Gas-phase methylation of phenol and anisole by CH3XCH3 +(X = F, Cl, or Br) ions

Abstract

Dimethylhalogenonium ions CH3XCH3+(X = F, Cl, or Br), obtained in the dilute gas state from the γ-radiolysis of the corresponding methyl halide CH3X, were allowed to react with representative ambident substrates, such as phenol and anisole, either neat or in competition with benzene, in the pressure range 100–760 Torr, and in the presence of a gaseous base (NH3). The competition between the n- and π-type nucleophilic centres of phenol is kinetically biased in favour of O-methylation, leading to the predominant (CH3FCH3+ 6 : 1; CH3ClCH3+ 5 : 1; CH3BrCH3+ 4 : 1) formation of anisole with respect to cresols under conditions, i.e. high CH3X pressure and in the presence of NH3, which favour kinetic control of the products. For ring methylation, both the selected substrates undergo preferential substitution at the ortho- and para-positions. In general, ortho-attack is favoured at low pressure and in the absence of NH3. The results of competition experiments show that phenol (S) reacts faster than benzene (B) with gaseous CH3XCH3+, the substrate selectivity of the electrophile increasing in the order CH3FCH3+(kS/kB 2.0) < CH3ClCH3+(kS/kB 13.8) < CH3BrCH3+(kS/kB 40.9). The same conclusion cannot be reached for anisole because of the occurrence of extensive O-methylation with formation of dimethylphenyloxonium ion, which turns out to be very stable under the conditions used. The results are consistent with a model involving kinetically predominant attack on the oxygen atom of the ambident substrate, which, in the case of CH3FCH3+ ion, can be preceded by an extensive electrostatic interaction with formation of a ‘chelated’ complex. The same interaction helps also explaining the relatively high reactivity of the ring position ortho to the n-donor substituent, observed at low pressure. The substrate and positional selectivity of the CH3XCH3+ ions toward ambident compounds is discussed and compared with related gas-phase results and with conventional Friedel–Crafts methylation data.