Photochemistry of highly alkylated dienes: Computational evidence for a concerted formation of bicyclobutane

In this report, high-level ab initio quantum chemical computations (MC-SCF and multireference Moller-Plesset perturbation theory) are used to compute the composite S-2 --> S-1 --> S-0 relaxation/reaction paths describing the photorearrangement of the highly alkylated diene 2,3-di-tert-butylbuta- 1,3-diene (1) and of the parent compound s-cis-buta-1,3-diene. Reaction path computations require, typically, hundreds of energy and gradient evaluations. For this reason, we have defined, validated, and employed a simple hybrid method designed to simulate a tert-butyl group at the computational cost of a methyl group. Despite the fact that the method only treats specific substituents (e.g. tert-butyl groups) embedded in a specific environment (e.g. a hydrocarbon skeleton) we show that it can be successfully employed in mechanistic studies where steric factors dominate. The analysis of the computed relaxation coordinate provides a mechanistic explanation for the different strained photoproducts generated by photolysis of the parent and substituted dienes. In particular, we show that while s-cis-buta-1,3-diene produces cyclobut-1-ene via a disrotatory ring-closure path, the two bulky tert-butyl substituents in 1 greatly enhance the production of a highly strained bicyclo[1.1.0]butane derivative (which forms only in traces when the parent compound is photolyzed) by driving the excited-state relaxation along a concerted and synchronous path characterized by a conrotatory rotation of the two terminal methylenes.