122924-35-0Relevant articles and documents
Rhodium-catalyzed reductive carbonylation of methanol
Moloy, Kenneth G.,Wegman, Richard W.
, p. 2883 - 2892 (2008/10/08)
In the presence of diphosphine ligands, CH3I, and synthesis gas, rhodium catalyzes the reductive carbonylation of methanol. With diphosphine = Ph2P(CH2)3PPh2, acetaldehyde is produced in selectivities approaching 90%; the remaining product is acetic acid. The reaction rates for the rhodium-diphosphine catalysts (up to 6 M h-1) rival those for the best previously reported catalysts. More importantly, these rates are achieved at much lower temperatures (130-150°C) and pressures (ca. 1000 psi) than normally employed for this chemistry (e.g., 175-220°C, 4000-8000 psi). If ruthenium is employed as a cocatalyst, acetaldehyde is hydrogenated in situ and ethanol is produced with the same high selectivity and rate. Thus, this catalyst is readily tuned to produce either acetaldehyde or ethanol under relatively mild conditions. The five-coordinate acetyl complexes Rh(diphosphine) (COCH3) (I)2 [diphosphine = R2PYPR2, where R = Ph, p-tol, or P-ClC6H5 and Y = (CH2)2, (CH2)3, CH(CH3) (CH2)2, or (CH2)2C(CH2)2] are the only rhodium and diphosphine-containing species detected at the end of catalysis experiments. They are isolable in nearly quantitative yield and have been characterized by standard methods. These complexes can, in turn be employed successfully as catalysts (e.g., no loss in rate or selectivity) and again be isolated quantitatively. Rh[Ph2P(CH2)3PPh2](COCH 3)(I)2 reacts quantitatively with H2 (100°C, 100 psi) yielding CH3CHO and the hydride Rh[Ph2P(CH2)3PPh2](H)(I) 2. Rh[Ph2P(CH2)3PPh2](H)(I)2 is converted to Rh[Ph2P-(CH2)3PPh2](COCH 3)(I)2 upon treatment with CO in CH3OH. This reaction likely proceeds via reductive elimination of HI to form Rh(I), followed by oxidative addition of CH3I (from HI + CH3OH) and migratory CO insertion. This possibility is verified by the observation that CH3I adds to Rh(diphosphine)(CO)(I) (diphosphine = Ph2P(CH2)nPPh2, n = 2 or 3) at room temperature, yielding the transient but detectable (1H, 31P NMR; IR) complexes Rh[Ph2P(CH2)nPPh2](CO)(I) 2(CH3) (two isomers). These Rh(III) methyl complexes are converted into Rh[Ph2P(CH2)nPPh2] (COCH3)(I)2 at a rate competitive with oxidative addition. Alternatively, treating Rh[Ph2P(CH2)3PPh2] (COCH3)(I)2 with CO in CH3OH results in the catalytic formation of CH3CO2H. Analysis of products from catalytic reactions employing the labeled compounds CH313CO2H, 13CH313CHO, and 13CH3I are consistent with a reaction sequence involving conversion of CH3OH to CH3I, followed by irreversible conversion of CH3I to CH3CHO or CH3CO2H. Kinetic studies on the catalytic reaction indicate a first-order dependence on acetyl concentration and zero-order dependence on CH3I. These results are discussed in terms of a catalytic cycle wherein the acetyl complexes are involved in a rate and selectivity determining reaction with either H2 or CO.
