Visible Light-Induced Regio- and Enantiodifferentiating [2 + 2] Photocycloaddition of 1,4-Naphthoquinones Mediated by Oppositely Coordinating 1,3,2-Oxazaborolidine Chiral Lewis Acid
journal contributionposted on 02.06.2022, 06:13 authored by Nao Shimizu, Hajime Shigemitsu, Toshiyuki Kida, Thorsten Bach, Tadashi Mori
A range of asymmetric photochemical transformations using visible light have recently become considerably attractive. Among the various approaches, chiral Lewis acid association to enones for [2 + 2] and ortho photocycloadditions and oxadi-π-methane rearrangements have shown to be very promising. Naturally, chiral Lewis acid coordination protects one of the prochiral faces of the CC double bond, which enables an effective enantiodifferentiation in the following bond-forming process(es). Here, we studied regio- and enantiodifferentiating [2 + 2] photocycloaddition reactions of naphthoquinone derivatives mediated by chiral oxazaborolidines. A stereochemical control was quite challenging for the 2-ene-1,4-dione substrate, as a double coordination of Lewis acid essentially cancels out the face selectivity, and a mono-coordination to each carbonyl group leads to an opposite stereochemical outcome. Furthermore, a stepwise coordination in the ground state of Lewis acid in a 1:1 fashion was practically inaccessible. We found that the excited-state decomplexation is a key to accomplish high regio- and enantioselectivities in the photocycloaddition of an ene-dione.
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carbonyl group leadsopposite stereochemical outcomenaphthoquinone derivatives mediatedaccomplish high regiooppositely coordinating 1stereochemical controlnaphthoquinones mediatedstudied regioinduced regiovisible lightvarious approachesstate decomplexationquite challengingprochiral facespractically inaccessibleortho </methane rearrangementslewis acidground statefollowing bondface selectivityeffective enantiodifferentiationchiral oxazaborolidines1 fashion