Solvate dihydrides formation

Formation of solvate dihydrides Experimentally formation of solvate dihydrides has been observed for several Rh-diphosphine catalysts. Thus, two diastereomeric dihydrides 28a, b were obtain via the reversible oxidahve addition of H2 to the solvate complex [Rh(f-Bu-BisP )(CD30D)2]+ (27) and have been characterized by NMR (Scheme 1.9). 

Similar observations of reversible formation of solvate dihydrides were made for other BisP -Rh complexes, for the TangPhos-Rh com-plex, and for the PHANEPhos-Rh complex. ... 

Scheme 1.12 Irreversible formation of solvate dihydrides. (Gridnev, I. D. and Imamoto, T., Chem. Commun., 7447-7464, 2009. Reproduced by permission of The Royal Society of Chemistry.)...

Importantly, the formation of 50 was stereoselective. On the other hand, 50 was shown to exist in an equilibrium with the catalyst-substrate complex 49, the solvate dihydride 47 along with the mixture of catalyst 48 and MAC. Stereochemistry of the hydrogenation product is completely defined in 50, but the reverse reaction can bring it back to numerous interconverting intermediates with flexible stereochemistry. Nevertheless, when 50 is formed again, it results in exactly the same structure that clearly defines the absolute configuration of the hydrogenation product. 

The rapidness of the low-temperature reaction between 28 and 4 was further illustrated by the experiments using monodeuterated solvate dihydrides (Scheme 1.21). These experiments showed that the initial imbalance in the deuterium distribution in favor of the axial position is roughly conserved within low-temperature reactions of the HD solvate dihydrides with the substrate and is equal to that observed xmder the catalytic conditions. This is reflected in the preferential formation of a-deuterated product. Therefore, the formation of the monohydride 69 via 71 must have t en place much faster than the scrambling of the positions of the hydride and the deuteride ligands (the latter has been measured as 1.4 s"i at -80 C for the H2 case). ... 

NMR Studies showed that removal of cod ligand leads to formation of a dimer existing in dynamic equilibrium with monomeric solvate dihydrides. Computational study has been carried out assuming monomeric catalyst (since the substrate could not be accommodated on the dimer) and Ir /Ir " mechanism (since stable molecular H2 complexes were not located). 

On the other hand, the irreversible formation of the two diastereo-meric dihydrides 31a, b was observed when the catalytic precursor having an o-xylyl backbone (30) was hydrogenated. Similarly, the stereoselective irreversible formation of a tetrahydride species 33 was observed in the hydrogenation of the binuclear tetraphosphine-Rh complex 32 (Scheme 1.12). No intermediate solvate complexes were detected in these two hydrogenation experiments. 

Computations showed that indeed in that case the oxidative addition of H2 to the re-coordinated complex, for example, 14, is the fastest mechanism of dihydrogen activation among all conceivable processes (hydrogenations of solvate complex, of nonchelating catalyst-substrate complexes and of sf-coordinated catalyst-substrate complexes are slower). Nevertheless, after formation of the dihydride intermediate 58, the double bond dissociation via the TS3 is much faster than migratory insertion via the TS2 (Scheme 1.17). As a result, the nonchelating dihydride intermediate 60 is formed, and (R)-57a is formed after stereoselective double bond coordination in a Rh(III) octahedral nonchelating complex vide infra). ° ... 

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