An NMR study of cobalt-catalyzed hydroformylation using para-hydrogen induced polarisation

Abstract

The syntheses of Co(eta3-C3H5)(CO)2PR2R' (R, R' = Ph, Me; R, R' = Me, Ph; R = R' = Ph, Cy, CH2Ph) and Co(eta3-C3H5)(CO)(L) (L = dmpe and dppe) are described, and X-ray structures for Co(eta3-C3H5)(CO)(dppe) and the PPh2Me, PCy3 derivatives reported. The relative ability of Co(eta3-C3H5)(CO)2(PR2R') to exchange phosphine for CO follows the trend PMe2Ph < PPh2Me < PCy3 < P(CH2Ph)3 < PPh3. Reactions of the allyl complexes with para-hydrogen (p-H2) lead to the observation of para-hydrogen induced polarisation (PHIP) in both liberated propene and propane. Reaction of these complexes with both CO and H2 leads to the detection of linear acyl containing species Co(COCH2CH2CH3)(CO)3(PR2R') and branched acyl complexes Co(COCH(CH3)2)(CO)3(PR2R') via the PHIP effect. In the case of PPh2Me, additional signals for Co(COCH2CH2CH3)(CO)2(PPh2Me)(propene) and Co(COCH(CH3)2)(CO)2(PPh2Me)(propene) are also detected. When the reactions of H2 and diphenylacetylene are studied with the same precursor, Co(CO)3(PPh2Me)(CHPhCH2Ph) is seen. Studies on how the appearance and ratio, of the PHIP enhanced signals vary as a function of reaction temperature and H2 : CO ratio are reported. These profiles are used to learn about the mechanism of catalysis and reveal how the rates of key steps leading to linear and branched hydroformylation products vary with the phosphine. These data also reveal that the PMe2Ph and PPh2Me based systems yield the highest selectivity for linear hydroformylation products.