A Combined DFT/IM-MS Study on the Reaction Mechanism of Cationic Ru(II)-Catalyzed Hydroboration of Alkynes

Recently, the Furstner group reported the first general trans-hydroboration of internal alkynes by using a cationic ruthenium(II) complex, [Cp*Ru(MeCN)(3)]PF6, as the catalyst. Density functional theory (DFT) calculations have been carried out to elucidate the reaction mechanism and the origin of stereo selectivity. The reaction mechanism was suggested to initiate with the rate-determining oxidative hydrogen migration to stereo selectively form a metallacyclopropene intermediate (that determines the trans selectivity), followed by a reductive boryl migration to form the unusual trans-addition alkenyl-borane product. A combined ion-mobility mass spectrometry (IM-MS) and DFT study has also been employed to investigate the unsuccessful reaction with terminal alkynes. Key oxidative-coupling intermediates have been identified. Our results suggest that [2 + 2 + 2] cycloaddition of terminal alkynes to form a very stable arene compound could be the reason for the unsuccessful hydroboration of the terminal alkynes. Moreover, unreactive catecholborane reagent attributes the strong coordination of its arene part with the catalyst. Our proposed nonclassical mechanism also accounted for the other related Ru(II)-catalyzed reactions (such as hydrogenation and hydrogermylation). Our combined computational and experimental study provides in-depth mechanistic understanding and insights on the unusual trans-addition catalyzed by the cationic ruthenium(II) complexes and could help design the other trans-addition reactions.