Hydroalumination versus Deprotonation of Alkynes with Sterically Demanding Substituents

Treatment of sterically highly shielded terminal alkynes, H-C C-aryl, with dialkylaluminium and dialkylgallium hydrides, R2E-H, afforded by hydrogen release dimeric dialkylelement alkynides with a four-membered E2C2 heterocycle independent of the bulk of the aryl groups. A rare example of a monomeric allcynylaluminium compound was only obtained with very bulky CH(SiMe3)(2) groups attached to the metal atoms and by salt elimination reaction. The steric shielding by the bulky aryl groups did not prevent condensation reactions. Hydroalumination of 1-(trimethylsilyl)-2-(2,6-dimethylphenyl)ethyne using Me2Al-H resulted in a divinyl compound by elimination of trimethyl-aluminium.