Free Radical Chemistry of β-Lactones. Arrhenius Parameters for the Decarboxylative Cleavage and Ring Expansion of 2-Oxetanon-4-ylcarbinyl Radicals. Facilitation of Chain Propagation by Catalytic Benzeneselenol

Abstract

2-Oxetanon-4-ylcarbinyl radicals undergo facile ring opening with cleavage of the C−O bond to give 3-butenoxyl radicals which in turn suffer loss of carbon dioxide to provide allyl radicals. When the initial radical is generated from a bromolactone with Bu 3 SnH and AIBN, chain propagation is poor owing to the relatively slow abstraction of hydrogen from the stannane by the allyl radical. The inclusion of catalytic Ph 2 Se 2, reduced in situ to PhSeH, provides for much smoother cleaner reactions because of the better hydrogen donating capacity of the selenol. The oxetanon-4-ylcarbinyl radical derived from 6-benzyl-1-(bromomethyl)-8-oxa-7-oxobicyclo[4.2.0]octane is anomalous and undergoes a radical ring expansion in competition with the fragmentation process. Possible reasons for this anomaly are presented as are Arrhenius functions for the fragmentation and rearrangement. The Arrhenius function for the fragmentation of a simple 2-oxetanon-4-yl radical is also presented. Conditions are described under which the fragmentation of 2-oxetanon-4-yl radicals may be suppressed.