Abstract
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.