Electrophilic aromatic reactivities via pyrolysis of esters. Part 18. Pyrolysis of 1-aryl-1-methylethyl acetates: the high polarisability of the meta-methyl substituent

Abstract

Rates of pyrolysis of a range of 1-aryl-1-methylethyl acetates, measured between 590.8 and 529.6 K, give a very good correlation with σ+ values. The ρ factor (–0.743 at 550 K) is almost exactly the same as in pyrolysis of the much less reactive 1-arylethyl acetates and this contrasts with SN1 solvolyses of the corresponding chlorides where increased electron supply by the methyl groups to the side-chain α-carbocation produces less demand for stabilisation of the transition state by the aryl substituents and a smaller ρ factor. In the elimination the cation is only partially formed and the electron supply from the extra methyl groups of the tertiary esters facilitates C–O bond breakage and a more polar transition state; the opposing effects of the methyl substituents therefore produce no nett change in ρ factor. The transition-state charge difference is confirmed by the (statistically corrected) tertiary: secondary reactivity ratio of 77 which is much greater, relative to the ρ factor, than the difference between the rates of SN1 solvolysis of the corresponding tertiary and secondary chlorides.The meta methyl substituent is confirmed as being of exceptional polarisability in the absence of solvent; in both the present reaction and phenyl carbonate pyrolysis a σ+ value of –0.13 is required to correlate its effect.The reactivity of 1-(2-pyridyl)-1-methylethyl acetate is less than predicted from the corresponding data for 1-arylethyl acetates and 1-arylethyl methyl carbonates, and reasons for this are considered. This anomaly apart, data from all three ester types confirm that the σ+ values for the pyridine free base are (positions in parentheses): 0.77(2), 0.295(3), and 0.86(4) and that the larger values obtained in most solution reactions refer to the hydrogen-bonded species.