Electrophilic aromatic substitution. Part 30. The effects of the p-bicyclo[2.2.2]octan-1-yl, adamantan-1-yl, exo- and endo-norbornan-2-yl, and neopentyl substituents in detritiation. Steric acceleration of hyperconjugation

Abstract

Rate coefficients have been measured for detritiation of the compounds [p-³H]C₆H₄R by anhydrous trifluroacetic acid at 70 °C, and lead to the following partial rate factors (R =): bicyclo[2.2.2]octan-1-yl, 2 650; adamantan-1-yl, 2 000; exo-norbornan-2-yl, 1 750; endo-norbornan-2-yl, 1 340; and neopentyl, 472. The corresponding σ⁺ values are –0.391, –0.377, –0.371, –0,358, and –0.306. These bulky substituents are much more electron releasing than they are in reactions carried out in highly solvating media where steric hindrance to solvation masks their true electronic effect; in trifluoroacetic acid, solvation is very poor, and steric hindrance to solvation therefore largely absent. Among alkyl groups, bicyclo[2.2.2]octan-1-yl is second only to cyclopropyl in its electron releasing ability which probably derives in part from steric acceleration of hyperconjugation. The neopentyl substituent is more electron releasing than methyl due to the greater importance of C–C over C–H hyperconjugation. The concept of ‘secondary hyperconjugation’ proposed to account for the low activation by neopentyl in molecular bromination is disproved, the phenomenon arising simply from steric hindrance to solvation. Detritiation of endo-norbornan-2-ylbenzene is accompanied by protiodealkylation, which is sterically accelerated by the interaction between the aryl-ring ortho-hydrogen, and the endo-hydrogen on C-6.