Rhodium dihydrides

Scheme 2.2 The formation of the rhodium dihydride complex via a cyclometallated intermediate...

In-Situ PHIP-NMR Observation of Mono- and Binuclear Rhodium Dihydride Complexes... 

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... 

The problems which remain in understanding and interpreting rhodium asymmetric hydrogenation arise from a persistent lack of information on the presumed rhodium dihydride without which the pathway between the enamide complex and the turnover limiting TS for H2 addition (i.e. the step in which the enantioselectivity of the reaction is set) remains opaque, and hence the overall understanding is elusive. [52]... 

Figure 9.17 The first rhodium dihydrides in enantioselective enamide hydrogenation that were detected using PHIP NMR spectroscopy. A Experimental spectrum. B Computer simulation [36].

With this system, we finally succeeded in characterizing the first rhodium dihydride species in the asymmetric hydrogenation of enamides. Additionally, we succeeded afterwards in the characterization of all the possible catalyst dihydride species [39]. In subsequent work, now knowing what to look for and where to look, all transient complexes in the asymmetric enamide hydrogenation with the Rh(PHA-NEPHOS) catalyst could also be observed with classical NMR techniques [37]. 

With respect to the formation of mono- and dihydride sites, Schrock and Osborn (16) observed an interesting equilibration between a rhodium monohydride complex and a rhodium dihydride complex as expressed by Eqs. (7) and (8). 

Halpern et al. found that the kinetics of the hydrogenation of cyclohexene by the rhodium dihydride are consistent with Scheme 5.28, with = 3.4x10" and itg = 0.2 s" at 25°C in benzene. 

Heinrich, H. Giernoth, R. Bargon, J. Brown, J. M. Observation of a stable cis-diphosphine solvate rhodium dihydride derived from PHANEPHOS. Chem. Commun. 2001, 1296-1297. 

These systems use BLs that provide intercomponent coupling and a photoreactive rhodium dihydride system. The authors demonstrated that these complexes undergo photoinduced reductive elimination of hydrogen when irradiated at wavelengths < 436 nm (Fig. 10). ... 

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