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⁺(k_S/k_B 2.0) < CH₃ClCH₃⁺(k_S/k_B 13.8) < CH₃BrCH₃⁺(k_S/k_B 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.