Esters as Radical Acceptors: -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 -borylalkenyl radical to the ester carbonyl group; 3)-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.