Comparative regiochemistry of protonation of (η6-C6H5R)(1,1′,1″-tris(2-diphenylphosphinomethyl)ethane)Mo(0) (R=H, Me and SiMe3)

Our previous mechanistic studies of 1 (R=H) demonstrate it is protonated to give a metal-hydride via a mechanism that involves indirect exo attack on the arene ligand followed by endo proton transfer to the metal. The toluene derivative 1 (R=Me) reacts via a similar mechanism with unusual ortho/meta/para=61:36:3% selectivity. In contrast, the first equivalent of proton hydrolyzes the TMS group of 1 (R=SiMe3) under surprisingly mild conditions and the second equivalent reacts with the product 1 (R=H) to give the metal-hydride via the aforementioned indirect mechanism.