The carbonylation of allylic halides

Abstract

[RhCl(CO)(Pet₃)₂] or a compound prepared in situ from [Rh₂(OAc)₄] and Pet₃ have been shown to be active catalysts for the carbonylation of allylic halides under relatively mild conditions. This reaction is of considerable significance since it is a rare example of a system in which C-Cl bonds can be carbonylated using a rhodiun based system. The reaction occurs in the absence of added base and there is little isomerisation of the double bond when forming the ester. Using either 3-chlorobut-l-ene or l-clilorobut-2-ene, the products obtained are identical. The oxidative addition of 1-clilorobut-2-ene occurs via an S[sub]N2 mechanism whereas an S[sub]N2’ mechanism operates for the 3-chlorobut-1-ene because of the steric effects of the methyl group adjacent to the chlorine. Extensive mechanistic studies have been carried out and many of the intermediates have been characterised using multinuclear variable temperature, high pressure NMR and high pressure IR as well as isolation of the intermediates. The oxidative addition and migratory insertion complexes have both been characterised using the above methods enabling the mechanism of the carbonylation reaction to be elucidated. Supercritical carbon dioxide has been used to replace ethanol as the solvent to increase the concentration of carbon monoxide in the solution in an attempt to achieve a greater ester:ether ratio. However, catalyst solubility was a problem in these reactions. In an attempt to solve this problem phosphine ligands containing organo-fluorine groups were investigated. With the fluorinated groups present on the phosphine ligands the rhodium complex was soluble in the supercritical carbon dioxide. The ethylene spacer between the phosphorus atom and the fluorinated chain ensured that the fluorinated phosphines had a similar basicity to that of triethylphosphine. However, only low yields were obtained from the catalytic reactions possibly due the failure of the supercritical carbon dioxide to stabilise the ionic intermediates of the oxidative addition reaction.