Asymmetric Catalysis with Chiral Oxazaborolidinium Ions (COBIs): from Cyclization to Radical Reactions

Jae Yeon Kim,^a Su Yong Shim,^*,b and Do Hyun Ryu^*,a

^aDepartment of Chemistry, Sungkyunkwan University 300, Cheoncheon Jangan, Suwon, 16419, Republic of Korea

^bInfectious Diseases Therapeutic Research Center, Division of Medicinal Chemistry and Pharmacology, Korea Research Institute of Chemical Technology (KRICT) KRICT School, University of Science and Technology, Daejeon 34114, Republic of Korea

dhryu@skku.edu
sshim@krict.re.kr

Abstract

Chiral Lewis acid catalysts are some of the most powerful and efficient catalysts for asymmetric synthesis. Among the various Lewis acid catalysts known, the chiral oxazaborolidinium ion (COBI) has proven widely applicable to a variety of asymmetric transformations, such as the Diels–Alder reaction, nucleophilic additions, and cycloadditions. In this review, we introduce our recent work on COBI-catalyzed asymmetric reactions, including cyclopropanation, epoxidation, and radical reactions.

1. Introduction

Asymmetric synthesis is one of the most important types of organic synthesis, because the majority of therapeutic compounds and bioactive natural products exist in one enantiomeric form.¹ The utilization of chiral auxiliaries or chiral reagents can be a good strategy for the asymmetric synthesis of chiral compounds,² but suffers from the need to use stoichiometric amounts of the auxiliary or reagent. For this reason, asymmetric catalysis is an attractive and advantageous alternative for accessing enantiomerically enriched/pure compounds.³

Lewis acids have a long history of being employed as catalysts in the field of organic synthesis.⁴ However, their true power and relevance as highly efficient asymmetric catalysts have only been recognized in the past several decades. Consequently, a large number of chiral Lewis acid catalysts have been developed and cover almost all the metals in the periodic table.⁵

In particular, after the first report in 1976 of an enantioselective catalytic Diels–Alder reaction with a chiral boron complex,⁶ numerous chiral boron complexes have been designed and used extensively for catalyzing various cycloadditions, cyclizations, carbonyl reductions, and rearrangement reactions.⁷

The asymmetric reduction of prochiral ketones catalyzed by proline-derived chiral oxazaborolidines of type 1 was developed independently by Itsuno and Corey in 1981 and 1987, respectively (Scheme 1).^8,9 This process, commonly referred to as the Corey–Itsuno reduction or Corey–Bakshi–Shibata (CBS) reduction, has proven effective in a large number of synthetic applications that have been reported in subsequent decades.^10,11

Scheme 1. Corey–Itsuno or Corey–Bakshi–Shibata (CBS) Reduction catalyzed by chiral oxazaborolidines (1) . (Ref. 8,9)

Since it was first reported by Corey in 2002,^12,13 the chiral oxazaborolidinium ion (COBI) has been used as a strong Lewis acid. COBI catalysts activated by Brønsted^12,14 or Lewis acids,¹⁵—referred to as combined acid catalysts^13c—exhibit higher catalytic activity and stereoselectivity than the individual acid catalysts through enhancement of their acidity by attachment of the Brønsted or Lewis acid. Representative structures of COBI catalysts that we discuss in this review are shown in Figure 1. 

A large number of enantioselective Diels–Alder reactions were developed using cationic oxazaborolidines to produce enantioenriched cyclized products, and these results were reviewed in 2002 and 2009 by E. J. Corey.^13a,b In addition, various asymmetric nucleophilic 1,2- or 1,4-additions to carbonyl compounds have been developed with COBI catalysts, and the results were reviewed in 2019 by our group.¹⁶ In this review, we highlight recent advances in the use of COBI catalysts to carry out the asymmetric formation of cyclic compounds (excluding Diels–Alder adducts) as well as enantioselective radical reactions.

All asymmetric reactions in this review have been shown to proceed via one of three pretransition-state assembly models of the COBI catalyst with carbonyl compounds (Figure 2).¹⁶ Thus, the stereochemistry of the asymmetric reactions can be rationalized based on one of these assembly models.

