M. Garavelli et al., Photochemistry of highly alkylated dienes: Computational evidence for a concerted formation of bicyclobutane, J AM CHEM S, 121(7), 1999, pp. 1537-1545
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 c
omputations 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 o
f a methyl group. Despite the fact that the method only treats specific sub
stituents (e.g. tert-butyl groups) embedded in a specific environment (e.g.
a hydrocarbon skeleton) we show that it can be successfully employed in me
chanistic studies where steric factors dominate. The analysis of the comput
ed relaxation coordinate provides a mechanistic explanation for the differe
nt strained photoproducts generated by photolysis of the parent and substit
uted dienes. In particular, we show that while s-cis-buta-1,3-diene produce
s cyclobut-1-ene via a disrotatory ring-closure path, the two bulky tert-bu
tyl substituents in 1 greatly enhance the production of a highly strained b
icyclo[1.1.0]butane derivative (which forms only in traces when the parent
compound is photolyzed) by driving the excited-state relaxation along a con
certed and synchronous path characterized by a conrotatory rotation of the
two terminal methylenes.