Steric and electronic contributions to barriers of internal rotation in 1,8-dibenzoylnaphthalenes

Abstract

Restricted rotation about the naphthalenylcarbonyl bonds in the title compounds resulted in mixtures of cis and trans rotamers, the equilibrium and the rotational barriers depending on the substituents. For 2,7-dimethyl-1,8-di-(p-toluoyl)-naphthalene ( 1) ΔH° = 3.66 ± 0.14 kJ mol −1, ΔS° = 1.67 ± 0.63 J mol −1 K −1, ΔH ≠ ct = 55.5 ± 1.3 kJ mol −1, ΔH ≠ ct = 51.9 ± 1.3 kJ mol −1, ΔS ≠ ct = −41.3±4.1 J mol −1 K −1 and ΔS ≠ ct = −42.9±4.1 J mol −1 K −1. The rotation about the phenylcarbonyl bond requires ΔH ≠ = −56.9±4.4 kJ mol −1 and ΔS ≠ = −20.5±15.3 J mol −1 K −1 for the cis rotamer, and ΔH ≠ = 43.5Δ0.4 kJ mol −1 and ΔS ≠ =± −22.4Δ1.3 J mol −1 K −1 for the trans rotamer. The role of electronic factors is likely to be virtually the same for both these rotamers but steric interaction between the two phenyl rings occurs in the cis rotamer only. Hence, the difference of the activation enthalpies obtained for the cis and trans rotamers, ΔΔH −1 = 13.4 kJ mol −1, provides a basis for the estimation of the role of steric factors in this rotation. For the tetracarboxylic acid 2 and its tetramethyl ester 3 the equilibrium is even more shifted towards the trans form because of enhanced steric and electrostatic interactions between the substituents in the cis form. The barriers for the rotation around the phenylcarbonyl bond and the cis-trans isomerization are lowered; an explanation for this result is presented.