Highly regioselective synthesis of gem-difluoroallenes through magnesium organocuprate SN2' substitution.

ABSTRACT

[reaction see text]. The reaction of gem-difluoropropargyl electrophiles with Grignard reagents is complicated by the inherent difficulty of executing nucleophilic substitutions on a CF2 group, and the facile formation of carbenoid intermediates arising from alpha-elimination of fluoride. In the presence of an excess amount of a copper salt, a Grignard reagent reacts with gem-difluoropropargyl bromide via an S(N)2' mechanism to produce gem-difluoroallene in high yield. If desired, the resulting difluoroallene can undergo a second nucleophilic attack on the CF2 terminus to yield a trisubstituted monofluoroallene through an addition-elimination mechanism.