Reactions of terminal alkynes with a bulky dialkylaluminum hydride:: Hydroalumination versus deprotonation

In recent investigations we found an easy access to dialkyl(alkynyl)aluminum compounds by the deprotonation of phenylethyne with di(tert-butyl)- and dimethylaluminum hydride with evolution of molecular hydrogen. In contrast, the reactions of the bulky hydride R2Al-H [R = CH(SiMe3)(2)] with different alkynes R-C C-H (R = CMe3, SiMe3, C6H5, H) yielded vinylic aluminum compounds, R2Al-CH=CH-R', by hydroalumination. Due to the effective steric shielding of the bulky alkyl substituents, the products were monomeric even in the solid state, with the alkenyl groups bonded to coordinatively unsaturated, tricoordinated aluminum atoms. Quantum-chemical calculations verified that there was no interaction between the pi-electrons of the C=C bond and the vacant p-orbital at the coordinatively unsaturated aluminum atoms.