Catalysis with rhodium complexes of monophosphines

Further research showed convincingly that the RhL2 species are likely to be responsible for the enantioselective catalysis that makes the mechanisms of the asymmetric hydrogenation catalyzed by Rh complexes with mono- or diphosphine ligands similar. 

Computations confirmed that 155a is 6-8 kcal/mol less stable than 156a (depending on the conformation), hence the former cannot be detected in the NMR spectra, but is nevertheless kinetically accessible. 

In less polar CD2CI2 hydrogenation stopped at the stage of the solvate complex 154 which reversibly formed dimer 157 and gave diastereomeric chelate catalyst-substrate complexes re-158 and si-158 in a 4 1 ratio upon addition of MAC.  

If the reaction product could be obtained directly via oxidative addition of H2 to sz-158 and re-158, then their hydrogenation at decreased 

Therefore, fhe sfereoselecfion occurs on a later stage of the catalytic cycle, after selective coordination of fhe double bond with one of its pro-chiral planes to the octahedral Rh(lll) complex that already contains activated hydrogen. The migratory insertion step is characterized by very low activation barriers, hence it takes place immediately after the double bond acquires the appropriate conformation coplanar wito the P -Rh-H bond. 

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