Theoretical study on cyclopropanation of endo-dicyclopentadiene with zinc carbenoids: effects of solvent and (ICH2)2Zn.

A computational study using a hybrid DFT method (M06) on the cyclopropanation of endo-dicyclopentadiene with Simmons-Smith zinc carbenoids is reported. Each channel proceeds via the methylene-transfer mechanism with a reactant complex (RC) and subsquently a asynchronous transition state (TS). The channels with monomeric IZnCH(2)I attacking the double bonds from the exo-face have a much lower barrier (about 16.17-18.43 kcal/mol) in the gas phase, compared with the channels from the endo-face (21.80-31.13 kcal/mol). Thus, P1 and P3 are the primary cyclopropanated compounds, and P5 is the sole final product, representing remarkable stereospecificity. When considering the bulk solvent effect of diethyl ether, the barriers are decreased about 0.50-7.77 kcal/mol due to more "destabilization" of RC than TS. The solvated (ICH(2))(2)Zn can further reduce the barriers about 0.18-2.30 kcal/mol. In addition, the solvated IZnCH(2)I and (ICH(2))(2)Zn do not change the reaction pathways and retain the stereospecificity. Our computational results agree with the experimental observations quite well and suggest that both IZnCH(2)I and (ICH(2))(2)Zn might be the active species in the real reaction system. Regarding the solvent effect, the polar continuum model is more realistic than the direct involvement of diethyl ether molecules.