β-Diketiminate Zinc Complexes for the Hydroamination of Alkynes

A series of electronically and sterically modified beta-diketiminate methylzinc complexes [{N,N'-bis(2,6-diisopropylphenyl)-beta-diketiminato}methylzinc] (1), [{N,N'-bis(2-isopropylphenyl)-beta-diketiminato}methylzinc] (2), [{N,N'-bis(2,4,6-trimethylphenyl)-beta-diketiminato}methylzinc] (3), [{N,N'-bis(4-methoxyphenyl)-beta-diketiminato}methylzinc] (4), a bis(beta-diketiminato)zinc compound [bis{N,N'-bis(4-methoxyphenyl)-beta-diketiminato}zinc] (5), and a beta-diketiminate bis(trimethylsilylamido)zinc complex [{2-(1-cyclohexylimidazolidine-2-ylidene)-1-methylethylidene-(2,6-diisopropylphenyl)aminato}-bis(trimethysilylamido)zinc] (6) were prepared and structurally characterized by single-crystal X-ray diffraction. All methylzinc compounds exhibit a trigonal-planar coordinated zinc atom, with the exception of polymeric compound 4, in which intermolecular coordination of the OMe group to the zinc atom additionally takes place. All zinc complexes were investigated as catalysts in the intramolecular hydroamination of nonactivated alkynes and show high catalytic activity in the presence of the cocatalyst [PhNMe2H][SO3CF3]. The zinc complex 1 shows the highest catalytic activity in most of the cyclization reactions. In this context, it was possible to isolate the intermediate, which was formed by the reaction of the precatalyst 1 and the cocatalyst [PhNMe2H][SO3CF3]. The resulting zinc catalyst [{N,N'-bis(2,6-diisopropylphenyl)-beta-diketiminato}(trifluoromethane-sulfonate)zinc]center dot thf (7) was structurally characterized by single-crystal X-ray diffraction. The zinc complex 7 show the same catalytic activity in the hydroamination when an equimolar mixture of the precatalyst 1 and the cocatalyst was used. It is supposed that the high steric bulk of the precatalyst 1 protects the Lewis acid zinc center from polar functional groups but allows interactions for the cyclization process.