO (0.08 mmol, addition as 8 portions in 30-min intervals over 3.5 h), and C12 (5 mol%) in ethyl acetate (1.0 ml) at rt for 4 h. aReaction with 0.5 mmol rac-1. bReaction with 1.0 gram of rac-1.

Indoline represents the other type of biologically important benzo-fused nitrogen-containing heterocycle. Accordingly, the applicability of the oxidative KR strategy in enantioselective access to indoline-based azides was next explored (Fig. 3). When Mn(salen) C7 was used as catalyst, oxidative KR of cyclic benzylic azide rac-3a proceeded, furnishing 2-aminobenzonitrile 4a as oxidized product together with recovered (S)-3a in 48% yield with 96% ee (s = 65) (The absolute configuration of recovered 3a was determined by X-ray diffraction analysis. See the Supporting Information for details). Notably, the reaction exhibited excellent site selectivity at the C3–H bond adjacent to azido moiety over C2–H bond α to amide motif. Indolines rac-3b-3h bearing electronically varied groups around the arene moiety were tolerated with a high level of chiral recognition (s = 36–95). Selectivities were not impaired for reaction on a 0.5 mmol scale. Substrates bearing other N-acyl groups, such as propanoyl (3i) and butyryl (3j), were also tolerated, though no reaction was observed for pivaloyl one (3k).Fig. 3 Kinetic resolution of indoline-based organic azides.

Conditions: rac-3 (0.1 mmol), PhIO (0.08 mmol, addition as 8 portions in 30-min intervals over 3.5 h), and C7 (5 mol%) in ethyl acetate (1.0 ml) at rt for 4 h. aReaction with 0.5 mmol rac-3.

Synthetic applications

Manipulating the azido moiety through copper-catalyzed AAC would allow facile integration of the other biologically important molecules into N-heterocycles for drug discovery. For example, vitamin E derivative (5) with potent antioxidation activity and estrone derivative (7) for treating abnormalities associated with menopause are efficiently installed into THQ skeleton using triazole as a linker, respectively (Fig. 4).Fig. 4 Synthetic applications.

A CuSO4-catalyzed AAC of optically pure THQ-based organic azide and vitamin E derivative. B CuI-catalyzed AAC of optically pure THQ-based organic azide and estrone derivative.

Mechanistic and DFT studies

Control experiments were conducted to get a preliminary understanding of the reaction mechanism (Fig. 5). The relationship between ee values of Mn(salen) catalyst and recovered substrate was explored, showing that the enantioselectivity of recovered 3a is proportional to the ee of C7 (Fig. 5A). The absence of nonlinear effects indicated that the reaction might not involve heterochiral aglomerates57,58. No reaction was observed for stoichiometric Mn(salen) C12 mediated reaction in the absence of PhIO, suggesting that oxoMn(V) might be the species in charge of C–H oxidation (Fig. 5B). A competition deuterium kinetic isotope effect (KIE) study, using a mixture of rac-1a and [D]-rac-1a, revealed a KIE of 2.7 (Fig. 5C). The observation implied that C−H bond cleavage might be involved in the rate-determining step. The substituent effect of different acyl groups on THQ-based azides was explored (Fig. 5D). Several aspects of the data merit further comment. Firstly, no reaction was observed for N-acyl substituted 9a. Secondly, the oxidized reactivity was gradually enhanced as the increasing numbers of methyl groups at α-position of the carbonyl moiety (9a-9c and 1a). Thirdly, the oxidative reactivity was lost when placing an oxygen atom between the carbonyl and tBu groups. In general, more sterically hindered substrates should exhibit lower reactivity than that of less sterically hindered ones. We speculated that the opposite trend observed for THQ-based substrates in Fig. 5D might originate from the non-covalent interaction between the α-alkyl group of carbonyl moiety and Mn(salen) catalyst. The deuteration effect of the N-acyl moiety of indoline-based substrates was next evaluated (Fig. 5E). No oxidative conversion was observed for [D]-3a, indicating that the sp3 C–H bond at α-position of carbonyl motif is crucial to the reactivity of substrate 3a.Fig. 5 Control experiments.

A Plot of enantiomeric excess of recovered 3a versus the enantiomeric excess of C7 at 20% conversion. The dotted line symbolized the linear correlation. B Stoichiometric Mn(salen) C12 mediated control experiment in the absence of PhIO. C The intermolecular kinetic isotope effect. D The N-acyl substituent effect for THQ-based azides. E Deuterated control experiment of indoline-based azide.