Figure 2. Pretransition-state assembly models for reactive complexes of COBI and carbonyl compounds. (Ref. 16)

Complexes of COBI with carbonyl compounds are proposed to possess a Lewis base–acid interaction between the carbonyl oxygen and boron atom of the COBI catalyst. Synergistically, the formyl CH…O (4) and α-CH…O (5 and 6) hydrogen bonding restricts the rotation of the bond between the boron atom and the carbonyl oxygen atom, which results in more rigid COBI-carbonyl complexes.¹⁷ In the pretransition-state models, the electron-deficient carbonyl carbon or the β-carbon of the α,β-unsaturated carbonyl compound is positioned above one of the geminal aromatic groups of COBI through a π–π donor–acceptor interaction. Moreover, the pseudoaxial aromatic ring of the catalyst effectively shields the rear face of the carbonyl compound from attack by nucleophiles and directs the addition of the nucleophile to the front face.

4. Conclusion

Asymmetric catalysis has been an important tool in organic synthesis. In particular, chiral Lewis acid catalysts have been applied to various synthetic methodologies and the total synthesis of natural products and pharmaceuticals. Among the chiral Lewis acid catalysts reported, chiral oxazaborolidinium ions (COBIs) are some of the most efficient catalysts for the activation of carbonyl compounds in such asymmetric reactions as dipolar cycloadditions, cyclopropanations, epoxidations, and visible-light-induced radical additions. We aimed in this review to, not only introduce various enantioselective methodologies that employ COBIs, but also provide a deeper understanding of the details of their catalytic action such as in pretransition state models or ternary EDA complexes. Further studies utilizing COBIs are underway in our laboratory to develop novel catalytic asymmetric methodologies for different types of nucleophilic addition reactions, rearrangements, radical reactions, and total syntheses.

5. Acknowledgments

We sincerely thank all the co-workers for their significant contributions to the research described in this review. We also acknowledge the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (No. RS-2023- 00219859 and 2022R1A6C101A751) and the R&D program of the institutional Research Program of KRICT (KK2332-10 and BSK23-415).

References

1.

(a) Li L, Chen Z, Zhang X, Jia Y. 2018. Divergent Strategy in Natural Product Total Synthesis. Chem. Rev. 118(7):3752-3832. https://doi.org/10.1021/acs.chemrev.7b00653 (b) Farina V, Reeves JT, Senanayake CH, Song JJ. 2006. Asymmetric Synthesis of Active Pharmaceutical Ingredients. Chem. Rev. 106(7):2734-2793. https://doi.org/10.1021/cr040700c (c) Koskinen AMP. 2023. Asymmetric Synthesis of Natural Products, 3rd edition. Wiley: Chichester, U.K.

2.

(a) Diaz-Muñoz G, Miranda IL, Sartori SK, de Rezende DC, Alves Nogueira Diaz M. 2019. Use of chiral auxiliaries in the asymmetric synthesis of biologically active compounds: A review. Chirality. 31(10):776-812. https://doi.org/10.1002/chir.23103 (b) Paquette LA. Ed. 2003. Handbook of Reagents for Organic Synthesis: Chiral Reagents for Asymmetric Synthesis; Wiley: Chichester, U.K.

3.

(a) Adly FG, Ghanem A. 2019. Asymmetric Catalysis in Organic Synthesis. Catalysts. 9(9):775. https://doi.org/10.3390/catal9090775 (b) Yoon TP. 2017. Strategies in asymmetric catalysis. Beilstein J. Org. Chem. 13(1):63-64. https://doi.org/10.3762/bjoc.13.8

4.

(a) Schinzer D. Ed. 1989. Selectivities in Lewis Acid Promoted Reactions; Springer Dordrecht: Dordrecht. https://doi.org/10.1007/978-94-009-2464-2 (b) Santelli M, Pons J.M. Eds. 1996. Lewis Acids and Selectivity in Organic Synthesis. CRC Press: Boca Raton, FL (c) Leach MR. 1999. Lewis Acid/Base Reaction Chemistry; Meta- Synthesis: Brighton, U.K.

5.

(a) Yamamoto H. Ed. 2000. Lewis Acids in Organic Synthesis; Wiley- VCH: Weinheim (b) Yamamoto H. Ed. 1999. Lewis Acid Reagents: A Practical Approach; Oxford University Press: Oxford (c) Mlynarski J. Ed. 2017. Chiral Lewis Acids in Organic Synthesis; Wiley- VCH: Weinheim, Germany.

