Carbene chemistry. Part X. Insertion reactions of 1,2,2-trifluoroethylidene into C–H bonds of alkanes containing the t-butyl group and some addition and insertion reactions of 2-chloro-1,2,2-trifluoroethylidene

Abstract

1,2,2-Trifluoroethylidene, generated by pyrolysis of trifluoro-(1,1,2,2-tetrafluoroethyl)silane, inserts exclusively into the tertiary C–H bond of the alkane Me₂CH·CMe₃, into both the primary and secondary C–H bonds (ratio 1 : 2) of the ethyl group in the alkane EtCMe₃, and into both the tertiary and secondary C–H bonds (ratio ca. 8 : 1) of the isobutyl group in the alkane Me₂CH·CH₂·CMe₃. (2-Chloro-1,1,2,2-tetrafluoroethyl)trifluorosilane is best prepared by fluorination of trichloro-(1,1,2,2-tetrafluoroethyl)silane followed by vapour-phase photochemical chlorination. On pyrolysis the silane affords 2-chloro-1,2,2-trifluoroethylidene, which inserts into the Si–H bond of trimethylsilane and reacts with allene to afford 1-chlorodifluoromethyl-1-fluoro-2-methylenecyclopropane. Stereospecific addition of the carbene to the double bond of trans-but-2-ene takes place to give r-1-chlorodifluoromethyl-1-fluoro-c-2,t-3-dimethylcyclopropane, but reaction with cis-but-2-ene is not stereospecific and affords a mixture of the c-2,c-3- and t-2,t-3-dimethylcyclopropanes and the c-2, t-3-dimethylcyclopropane in the ratio 63 : 18.