The influence of the leaving group X (XÂ =Â F, Cl, Br, I) on the carbenoid nature of the carbenoids X2AlCH2X - A theoretical study

The cyclopropanation reaction of ethene with aluminum carbenoid has been studied by means of the B3LYP hybrid density functional method. The reaction goes through two pathways: methylene transfer and carbometalation. In methylene transfer pathway, a quantum-chemical investigation shows that the reactions of the carbenoids X2AlCH2X 1-X, (X = F, Cl, Br, I) with ethene 2 to cyclopropane 3 + lX3 profit from a weakening of the C-X bonds by the C-Al bonds in the carbenoids 1-X and in the complex [1-X∗2]. The C-F bond is more affected than the C-I bond. Since in the transition states 3[1-X∗2]‡ AlHal is strongly decomplexed, the cleavage of the C-Hal bond is essential compensated by the formation of the Al-Hal bonds, which leads to almost equal transition state energy for the reactions of 1-X with 2. In contrast with methylene transfer, the cyclopropanation reaction of the carbometalation pathway profit from a weakening of the C-Al bonds by the C-X bonds.