N.m.r. studies of a rate process in a bridged biphenyl: Resolution of a discrepancy between n.m.r. and polarimetric kinetic data

Abstract

AbstractTotal bandshape analysis of the temperature‐dependent n.m.r. spectrum of the bridge methylene protons in 6,6‐dicarbethoxy‐1,2,3,4‐dibenzcyclohepta‐1,3‐diene has been carried out to yield the following activation parameters: ΔG = 60.96 ± 0.17 kJmol−1 (14.57 ± 0.04 kcal mol−1), ΔH‡ = 47.3 ± 0.8 kJ mol−1 (11.3 ± 0.2 kcal mol−1) and ΔS‡ = −45.6 ± 3.3 J mol−1 K−1 (−10.9 ± 0.8 cal mol−1 K−1). A value of ΔG‡ in agreement with the above may be obtained from a bandshape analysis (at a suitable temperature) of the spectrum of the methylene protons in the carbethoxy groups. In our opinion, the rate process described by these activation parameters must be the configurational inversion of the biphenyl system, and thus the polarimetric data on this compound reported by Iffland and Siegel [J. Am. Chem. Soc. 80, 1947 (1958); half‐life of 80 min at room temperature] are seriously called to question, as previously noted by Sutherland and Ramsay [Tetrahedron 21, 3401 (1965)]. We have attempted unsuccessfully to repeat the polarimetric work, and have been able to show by means of thin‐layer chromatography in the cold, mass spectral analysis and polarimetry that an optically active impurity (as yet unidentified) is the most probable cause of the discrepancy.