According to the generally accepted mechanism of manganese-catalyzed C(sp3)−H oxidation and the control experiments, a plausible mechanistic pathway for oxidative KR of benzylic azides was suggested (Fig. 6)59. Chiral MnIII catalyst is first oxidized by PhIO affording oxoMnV intermediate. THQ rac-1a underwent hydrogen atom transfer (HAT) to oxoMnV, giving benzylic radical 10 and MnIV−OH. Finally, azide 10 decomposed by losing molecular nitrogen to form iminyl radical 11, which immediately dimerized to provide 2a60. MnIV−OH dimerized by releasing H2O to generate μ-oxo bridged dimer MnIVOMnIV, which might undergo disproportionation reaction regenerating MnIII precursor and oxoMnV species for the catalytic cycle61–63. Based on the absolute configuration of recovered THQs, (R)-1a should be oxidized more preferentially than (S)-1a. With respect to the oxidative KR of indoline-based benzylic azides, rac-3a might proceed through a similar HAT process to oxoMnV species producing benzylic radical 12, which underwent azide collapse followed by C−C bond cleavage forming radical 14. Alkyl radical 14 underwent oxygen rebound with MnIV−OH followed by hemiaminal decomposition, generating 2-aminobenzonitrile 4a together with MnIII precursor for the catalytic cycle. Based on the absolute configuration of recovered indolines, (R)-3a should be oxidized more preferentially than (S)-3a.Fig. 6 Proposed reaction mechanism.

The possible reaction pathway based on our studies and the previous literatures.

To elucidate the origin of the high level of chiral recognition of azide rac-1a, density functional theory (DFT) calculations were performed for the stereo-determining HAT process (Fig. 7). The Gibbs free energies of corresponding transition states follow the spin ordering of triplet < quintet < singlet, and the triplet state was determined to be the ground state (see Table S1 in the Supporting Information). 3TSR is 1.8 kcal/mol more favorable than 3TSS, which is consistent with experimentally observed stereoselectivity. The effective chiral recognition arises from additional CH···F hydrogen bonding interaction between tert-butyl group of (R)-1a and 2,4-difluorophenyl moiety of catalyst C12 in 3TSR. This CH···F hydrogen bonding interaction is further confirmed by independent gradient model analysis64.Fig. 7 Geometries and the relative Gibbs free energies of stereoselectivity-determining transition states.

Trivial hydrogen atoms are omitted for clarity. The isosurface of IGM analysis is 0.005. The bond distances are given in Å. All energies are given in kcal/mol.

In this work, the KR of organic azides through site- and enantioselective C(sp3)–H oxidation is described. The practical manganese catalyzed reaction exhibits good functional group tolerance, and is applicable to a variety of cyclic benzylic azides bearing pharmacologically significant nitrogen-containing heterocycle skeletons with extremely efficient site- and enantio-discrimination. The usefulness of products has also been demonstrated in synthetic applications. Detailed computational studies elucidate the origins of effective chiral recognition involving a hydrogen bonding interaction between substrate and catalyst. This strategically different approach would unlock opportunities for topologically straightforward synthetic planning for KR reactions relying on the reactivity of C(sp3)–H bonds.

Methods

General procedure

To a solution of rac-1a (0.1 mmol, 1.0 equiv) in ethyl acetate (1.0 ml) was added C12 (0.005 mmol, 0.05 equiv) at room temperature. Then PhIO (0.08 mmol, 0.8 equiv) was added as eight portions in 30-min intervals over 3.5 h. After that, the solvent was removed under vacuum and the residue was purified by flash chromatography on silica gel using ethyl acetate/petroleum ether as eluent to give the product (S)-1a.

Supplementary information

Supplementary Information

Peer Review File

Description of Additional Supplementary Files

Supplementary Dataset 1

Supplementary Dataset 2

Supplementary Dataset 3

Supplementary information

The online version contains supplementary material kimberlyibarra@example.com/s41467-022-29319-z.

Acknowledgements

We gratefully acknowledge the National Science Foundation of China (92156008, 22161142016, 21971148) and Youth Interdiscipline Innovative Research Group of Shandong University (2020QNQT009). The scientific calculations in this paper have been done on the HPC Cloud Platform of Shandong University.

