Perfluorocarbenes produced by thermal cracking. Barriers to generation andrearrangement

Thermal crackings of substituted oxiranes to generate various perfluoroalky lcarbenes are examined using ab initio density functional theory. Such reac tions are generalizations of a current technology for the preparation of di fluorocarbene. Barriers for the generation of fluoro(perfluoroalkyl)carbene s by this approach are computed to be higher than those for generation of d ifluorocarbene; the difference is attributed primarily to the lower stabili ty of the respective singlet carbenes. Once generated, however, the carbene s are reasonably stable with respect to unimolecular rearrangement, so that high selectivity for reaction with olefins, such as might be present in an unsaturated polymer, may be expected under typical; experimental condition s. With respect to rearrangements of perfluorocarbenes, 1,2-alkyl shifts ar e lowest in energy, 1,2-fluorine atom shifts are higher, and 1,3-fluorine a toms shifts are highest of all; this ordering reflects the relative orbital energies of the a bonds broken in the respective migrations. In instances where thermal conditions required for oxirane cracking may be incompatible with other functionalities in the reaction medium, theory predicts that ext rusion of SiF4 from perfluorosilylalkanes may be an attractive alternative for perfluorocarbene generation.