The influence of the leaving group X (X=F, Cl, Br, I, OH) on the carbenoid nature of the carbenoids LiCH2X and XZnCH2X -: A theoretical study

A quantum-chemical investigation shows that the reactions of the carbenoids LiCH2X 1-X, X=F, Cl, Br, I and OH, with ethene 3 to cyclopropane 4+LiX profit from a weakening of the C-X bonds by the C-Li bonds in the carbenoids 1-X and;in the complexes [1-X.3]. The C-F bond is more affected than the C-I bond. Since in the transition states [1-X.3](double dagger) LiHal is strongly decomplexed, the cleavage of the C-Hal bonds is essentially compensated by the formation of the Li-Hal bonds, which leads to almost equal transition state energies for the reactions of 1-Hal with 3. The higher energy for the reaction of 1-OH with 3 results from the high C-OH bond energy. In the reactions of XZnCH2X 2-X with 3 to 4, the C-ZnX bonds cause almost no elongation of the C-X bonds. This leads to higher activation energies, which is in agreement with experimental results. Furthermore, 2-F is calculated to have a much higher transition state energy than 2-I, the normally used Simmons-Smith reagents, again in excellent agreement with the experiment. The latter result is due to the much higher C-F (110.9 kcal/mol) than C-I bond energy (58.2 kcal/mol). (C) 2000 Elsevier Science Ltd. All rights reserved.