A density functional theory study of aluminum carbenoid (CH 3)2A1CH2X (X = Cl, Br, I) promoted cyclopropanation reactions compared to IMCH2i (M = Li, Sm, Zn) carbenoids

Abstract

Density functional theory calculations are reported for the cyclopropanation reactions of selected aluminum carbenoids with ethylene for two reaction channels: methylene transfer and carbometalation. The aluminum carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier (11-15 kcal/mol). In contrast, the reaction barriers for cyclopropanation via the carbometalation are much higher (about 30 kcal/mol). These computational results are in good agreement with experimental results, and this suggests that the methylene transfer process is favored and the competition from the carbometalation pathway is negligible. The (CH3)2AlCH2Cl carbenoid (reaction barrier of 11.3 kcal/mol) is found to be the most reactive carbenoid in the (CH3)2AlCH2X (X = Cl, Br, I) series of carbenoids, and the (CH3)2AlCH2I carbenoid is the least reactive one. The present computational results are briefly compared with previously reported results for related lithium, samarium, and zinc carbenoids. The trend of the cyclopropanation reaction barrier of the carbenoids compared is LiCH2I (6.8 kcal/mol) ≈ ISmCH2I (5.5 kcal /mol) < (CH3)2AlCH2I (12.8 kcal/mol) < IZnCH2I (21.2 kcat/mol). These results are qualitatively consistent with the agreement between carbenoid character and experimental conditions needed for efficient reaction. © 2006 American Chemical Society.