cis / trans Isomerism of Hydroalumination and Hydrogallation Products—Reflections on Stability and Rearrangement Mechanism

Treatment of (silylalkynyl)benzenes with (Me 3 C) 2 GaH afforded stable cis ‐addition products, for example, (Me 3 C) 2 GaC(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 AlH with phenyltrimethylsilylethyne. In this case the cis isomer ( 17 ) was detected only at low temperature, while the trans product ( 18 ) formed quantitatively above 0 °C. Quantum‐chemical calculations showed that the trans forms are thermodynamically favored, essentially caused by a better mesomeric interaction of the CC 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 CH bonds of the aromatic rings. The rotation about the CC double bonds follows a zwitterionic mechanism, and the relatively small rotational barrier is further lowered by an interaction to a Lewis acidic lithium cation.