Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins and Its Application for the Syntheses of Coumarins

Abstract

Photocatalytic isomerization of thermodynamically stable E-alkene to less stable Z-alkene has been the subject of numerous studies, being successfully achieved mainly under UV irradiation. Recent development of visible light photoredox catalysis has witnessed it emerging as a powerful tool for the access of new structural complexity and many challenging targets. Herein, we report a visible light-promoted E to Z isomerization of cinnamates. When E-isomer of cinnamates was irradiated with blue light in the presence of an organo-photocatalyst, fac-Ir(ppy) 3 , Z-isomer was exclusively obtained in high yields and with good selectivity. The mild, convenient reaction condition has made this protocol an effective synthetic methodology, which was subsequently implemented in an efficient synthesis of coumarins.

Photoinduced C(sp3)−N Bond Cleavage Leading to the Stereoselective Syntheses of Alkenes

“…[7h, 18a] Cation intermediate C then undergoes elimination to form alkenep roducts 3 with the kinetically and thermodynamically favored E configuration. Here, the CÀC bond-forming step involves ar adicala ddition step rather than at ransition-metal-mediated insertion/extrusionp rocess, thus avoidingt he undesired b-H elimination.O nt he other hand, although E-alkenes are thermodynamically more stable than their Z isomers, the uphill E-to-Z isomerization can be efficiently realized photochemically.T aking account especially of the recently reported photocatalytico rthogonal E-to-Z approaches, [20] we also considered that the E-alkene product might be isomerized in situ to the Z form in the presence of a suitable photocatalyst, thus enabling controlo ft he E/Z selectivity (Scheme 2).…”

Selective Synthesis of Z‐Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light‐Promoted Photocatalytic E to Z Isomerization

“…In recent years, photocatalysts such as iridium‐base triplet photosensitisers, organic dyes such as riboflavin, aromatic keto compounds such as 2‐iodo‐9‐fluorenone, and covalent organic frameworks (COF) have been employed to promote the E to Z isomerization of alkenes (Scheme b). Detailed investigations by Weaver, Rueping, Gilmour, Wang, Balaraman and others, carefully delineated the sensitized isomerization mechanisms and delivered the E to Z isomerization of stilbenes, allylamines, α,β‐unsaturated esters, cinnamonitriles, allylic alcohols, styrenyl boron species and vinyl silanes . Inspired by these seminal investigations, we aim to realize stereoselective synthesis of cis ‐cinnamyl ethers via photocatalytic isomerization of the easily accessed trans ‐isomers with high fidelity (Scheme c).…”

Photochemical Alkene Isomerization for the Synthesis of Polysubstituted Furans and Pyrroles under Neutral Conditions

“…Buildingo nt he seminal report of House, [10] Alexakis [11] and Gilmour [12] independently demonstrated the photochemically enabledi somerizationr eactiono fa-b unsaturated carbonyls from their more stable E geometry to the Z isomer in synthetically useful yields (Scheme 1, middle). [14][15][16] Alexakis and coworkersa ccessed ap hotostationary equilibrium between E and Z enone isomerst hrough directe xcitation of the alkene with am ercury lamp,f rom which the Z isomer was isolated in 42 %y ield. Gilmour and co-workerss uccessfully effected selective E-to-Z acrylate isomerization using (À)-riboflavin as at riplet sensitizer.B ye xploiting the stericc lash between the phenyl and carbonyl substituents of the Z isomer they could deconjugate the chromophore and prevent the reverse isomerization.…”

“…This controlr eaction suggests that any decomposition of the chloroform solvent by UV light to HCl and subsequent Brønsted acid-mediated isomerization is not ac ontributing reaction pathway. [16] Full consumption of enone 3a was observed when conducting the reaction with a2 54 nm source but furan 4a was isolated in only 68 %y ield (Entry 10 vs. 2). The remainder of the mass balance was made up of decomposition side-products, as expected by the absorbance at this wavelength by the furan product 4a.P leasingly,t he key reaction could also be scaled up to 3.5 mmol with minimal reduction in yield, althoughalonger reactiont ime was required (Entry 11).…”