A new, efficient, catalytic
difluorocarbenation of olefins to give 1,1-difluorocyclopropanes is presented.
The catalyst, an organobismuth complex, uses TMSCF<sub>3</sub> as a stoichiometric
difluorocarbene source. We demonstrate both the viability and robustness of this
reaction over a wide range of alkenes and alkynes, including electron-poor alkenes,
to generate the corresponding 1,1-difluorocyclopropanes and
1,1-difluorocyclopropenes. Ease of catalyst recovery from the reaction mixture
is another attractive feature of this method. In depth experimental and
theoretical studies showed that the key difluorocarbene-generating step
proceeds through a bismuth non-redox synchronous mechanism generating a highly
reactive free CF<sub>2</sub> in an endergonic pre-equilibrium. It is the reversibility
when generating the difluorocarbene that accounts for the high selectivity,
while minimizing CF<sub>2</sub>-recombination side-reactions.
Experimental evidence for the existence of a second partially-ordered phase of ice VI