Author contributions

P.Y. conducted the asymmetric oxygenation experiments and mechanistic studies; A.F. and R.Z. performed the DFT calculations; L.W. prepared the substrates; M.C. initially developed the reaction; L.L. designed the experiments and wrote the paper.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files. Extra data are available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in Supplementary Information have been deposited at the Cambridge Crystallographic Data Center (S1: CCDC 2009823, S2: CCDC 2009831). These data could be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

Competing interests

The authors declare no competing interests.

Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Pengbo Ye, Aili Feng, Lin Wang.
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References

1. Keith JM Larrow JF Jacobsen EN Practical considerations in kinetic resolution reactions Adv. Synth. Catal. 2001 343 5 26
2. Breuer M Industrial methods for the production of optically active intermediates Angew. Chem. Int. Ed. 2004 43 788 824
3. Vedejs E Jure M Efficiency in nonenzymatic kinetic resolution Angew. Chem. Int. Ed. 2581-61-7517 4001
4. Pellissier H Catalytic non-enzymatic kinetic resolution Adv. Synth. Catal. 2011 353 1613 1666
5. Ren J Kinetic and dynamic kinetic resolution of racemic tertiary bromides by pentanidium-catalyzed phase-transfer azidation Angew. Chem. Int. Ed. 2581-61-7517 9058
6. Bräse, S. & Banert, K. Organic Azides: Synthesis and Applications (Wiley, 2009).
7. Scriven EFV Turnbull K Azides: their preparation and synthetic uses Chem. Rev. 1988 88 297 368
8. Jung N Bräse S Vinyl and alkynyl azides: well-known intermediates in the focus of modern synthetic methods Angew. Chem. Int. Ed. 2581-61-75179 12171
9. Chiba S Application of organic azides for the synthesis of nitrogen-containing molecules Synlett 2012 1 21 44
10. Thirumurugan P Matosiuk D Jozwiak K Click chemistry for drug development and diverse chemical–biology applications Chem. Rev. 2013 113 4905 4979 23531040
11. Grammel M Huang HC Chemical reporters for biological discovery Nat. Chem. Biol. 2013 9 475 484 23868317
12. Best MD Click chemistry and bioorthogonal reactions: unprecedented selectivity in the labeling of biological molecules Biochemistry 2581-61-7517 6584 19485420
13. Pathak T Azidonucleosides: synthesis, reactions, and biological properties Chem. Rev. 2002 102 1623 1668 11996546
14. Binder WH Kluger C Azide/Alkyne-"Click" reactions: applications in material science and organic synthesis Curr. Org. Chem. 2581-61-7517 1815
15. Xi W Scott TF Kloxin CJ Bowman CN Click chemistry in materials science Adv. Funct. Mater. 2581-61-7517 2590
16. Bräse S Gil C Knepper K Zimmermann V Organic azides: an exploding diversity of a unique class of compounds Angew. Chem. Int. Ed. 2581-61-7517 5240
17. Ding P Hu X Zhou F Zhou J Catalytic enantioselective synthesis of α-chiral azides Org. Chem. Front. 2018 5 1542 1559
18. Liu W Iron-catalyzed enantioselective radical carboazidation and diazidation of α,β-unsaturated carbonyl compounds J. Am. Chem. Soc. 2021 143 11856 11863 34296601
19. Deng Q-H Bleith T Wadepohl H Gade LH Enantioselective iron-catalyzed azidation of β‑keto esters and oxindoles J. Am. Chem. Soc. 2013 135 5356 5359 23537339
20. Martínez LE Leighton JL Carsten DH Jacobsen EN Highly enantioselective ring opening of epoxides catalyzed by (salen)Cr(III) complexes J. Am. Chem. Soc. 1995 117 5897 5898
21. Seidl FJ Min C Lopez JA Burns NZ Catalytic regio- and enantioselective haloazidation of allylic alcohols J. Am. Chem. Soc. 2018 140 15646 15650 30403852
