Solvolysis of Tricyclo[4.1.0.02,7]hept‐4‐en‐3‐yl and Tricyclo[4.1.0.02,7]hept‐3‐yl Methanesulfonates and p‐Nitrobenzoates – Remarkably Small Enthalpy Difference between the C7H7 Cations Tricyclo[4.1.0.02,7]hept‐4‐en‐3‐yl and 7‐Norbornadienyl

Abstract

AbstractIn an extension of previous work, tricyclo[4.1.0.02,7]heptan‐3‐ol (5) was synthesised in four steps from benzvalene. Methanesulfonates 12 and 6b, c, prepared from 5 and 4‐halotricyclo[4.1.0.02,7]hept‐4‐en‐3‐ols 4b, c, were too unstable to be isolated, but were unambiguously characterised by their NMR spectra. On treatment of the unsubstituted tricycloheptenol 4a with methanesulfonyl chloride, the expected mesylate 6a could not be observed. Even at −40°C, only its consecutive products 3‐chlorotricyclo[4.1.0.02,7]hept‐4‐en (7a) and 7‐chloronorbornadiene (8a) were discernible. The analogous reaction of 6b occurred only at room temperature, with formation of the dichlorotricycloheptene 7b and the dichloronorbornadiene 8b. Rearrangement products of 6b and 12 were the chloronorbornadiene mesylate 10b and anti‐7‐norbornenyl mesylate (13), respectively. – The solvolysis of 6b, c in aqueous ethanol and of 6b in 2,2,2‐trifluoroethanol (TFE) gave nonrearranged products exclusively, i.e. the alcohols 4b, c, the ethyl ethers 17b, c, and the trifluoroethyl ether 18b. In contrast, from 12 only rearranged products arose, namely anti‐7‐norbornenol (30) and its ethyl ether 31. The solvolysis of the unsubstituted tricycloheptenyl p‐nitrobenzoate 11a in 80% aqueous ethanol proceeded about equally fast in parallel reactions with cycloalkyl‐oxygen and acyl‐oxygen cleavage. In addition to the tricycloheptenol 4a, the tricycloheptenyl ethyl ether 17a, ethyl 7‐norbornadienyl ether (19), 7‐norbornadienyl p‐nitrobenzoate (20), and ethyl p‐nitrobenzoate were formed. No acyl‐oxygen cleavage took place in TFE as solvent, where the tricycloheptenyl trifluoroethyl ether 18a, 20, 7‐norbornadienyl trifluoroethyl ether (21), and the cyclopentadienylvinyl trifluoroethyl ethers 22/23 were produced. A new mechanism is proposed for the route to the major products 22/23 via the intermediate 7‐norbornadienyl cation (9a), which was attacked by TFE at the two‐membered bridge stabilising the positive charge. Thus formed, tricyclo‐[3.2.0.02,7]hept‐3‐en‐endo‐6‐yl trifluoroethyl ether (24) underwent a Diels‐Alder cycloreversion generating the cyclopentadienylvinyl trifluoroethyl ether 25, the immediate precursor of 22/23. – From kinetic data for these processes and thermochemical data for appropriate hydrocarbons, the enthalpy difference between the C7H7 cations, tricyclo‐[4.1.0.02,7]hept‐4‐en‐3‐yl (3a) and 7‐norbornadienyl (9a), was calculated to be only 8 kcal mol−1.