Investigations on the Lewis-Acids-Catalysed Electrophilic Aromatic Substitution Reactions of Thionyl Chloride and Selenyl Chloride,the Substituent Effects,and the Reaction Mechanisms

Abstract

The previously established aluminium-chloride-(AlCl₃)-catalysed electrophilic aromatic substitution (EAS) of benzene (PhH) with thionyl chloride (SOCl₂) has been extended to toluene (PhCH₃), chlorobenzene (PhCl), and phenol (PhOH). –CH₃ was found to be mainly a para-director with a minor ortho-directing effect on the EAS reactions giving diaryl sulfoxides (Ar₂SO). –Cl was found to be exclusively a para-director for formation of Ar₂SO. All the –CH₃, –Cl, and –OH groups were shown to be exclusive para-directors for formation of diaryl sulfides (Ar₂S) from the EAS reactions. Although the reactions of PhH and PhCH₃ with SOCl₂ in the presence of AlCl₃ gave the major Ar₂SO and minor Ar₂S at ambient temperature, the phenol (PhOH) reaction was shown to give only the reduced sulfide (p-HOC₆H₄)₂S with no sulfoxide (p-HOC₆H₄)₂SO formed. The mixed diaryl sulfoxides ArSOAr’ (Ar, Ar'=C₆H₅, p-CH₃C₆H₄; C₆H₅, o-CH₃C₆H₄; and C₆H₅, p-ClC₆H₄) were produced in the AlCl₃-catalysed reactions of SOCl₂ with molar 1:1 PhH–PhX mixtures (X=CH₃ and Cl). Efforts to enhance the yield of S-aryl arenesulfonothioates ArSO₂SAr (Ar=Ph, p-CH₃C₆H₄, and p-ClC₆H₄) from the AlCl₃-catalysed EAS reactions of SOCl₂ were made, showing that decreasing the molar ratios of ArH/SOCl₂ or lowering the temperature resulted in an increase in the product yield. A detailed mechanism has been proposed to account for the formation of ArSO₂SAr. The Lewis-acid-MCl₃-(M=Al and Fe)-catalysed EAS reactions of PhH with selenyl chloride (SeOCl₂) were demonstrated to give the reduced diphenyl selenide (Ph₂Se) and diphenyl diselenide (PhSeSePh) via novel auto-redox processes in selenium of the key EAS intermediates.