22. Meng J-C Fokin VV Finn MG Kinetic resolution by copper-catalyzed azide–alkyne cycloaddition Tetrahedron Lett. 2581-61-7517 4546
23. Alexander JR Ott AA Liu E-C Topczewski JJ Kinetic resolution of cyclic secondary azides, using an enantioselective copper-catalyzed azide–alkyne cycloaddition Org. Lett. 2581-61-7517 4358 31117717
24. Zhou F Asymmetric copper(I)-catalyzed azide−alkyne cycloaddition to quaternary oxindoles J. Am. Chem. Soc. 2013 135 10994 10997 23855917
25. Brittain WDG Buckley BR Fossey JS Asymmetric copper-catalyzed azide−alkyne cycloadditions ACS Catal. 2016 6 3629 3636
26. Liu E-C Topczewski JJ Enantioselective copper catalyzed alkyne−azide cycloaddition by dynamic kinetic resolution J. Am. Chem. Soc. 2019 141 5135 5138 30888164
27. Ott AA Goshey CS Topczewski JJ Dynamic kinetic resolution of allylic azides via asymmetric dihydroxylation J. Am. Chem. Soc. 2017 139 7737 7740 28574252
28. Yang X Birman VB Kinetic resolution of α-substituted alkanoic acids promoted by homobenzotetramisole Chem. Eur. J. 2581-61-75176 11304 21922562
29. Pramanik S Ghorai P Synthesis and asymmetric resolution of α-azido-peroxides Org. Lett. 2581-61-7517 3835 23855809
30. Newhouse T Baran PS If C-H bonds could talk: selective C-H bond oxidation Angew. Chem. Int. Ed. 2581-61-7517 3374
31. Milan M Bietti M Costas M Enantioselective aliphatic C–H bond oxidation catalyzed by bioinspired complexes Chem. Commun. 2581-61-7517 9570
32. Groves JT Viski P Asymmetric hydroxylation by a chiral iron porphyrin J. Am. Chem. Soc. 1989 111 8537 8538
33. Hamachi K Irie R Katsuki T Asymmetric benzylic oxidation using a Mn-salen complex as catalyst Tetrahedron Lett. 1996 37 4979 4982
34. Zhang, R., Yu, W.-Y., Lai, T.-S. & Che, C.-M. Enantioselective hydroxylation of benzylic C–H bonds by D4-symmetric chiral oxoruthenium porphyrins. Chem. Commun. 1791–1792 (1999).
35. Srour H Maux PL Simonneaux G Enantioselective manganese-porphyrin-catalyzed epoxidation and C–H hydroxylation with hydrogen peroxide in water/methanol solutions Inorg. Chem. 2581-61-7517 5856 22568436
36. Talsi EP Samsonenko DG Ottenbacher RV Bryliakov KP Highly enantioselective C−H oxidation of arylalkanes with H2O2 in the presence of chiral Mn-aminopyridine complexes ChemCatChem 2017 9 4580 4586
37. Burg F Gicquel M Breitenlechner S Pöthig A Bach T Site- and enantioselective C−H oxygenation catalyzed by a chiral manganese porphyrin complex with a remote binding site Angew. Chem. Int. Ed. 2581-61-7517 2957
38. Abazid AH Clamor N Nachtsheim BJ An enantioconvergent benzylic hydroxylation using a chiral aryl iodide in a dual activation mode ACS Catal. 2581-61-7517 8048
39. Miyafuji A Katsuki T Asymmetric desymmetrization of meso-tetrahydrofuran derivatives by highly enantiotopic selective C-H oxidation Tetrahedron 1998 54 10339 10348
40. Murahashi S-I Noji S Hirabayashi T Komiya N Manganese-catalyzed enantioselective oxidation of C–H bonds of alkanes and silyl ethers to optically active ketones Tetrahedron. Asymmetry 2581-61-7517 3535
41. Frost JR Huber SM Breitenlechner S Bannwarth C Bach T Enantiotopos-selective C-H oxygenation catalyzed by a supramolecular ruthenium complex Angew. Chem. Int. Ed. 2015 54 691 695
42. Milan M Bietti M Costas M Highly enantioselective oxidation of nonactivated aliphatic C–H bonds with hydrogen peroxide catalyzed by manganese complexes ACS Cent. Sci. 2017 3 196 204 28386597
43. Qiu B Highly enantioselective oxidation of spirocyclic hydrocarbons by bioinspired manganese catalysts and hydrogen peroxide ACS Catal. 2018 8 2479 2487
