Hantzsch Ester-Mediated Visible-Light-Induced Radical Ethoxycarbonyldifluoromethylation of Aryl Alkynes: Kinetic-Controlled Stereoselective Synthesis of Z - gem -Difluoroallyl Esters.

A kinetic-controlled photocatalyst-free visible-light-induced stereoselective synthesis of α,α- gem -difluoro- Z -allyl esters was succeeded at room temperature in moderate to excellent yields with up to Z -isomer-only stereoselectivities through a reductive radical ethoxycarbonyldifluoromethylation of terminal aryl alkynes with blue LED (465 nm)-excited Hantzsch ester. The geometry optimizations obtained by the DFT/B3LYP calculation with a standard 6-31G(d,p) basis set were also employed for the mechanism study on the formation of a key α,α-difluoroallyl ester radical intermediate, which was generated from the addition of the ethoxycarbonyldifluoromethyl radical to aryl alkyne substrates and the stabilization effect of the neighboring group. Subsequent steric hindrance-determined hydrogen generated from the Hantzsch ester radical cation led to the formation of final aryl-substituted Z -difluoroallyl esters. This methodology provided convenient access to the synthetically important Z -configured gem -difluoroallylic building blocks.