CATALYTIC ASYMMETRIC HYDROGENATION OF IMINES - USE OF RHODIUM(I)/PHOSPHINE COMPLEXES AND CHARACTERIZATION OF RHODIUM(I)/IMINE COMPLEXES

An in situ Rh(I)(P-P) catalyst formed from [Rh(NBD)Cl]2 and cycphos (P-P = 1,2-bis(diphenylphosphino)-1-cyclohexylethane) effects asymmetric hydrogenation of some commercially important and model imines in 1:1 benzene/methanol under 1000-1500 psig H2 from -25 to +25-degrees-C; a maximum of 91% ee is obtained for ArC(Me) = NCH2Ph (Ar = 4-MeOC6H4) in the presence of iodide cocatalyst at -25-degrees-C. Two [Rh(diphos)(imine)2]BF4 complexes have been isolated (diphos = 1,2-bis(diphenyl-phosphino)ethane): imine 8 = 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (complex 11) or imine 4 with Ar = 2-MeOC6H4 (complex 16). Complex 11 crystallizes in the triclinic system, space group P1BAR, with a = 12.564 (1) angstrom, b = 21.446 (2) angstrom, c = 10.521 (1) angstrom, alpha = 100.655 (9)-degrees, beta = 110.539 (8)-degrees, gamma = 79.102 (7)-degrees, and Z = 2, the structure refining to R = 6.8% and R(w) = 8.2% for 5881 reflections; the eta(1)-imines bind via nitrogen in a syn arrangement at the essentially square-planar Rh, while in solution an anti isomer is also evident. Other species [Rh(diphos)(eta(1)-imine)(MeOH)]+ and Rh(P-P)Cl(eta(1)-imine), where P-P = diphos or chiral bis(tertiary phosphines), have been characterized in solution, and Rh(diop)(Cl) (8) has been isolated (diop = 2,3-o-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane). Complexes [Rh(diphos)(imine)]+ containing a chelated imine (via the nitrogen lone pair and oxygen of a methoxy functionality) exist, but chelation is not essential for effective asymmetric induction. The catalytic hydrogenations appear to occur via an unsaturate route; a plausible intermediate is Rh(P-P)(X)-(MeOH)(imine), where X = halide and the alcohol facilitates eta(2)-(C = N) binding.