Promotion of Iridium-Catalyzed Methanol Carbonylation: Mechanistic Studies of the Cativa Process
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- 作者：Anthony Haynes ; Peter M. Maitlis ; George E. Morris ; Glenn J. Sunley ; Harry Adams ; Peter W. Badger ; Craig M. Bowers ; David B. Cook ; Paul I. P. Elliott ; Talit Ghaffar ; Helena Green ; Tim R. Griffin ; Marc
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：March 10, 2004
- 年：2004
- 卷：126
- 期：9
- 页码：2847 - 2861
- 全文大小：272K
- 年卷期：v.126,no.9(March 10, 2004)
- ISSN：1520-5126
文摘
The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonylcomplexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu4NI)are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H2O levels 
5%w/w) is fac,cis-[Ir(CO)2I3Me]-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)2I3(COMe)]-, 6, isaccelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)3I2]2, [Ru(CO)2I2]n,InI3, GaI3, and ZnI2). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)3I3]-, [Ru(CO)2I4]2-) do not promotecarbonylation of 2 and Bu4NI is an inhibitor. Mechanistic studies indicate that the promoters acceleratecarbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give[Ir(CO)3I2Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700times faster for 4 than for 2 (85 
C, PhCl), representing a lowering of 
G
by 20 kJ mol-1. Ab initio calculationssupport a more facile methyl migration in 4, the principal factor being decreased 
-back-donation to thecarbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)2I3(COMe)]-, 6a (as its Ph4As+ salt), wascharacterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)mIn](M = Ru, In; m = 3, 0; n = 2, 3) binds I- to form [M(CO)mIn+1]-H3O+ and catalyzes the reactionHI(aq) + MeOAc 
MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalyticturnover via neutral 4.
5%w/w) is fac,cis-[Ir(CO)2I3Me]-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)2I3(COMe)]-, 6, isaccelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)3I2]2, [Ru(CO)2I2]n,InI3, GaI3, and ZnI2). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)3I3]-, [Ru(CO)2I4]2-) do not promotecarbonylation of 2 and Bu4NI is an inhibitor. Mechanistic studies indicate that the promoters acceleratecarbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give[Ir(CO)3I2Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700times faster for 4 than for 2 (85 C, PhCl), representing a lowering of G by 20 kJ mol-1. Ab initio calculationssupport a more facile methyl migration in 4, the principal factor being decreased -back-donation to thecarbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)2I3(COMe)]-, 6a (as its Ph4As+ salt), wascharacterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)mIn](M = Ru, In; m = 3, 0; n = 2, 3) binds I- to form [M(CO)mIn+1]-H3O+ and catalyzes the reactionHI(aq) + MeOAc MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalyticturnover via neutral 4.