cis/trans Isomerism of hydroalumination and hydrogallation products-reflections on stability and rearrangement mechanism.

Treatment of (silylalkynyl)benzenes with (Me(3)C)(2)Ga-H afforded stable cis-addition products, for example, (Me(3)C)(2)Ga-C(SiMe(3))=C(H)-C(6)H(5) (1), while spontaneous cis/trans rearrangement was observed for sterically less shielded gallium hydrides. The corresponding trans-di(tert-butyl)gallium compounds (13, 14) were obtained by the reaction of C(6)H(6-n)[C(H)=C(SiMe(3))GaCl(2)](n) (11, 12) with LiCMe(3). In contrast, spontaneous isomerization took place upon reaction of (Me(3)C)(2)Al-H with phenyltrimethylsilylethyne. In this case the cis isomer (17) was detected only at low temperature, while the trans product (18) formed quantitatively above 0 degrees C. Quantum-chemical calculations showed that the trans forms are thermodynamically favored, essentially caused by a better mesomeric interaction of the C==C double bonds with the phenyl groups, a smaller steric stress in the molecules, and a short bonding contact of the coordinatively unsaturated Al or Ga atoms to C-H bonds of the aromatic rings. The rotation about the C=C double bonds follows a zwitterionic mechanism, and the relatively small rotational barrier is further lowered by an interaction to a Lewis acidic lithium cation.