From phenyl chlorides to α,n-didehydrotoluenes via phenyl cations. A CPCM-CASMP2 investigation.

Calculations with the complete active space self-consistent field (CASSCF) method were carried out for rationalizing the photochemical generation of the three isomeric didehydrotoluenes (DHTs) from the corresponding (n-chlorobenzyl)trimethylsilanes. Moreover, the original CASSCF energies were corrected through the introduction of the dynamic electron correlation term (at the MP2 level) and of an appropriate solvent model (CPCM). The work demonstrated the viability of intersystem crossing (conical intersection located) leading to the lowest lying triplet state of the silanes that fragments to give the corresponding triplet phenyl cations. The para- and ortho-isomers desilylate directly from such states of radical/radical cation character and yield the corresponding DHTs in their triplet state. Different from the other isomers, the meta-cation has a radical/radical cation structure in both spin states and thus two potential accesses to the different spin states of the corresponding DHT.