Reactivity and selectivity in the cyclization of sila-5-hexen-1-yl carbon-centered radicals

Abstract

A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri- n -butyltin hydride (deuteride). The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially exo-trig ) cyclization compared to the all-carbon system. The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system. The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate via the expected exo-trig mode and and from a significantly enhanced rate of hydrogen abstraction from TBTH . Both the α- and γ-silyl radicals cyclized via the endo-trig mode at rates close to that of the parent 5-hexen-1-yl radical itself. The cyclizations studied were demonstrated to be irreversible. The kinetic control thus shown by the preferred formation of endo cyclized product from the α- and β-silyl radicals is highly unusual and represents the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule exo-trig cyclization preferred by 5-hexen-1-yl radicals). Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors. The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no exo mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet. Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.