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]‡ 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).