Photosensitised regioselective [2+2]-cycloaddition of cinnamates and related alkenes

Abstract

An efficient method for the synthesis of substituted cyclobutanes from cinnamates, chalcones, and styrenes has been developed utilizing a visible-light triplet sensitisation mode. This reaction provides a diverse range of substituted cyclobutanes in high yields under mild conditions without the need of external additives. Good regioselectivity is obtained due to strong π-π-stacking of arene moieties, whereas diastereoselectivity relies on the electronic effects or ortho-substitution of the arene substrate. The utility of this transformation is demonstrated by the formal synthesis of the lignane natural product (±)-Tanegool.

Formal Giese addition of C(sp3)–H nucleophiles enabled by visible light mediated Ni catalysis of triplet enone diradicals

“…To overcome such limitations, we envisioned an unprecedented utilization of triplet excited enones, generated by Ir-mediated energy transfer, 13 based on the fact that the two resulting radicals have different electronic characters; the α-radical is electrophilic and the β-radical is nucleophilic. 14 In such a system, the nucleophilic β-radical could be selectively trapped by a Ni catalyst, while the electrophilic α-radical could participate in hydrogen atom transfer (HAT) involving the most hydridic C–H bond of the Giese donor to generate a donor radical.…”

Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization

“…As a green and clean procedure for chemical process and production that are contemporarily demanded by sustainable development, the visible-light sensitized intra-and intermolecular [2 + 2] cycloaddition of olefins has drawn great attention of chemical society in past decades [10][11][12][13][14][15] , leading to fast progress in [2 + 2] photocycloaddition of nonrigid olefins with excellent regio-and stereoselectivities [16][17][18][19][20] . Nevertheless, owing to steric preference and relative radical stability of excited-state intermediates, hitherto, the stereochemical control is mainly achieved for the thermodynamic favored anti-diastereomers from acyclic olefins, e.g., styrenes, chalcones, and cinnamates 16,[21][22][23][24] . A general synthetic strategy for thermodynamic unfavored syn-diastereomers, which present as the important backbones of many biological active natural products [1][2][3] , remains underdeveloped and desired for a long time 25,26 .…”

A Retrosynthetic Approach for Photocatalysis

“…Functional group tolerance comprises methoxy, bromo, and CF 3 . Using similar activation modes the intermolecular [2+2] homocoupling of styrenes, cinnamates, and chalcones, as well as the heterocoupling between cinnamates and 1,1‐diarylethylenes is possible . Bach and co‐workers used chiral xanthone and thioxanthone for inter‐ and intramolecular [2+2] photocycloaddition reactions at 366 nm .…”