The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu(4)NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H(2)O levels > or = 5% w/w) is fac,cis-[Ir(CO)(2)I(3)Me](-), 2. The stoichiometric carbonylation of 2 into [Ir(CO)(2)I(3)(COMe)](-), 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)(3)I(2)](2), [Ru(CO)(2)I(2)](n), InI(3), GaI(3), and ZnI(2)). 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)(3)I(3)](-), [Ru(CO)(2)I(4)](2)(-)) do not promote carbonylation of 2 and Bu(4)NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)(3)I(2)Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 degrees C, PhCl), representing a lowering of Delta G(++) by 20 kJ mol(-1). Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased pi-back-donation to the carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)(2)I(3)(COMe)](-), 6a (as its Ph(4)As(+) salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)(m)I(n)] (M = Ru, In; m = 3, 0; n = 2, 3) binds I(-) to form [M(CO)(m)I(n+1)](-)H(3)O(+) and catalyzes the reaction HI(aq) + MeOAc --> MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic turnover via neutral 4.

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In ethanol, [RhX(CO)(PEt 3 ) 2 ] added directly or formed in situ from [Rh 2 (OAc) 4 ]ؒ2MeOH (OAc = O 2 CMe) and PEt 3 or [Rh(OAc)(CO)(PEt 3 ) 2 ] catalysed the carbonylation of CH 2 ᎐ ᎐ CHCH 2 X (X = Cl, Br or I) to ethyl but-3enoate with CH 2 ᎐ ᎐ CHCH 2 OEt as a side product. Small amounts of the isomerisation product, ethyl but-2-enoate were produced but no base was required for the reaction. The selectivity of the reaction is in the order Cl > Br > I and prop-2-en-1-ol can be successfully carbonylated to prop-2-enyl but-3-enoate by the same system using 3-chloroprop-1-ene as a promoter. 3-Fluoropropene was not carbonylated, but in the presence of H 2 underwent hydroformylation to produce acetals. 3-Chlorobut-1-ene and 1-chlorobut-2-ene both produced ethyl pent-3-enoate and 3-ethoxybut-1-ene. In situ and ex situ NMR and IR spectroscopic studies have been used to show that the first step of the reaction is oxidative addition to give [Rh(CH 2 CH᎐ ᎐ CH 2 )Cl 2 (CO)(PEt 3 ) 2 ] for which thermodynamic parameters have been obtained. Both 3-chlorobut-1-ene and 1-chlorobut-2-ene give [Rh(CH 2 CH᎐ ᎐ CHMe)Cl 2 -(CO)(PEt 3 ) 2 ] but with different E : Z ratios. The detailed mechanism of the oxidative addition is discussed. The CO inserts into the Rh᎐C bond to give [Rh(COCH 2 CH᎐ ᎐ CH 2 )Cl 2 (CO)(PEt 3 ) 2 ], from which but-3-enoyl chloride reductively eliminates to react with ethanol to give the observed products. High-pressure IR and high-pressure NMR studies reveal that [RhX(CO)(PEt 3 ) 2 ] (X = Cl or Br) reacts with CO to give [RhX(CO) 2 (PEt 3 ) 2 ], which exists as two isomeric forms. The compound [Rh(OAc)(CO)(PEt 3 ) 2 ] catalyses the formation of prop-2-enyl ethanoate from 1-chloroprop-2-ene and sodium ethanoate. A mechanism is proposed. ExperimentalThe NMR spectra were recorded on a Brüker AM300 spectrometer operating in the Fourier-transform mode with, † In honour of Sir Geoffrey Wilkinson, FRS, a great friend and mentor as well as a superb chemist.

Using rhodium complexes of tertiary phosphines with carbonyl groups b to the P atom, ethene and CO react in methanol to give products involving increased chain growth (octane-3,6-dione, methyl 4-oxohexanoate) compared with PEt 3 complexes and unsaturated products (methyl propenoate, penten-3-one and 1-methoxypentan-3-one from addition of methanol to penten-3-one); mechanistic studies suggest that the ligand carbonyl group prevents coordination of the keto group in the growing chain.