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

Abstract

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 SnMe₃ca. 10 kcal mol⁻¹ for the triplets and 20 kcal mol⁻¹ 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 π 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.