A New Stepwise Mechanism for Formation of a Metallacyclobutane via a Singlet Diradical Intermediate

Abstract

Density functional calculations (M06/def2-TZVP//M06/LACVP** in THF solvent) reveal that the formation of a perfluorometallacyclobutane from tetrafluoroethylene, C 2 F 4 (TFE), and the difluorocarbene complex CoCp(PPh 2 Me)(CF 2 ) is highly exothermic and involves a stepwise [2 + 2] cycloaddition mechanism with a 1,4-singlet diradical intermediate. This pathway is significantly lower in energy than pathways involving initial η 1 or η 2 binding of TFE to cobalt. The stability of the singlet-diradical intermediate results from the formation of a strong CF 2 –CF 2 bond coupled with the radical stabilizing effect of a difluoromethylene group. A concerted [2 + 2] transition state between the 18-electron complex CoCp(PPh 2 Me)(CF 2 ) and TFE is very high in energy and essentially forbidden.