Kinetic and mechanistic aspects of the binding of dihydrogen by bis(ditertiaryphosphine)rhodium(I) tetrafluoroborate complexes, and activity of the dihydrides for catalytic asymmetric hydrogenation of prochiral olefinic acids

The kinetics of the reversible binding of H₂ by solutions of the cationic bis(ditertiaryphosphine)rhodium(I) complexes Rh(dppp)₂⁺ BF₄^- and Rh[(+)-diop]₂⁺ BF₄^-, where dppp = 1,3-bis(diphenylphosphino)propane and (+)-diop = 4S,5S-bis((diphenylphosphino)methyl)-2,2-dimethyl-1,3-dioxolane, are reported. Reversible formation of RhH₂dppp)₂⁺, monitored by the stopped-flow method, analyzes for the expected second-order forward reaction opposed by a first-order off-rate. Formation of RhH₂(diop)₂⁺ proceeds, however, via a two-stage process which is interpreted in terms of intermediate solvated species containing monodentate diphosphine. Kinetic and spectroscopic studies on catalytic hydrogenation of the prochiral olefinic substrate, 2-methylenesuccinic acid, indicate a mechanism involving olefin coordination to the rapidly formed dihydride and subsequent hydrogen transfer; the (+)-diop system shows an unusual increase in optical yield of the R-2-methylsuccinic acid with temperature. Hydrogenation of olefinic acids with bis(diop) species is much slower than with comparable mono(diop) systems, but can lead to higher optical yields (e.g. up to 94% e.e. with (Z)-N-acetamidocinnamic acid).