6.

Guseinov MM, Akhmedov IM, Mamedov EC, Azerb. khim. Zh. 1976. 1. 46.

7.

(a) Ishihara K. 2000. Chiral B(III) Lewis Acids. In Lewis Acids in Organic Synthesis; Yamamoto H. Ed. Wiley-VCH: Weinheim. Vol. 1. pp 135−190 (b) Deloux L, Srebnik M. 1993. Asymmetric boron-catalyzed reactions. Chem. Rev. 93(2):763-784. https://doi.org/10.1021/cr00018a007 (c) Nori V, Pesciaioli F, Sinibaldi A, Giorgianni G, Carlone A. 2022. Boron-Based Lewis Acid Catalysis: Challenges and Perspectives. Catalysts. 22(1):5. https://doi.org/10.3390/catal12010005

8.

Hirao A, Itsuno S, Nakahama S, Yamazaki N. 1981. Asymmetric reduction of aromatic ketones with chiral alkoxy-amineborane complexes. J. Chem. Soc., Chem. Commun.(7):315. https://doi.org/10.1039/c39810000315

9.

Corey EJ, Bakshi RK, Shibata S. 1987. Highly enantioselective borane reduction of ketones catalyzed by chiral oxazaborolidines. Mechanism and synthetic implications. J. Am. Chem. Soc. 109(18):5551-5553. https://doi.org/10.1021/ja00252a056

10.

Cho BT. 2006. Recent advances in the synthetic applications of the oxazaborolidine-mediated asymmetric reduction. Tetrahedron Lett. 62(33):7621-7643. https://doi.org/10.1016/j.tet.2006.05.036

11.

Corey EJ, Helal CJ. 1998. Angew. Chem., Int. Ed. 37(15):1986-2012. https://doi.org/10.1002/(SICI)1521-3773(19980817)37:15<1986::AID-ANIE1986>3.0.CO;2-Z

12.

Corey EJ, Shibata T, Lee TW. 2002. Asymmetric Diels−Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine. J. Am. Chem. Soc. 124(15):3808-3809. https://doi.org/10.1021/ja025848x

13.

(a) Corey EJ. 2002. Catalytic Enantioselective Diels–Alder Reactions: Methods, Mechanistic Fundamentals, Pathways, and Applications. Angew. Chem., Int. Ed. 41(10):1650-1667. https://doi.org/10.1002/1521-3773(20020517)41:10<1650::AID-ANIE1650>3.0.CO;2-B (b) Corey EJ. 2009. Enantioselective Catalysis Based on Cationic Oxazaborolidines Angew. Chem., Int. Ed. 48(12):2100-2117. https://doi.org/10.1002/anie.200805374 (c) Yamamoto H, Futatsugi K. 2005. “Designer Acids”: Combined Acid Catalysis for Asymmetric Synthesis. Angew. Chem., Int. Ed. 44(13):1924-1942. https://doi.org/10.1002/anie.200460394

14.

Ryu DH, Corey EJ. 2003. Triflimide Activation of a Chiral Oxazaborolidine Leads to a More General Catalytic System for Enantioselective Diels−Alder Addition. J. Am. Chem. Soc. 125(21):6388-6390. https://doi.org/10.1021/ja035393r

15.

Liu D, Canales E, Corey EJ. 2007. Chiral Oxazaborolidine−Aluminum Bromide Complexes Are Unusually Powerful and Effective Catalysts for Enantioselective Diels−Alder Reactions. J. Am. Chem. Soc. 129(6):1498-1499. https://doi.org/10.1021/ja068637r

16.

Shim SY, Ryu DH. 2019. Enantioselective Carbonyl 1,2- or 1,4-Addition Reactions of Nucleophilic Silyl and Diazo Compounds Catalyzed by the Chiral Oxazaborolidinium Ion. Acc. Chem. Res. 52(8):2349-2360. https://doi.org/10.1021/acs.accounts.9b00279

17.

Corey EJ, Lee TW. 2001. The formyl C–H⋯O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes. Chem. Commun.(15):1321-1329. https://doi.org/10.1039/b104800g

18.

