Dihydride route, rhodium complexes

Coordination of the -SOT group of the ligand to the rhodium may, indeed, be important in the observed effect. It was found by Buriak and Osborn [146,147] that in microemulsions, prepared with the surfactant AOT (Scheme 3.11) the sulfonate group of AOT did coordinate to rhodium in the [Rh (-)-DBPP (NBD)]+ complex. It was suggested that this led to an easier deprotonation of an intermediate dihydride species in case of RS03 than for example in case of T (Scheme 3.28), i.e. to a switch from a dihydride route of hydrogenation to a monohydride pathway. How this would lead to high enantioselection still remains elusive. 

Whatever the route to a rhodium dihydride alkene complex, the hydrogen must be transferred sequentially to the double bond. It had always been assumed that the first C-H bond is formed / to the amido-group, so that the more stable Rh-substrate chelate is formed. This is the alkylhydride isomer observed in stoichiometric NMR studies at low temperatures, and is supported by studies under catalytic turnover conditions, assuming a normal isotope effect... 

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