posted on 2007-12-26, 00:00authored byValentina Dichiarante, Andrea Salvaneschi, Stefano Protti, Daniele Dondi, Maurizio Fagnoni, Angelo Albini
Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS)
group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar−Cl bond
and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The β effect of
silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents
with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level
in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides,
while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the
singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem
crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these
compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or
arylation (from the triplet). In the case of the silylated 4-chloro-N,N-dimethylaniline, the triplet cation remained
in the ground state and trapping by π nucleophiles remained efficient, though slowed by the steric bulk of
the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the
irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed
shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.