The First Complete Identification of a Diastereomeric Catalyst-Substrate (Alkoxide) Species in an Enantioselective Ketone Hydrogenation. Mechanistic Investigations
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- 作者：Christopher J. A. Daley and Steven H. Bergens
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：April 10, 2002
- 年：2002
- 卷：124
- 期：14
- 页码：3680 - 3691
- 全文大小：187K
- 年卷期：v.124,no.14(April 10, 2002)
- ISSN：1520-5126
文摘
The enantioselective hydrogenations of the dialkyl 3,3-dimethyloxaloacetate ketone substrates(2, 3, and 4; alkyl = Me, iPr, and tBu, respectively) were catalyzed by [Ru((R)-BINAP)(H)(MeCN)n(sol)3-n](BF4) (1, n = 0-3, sol = THF or MeOH, (R)-BINAP = (R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) inup to 82% ee (R). Reaction of the active catalyst 1 with 1 equiv of substrate (2, 3, or 4) in THF or MeOHsolution formed the diastereomeric catalyst-alkoxide complexes [Ru((R)-BINAP)(MeCN)(OCH(CO2R)-(C(CH3)2CO2R))](BF4) (5/6 R = Me, 8/9 R = iPr, and 10 R = tBu, respectively) via hydride addition to theketone carbonyl carbon and ruthenium addition to oxygen. The absolute configurations at the alkoxidegroups ((R)- for the major diastereomers 5, 8, and 10) were determined via cleavage of the ruthenium-alkoxide bond with 1 equiv of HBF4·OEt2. The solution structures of the major diastereomer catalyst-alkoxide complexes (5, 8, and 10) were unambiguously determined by variable-temperature NMRspectroscopy. The major diastereomers (5, 8, and 10) had the same absolute configuration as the majorproduct enantiomers from the catalytic hydrogenation of 2, 3, and 4 with 1 as catalyst. The ratio of majorto minor alkoxide diastereomers was similar to the ee of the catalytic hydrogenation. The catalyst-alkoxidecomplexes are formed at temperatures as low as -30 
C with no other precursors or intermediates observedby NMR showing that ketone-hydride insertion is likely not the turnover limiting step of the catalytichydrogenation. Results from the stoichiometric hydrogenolysis of 5/6, 8/9, or 10 indicate that their formationis 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 thestereoselectivities of the respective catalytic hydrogenations of 2, 3, and 4. The rates of the hydrogenolysiswere consistent with these diastereomers being true catalytic intermediates.
C with no other precursors or intermediates observedby NMR showing that ketone-hydride insertion is likely not the turnover limiting step of the catalytichydrogenation. Results from the stoichiometric hydrogenolysis of 5/6, 8/9, or 10 indicate that their formationis 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 thestereoselectivities of the respective catalytic hydrogenations of 2, 3, and 4. The rates of the hydrogenolysiswere consistent with these diastereomers being true catalytic intermediates.