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. 2018 Apr 27;9(20):4562-4568.
doi: 10.1039/c8sc01219a. eCollection 2018 May 28.

A chiral nickel DBFOX complex as a bifunctional catalyst for visible-light-promoted asymmetric photoredox reactions

Xiang Shen  1 , Yanjun Li  1 , Zhaorui Wen  1 , Shi Cao  1 , Xinyi Hou  1 , Lei Gong  1
Affiliations

A chiral nickel DBFOX complex as a bifunctional catalyst for visible-light-promoted asymmetric photoredox reactions

Xiang Shen et al. Chem Sci. .

Abstract

The enantioselective photoredox reaction of α,β-unsaturated carbonyl compounds and tertiary/secondary α-silylamines was enabled by a readily available single NiII-DBFOX catalyst (DBFOX = 4,6-bis((R)-4-phenyl-4,5-dihydrooxazol-2-yl)dibenzo[b,d]furan) under visible light conditions. The non-precious chiral catalyst is involved in the photochemical process to initiate single electron transfer and at the same time provides a well-organized chiral environment for the subsequent radical transformations. Good to excellent enantioselectivities (80-99% ee) were obtained for the formation of chiral γ-amino carboxylic acid derivatives and γ-lactams.

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Figures

Fig. 1
Fig. 1. Previous strategies for enantioselective radical conjugate additions and that developed in this study.
Fig. 2
Fig. 2. (a) Left: UV-Vis absorption spectra recorded on a Shimadzu UV-2550 in a 10.0 mm quartz cuvette. Middle: I. Substrate 1c in THF (0.030 M). II. Substrate 2a in THF (0.030 M). III. Ni–L1 in THF (0.030 M). IV. Ni–L1–1c in THF (0.030 M). Right: A THF solution of nickel catalyst Ni–L1 (0.030 M) in the dark and in the light. (b) Cyclic voltammogram of nickel catalyst Ni–L1 (0.030 M) and potential intermediate complex Ni–L1–1c (0.030 M) in TBAPF6 (0.10 M) in CH3CN. Sweep rate: 20 mV s–1. A Pt electrode was used as the working electrode, a SCE as the reference electrode, and Pt wire as the auxiliary electrode. EpA is the anodic peak potential. ECp is the cathodic peak potential. (c) Calculated reductive potentials of nickel catalyst Ni–L1 and potential intermediate complex Ni–L1–1c in the excited states.
Fig. 3
Fig. 3. Mechanistic investigation. (a) Intervals of irradiation and dark periods for the nickel-catalyzed reaction 1c + 2a → 3c. (b) Radical trapping experiment in the nickel-catalyzed reaction 1c + 2d → 3r.
Fig. 4
Fig. 4. A proposed reaction mechanism for the nickel-catalyzed enantioselective photoredox reaction of α,β-unsaturated carbonyl compounds and α-silylamines.
Fig. 5
Fig. 5. proposed transition state for radical addition.
Fig. 6
Fig. 6. Substrate scope with respect to α,β-unsaturated N-acyl pyrazoles.
Fig. 7
Fig. 7. Substrate scope with respect to tertiary α-silylamines.
Fig. 8
Fig. 8. Synthetic applications of the methodology.
See this image and copyright information in PMC

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