π–π Interaction Energies as Determinants of the Photodimerization of Mono‑, Di‑, and Triazastilbenes

We describe the quantitative [2 +
2] photocycloaddition of crystalline trans-2,4-dichloro-6-styrylpyrimidine
to produce the corresponding
htt r-ctt cyclobutane dimer, and we present 1H NMR analysis of the photolysis of this and six other mono-,
di-, and triazastilbenes in solid and solution states. Density functional
(M06-2X) and correlated ab initio (MP2) calculations were used to
obtain interaction energies between two monomers of each azastilbene.
These energies mirror the relative polarization of the stilbene moieties
and can be quantitatively correlated with the rate of reaction and
selective formation of the htt r-ctt dimers. In the
solid state, poor correlation is observed between interaction energy
and reactivity/selectivity. This lack of correlation is explained
through X-ray analysis of the azastilbene monomers and is shown to
be in accordance with the principles of Schmidt’s topochemical
postulate. Conversely, in solution there is a strong positive correlation
(R2 = 0.96) between interaction energies
and formation of the htt r-ctt dimer. These results
are the first to show this correlation and to demonstrate the utility
of calculated interaction energies as a tool for the prediction of
stereo- and regioselectivity in solution-state stilbene-type photocycloadditions.