Computational Studies and Additional Experiments

Thus the two plausible catalytic cycles have been considered, one via an Ir dihydride complex A and the other via an IrH2(ri -H2) complex B (Fig. 3). The first is analogous to the well-established mechanism for rhodium diphosphine-catalyzed hydrogenation of olefins going through Ir(I) and Ir(III) intermediates [26-29]. 

Experimental support for an Ir(I)-Ir(III) mechanism was provided by Chen and Dietiker [30]. They reported an elegant experimental investigation of the hydrogenation of styrene with [Ir(PHOX)(COD)]BArp in the gas phase by means of electrospray ionization tandem mass spectrometry. By means of reversible deuterium labeling, the investigators found masses corresponding only to intermediates with a mass corresponding to a dihydride complex with no presence of a trihydride species and concluded that no Ir(V) species with PHOX could be present in the catalytic reaction. 

Based on DFT calculations Brandt et al. proposed a catalytic cycle via Ir(III) and Ir(V) intermediates, in which an additional dihydrogen molecule coordinated to an Ir-dihydride undergoes oxidative addition during migratory insertion [31]. However, since an extremely truncated model for the ligand and substrate (ethylene) was used which neglected the severe steric interactions present in the actual catalysts it 

Fan et al. also reported DFT calculations on the complete hgand and substrate structures with an iridium carbene-based complex for an Ir(III)-Ir(V) catalytic cycle, which reproduced the correct selectivity order for three different substrates [33]. Calculations on complete ligands and real substrates that reproduce experimentally determined enantioselectivities give plausible credence to these authors computational experiments. 

additional experimental and computational studies will be needed to draw definitive conclusions regarding the mechanism of Ir-catalyzed asymmetric hydrogenation. The Ir(I)-Ir(III) and Ir(III)-Ir(V) cycles seem to be similar in energy, so it may well be that depending on the catalyst, substrate, and the hydrogenation conditions, one or the other pathway will be preferred or both cycles could operate in parallel. 

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