SELECTIVITY AND MECHANISM IN THE SIDE-CHAIN HALOGENATION OF METHYLBENZENES PROMOTED PHOTOCHEMICALLY AND BY METAL COMPLEXES IN THE PRESENCE OF HALIDE IONS

Article abstract of DOI:10.1016/S0040-4020(01)89841-0

The intramolecular selectivity in a variety of side-chain halogenations of alkyl-aromatics has been determined in AcOH by measuring the isomeric distribution in the reactions of 4-t-butyl- and 4-chloro-1,2-dimethylbenzene (1 and 2, respectively) with: Br2/hν, CAN/Br^-, CAN=cerium(IV) ammonium nitrate, cobalt(III) acetate/Br^-, S2O8⁼/Br^-, N-bromosuccinimide (in CCl4), Cl2/hν, CAN/Cl^-, cobalt(III) acetate/Cl^-.In the bromination reactions selectivity is independent of the reaction conditions, thus suggesting that in all brominating systems Br^. is the actual reacting species.Very surprisingly, with 1 as the substrate, Cl2/hν is a more selective system than Br2/hν.With 2 the two systems display similar selectivity.It has been suggested that in AcOH the transition state for photochlorination has an electron transfer character which increases as the substrate becomes more electron rich.The idea of a "variable" transition state for the photochlorination in AcOH is supported by data of relative reactivity of substituted toluenes indicating that the effect on the rate increases as the substituent becomes more electron donor.AcOH must have an essential role in this respect since in CCl4 situation returns to be "normal" with chlorination less selective than bromination.Selectivity of CAN/Cl^- is very similar to that of Cl2/hν, whereas significant differences are observed with cobalt(III) acetate/Cl^-.Probably Cl^. and a cobalt(III) chloride complex are the reacting species in CAN/Cl^- and cobalt(III) acetate/Cl^-, respectively.