Visible-Light-Activated Divergent Reactivity of Dienones: Dimerization in Neat Conditions and Regioselective E to Z Isomerization in the Solvent

Abstract

2,4-Dienones undergo visible-light-promoted, photocatalyst-free dimerization in neat conditions to provide cyclohexene derivatives stereoselectively through cascade rearrangement pathways, whereas regioselective E → Z isomerization of the more dienophilic double bond takes place exclusively in nitromethane. On the basis of intermediate isolation and computational DFT studies, the dimerization reaction is proposed to proceed via s-trans to s-cis isomerization/regioselective E → Z isomerization/Diels−Alder cycloaddition.

C3‐Thioester/‐Ester Substituted Linear Dienones: A Pluripotent Molecular Platform for Diversification via Cascade Pericyclic Reactions

“…During our previous study under photochemical conditions also, we had not detected/isolated any dimerized product 3 arising from intermediate I . This may be ascribed to the fact that photoabsorption by the E ‐isomer excites the molecule to a higher energy state from where the formation of Z ‐isomer via the biradical mechanism may be a facile and viable process than oxa‐6π‐electrocyclization; subsequently the Z ‐isomer underwent rapid dimerization to afford 2 as the only isolated product . These results support the rationale behind the isolation of dimerized products 2 (previous work) and 3 (this work) exclusively under photochemical and thermal conditions, respectively.…”

“…We have quite recently reported that intermediate I underwent dimerization under photochemical conditions to yield 2 through an uphill E / Z isomerization of C 2 −C 3 double bond followed by a Diels‐Alder cycloaddition . In the present study carried out under thermal conditions, we observed the formation of dimerized product 3 exclusively.…”

“…Therefore, intermediate I could not follow the E / Z isomerization pathway to deliver 2 under thermal conditions (Scheme ). During our previous study under photochemical conditions also, we had not detected/isolated any dimerized product 3 arising from intermediate I . This may be ascribed to the fact that photoabsorption by the E ‐isomer excites the molecule to a higher energy state from where the formation of Z ‐isomer via the biradical mechanism may be a facile and viable process than oxa‐6π‐electrocyclization; subsequently the Z ‐isomer underwent rapid dimerization to afford 2 as the only isolated product .…”

“…In the present study carried out under thermal conditions, we observed the formation of dimerized product 3 exclusively. Concerning the differences in reactivity between the photochemical and thermal processes and subsequent formation of the different dimerized products 2 and 3 , respectively, we presume that a barrier of 35 kcal/mol (for 1 o ) between E and Z isomers proves to be too large to be surmounted by the thermal events (at 50 °C reaction temperature), particularly in the absence of any catalyst and solvent. Therefore, intermediate I could not follow the E / Z isomerization pathway to deliver 2 under thermal conditions (Scheme ).…”

“…Although substantial progress has been made with the dienals, reagent‐based cascade reactions of the corresponding C 3 ‐carbonyl substituted dienones have not been reported until recently due to the paucity of methods available to access them . Over the past few years, we have been involved in the development of synthetic methods for cyclopropanated furans, regioselective E → Z isomerization of olefins, and butenolides from dienones 1 . Recently, we have reported that neat 1 underwent dimerization due to a substituent‐driven acceleration effect via sequential s‐trans to s‐cis isomerization/regioselective E/Z isomerization/Diels‐Alder cycloaddition to provide cyclohexene derivatives 2 under direct excitation of the reactant by visible light absorption (Scheme ) .…”