40. “The Acetolysis of exo- and endo-5,6-Dimethylidene-2-Norbornyl p-Bromobenzenesulfonates and of their Optically Active and Deuterium-Labelled Derivatives”
    Sonney, J.-M.; Vogel, P.; Burger, U.
    Helv. Chim. Acta 1980, 63, 1016-1033.

The buffered (AcOK) acetolyses of exo, (11) and endo-5, 6-dimethylidene-2-norbornyl brosylate (12) yielded exo5, 6-dimethylidene-2-norbornyl (16) and (3-methylidene-2-nortricyclyl)methyl acetates (18). Endo-5, 6-dimethylidene-2-norbornyl (17) and 2-methylidene-3-tricyclo [3.2.1.0^3,6]octyl acetates (20) could not be detected. The titrimetric rate constants of the acetolysis of 11 (k_t(exo)=4.49 ± 0.02) · 10⁻⁵s⁻¹ at 25°, ΔH^≠=23.6 ±0.7 kcal mol⁻¹, ΔS^≠=0.7 ±2 calmol⁻¹ K⁻¹ and 12 (k_t(endo)=1.9 ±0.08) · 10⁻⁹ s⁻¹ at 25°, ΔH^≠=27 ±1 kcal mol⁻¹, ΔS^≠=-8 ±2.5 calmol⁻¹ K⁻¹) were measured and compared with the polarimetric rate constants (k_α/k_(exo)=6.8 at 25°,(k_α/k_(exo)=1.0 at 121°) of the buffered acetolyses of the optically active brosylates (+)-11 and (+)-12 . Neither a common-ion (KOBs) nor a special ion effect (LiClO₄) onk_t(endo) could be detected, although external return might well intervene as some exo-5,6-dimethylidene-2-norbornyl tosylate (21) was formed upon solvolysis in the presence of KOTs. Acetolysis of (+)-11 yielded completely racemized products, whereas (+)-12 led to incomplete racemization. The buffered acetolysis of exo-(3exo-D)-5,6-dimethylidene-2-norbornyl brosylate (24) furnished (3exo-D)-(26:37.5%), exo-(7syn-D)-5,6-dimethylidene-2-norbornyl brosylate (27: 37.5%) and [(5anti-D)-3-methylidene-2-nortricyclyl]methyl acetates(28:25%). The acetolysis of endo-(2exo-D)-5,6-dimethylidene-2-norbornyl brosylate (25) yielded (2endo-D)-(29: 54%), exo-(1-D)-5,6-dimethylidene-2-norbornyl (30: 36%) and [(6-D)-3-methylidene-2-nortricyclyl]methyl acetates (31: 10%). Product analysis and deuterium label distribution was established by a combination of GC., ¹H-NMR., ²H-{¹H}-NMR. and MS. techniques. The results are rationalized by invoking anchimerically assisted ionization of the exo-brosylate 11 to symmetrical ion-pairs (cyclopropylcarbinyl cation intermediates) which undergo internal (and probably also external) return. Acetolysis of the endo-brosylate 12 is not anchimerically assisted and leads initially to non-symmetrical ion pairs. These evolve to symmetrical ion pair intermediates or, to a minor extent, are intercepted by solvent.

DOI : 10.1002/hlca.19800630430 

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