Controlling Selectivity in Intermolecular Alkene or Aldehyde Hydroacylation Reactions Catalyzed by {Rh(L2)}+ Fragments

Rhodium(III) dihydrido complexes [Rh(L-2)(H)(2)(acetone)][BAr4F] (Ark = C6H3(CF3)(2)) containing the potentially hemilabile ligands L-2 = 9,9-dimethy1-4,5-bis(diphenylphosphino)xanthene (Xantphos) and [Ph2P(CH2)(2)](2)O (POP') have been prepared from their corresponding norbornadiene rhodium(I) precursors. In solution these complexes are fluxional by proposed acetone dissociation, which can be trapped out by addition of MeCN to form [Rh(L-2)(H)(2)(NCMe)][BAr4F], which have been crystallographically characterized. Addition of alkene (methyl acrylate) to these complexes results in reduction to a rhodium(I) species and when followed by addition of the aldehyde HCOCH2CH2SMe affords the new acyl hydrido complexes [Rh(L-2)(COCH2CH2SMe)H][BAr4F] in good yield. The solid-state and solution structures show a tight binding of the POP' and Xantphos ligands, having a trans-arrangement of the phosphines with the central ether linkage bound. This is similar to the previously reported complex [Rh(DPEphos)(COCH2CH2SMOHNBAr4F] (DPEphos = [Ph2P(C6H4)](2)O). Unlike the DPEphos complex, the Xantphos and POP' ligated complexes are not effective catalysts for the hydroacylation reaction between methyl acrylate and HCOCH2CH2SMe. This is traced to their inability to dissociate the central ether link in a hemilabile manner to reveal a vacant site necessary for alkene coordination. Consistent with this lack of availability of the vacant site, these complexes also are stable toward reductive decarbonylation. Complexes [Rh(Ph2P(CH2)PPh2)(acetone)(2)][BAr4F] (n = 2-5) have also been studied as catalysts for the hydroacylation reaction between methyl acrylate and HCOCH2CH2SMe at 22 degrees C. As found previously, for n=2 this affords the product of alkene hydroacylation, but as the chain length is progressively increased to n= 5, the reaction also progressively changes to favor the product of aldehyde hydroacylation. This is suggested to occur by a decrease in the accessibility of the metal site on increasing the bite angle of the chelate ligand, so that alkene coordination to a putative Rh(III)-acyl hydrido intermediate is progressively disfavored and aldehyde coordination (followed by hydride transfer) is progressively favored. These, and previous, results show that the overall conversion in the hydroacylation reaction can be controlled by the hemilabile nature of the chelating phosphine in the catalyst (e.g.. DPEphos versus Xantphos), and the course of the reaction can also be tuned by changing the bite angle of the phosphine, cf. Ph2P(CH2)(2)PPh2 and Ph2P(CH2)(5)PPh2).