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 S_N1 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 S_N1 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.