(a) Ma S. Ed. 2009. Handbook of Cyclization Reactions; Wiley- VCH: Weinheim (b) Nicolaou KC, Vourloumis D, Winssinger N, Baran PS. 2000. The Art and Science of Total Synthesis at the Dawn of the Twenty-First Century. Angew. Chem., Int. Ed. 39(1):44-122. https://doi.org/10.1002/(SICI)1521-3773(20000103)39:1<44::AID-ANIE44>3.0.CO;2-L

19.

(a) Liu X, Zheng H, Xia Y, Lin L, Feng X. 2017. Asymmetric Cycloaddition and Cyclization Reactions Catalyzed by Chiral N,N′-Dioxide–Metal Complexes. Acc. Chem. Res. 50(10):2621-2631. https://doi.org/10.1021/acs.accounts.7600377 (b) Moyano A, Rios R. 2011. Asymmetric Organocatalytic Cyclization and Cycloaddition Reactions. Chem. Rev. 111(8):4703-4832. https://doi.org/10.1021/cr100348t (c) Jo J, Kim SH, Han YT, Kwak JH, Yun H. 2017. Recent Advances in Substrate-Controlled Asymmetric Cyclization for Natural Product Synthesis. Molecules. 22(7):1069. https://doi.org/10.3390/molecules22071069

20.

(a) Fujimura O, Grubbs RH. 1998. Asymmetric Ring-Closing Metathesis Catalyzed by Chiral Molybdenum Alkylidene Complexes. J. Org. Chem. 63(3):824-832. https://doi.org/10.1021/jo971952z (b) Sattely ES, Cortez GA, Moebius DC, Schrock RR, Hoveyda AH. 2005. Enantioselective Synthesis of Cyclic Amides and Amines through Mo-Catalyzed Asymmetric Ring-Closing Metathesis. J. Am. Chem. Soc. 127(23):8526-8533. https://doi.org/10.1021/ja051330s

21.

Hashimoto T, Maruoka K. 2015. Recent Advances of Catalytic Asymmetric 1,3-Dipolar Cycloadditions. Chem. Rev. 115(11):5366-5412. https://doi.org/10.1021/cr5007182

22.

(a) Kagan HB, Riant O. 1992. Catalytic asymmetric Diels Alder reactions. Chem. Rev. 92(5):1007-1019. https://doi.org/10.1021/cr00013a013 (b) Jørgensen KA. 2000. Catalytic Asymmetric Hetero-Diels–Alder Reactions of Carbonyl Compounds and Imines. Angew. Chem., Int. Ed. 39(2):3558-3588. https://doi.org/10.1002/1521-3773(20001016)39:20<3558::AID-ANIE3558>3.0.CO;2-I

23.

(a) Shimada 2011. N, Stewart C, Tius MA. 2011. Asymmetric Nazarov cyclizations. Tetrahedron Lett. 67(33):5851-5870. https://doi.org/10.1016/j.tet.2011.05.062 (b) Frontier AJ, Hernandez JJ. 2020. New Twists in Nazarov Cyclization Chemistry. Acc. Chem. Res. 53(9):1822-1832. https://doi.org/10.1021/acs.accounts.Oc00284

24.

Miyabe H, Kawashima A, Yoshioka E, Kohtani S. 2017. Progress in Enantioselective Radical Cyclizations. Chem.—Eur. J. 23(26):6225-6236. https://doi.org/10.1002/chem.201603124

25.

Haider K, Shafeeque M, Yahya S, Yar MS. 2022. A comprehensive review on pyrazoline based heterocyclic hybrids as potent anticancer agents. Eur. J. Med. Chem. Rep. 5100042. https://doi.org/10.1016/j.ejmcr.2022.100042

26.

Kanemasa S, Kanai T. 2000. Lewis Acid-Catalyzed Enantioselective 1,3-Dipolar Cycloadditions of Diazoalkane:  Chiral Ligand/Achiral Auxiliary Cooperative Chirality Control. J. Am. Chem. Soc. 122(43):10710-10711. https://doi.org/10.1021/ja002670a

27.

(a) Kano T, Hashimoto T, Maruoka K. 2006. Enantioselective 1,3-Dipolar Cycloaddition Reaction between Diazoacetates and α-Substituted Acroleins: Total Synthesis of Manzacidin A. J. Am. Chem. Soc. 128(7):2174-2175. https://doi.org/10.1021/ja056851u (b) Hashimoto, T.; Maruoka, K. Org. Biomol. Chem. 2008, 6, 829, DOI: 10.1039/b716062c. (c) Sibi MP, Stanley LM, Soeta T. 2007. Enantioselective 1,3-Dipolar Cycloadditions of Diazoacetates with Electron-Deficient Olefins. Org. Lett. 9(8):1553-1556. https://doi.org/10.1021/ol070364x

28.

