Direct formation of alcohols by hydrocarbonylation of alkenes under mild conditions using rhodium trialkylphosphine catalysts

Abstract

The complex[RhH(PEt₃)₃] catalysed the hydroformylation of hex-1-ene to heptanal and 2-methylhexanal in toluene, but heptanol and 2-methylhexanol were significant products in tetrahydrofuran especially over long reaction times(16h). In protic solvents only alcohols were produced even after short reaction times. The reactions are very rapid and also occur readily with alkenes such as hex-2-ene, propene, ethene, styrene and 3,3-dimethylbutene. The highest rates observed are for ethene (54 000 turnovers h^–1) and the products in all cases are alcohols. Other phosphines containing primary alkyl groups also produced alcohols, but in contrast reactions in ethanol using rhodium complexes containing PPh₃, PPh₂Et, PPhEt₂ or PPrⁱ₃ produced significant amounts of aldehydes and/or acetals whilst Me₂PCH₂CH₂PMe₂ inhibited the reaction. The NMR studies showed that species present in equilibrium in ethanol solution are [RhH(CO)(PEt₃)₃], [RhH(CO)₂(PEt₃)₂], [Rh₂(CO)₄(PEt₃)₄], [Rh₂(CO)₂(PEt₃)₆] and PEt₃ but that [RhH(CO)(PEt₃)₃] predominates under the catalytic conditions. Reactions carried out under D₂–CO in EtOH produced, 90% BuCHDCH₂CD₂OH/D and 10% BuCHDCH₂CHDOH/D but hydrogenation of heptanal under the same conditions gave a mixture of C₆H₁₃CHDOH/D (39%) and C₆H₁₃CH₂OH/D (61%). These results are interpreted to indicate that the alcohols produced from hex-1-ene are primary reaction products and not produced via intermediate aldehydes. A new mechanism for this direct hydrocarbonylation is proposed in which the key acyl intermediate becomes protonated by the alcoholic solvent because of the high electron density it bears as a result of the presence of the electron-donating trialkylphosphines. Oxidative addition of H₂ followed by two H-atom transfers leads directly to the alcohol. High pressure NMR studies showed that [Rh{C(O ⋯ HOEt)Et}(CO)₂(PEt₃)₂] is present during catalytic hydrocarbonylation of ethene in ethanol. Two different cycles are proposed to explain the products obtained from the catalytic reaction of heptanal with D₂–CO. Again, protonation, this time of the metal, appears to be important.