Electrophilic aromatic reactivities via pyrolysis of esters. Part 20. The electronic effect of large alkyl groups in the gas phase

Abstract

The compounds p-RC₆H₄CH(OAc)CH₃, where R = adamanlan-1-yl, neopentyl, endo-norbornan-2-yl, and exo-norbornan-2-yl, have been prepared and pyrolysed over a temperature range of at least 50° for each compound, between 617 and 678 K. The data give the corresponding p-R σ⁺ values as –0.425, –0.35, –0.39, and –0.42, which show that these bulky substituents are all substantially more activating than in solution reactions in highly solvating media, where their effect is masked by steric hindrance to solvation. Each substituent is more activating than in detritiation of [p-³H]PhR in trifluoroacetic acid (which is relatively poorly solvating), but the differential effect is smaller than the difference in effects between detritiation and solvolyses of 1-aryl-1-methylpropyl chlorides in aqueous acetone (which is highly solvating). This result exactly parallels that noted previously for other bulky groups, viz. t-butyl, cyclohexyl, trimethylsilyl, and phenyl. The data confirm that p-exo-norbornan-2-yl is unambiguously more electron releasing than p-endo-norbornan-2-yl and this is interpreted as a conformational effect. These alkyl groups release electrons mainly via C–C hyperconjugation, but in the endo-compound the aryl p-orbitals are prevented from becoming coplanar with the 2,3-bond because of steric interaction between the aryl Ortho-hydrogen and the endo-6-hydrogen in the substituent: this interaction is the same as that which facilitates protiodealkylation of endo-norbornan-2-ylbenzene.