Singlet/triplet phenyl cations and benzyne from the photodehalogenation of some silylated and stannylated phenyl halides

The photodehalogenation of fluoro or chlorobenzene derivatives smoothly generates triplet and singlet phenyl cations (3,1Ar+) and potentially benzyne. These intermediates lead to different products, which warrants exploring the direct effect of substituents. SiMe3 and SnMe3 groups have been found to be convenient probes since they affect neither the photophysics nor the primary dehalogenation of 4-chloroanisole and 4-chloro (or 4-fluoro) N,N-dimethylaniline (except for the case of 4-fluoro-3-(trimethylstannyl)aniline, which was preferentially demetallated). The stabilization by these groups (with SnMe3 ca. 10 kcal mol1 for the triplets and 20 kcal mol1 for the singlets, as computed by using (U)B3LYP DFT method with LANL2DZ basis set) made 1Ar+ the lowest state with stannylated and silylated anisoles and solvolysis the main process. On the other hand, donating substituents and an acidic solvent favored the triplet cation chemistry, a quite general process leading to reduction or (in the presence of p bond nucleophiles) to arylation. Noteworthily, the stannylated 4-N,N-dimethylaminophenyl cation eliminated the Me3Sn+ group, opening an unprecedented path to the corresponding benzyne. Apart from the control of the chemical output, the photostabilizing effect found with a silyl group may be useful for designing less phototoxic fluorinated drugs.