Reactivity in Methyl Transfer Reactions. 4. Powerful Methylating Agents with Neutral Nucleophiles

Article abstract of DOI:10.1021/ja00521a048

Rates and activation energies for methyl transfer to a series of substituted phenyl methyl sulfides from methyl trifluoromethanesulfonate (triflate) and from trimethyloxonium tetrafluoroborate are reported, along with rates for reaction of a few substituted dimethylanilines and pyridine with methyl triflate and dimethyl sulfate.The rate constants (in sulfolane) are not very high; varying from about 70 for pyridine and methyl triflate to 9.6*10^-5 M^-1s^-1 for dimethyl sulfate with N,N-dimethyl-m-nitroaniline (at 30 deg C) and do not provide consistent support for a loss of selectivity with increasing reactivity.With the uncharged phenyl methyl sulfide, trimethyloxonium ion is less than three times more reactive than methyl triflate, in contrast to the factor of 75 reported earlier for the p-nitrothiophenoxide ion.The substantial values of the Hammett ρ, the fairly large activation energies, and the persistence of considerable selectivity all suggest that these reactions do not have highly reagent-like transition states.The similarities of rates and activation energies for the reaction of the sulfides with the charged trimethyloxonium ion and the neutral methyl triflate, however, suggest that product solvation is not far along at the transition state.