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
作者:Roberta Settambolo
DOI:10.1055/s-0030-1257860
日期:2010.9
Rhodium-catalyzed hydroformylation of the chiral (S)-3-alkyl-3-pyrrol-1-ylprop-1-enes at 100 atmospheres total pressure and 25 ËC led to the preferential formation of the branched 3-alkyl-2-methyl-3-pyrrol-1-ylpropanals. At 30 atmospheres and 125 ËC, 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.
铑催化的手性 (S)-3-烷基-3-吡咯-1-基丙-1-烯在100大气压和25°C下反应,优先生成支链的 3-烷基-2-甲基-3-吡咯-1-基丙醛。在30大气压和125°C时,得到线性 4-烷基-4-吡咯-1-基丁醛:这些醛类不是最终产物,而是通过多米诺环化-脱水过程演变为更稳定的 5,6-二氢吲哚啉,且与起始烯烃具有相同的光学纯度。根据普遍接受的铑催化加氢反应机制,可以合理解释区域选择性以及最终的化学选择性,因为在常温下没有发生β-消除,而在高温下,β-消除仅涉及支链铑-烷基中间体。