Hydrogenation of styrene catalyzed by TpRuH(PPh3)(CH3CN) [Tp＝hydrotris(pyrazolyl)borate] with THF, H2O/THF, CH3CN/THF and ROH/THF (R＝Me, Et, iso-Pr, tert-Bu) as solvents was studied, respectively. It was found that addition of trace amount of small molecule H2O, CH3CN or ROH in anhydrous THF effectively promoted the catalytic reaction. The catalytic mechanism is proposed that PPh3 ligand in TpRuH(PPh3)(CH3CN) is first replaced by H2O, CH3CN or ROH to form intermediates TpRuH(CH3CN)L (L＝H2O, CH3CN or ROH) to reduce the steric hindrance of PPh3, and CH3CN is then replaced by styrene to produce TpRuH(H2C＝CHPh)L. The insertion of η2-styrene into the Ru—H bond followed by reaction with H2 leads to the formation of Ru-alkyl intermediates TpRu(CH2CH2Ph)(H2)L and TpRu[CH(CH3)Ph](H2)L. Intramolecular protonation of the adjacent alkyl ligand by an η2-H2 ligand produces ethylbenzene to fulfill the catalytic cycle.

Key words: styrene, steric hindrance, catalytic hydrogenation, ruthenium hydride complex