Ab initio study on the mechanism of forming a silapolycyclic compound between silylidene and formaldehyde

The mechanism of the cycloaddition reaction of forming a silapolycyclic compound between singlet silylidene and formaldehyde has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the cycloaddition reaction process of forming the silapolycyclic compound (P₂) for this reaction consists of four steps: (I) the two reactants first form a semi-cyclic intermediate INT1a through a barrier-free exothermic reaction of 32.5 kJ mol^?1; (II) this intermediate then isomerizes to an active four-membered ring intermediate INT1 via a transition state TS1a with an energy barrier of 30.8 kJ mol^?1; (III) INT1 further reacts with formaldehyde to form an intermediate INT2, which is also a barrier-free exothermic reaction of 30.1 kJ mol^?1; (IV) INT2 isomerizes to a silapolycyclic compound P₂ via a transition state TS2 with a barrier of 50.6 kJ mol^?1. Comparing this reaction path with other competitive reaction paths, we can see that this cycloaddition reaction has an excellent selectivity.