Gao L, Hwang G, Lee MY, Ryu DH. 2009. Catalytic enantioselective 1,3-dipolar cycloadditions of alkyl diazoacetates with α,β-disubstituted acroleins. Chem. Commun.(36):5460. https://doi.org/10.1039/b910321j

29.

Lee SI, Kim KE, Hwang G, Ryu DH. 2015. Highly enantioselective catalytic 1,3-dipolar cycloadditions of α-alkyl diazoacetates: efficient synthesis of functionalized 2-pyrazolines. Org. Biomol. Chem. 13(9):2745-2749. https://doi.org/10.1039/c4ob02372b

30.

Liu D, Hong S, Corey EJ. 2006. Enantioselective Synthesis of Bridged- or Fused-Ring Bicyclic Ketones by a Catalytic Asymmetric Michael Addition Pathway. J. Am. Chem. Soc. 128(25):8160-8161. https://doi.org/10.1021/ja063332y

31.

Gao L, Hwang G, Ryu DH. 2011. Oxazaborolidinium Ion-Catalyzed Cyclopropanation of α-Substituted Acroleins: Enantioselective Synthesis of Cyclopropanes Bearing Two Chiral Quaternary Centers. J. Am. Chem. Soc. 133(51):20708-20711. https://doi.org/10.1021/ja209270e

32.

Shim SY, Kim JY, Nam M, Hwang G, Ryu DH. 2016. Enantioselective Cyclopropanation with α-Alkyl-α-diazoesters Catalyzed by Chiral Oxazaborolidinium Ion: Total Synthesis of (+)-Hamavellone B. Org. Lett. 18(2):160-163. https://doi.org/10.1021/acs.orglett.5b02970

33.

Lee SI, Hwang G, Ryu DH. 2013. Catalytic Enantioselective Carbon Insertion into the β-Vinyl C–H Bond of Cyclic Enones. J. Am. Chem. Soc. 135(19):7126-7129. https://doi.org/10.1021/ja402873b

34.

Kim T, Kim JY, Park KY, Ryu DH. 2021. Asymmetric Synthesis of (−)‐Dictyopterene C' and its Derivatives via Catalytic Enantioselective Cyclopropanation. Bull. Korean Chem. Soc. 42(4):675-678. https://doi.org/10.1002/bkcs.12250

35.

(a) Sperling D, Reißig HU, Fabian J. 1999. [1,3]-Sigmatropic Rearrangements of Divinylcyclopropane Derivatives and Hetero Analogs in Competition with Cope-Type Rearrangements – A DFT Study. Eur. J. Org. Chem. 1999(5):1107-1114. https://doi.org/10.1002/(SICI)1099-0690(199905)1999:5<1107::AID-EJOC1107>3.0.CO;2-E (b) Nasveschuk CG, Rovis T. 2005. Stereoselective Lewis Acid Mediated [1,3] Ring Contraction of 2,5-Dihydrooxepins as a Route to Polysubstituted Cyclopentenes. Angew. Chem., Int. Ed. 44(21):3264-3267. https://doi.org/10.1002/anie.200500088

36.

Shim SY, Cho SM, Venkateswarlu A, Ryu DH. 2017. Catalytic Enantioselective Synthesis of 2,5‐Dihydrooxepines. Angew. Chem., Int. Ed. 56(30):8663-8666. https://doi.org/10.1002/anie.201700890

37.

Shim SY, Choi Y, Ryu DH. 2018. Asymmetric Synthesis of Cyclobutanone via Lewis Acid Catalyzed Tandem Cyclopropanation/Semipinacol Rearrangement. J. Am. Chem. Soc. 140(36):11184-11188. https://doi.org/10.1021/jacs.8b06835

38.

Pandit RP, Kim ST, Ryu DH. 2019. Asymmetric Synthesis of Enantioenriched 2‐Aryl‐2,3‐Dihydrobenzofurans by a Lewis Acid Catalyzed Cyclopropanation/Intramolecular Rearrangement Sequence. Angew. Chem., Int. Ed. 58(38):13427-13432. https://doi.org/10.1002/anie.201906954

39.

