Organomagnesium-catalyzed isomerization of terminal alkynes to allenes and internal alkynes.

Organomagnesium complexes 2 were synthesized from N,N-dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3-Aryl-1-propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium-catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl-, allenyl-, and propargylmagnesium species.