Ab initio MC-SCF study of thermal and photochemical [2 + 2] cycloadditions

Abstract

The results of a study of few prototypical polar and non-polar [2 + 2] cycloadditions are presented at a level of theory where biradical and zwitterionic mechanisms can be treated in a balanced way (CAS-SCF). The polarity of the reactants is modelled either through the inclusion of donor/acceptor substituents (cycloaddition of dicyanoethene to hydroxyethene) or through the replacement of a carbon atom of the ethylenic CC double bond with a heteroatom (silaethene and formaldehyde). Two different types of reaction mechanism are documented in each case: (i) a two-step biradical mechanism and (ii) a concerted mechanism. The surface topology describing the two-step mechanism turns out to be almost independent of the polarity introduced by the substituents or by heteroatoms. In contrast, the topology of the surface describing the concerted mechanism is sensitive to substituents and heteroatoms. In non-polar systems (e.g. ethene dimerization) a concerted synchronous or asynchronous supra–supra pathway does not exist; however, in polarized π-systems we have been able to locate a true supra–supra transition state. Since this concerted transition state is at high energy, it will eventually become important only when solvent effects are considered.

A study of the photochemical [2 + 2] dimerization occuring via the lowest excited singlet (S₁) and triplet states (T₁) is also presented. The computations demonstrate that the lowest-energy region of the S₁ potential-energy surface is centered on a singularity (i.e. a cusp) corresponding to a conical intersection between the S₁ and S₀ surfaces. The presence of this topological feature seems to be related to the highly stereospecific formation of cyclobutane from simple 2,3-disubstituted alkenes.

Both the change in potential surface topology on moving from non-polar to polar reactants and the nature of the excited-state decay process are rationalized using the same simple VB model.