Intramolecular and intermolecular reactions of alkenylsilyl radicals

Abstract

The radicals formed during photolysis of di-t-butyl peroxide and a number of alkenyldimethylsilanes have been examined by e.s.r. spectroscopy. Only carbon-centred radicals were observed. These were either secondary alkyl radicals formed by the addition of the initially formed silyl radical to a double bond and/or allyl radicals formed by hydrogen-atom abstraction from the alkenyl group. In most cases addition to the double bond was an intermolecular process. However, pent-4-enylsilyl radicals undergo intramolecular addition with endo-cyclization to form the six-membered ring being strongly favoured over exo-cyclization to form the five-membered ring. This preference was confirmed by product studies on 3,3-dimethylpent-4-enyldimethylsilane. The factors that control homolytic exo : endo cyclization rates for species in which the radical centre is located on a second row element are adumbrated.