Synthesis of Z-alkenes via visible light promoted photocatalytic E → Z isomerization under metal-free conditions

Abstract

A green approach for the synthesis of thermodynamically less stable Z-alkenes has been presented through the exploration of visible light promoted photocatalytic E-to-Z isomerization. This protocol features metal-free conditions, which is operationally simple, remarkably clean, and allows the synthesis of both electron-rich and electron-deficient Z-alkenes with diverse functional groups.

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).…”

“…[13] Of which, visible-light-mediated E!Z isomerization of simple alkenes can be achieved by using photosensitizers as a safe and clean energy source (Scheme 1b). [14] This approach has significant appeal both in terms of atom economy and environmentally benign nature.I nr ecent years, photocatalysts such as iridium-base triplet photosensitisers, [15] organic dyes such as riboflavin, [16] aromatic keto compounds such as 2-iodo-9-fluorenone, [17] and covalent organic frameworks (COF) [18] have been employed to promote the E to Z isomerization of alkenes (Scheme 1b). Detailed investigations by Weaver,R ueping, Gilmour,W ang, Balaraman and others, carefully delineated the sensitized isomerization mechanisms and deliveredt he E to Z isomerization of stilbenes, [15b, 17, 18] allylamines, [15a] a,b-unsaturatede sters, [15d, 16a] cinnamonitriles, [16c] allylic alcohols, [19] styrenyl boron species [15f] andv inyl silanes.…”

Pyrimidopteridine N‐Oxide Organic Photoredox Catalysts: Characterization, Application and Non‐Covalent Interaction in Solid State

“…Herein, we showcase the catalytic activity of the synthesized pyrimidopteridine N ‐oxides in representative reactions that have previously been reported using (−)‐riboflavin as organic photoredox catalyst (Scheme ). For instance, the PPTNO photocatalyst demonstrates a similar behavior as compared to the (−)‐riboflavin‐based system by inducing a contra‐thermodynamic E → Z isomerization of cinnamic acid derivatives via productive energy transfer ( E T ) . Methyl cinnamate ( 6 ) was isomerized to a mixture of E ‐ and Z ‐methyl cinnamate ( 7 ) in a 43:57 ratio using phenyl derivative 4 .…”

“…For instance, the PPTNO photocatalyst demonstrates as imilarb ehavior as compared to the (À)-riboflavin-based system by inducingacontra-thermodynamic E!Z isomerization of cinnamica cid derivatives via productive energy transfer (E T ). [14,15] Methyl cinnamate (6)w as isomerized to am ixture of E-a nd Z-methyl cinnamate (7)i na 43:57 ratio using phenyl derivative 4.I nc omparison, (À)-riboflavin promoted the same reaction in a4 1:59 isomeric ratio. The isomerization of E to Z ethyl 3-(4-bromophenyl)but-2enoate (8)p roceeds in lower selectivity of am aximum of 21:79 (E:Z)u sing MePTTNO( 1)a sc ompared to riboflavin system but still exemplifies the proof of concept.…”

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.…”