Highly selective aromatic chlorination. Part 4. The chlorination of aromatic hydrocarbons with N-chloroamines in acidic solution

Benzene, toluene, some polymethylbenzenes, and naphthalene have been treated with N-chlorotrialkylammonium salts and N-chlorodialkylamines in trifluoroacetic acid at room temperature. With benzene, toluene, and 1,3,5-trimethylbenzene the major products arise from aromatic chlorination whereas with the other polymethylbenzenes side-chain reactions predominate. By controlling the acidity of the reaction and the nature of the N-chloroamine, the chlorination of toluene can be made to give preferentially 2- or 4-chlorination. However, the selectivities are not as great as reported previously with electron-rich aromatic compounds with a π-donor substituent. The products from the reaction of naphthalene are very dependent on the structure of the N-chlorinated amine. The bulky N-chlorotrialkylammonium salts selectively chlorinate the 1-position, but in low yield, whereas the less hindered N-chloropiperidine gives good yields of 1-(1-piperidino)naphthalene. The results from these studies are discussed in terms of arenium-ion and electron-transfer mechanisms.