Synthetic scope and mechanistic studies of Ru(OH)x/Al2O3-catalyzed heterogeneous hydrogen-transfer reactions.
Synthetic scope and mechanistic studies of Ru(OH)x/Al2O3-catalyzed heterogeneous hydrogen-transfer reactions.
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DOI：
10.1002/chem.200500539
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发表时间：
2005-11
期刊：
影响因子：
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通讯作者：
K. Yamaguchi;T. Koike;M. Kotani;Mitsunori Matsushita;Satoshi Shinachi;N. Mizuno
中科院分区：
文献类型：
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作者：
K. Yamaguchi;T. Koike;M. Kotani;Mitsunori Matsushita;Satoshi Shinachi;N. Mizuno
钌催化剂Ru(OH)x/Al2O3制备简单，价格低廉，可有效促进手性仲醇的外消旋反应、羰基化合物以2-丙醇为供氢体还原为醇以及烯丙醇异构化为饱和酮的三种氢转移反应。各种各样的底物，如芳香族、脂肪族和杂环醇或羰基化合物，可以在厌氧条件下以中等至优异的产量转化为所需的产品，而不需要添加诸如碱之类的添加剂。更大规模的无溶剂反应也得到了证实：当底物/催化剂比为20,000/1时，1-辛烯-3-醇的异构化反应显示出极高的翻转频率（TOF），为18,400 h(-1)，翻转数（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可以重复使用，而催化性能没有明显的损失。用单氘化底物考察了Ru(OH)x/ al2o3催化氢转移反应的反应机理。(S)-1-氘-1-苯乙醇在苯乙酮存在下外消旋化完成后，相应外消旋醇α位置的氘含量为91%，而(S)-1-苯乙醇- od外消旋时α位置没有氘的掺入。这些结果表明，手性仲醇的外消旋和羰基化合物还原为醇的直接碳-氢转移是通过金属一氢化物发生的。在异构化过程中，3-氘-1-辛烯-3-醇的α -氘被选择性地重新定位到相应酮的β位置（β位置有99% D），这表明一氢化钌参与了1,4加成到α， β -不饱和酮中间体中。一氢化钌和α、β -不饱和酮将通过醇的形成/消除而形成。动力学研究和动力学同位素效应表明，钌氢键裂解（氢化物转移）包含在速率决定步骤中。
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.