Perfluorocarbenes produced by thermal cracking. Barriers to generation and rearrangement

Thermal crackings of substituted oxiranes to generate various perfluoroalkylcarbenes are examined using ab initio density functional theory. Such reactions are generalizations of a current technology for the preparation of difluorocarbene. Barriers for the generation of fluoro(perfluoroalkyl)carbenes by this approach are computed to be higher than those for generation of difluorocarbene; the difference is attributed primarily to the lower stability of the respective singlet carbenes. Once generated, however, the carbenes 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 conditions. With respect to rearrangements of perfluorocarbenes, 1,2-alkyl shifts are lowest in energy, 1,2-fluorine atom shifts are higher, and 1,3-fluorine atoms shifts are highest of all; this ordering reflects the relative orbital energies of the u 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 extrusion of SiF₄ from perfluorosilylalkanes may be an attractive alternative for perfluorocarbene generation.