Hydrogallation of Trimethylsilylethynylbenzenes: Generation of Potential Di- and Tripodal Chelating Lewis Acids

Hydrogallation of 1,4-bis(trimethylsilylethynyl)benzene and 1,3,5-tris(trimethylsilylethynyl)benzene with dialkylgallium hydrides R2GaH (R = Et, nPr, iPr, neopentyl, tBu) afforded the corresponding addition products with intact GaR2 groups and two or three alkenyl substituents. In all products the gallium atoms attacked those carbon atoms that are attached to the trimethylsilyl groups. The expected cis arrangement of gallium and hydrogen atoms at the CC double bonds was detected only with di(tert-butyl)gallium residues. Smaller alkyl groups gave the spontaneous formation of the trans-addition products. Cis/trans isomerization is an inevitable step for the formation of effective chelating Lewis acids, and in particular the trisalkene derivatives form interesting chalice-like hollows containing three Lewis-acidic centers at their inner surfaces.