Esters as Radical Acceptors: β-NHC-Borylalkenyl Radicals Induce Lactonization by C-C Bond Formation/Cleavage on Esters.

Substituted propargyl acetates are converted into 4-boryl-2(5H)-furanones upon thermolysis in the presence of an N-heterocyclic carbene borane (NHC-borane) and di-tert-butyl peroxide. The acetyl methyl group is lost during the reaction as methane. Evidence suggests that the reaction proceeds by a sequence of radical events including: 1) addition of an NHC-boryl radical to the triple bond; 2) cyclization of the resultant β-borylalkenyl radical to the ester carbonyl group; 3) β-scission of the so-formed alkoxy radical to provide the 4-boryl-2(5H)-furanone and a methyl radical; and 4) hydrogen abstraction from the NHC-borane to return the initial NHC-boryl radical and methane.