Reduction of gem-Bromofluorocyclopropanes with Organosilicon Hydrides

Abstract

Abstract 7-Bromo-7-fluoronorcarane, 1-bromo-1-fluoro-2-phenylcyclopropane, and 7-bromo-7-fluoro-2-oxanorcarane were reduced with tri-n-butylsilane or di-n-butylsilane at 100–145 °C in the presence of a small amount of radical initiator, to afford the corresponding monofluorocyclopropanes. It has been found, by examining the stereochemistry of the reduction, that the extent of stereospecificity is dependent on the reaction temperature as well as on the kind of the silicon hydride used. Comparison of these results with those of the tri-n-butyltin hydride reduction shows that the reactivity of metal hydrides as a reducing agent of gem-dihalocyclopropanes is in the order Bu3SnH>Bu2SiH2>Bu3SiH. These observations are well explained by postulating that the α-fluorocyclopropyl radical has a pyramidal configuration and that the extent of stereospecificity is determined by the relative rates of its inversion of configuration and its hydrogen abstraction from the tin or silicon hydride. The reduction of 7,7-dibromonoracrane with tri-n-butylsilane or di-n-butylsilane gave a mixture of the two geometrical isomers of 7-bromonoracrane, whose composition (endo-Br/exo-Br\fallingdotseq3.0) was nearly identical irrespective of the reaction temperature and of the reactivity of silicon hydride.