Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation

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• 作者：Gary L. Williams ; Christopher M. Parks ; C. Robert Smith ; Harry Adams ; Anthony Haynes ; Anthony J. H. M. Meijer ; Glenn J. Sunley ; Sander Gaemers
• 刊名：Organometallics
• 出版年：2011
• 出版时间：November 28, 2011
• 年：2011
• 卷：30
• 期：22
• 页码：6166-6179
• 全文大小：674K
• 年卷期：v.30,no.22(November 28, 2011)
• ISSN：1520-6041

文摘

Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H₂. Residues from catalytic reactions contain a Rh(III) acetyl complex, [Rh(xantphos)(COMe)I₂] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a 鈥減incer鈥?魏³-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh(CO)₂I]₂. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh(xantphos)(COMe)I(L)]⁺, whereas in CH₂Cl₂ migratory CO deinsertion gives [Rh(xantphos)(Me)I(CO)]⁺ (4), which reacts with H₂ to liberate methane, as observed in catalytic reactions using syngas. A number of Rh(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh(xantphos)(CO)]⁺ is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh(xantphos)(CO)I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh鈥揙 distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh-鈥?鈥?O interaction is supported by DFT calculations for the S_N2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P鈥揗鈥揚 bite angle and M鈥揙 distance and division into two broad categories with bite angle <120掳 (cis) or >143掳 (trans).