Kang BC, Nam DG, Hwang G, Ryu DH. 2015. Catalytic Asymmetric Formal Insertion of Aryldiazoalkanes into the C–H Bond of Aldehydes: Synthesis of Enantioenriched Acyclic α-Tertiary Aryl Ketones. Org. Lett. 17(19):4810-4813. https://doi.org/10.1021/acs.orglett.5b02370

40.

Kim JY, Kang BC, Ryu DH. 2017. Catalytic Asymmetric Roskamp Reaction of Silyl Diazoalkane: Synthesis of Enantioenriched α-Silyl Ketone. Org. Lett. 19(21):5936-5939. https://doi.org/10.1021/acs.orglett.7b02928

41.

Nam DG, Shim SY, Jeong H, Ryu DH. 2021. Catalytic Asymmetric Darzens‐Type Epoxidation of Diazoesters: Highly Enantioselective Synthesis of Trisubstituted Epoxides. Angew. Chem., Int. Ed. 60(41):22236-22240. https://doi.org/10.1002/anie.202108454

42.

(a) Romero NA, Nicewicz DA. 2016. Organic Photoredox Catalysis. Chem. Rev. 116(17):10075-10166. https://doi.org/10.1021/acs.chemrev.6b00057 (b) Wang C, Lu Z. 2015. Catalytic enantioselective organic transformations via visible light photocatalysis. Org. Chem. Front. 2(2):179-190. https://doi.org/10.1039/c4qo00306c (c) Meggers E. 2015. Asymmetric catalysis activated by visible light. Chem. Commun. 51(16):3290-3301. https://doi.org/10.1039/c4cc09268f (d) Prier CK, Rankic DA, MacMillan DWC. 2013. Chem. Rev. 113(7):5322-5363. https://doi.org/10.1021/cr300503r (e) Skubi KL, Blum TR, Yoon TP. 2016. Dual Catalysis Strategies in Photochemical Synthesis. Chem. Rev. 116(17):10035-10074.

43.

Brenninger C, Jolliffe JD, Bach T. 2018. Chromophore Activation of α,β‐Unsaturated Carbonyl Compounds and Its Application to Enantioselective Photochemical Reactions. Angew. Chem., Int. Ed. 57(44):14338-14349. https://doi.org/10.1002/anie.201804006

44.

Schwinger DP, Bach T. 2020. Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions. Acc. Chem. Res. 53(9):1933-1943. https://doi.org/10.1021/acs.accounts.0c00379

45.

Daub ME, Jung H, Lee BJ, Won J, Baik M, Yoon TP. 2019. Enantioselective [2+2] Cycloadditions of Cinnamate Esters: Generalizing Lewis Acid Catalysis of Triplet Energy Transfer. J. Am. Chem. Soc. 141(24):9543-9547. https://doi.org/10.1021/jacs.9b04643

46.

Genzink MJ, Kidd JB, Swords WB, Yoon TP. 2022. Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis. Chem. Rev. 122(2):1654-1716. https://doi.org/10.1021/acs.chemrev.1c00467

47.

Ishitani O, Ihama M, Miyauchi Y, Pac C. 1985. Redox-photosensitised reactions. Part 12. Effects of magnesium(II) ion on the [Ru(bpy)3]2+-photomediated reduction of olefins by 1-benzyl-1,4-dihydronicotinamide: metal-ion catalysis of electron transfer processes involving an NADH model. J. Chem. Soc., Perkin Trans. 1.1527. https://doi.org/10.1039/p19850001527

48.

(a) Curran DP, Diederichsen U, Palovich M. 1997. Radical Cyclizations of Acylgermanes. New Reagent Equivalents of the Carbonyl Radical Acceptor Synthon. J. Am. Chem. Soc. 119(21):4797-4804. https://doi.org/10.1021/ja970219m (b) Wilsey S, Dowd P, Houk KN. 1999. Effect of Alkyl Substituents and Ring Size on Alkoxy Radical Cleavage Reactions. J. Org. Chem. 64(24):8801-8811. https://doi.org/10.1021/jo990652+ (c) Pitzer L, Sandfort F, Strieth-Kalthoff F, Glorius F. 2017. Intermolecular Radical Addition to Carbonyls Enabled by Visible Light Photoredox Initiated Hole Catalysis. J. Am . Chem. Soc. 139(39):13652-13655. https://doi.org/10.1021/jacs.7b08086

49.