44. Cianfanelli M Enantioselective C–H lactonization of unactivated methylenes directed by carboxylic acids J. Am. Chem. Soc. 2020 142 1584 1593 31881152
45. Hashiguchi S Kinetic resolution of racemic secondary alcohols by ruii-catalyzed hydrogen transfer Angew. Chem. Int. Ed. 1997 36 288 290
46. Jensen DR Pugsley JS Sigman MS Palladium-catalyzed enantioselective oxidations of alcohols using molecular oxygen J. Am. Chem. Soc. 2001 123 7475 7476 11472200
47. Ferreira EM Stoltz BM The palladium-catalyzed oxidative kinetic resolution of secondary alcohols with molecular oxygen J. Am. Chem. Soc. 2001 123 7725 7726 11481006
48. Sun W Wang H Xia C Li J Zhao P Chiral-Mn(Salen)-complex-catalyzed kinetic resolution of secondary alcohols in water Angew. Chem. Int. Ed. 2581-61-7517 1044
49. Radosevich AT Musich C Toste FD Vanadium-catalyzed asymmetric oxidation of α-hydroxy esters using molecular oxygen as stoichiometric oxidant J. Am. Chem. Soc. 2005 127 1090 1091 15669834
50. Pawar VD Bettigeri S Weng S-S Kao J-Q Chen C-T Highly enantioselective aerobic oxidation of α-hydroxyphosphonates catalyzed by chiral vanadyl(V) methoxides bearing N-salicylidene-α-aminocarboxylates J. Am. Chem. Soc. 2006 128 6308 6309 16683782
51. Arita S Koike T Kayaki Y Ikariya T Aerobic oxidative kinetic resolution of racemic secondary alcohols with chiral bifunctional amido complexes Angew. Chem. Int. Ed. 2581-61-7517 2449
52. Murakami K Highly enantioselective organocatalytic oxidative kinetic resolution of secondary alcohols using chiral alkoxyamines as precatalysts: catalyst structure, active species, and substrate scope J. Am. Chem. Soc. 2014 136 17591 17600 25412147
53. Saito K Shibata Y Yamanaka M Akiyama T Chiral phosphoric acid-catalyzed oxidative kinetic resolution of indolines based on transfer hydrogenation to imines J. Am. Chem. Soc. 2013 135 11740 11743 23865933
54. Lu R Cao L Guan H Liu L Iron-catalyzed aerobic dehydrogenative kinetic resolution of cyclic secondary amines J. Am. Chem. Soc. 2019 141 6318 6324 30916947
55. Larrow JF Jacobsen EN Kinetic resolution of 1,2-dihydronaphthalene oxide and related epoxides via asymmetric C-H hydroxylation J. Am. Chem. Soc. 1994 116 12129 12130
56. Sun S Ma Y Liu Z Liu L Oxidative kinetic resolution of cyclic benzylic ethers Angew. Chem. Int. Ed. 2021 60 176 180
57. Berrisford DJ Bolm C Sharpless KB Ligand-accelerated catalysis Angew. Chem. Int. Ed. 1995 34 1059 1070
58. Satyanarayana T Abraham S Kagan HB Nonlinear effects in asymmetric catalysis Angew. Chem. Int. Ed. 2009 48 456 494
59. Nam W Lee Y-M Fukuzumi S Hydrogen atom transfer reactions of mononuclear nonheme metal–oxygen intermediates Acc. Chem. Res. 2581-61-7517 2022 30179459
60. Wang Y-F Lonca GH Chiba S PhI(OAc)2-mediated radical trifluoromethylation of vinyl azides with Me3SiCF3 Angew. Chem. Int. Ed. 2581-61-7517 1071
61. Feichtinger D Plattner DA Direct proof for O=Mn"(salen) complexes Angew. Chem. Int. Ed. 1997 36 1718 1719
62. Feichtinger D Plattner DA Probing the reactivity of oxomanganese−salen complexes: an electrospray tandem mass spectrometric study of highly reactive intermediates Chem. Eur. J. 2001 7 591 599 11261656
63. Bryliakov KP Babushkin DE Talsi EP 1H NMR and EPR spectroscopic monitoring of the reactive intermediates of (Salen)MnIII catalyzed olefin epoxidation J. Mol. Catal. A Chem. 2000 158 19 35
64. Lefebvre C Accurately extracting the signature of intermolecular interactions present in the NCI plot of the reduced density gradient versus electron density Phys. Chem. Chem. Phys. 2581-61-75178 17936 28664951


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