Mechanistic Investigations on Hydrogenation, Isomerization and Hydrosilylation Reactions Mediated by a Germyl-Rhodium System.

We recently disclosed a dehydrogenative double C-H bond activation reaction in the unusual pincer-type rhodium-germyl complex [(Ar)ClGeRh] (Ar=CH-2,6-(CH-2,4,6-Me)). Herein we investigate the catalytic applications of this Rh/Ge system in several transformations, namely semihydrogenation of internal alkynes, -isomerization of olefins and hydrosilylation of alkynes. We have compared the activity and selectivity of this catalyst against other common rhodium precursors, as well as related sterically hindered rhodium complexes, being the one with the germyl fragment superior in terms of selectivity towards -isomers. To increase this selectivity, a tandem catalytic protocol that incorporates the use of a heterogeneous catalyst for the -semihydrogenation of internal alkynes has been devised. Kinetic mechanistic investigations provide important information regarding the individual catalytic cycles that comprise the overall -semihydrogenation of internal alkynes.