Xie S, Li D, Huang H, Zhang F, Chen Y. 2019. Intermolecular Radical Addition to Ketoacids Enabled by Boron Activation. J. Am. Chem. Soc. 141(41):16237-16242. https://doi.org/10.1021/jacs.9b09099

50.

Kim JY, Lee YS, Choi Y, Ryu DH. 2020. Enantioselective 1,2-Addition of α-Aminoalkyl Radical to Aldehydes via Visible-Light Photoredox Initiated Chiral Oxazaborolidinium Ion Catalysis. ACS Catal. 10(18):10585-10591. https://doi.org/10.1021/acscatal.0c02443

51.

Ruiz Espelt L, McPherson IS, Wiensch EM, Yoon TP. 2015. Enantioselective Conjugate Additions of α-Amino Radicals via Cooperative Photoredox and Lewis Acid Catalysis. J. Am. Chem. Soc. 137(7):2452-2455. https://doi.org/10.1021/ja512746q

52.

Cismesia MA, Yoon TP. 2015. Characterizing chain processes in visible light photoredox catalysis. Chem. Sci. 6(10):5426-5434. https://doi.org/10.1039/c5sc02185e

53.

Nakajima K, Miyake Y, Nishibayashi Y. 2016. Synthetic Utilization of α-Aminoalkyl Radicals and Related Species in Visible Light Photoredox Catalysis. Acc. Chem. Res. 49(9):1946-1956. https://doi.org/10.1021/acs.accounts.6b00251

54.

(a) Wang C, Qin J, Shen X, Riedel R, Harms K, Meggers E. 2016. Asymmetric Radical–Radical Cross-Coupling through Visible-Light-Activated Iridium Catalysis. Angew. Chem., Int. Ed. 55(2):685-688. https://doi.org/10.1002/anie.201509524 (b) Ma J, Harms K, Meggers E. 2016. Enantioselective rhodium/ruthenium photoredox catalysis en route to chiral 1,2-aminoalcohols. Chem. Commun. 52(66):10183-10186. https://doi.org/10.1039/c6cc04397f (c) Liu Y, Liu X, Li J, Zhao X, Qiao B, Jiang Z. 2018. Catalytic enantioselective radical coupling of activated ketones with N-aryl glycines. Chem. Sci. 9(42):8094-8098. https://doi.org/10.1039/c8sc02948b

55.

(a) Ryu DH, Corey EJ. 2005. Enantioselective Cyanosilylation of Ketones Catalyzed by a Chiral Oxazaborolidinium Ion. J. Am. Chem. Soc. 127(15):5384-5387. https://doi.org/10.1021/ja050543e (b) Kang BC, Shin SH, Yun J, Ryu DH. 2017. Highly Enantioselective Hydrosilylation of Ketones Catalyzed by a Chiral Oxazaborolidinium Ion. Org. Lett. 19(23):6316-6319. https://doi.org/10.1021/acs.orglett.7b03076

56.

Cho SM, Kim JY, Han S, Ryu DH. 2022. Visible Light-Mediated Enantioselective Addition of α-Aminoalkyl Radicals to Ketones Catalyzed by Chiral Oxazaborolidinium Ion. J. Org. Chem. 87(16):11196-11203. https://doi.org/10.1021/acs.joc.2c01527

57.

Kim JY, Lee YS, Ryu DH. 2021. Ternary Electron Donor–Acceptor Complex Enabled Enantioselective Radical Additions to α, β-Unsaturated Carbonyl Compounds. ACS Catal. 11(24):14811-14818. https://doi.org/10.1021/acscatal.1c04835

58.

Kochi JK. 1991. Charge-transfer excitation of molecular complexes in organic and organometallic chemistry. Pure Appl. Chem. 63(2):255-264. https://doi.org/10.1351/pac199163020255

59.

Aramaki Y, Imaizumi N, Hotta M, Kumagai J, Ooi T. 2020. Exploiting single-electron transfer in Lewis pairs for catalytic bond-forming reactions. Chem. Sci. 11(17):4305-4311. https://doi.org/10.1039/d0sc01159b