Structural effects on valence tautomerism. Part 5. A carbon-13 nuclear magnetic resonance. Study on the effects of the π-acceptor ability of aryl substituents on the valence tautomerism of 7-aryl-2,5-di-t-butyl-cyclohepta-1,3,5-trienes

Abstract

The cycloheptatriene (CHT)–norcaradiene (NCD) equilibria of eight 7-aryl-2,5-di-t-butylcyclohepta-1,3,5-trienes in CS₂–CD₂Cl₂ have been studied with the use of variable-temperature ¹³C n.m.r. The equilibrium constants (K=[NCD]/[CHT]) below the coalescence temperatures were calculated from signal intensities; those at higher temperatures were obtained by use of the equation K=(δ_CHT–δ)/(δ–δ_NCD). The enthalpy (ΔH^o) for the conversion of the CHT into the NCD tautomer decreases in the substituent order p-MeO > p-Me > H > p-Br ∼p-Cl > m-Cl > p-CF₃ > m,m′-Cl₂, with the ΔH^o values for p-MeO and m,m′-Cl₂ 2.64 ± 0.11 and –0.81 ± 0.13 kJ mol^–1, respectively. The ΔH^o values are linearly correlated not only with the Hammett σ-constants, but also with the π-acceptor ability (the D value) of the aryl carbon atom adjacent to C(7) as estimated by INDO calculations; this supports Hoffmann and Günther's donor–acceptor theory. The ¹³C signals of C(3) and C(4) in both the NCD and the CHT tautomer undergo downfield shifts as the D value of the aryl group increases; the shift of the former is 4.3 times more sensitive than that of the latter as evaluated by the Hammett plot. In addition, the difference in the ¹³C shift of the para-carbon atom between the NCD and the CHT tautomer is larger in the 7-(m,m′-dichlorophenyl) than in the 7-phenyl system. These results suggest that conjugative interaction between the cyclopropane ring in the NCD structure and the aryl group increases with increase in the π-acceptor ability of the aryl group.