The first complete identification of a diastereomeric catalyst-substrate (alkoxide) species in an enantioselective ketone hydrogenation. Mechanistic investigations.

Abstract

The enantioselective hydrogenations of the dialkyl 3,3-dimethyloxaloacetate ketone substrates (2, 3, and 4; alkyl = Me, (i)Pr, and (t)Bu, respectively) were catalyzed by [Ru((R)-BINAP)(H)(MeCN)(n)(sol)(3-n)](BF(4)) (1, n = 0-3, sol = THF or MeOH, (R)-BINAP = (R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) in up to 82% ee (R). Reaction of the active catalyst 1 with 1 equiv of substrate (2, 3, or 4) in THF or MeOH solution formed the diastereomeric catalyst-alkoxide complexes [Ru((R)-BINAP)(MeCN)(OCH(CO(2)R)-(C(CH(3))(2)CO(2)R))](BF(4)) (5/6 R = Me, 8/9 R = (i)Pr, and 10 R = (t)Bu, respectively) via hydride addition to the ketone carbonyl carbon and ruthenium addition to oxygen. The absolute configurations at the alkoxide groups ((R)- for the major diastereomers 5, 8, and 10) were determined via cleavage of the ruthenium-alkoxide bond with 1 equiv of HBF(4).OEt(2). The solution structures of the major diastereomer catalyst-alkoxide complexes (5, 8, and 10) were unambiguously determined by variable-temperature NMR spectroscopy. The major diastereomers (5, 8, and 10) had the same absolute configuration as the major product enantiomers from the catalytic hydrogenation of 2, 3, and 4 with 1 as catalyst. The ratio of major to minor alkoxide diastereomers was similar to the ee of the catalytic hydrogenation. The catalyst-alkoxide complexes are formed at temperatures as low as -30 degrees C with no other precursors or intermediates observed by NMR showing that ketone-hydride insertion is likely not the turnover limiting step of the catalytic hydrogenation. Results from the stoichiometric hydrogenolysis of 5/6, 8/9, or 10 indicate that their formation is rapid and only partially reversible prior to the irreversible hydrogenolysis of the ruthenium-oxygen bond. The stereoselectivities of the formation and hydrogenolysis of 5/6, 8/9, and 10 sum up to equal the stereoselectivities of the respective catalytic hydrogenations of 2, 3, and 4. The rates of the hydrogenolysis were consistent with these diastereomers being true catalytic intermediates.