Toluenes regioselective chlorination

Regioselective chlorination of toluene is not observed under the above conditions but the ortholpara ratio can be controlled over a reasonable range (0.52-2.85) by altering the acidity of the solvent766 and the nature of the parent amine767. 

Regioselective chlorination. This N-chloroammonium salt (1) as well as N-chlo-ropiperidine, chlorinates aromatic compounds substituted with an electron-donating group with high para-selectivity. Benzene and toluene are attacked only slowly. 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

The electrochemical nuclear chlorination of substituted aromatics in some cases allows to achieve better regioselectivities than the chemical alternatives. DOW 93 94) has shown that, in the anodic chlorination of toluene in aprotic electrolytes, the p/o ratio of the chlorotoluenes can be increased to about 2.2 (chemical alternatives 0.5-1, depending on substances added to the reaction mixture) ... 

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