Electrophilic Aromatic Substitution. 13.1 Kinetics and Spectroscopy of the Chloromethylation of Benzene and Toluene with Methoxyacetyl Chloride or Chloromethyl Methyl Ether and Aluminum Chloride in Nitromethane or Tin Tetrachloride in Dichloromethane. the Methoxymethyl Cation as a Remarkably Selective Common Electrophile

Article abstract of DOI:10.1021/jo961064o

Vacuum line kinetics studies have been made of the reaction in nitromethane between benzene and/or toluene, methoxyacetyl chloride (MAC), and AlCl₃ to produce benzyl or xylyl chlorides, CO, and a CH₃OH^- AlCl₃ complex. For both arenes, the rate law appears to be R = (k₃/[AlCl₃]₀) [AlCl₃]²-[MAC]. When chloromethyl methyl ether (CMME) is substituted for MAC, a similar rate law is obtained. Both chloromethylation reactions yielded similar, large k_T/k_B ratios (500-600) and similar product isomer distributions with low meta percentages (～0.4) which suggest CH₃OCH₂⁺ or the CH₃OCH₂⁺Al₂Cl₇ ^- ion pair as a common, remarkably selective, electrophile. The kinetics of MAC decomposition to CMME and CO in the presence of AlCl₃ yielded the rate law R = k₂[AlCl₃]₀[MAC]. Here AlCl₃ is a catalyst (no CH₃OH is formed), and thus the rate law is equivalent to the chloromethylation rate law. All three reactions have comparable reactivities, which is consistent with rate-determining production of the electrophile. Kinetics studies of benzene or toluene with SnCl₄ and MAC or CMME in dichloromethane were also completed. With MAC and benzene the rate law is R = k₃[SnCl₄]₀[MAC][benzene] and with toluene R = k₂[SnCl₄]₀[MAC]. MAC decomposition, again followed by CO production, was unaffected by the presence of either aromatic and obeyed the rate law R = k_2′ [SnCl₄]₀[MAC] where k₂ ≈ k_2′ Chloromethylation with CMME followed the rate law R = k₃[SnCl₄]₀[CMME][arene] for benzene and toluene and produced a k_T/k_B ratio and product isomer distributions very similar to those determined with AlCl₃ in nitromethane, further supporting a common electrophile. Low-temperature ¹³C and ¹¹⁹Sn FT-NMR and Raman spectroscopic studies suggest the existence of a weak 1:1 adduct between MAC and SnCl₄ of the type RCXO → SnCl₄, with electron donation to the metal through carboxy oxygen. Finally, an explanation is provided for the range of chloromethylation k_T/k_B values and product isomer percentages published in the literature.