Theoretical Study On The Mechanism Of Cycloaddition Reaction Of Unsaturated Silylene

This paper respectively takes the cycloaddition reactions between methylene silylene,dimethylmethylene silylene,dichloromethylene silylene and ethylene,formaldehyde,acetone,methylene silylene and cis,trans-2-butene as model reactions to investigate the reaction mechanisms of these cycloaddition reactions, which aims at revealing the reaction mechanisms of the cycloaddition reactions between unsaturated silylene or its derivatives and symmetric or asymmetricπ-bonded compounds.The main methods used in this paper are MP2 and B3LYP theories of quantum chemistry implemented in the Gaussian 98, which have respectively been employed to locate all the stationary points along the reaction pathways at the level of 6-31G*. Full optimization and vibrational analysis are done for the stationary points on the reaction profile. Zero-point energy and CCSD(T) corrections are included for energy calculations. In order to explicitly establish the relevant species, IRC is also calculated for all the transition states appearing on the cycloaddition energy surface profile.By studying on the mechanisms of cycloaddition reactions between methylene silylene,dimethylmethylene silylene,dichloromethylene and ethylene, we can know that the common reaction path of this kind of reaction is that: (1) the two reactants firstly form an energy-rich three-membered ring intermediate; (2) the three-membered ring intermediate isomerizes to a twisted four-membered ring intermediate; (3) the four-membered ring intermediate further reacts with ethylene to form a silapolycyclic product. The common reaction mechanism of this kind of reaction is that (shown in Figure 1) : firstly, the insertion of the 3p unoccupied orbital on Si atom of unsaturated silylene into theπorbital of ethylene, forms aπ→p donor-acceptor bond, which leads to the three-membered ring intermediate; subsequently, theπbond between C(1) and Si further reacts with theπorbital between C(2) and C(3) in the three-membered ring intermediate, leading to the four-membered ring intermediate, which results from the [2+2] cycloaddition effect; the 3p unoccupied orbital and the lone-pair sp electrons in the four-membered ring intermediate further reacts with theπorbital on C(4) and the antibondingπ* orbital on C(5) of ethylene (shown in Figure 2), respectively, forming theπ...