Theoretical studies of organic reactions

My M.Phil program is mainly on the studies of organic reactions using computational methods. The subjects include the substituent effects on the bond dissociation energy and the mechanism and stereochemical control of Cu(I)-catalyzed cyclopropanation. The origin of the substituent effect on the bond dissociation energy (BDE) of Si-H is first introduced in Chapter I. The calculated BDEs agree with experimental measurements and previous ab initio calculation. The calculated substituent effect is well correlated with the calculated Hirshfeld charge and spin density variations on the Si radical center. The results show that the substituent effect is mainly caused by the spin delocalization and inductive effect. In Chapter II, density functional studies of a series of 13 substituted silanes on the bond dissociation energies of para-substituted toluenes and a series of 33 substituted methanes are reported. Both local density functional and non-local density functional methods quite accurately calculate the substituent effect on the bond dissociation energy (BDE). Almost every substituent can cause a reduction in the C-H BDE in toluene and methane. This is mainly due to a dominant spin delocalization stabilization on the radical by the substituent. Once again, electron-withdrawing inductive effect increases the C-H BDE, and electron-donating inductive effect decreases the C-H BDE. In the case of substituted methanes, a steric effect also reduces the C-H BDE. It is demonstrated that the substituent effect on the bond dissociation energy can be correlated well with calculated variations in physical properties. In Chapter III, the catalytic cyclopropanation by using Copper (I) catalyst is studied. Quantum mechanics calculations on the reactions of ethylene with [(NH3)[2Cu(I)=CHR]+ (R=H, COOCH3) and [(NH=CH-CH2-CH=NH)Cu(I)=CH2+ reveal that a Cu(I)-carbenoid favors a perpendicular structure. The most favorable transition structure is derived from alkene addition to the perpendicular structure. The transition structure is in a triangular structure with the inner C---C bond formed to a larger extent. This transition structure leads directly to the formation of cyclopropane product without the formation of a metallocyclobutane intermediate. A molecular mechanics force-field has been developed based on the quantum mechanics transition structures. The stereoselectivities observed experimentally can be reproduced by the force-field calculations