BROMIDE ASSISTED ADDITION OF HYDROGEN BROMIDE TO ALKYNES AND ALLENES

The addition of 0.1 M quaternary ammonium bromide to a solution of 20% trifluoroacetic acid in methylene chloride causes a large rate increa se in the reaction of non-conjugated alkynes. The initial vinyl bromid e product reacts further to provide a mixture of isomeric vinyl bromid es and dibromides. At high salt concentrations however, the secondary reactions are prevented and only the initial vinyl bromide is found. I n contrast to the corresponding addition to alkenes, the predominant m echanism is proposed to involve a one-step, concerted, nucleophilic at tack by halide ion upon an acid-alkyne pi-complex which produces the v inyl halide directly. A minor path involving syn addition is also seen . At all salt concentrations, allenes produce significant amounts of r earrangement products suggesting the significant involvement of a cati onic mechanism.