Formation of C=C Double Bonds by Novel Insertion Reactions of Allenes, Heterocumulenes and Acid Amides into M=C Bonds of Fischer Carbene Complexes

Abstract

Methodologies for making carbon-carbon bonds are of central interest to organic synthesis. There are many methods for the transformation of M-C into C-C single bonds but only few for the transformation of M=C into C=C double bonds. Here we present three new types of insertion reactions into M=C bonds of Fischer carbene complexes, by which C=C bonds are obtained. Reaction of type 1 involves insertions of allenes into M=C bonds to give tri(methylene)methane complexes. These may further undergo [3+2] cycloadditions and form carbocyclic five-membered rings or undergo metal induced H-shifts to give dienes. Reactions of type 2 involve insertions of isocyanides into M=C bonds to give ketenimine complexes with highly variable N=C=C (= ketenimine) ligands. The latter have been used, e.g. for the synthesis of four-membered N-heterocycles by the addition of isocyanides and alkynes respectively. Reaction of type 3 involves an insertion of a C₂-unit into M=C bonds on condensation of carbene complexes with acid amides and leads to the formation of alkenylaminocarbene complexes. We further investigate reactions of C=N=C (= 2-azaallenyl) ligands, which may be obtained by condensation of aminocarbene complexes with a wide variety of organic carbonyl compounds.