The acid-catalyzed formation of carbon-carbon bonds from C1 precursors via CO insertion into chemisorbed methyl groups occurs selectively within eight-membered ring (8-MR) zeolite channels. This elementary step controls catalytic carbonylation rates of dimethyl ether (DME) to methyl acetate. The number of O-H groups within 8-MR channels was measured by rigorous deconvolution of the infrared bands for O-H groups in cation-exchanged and acid forms of mordenite (M,H-MOR) and ferrierite (H-FER) after adsorption of basic probe molecules of varying size. DME carbonylation rates are proportional to the number of O-H groups within 8-MR channels. Na+ cations selectively replaced protons within 8-MR channels and led to a disproportionate decrease in carbonylation turnover rates (per total H+). These conclusions are consistent with the low or undetectable rates of carbonylation on zeolites without 8-MR channels (H-BEA, H-FAU, H-MFI). Such specificity of methyl reactivity upon confinement within small channels appears to be unprecedented in catalysis by microporous solids, which typically select reactions by size exclusion of bulkier transition states.

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Methanol carbonylation is currently used to produce acetic acid with Rh and Ir organometallic complexes and iodide cocatalysts. [1][2][3] Zeolites [4][5][6][7][8][9] and Keggin polyoxometallate clusters [10][11][12][13] also catalyze the carbonylation of alcohols and ethers (Koch reaction) to form carboxylic acids and esters. Surface alkyl groups, formed by alcohol dissociation, alkene protonation, or alkane deprotonation at acidic sites react with CO to form acylium ions, which then form carboxylic acids and esters, as in the reactions of isobutane on sulfated zirconia [14] and tert-butyl alcohol [5,7,8] on acidic ZSM5, MOR, BEA, and Y-zeolites to form pivalic acid. Carbonylation of methanol and dimethyl ether (DME) on acidic zeolites and polyoxometallate clusters occurs concurrently with side reactions (Scheme 1) and significant catalyst deactivation. [4,6,[9][10][11][12][13] We report herein stable and highly selective (> 99 %) halide-free catalysts based on zeolites for DME carbonylation to methyl acetate at low temperatures (423-463 K). The reaction rates are much higher than for methanol carbonylation because water may adsorb competitively at CO binding sites and/or cause parallel methanol dehydration Scheme 1. Network of carbonylation, hydration, dehydration, and methanol-to-hydrocarbon chemistries.

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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The transformation of lower alkanes into higher homologues is still a great challenge in chemistry. [1][2][3] A highly electrophilic tantalum hydride supported on silica, [( SiO) 2 TaH], catalytically transforms a given alkane into its higher and lower homologues-an alkane metathesis reaction. [4,5] This reaction involves selective cleavage and formation of carbon-carbon bonds; for example, propane metathesis selectively gives ethane (45-50 %) and butanes (30-35 %) along with methane and minor amounts of higher alkanes. Moreover, it is possible to catalyze the reverse reaction, which allows methane to be incorporated into higher alkanes.[6] Similarly, the cross-metathesis of ethane and toluene is also catalyzed by [( SiO) 2 TaH]. [7] With [( SiO)Ta( = CHtBu)(CH 2 tBu) 2 ] as a catalyst precursor, [8] the ratio of cross-metathesis products and the selectivity in higher alkanes led us to propose that the key step in alkane metathesis involves metallocarbenes and metallacyclobutane intermediates as is the case with olefin metathesis.

The reaction of [Ta(=CHtBu)(CH2tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylated at 700 degrees C gives the corresponding monosiloxy surface complexes [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2] and [([triple bond]SiO)Ta(CH3)3Cp*] by eliminating a sigma-bonded ligand as the corresponding alkane (H-CH2tBu or H-CH3). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') and [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a'). In the case of [(SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (JC-H = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') as a catalyst precursor and the inactivity of the surface complex [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.

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