Loss of CO2 from the ortho isomer of deprotonated methyl phenyl carbonate involves a methyl migration

The collision induced decompositions of the ortho, meta, and para- (M - H)- anions of methyl phenyl carbonate were studied to see whether there is los s of CO2 Occurring by the benzyne cine-substitution {(C6H4)(-) - OCO2Me --> [(C6H4)MeOCO2-] --> (C6H4)(-) - OMe + CO2}. Loss of CO2 is observed from t he ortho isomer, but the process does not involve a benzyne cine substituti on. It is a methyl migration through a six-centre transition state to give a cresol (M - H) ion, probably ortho-MeC6H4 - O-. Theoretical calculations [at the B3LYP/6-311 + +G(d,p)//HF/3-21+G(d) level of theory] indicate the m ethyl migration is a stepwise process, with the barrier for the first (and rate determining) step being 191 kJ mol(-1). The overall process is calcula ted to be exothermic by 276 kJ mol(-1). (C) 2001 Elsevier Science B.V.