Crucial Role of β-Elimination in Determining Regio- and Chemoselectivity of the Rhodium-Catalyzed Hydroformylation of N-Allylpyrroles: A New Approach to 5,6-Dihydroindolizines

Rhodium-catalyzed hydroformylation of the chiral (S)-3-alkyl-3-pyrrol-1-ylprop-1-enes at 100 atmospheres total pressure and 25 ˚C led to the preferential formation of the branched 3-alkyl-2-methyl-3-pyrrol-1-ylpropanals. At 30 atmospheres and 125 ˚C, the linear 4-alkyl-4-pyrrol-1-ylbutanals were obtained: these aldehydes are not the final products, but evolve into more stable 5,6-dihydroindolizines, with the same optical purity as the starting olefins, via a domino cyclization-dehydration process. According to the generally accepted mechanism for rhodium-catalyzed hydroformylation, the regioselectivity, and then the final chemoselectivity, can be rationalized by taking into account that while at room temperature no β-elimination occurs, at high temperature the β-elimination involves the branched rhodium-alkyl intermediate only.