SYNTHESIS OF THE NEW CHIRAL AMINODIPHOSPHINE LIGANDS (R)- AND (S)-(ALPHA-METHYLBENZYL)BIS(2-(DIPHENYLPHOSPHINO)ETHYL)AMINE AND THEIR USE IN THE ENANTIOSELECTIVE REDUCTION OF ALPHA,BETA-UNSATURATED KETONES TO ALLYLIC ALCOHOLS BY IRIDIUM CATALYSIS

摘要

Synthesis of the potentially tridentate, optically pure ligands C6H5C*H(Me)N(CH(2)CH(2)PPh(2))(2) (R)-(+)- and (S)-(-)-PNP*; 5a,b) from their corresponding chiral primary amines was achieved with excellent yields. The catalytic hydrogen-transfer reduction of PhCH=CHCOMe and other alpha,beta-unsaturated or unsymmetrical ketones in the presence of [Ir(COD)(OMe)](2) and 5a,b is reported. These catalytic systems show high activity and chemoselectivity for the reduction of the carbonyl group (up to 94%). Due to the presence of the stereogenic center on the PNP* ligands 5a,b, asymmetric induction was also achieved with enantioselectivities up to 54%. Discussion concerning the catalytic pathway and the intermediates involved is reported in conjunction with independent reactions of isolated, optically active compounds. Some of these are the iridium(I) cyclooctadiene complexes (R)- and (S)-[IrH(COD){C6H5C*H(Me)N(CH(2)CH(2)PPh(2))(2)}] (8a,b) and the iridium(III) ortho-metalated dihydrides fac-exo-(R)- fac-exo-(S)-, fac-endo-(R)-, and fac-endo-(S)[IrH2{C6H4C*:H(Me)N(CH(2)CH(2)PPh(3))(2)}] 10a,b and 11a,b). X-ray crystal data are given for fac-exo-(R)-[IrH2-{C6H4C*H(Me)N(CH(2)CH(2)PPh(2))(2)}] (10a), a distorted octahedron. The (R)-PNP* ligand facially coordinates to the iridium metal center, with the two hydride ligands cis to one another. The sixth coordination site is occupied by the ortho carbon atom of the phenyl substituent bound to the stereocenter. Crystal data: monoclinic, P2(1), with a = 12.623(2) Angstrom, b = 18.265(3) Angstrom, c = 14.032(3) Angstrom, beta = 92.55(1)degrees, V = 3232 Angstrom(3), Z = 4, R = 0.037, and R(w) = 0.041. [References: 51]