Rhodium catalysed hydrocarbonylation reactions

Abstract

The one step hydrohydroxymethylation which can be used to transform C[sub]n alkenes into C [sub](n+1) alcohols has been explored for functionalised alkenes. The catalyst system used for this study was generated in situ from [Rh₂(OAc)₄], Pet₃, CO/ H₂ and ethanol. The main alkene investigated was 2-propen-1-ol, because of the potential to produce 1,4-butanediol by a novel route, which it indeed does in reasonable yield. Of interest the branched chain product was not the expected 2-methyl-1,3-propanediol, but 2-methylpropan-1-ol. 1,4-butanediol and 2-methylpropan-1-ol make up the majority of the final products, no aldehydic intermediates from the possible hydroformylation reaction were detected. A mechanism is proposed for the formation of 2-methylpropan-1-ol, the key steps of which are; protonation of a metal acyl species (on the acyl oxygen) to form a metal hydroxycarbene intermediate, dehydration of the hydroxycarbene species (conjugation being the driving force for this step), oxidative addition of hydrogen to the metal centre, a single hydrogen atom transfer to the carbene carbon, a sigma-pi allylic rearrangement of the substrate based ligand, followed by reductive elimination of a vinyl alcohol. This rapidly rearranges to 2-methylpropanal, which is hydrogenated by this system to give 2-methylpropan-1-ol. The mechanism was studied using 'black box' studies, recently developed deuterium labelling techniques (in one experiment 18 different isotopomers were formed, this technique could quantify them all) and some model studies. The effect on product distribution was examined by systematic variation of the reaction conditions and ligands. The reaction was expanded to other substrates such as propenyl halide, propenyl ethers, propenyl cyanide and ketones. Propenyl ethers gave similar reaction products to 2-propen-1-ol, whilst propenyl halides gave esters and ethers.