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

Listen

Translate article

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=(δCHT–δ)/(δ–δNCD). The enthalpy (ΔHo) 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 ΔHo values for p-MeO and m,m′-Cl2 2.64 ± 0.11 and –0.81 ± 0.13 kJ mol–1, respectively. The ΔHo 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 Gunther'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.