Yamaguchi Kazuya, Koike Takeshi, Kotani Miyuki, Matsushita Mitsunori, Shinachi Satoshi, Mizuno Noritaka
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Chemistry. 2005 Nov 4;11(22):6574-82. doi: 10.1002/chem.200500539.
Three kinds of hydrogen-transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH)x/Al2O3. A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent-free reaction is also demonstrated: the isomerization of 1-octen-3-ol with a substrate/catalyst ratio of 20,000/1 shows a very high turnover frequency (TOF) of 18,400 h(-1), with a turnover number (TON) that reaches 17,200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH)x/Al2O3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions were examined with monodeuterated substrates. After the racemization of (S)-1-deuterio-1-phenylethanol in the presence of acetophenone was complete, the deuterium content at the alpha-position of the corresponding racemic alcohol was 91%, whereas no deuterium was incorporated into the alpha-position during the racemization of (S)-1-phenylethanol-OD. These results show that direct carbon-to-carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the alpha-deuterium of 3-deuterio-1-octen-3-ol was selectively relocated at the beta-position of the corresponding ketones (99% D at the beta-position), suggesting the involvement of a 1,4-addition of ruthenium monohydride species to the alpha,beta-unsaturated ketone intermediate. The ruthenium monohydride species and the alpha,beta-unsaturated ketone would be formed through alcoholate formation/beta-elimination. Kinetic studies and kinetic isotope effects show that the Ru-H bond cleavage (hydride transfer) is included in the rate-determining step.
三种氢转移反应，即手性仲醇的外消旋化、以2-丙醇作为氢供体将羰基化合物还原为醇以及烯丙醇异构化为饱和酮，可通过易于制备且价格低廉的负载型钌催化剂Ru(OH)x/Al2O3有效促进。在厌氧条件下，各种底物，如芳香族、脂肪族和杂环醇或羰基化合物，均可转化为所需产物，产率适中至优异，且无需碱等添加剂。还展示了更大规模的无溶剂反应：底物/催化剂比例为20,000/1时，1-辛烯-3-醇的异构化表现出非常高的周转频率（TOF）为18,400 h(-1)，周转数（TON）达到17,200。这些反应的催化本质上是多相的，反应后回收的Ru(OH)x/Al2O3可重复使用，且催化性能无明显损失。用单氘代底物研究了目前Ru(OH)x/Al2O3催化的氢转移反应的机理。在苯乙酮存在下(S)-1-氘代-1-苯乙醇的外消旋化完成后，相应外消旋醇α位的氘含量为91%，而在(S)-1-苯乙醇-OD的外消旋化过程中，α位未掺入氘。这些结果表明，对于手性仲醇的外消旋化和羰基化合物还原为醇，直接的碳-碳氢转移通过金属氢化物发生。对于异构化，3-氘代-1-辛烯-3-醇的α-氘选择性地重新定位在相应酮的β位（β位99% D），表明钌氢化物物种向α,β-不饱和酮中间体发生1,4-加成。钌氢化物物种和α,β-不饱和酮将通过醇盐形成/β-消除形成。动力学研究和动力学同位素效应表明，Ru-H键断裂（氢化物转移）包含在速率决定步骤中。
