The acetolysis of exo- and endo-5,6-Dimethylidene-2-Norbornyl p-Bromobenzenesulfonates and of their Optically Active and Deuterium-Labelled Derivatives

Article abstract of DOI:10.1002/hlca.19800630430

The buffered (AcOK) acetolyses of exo (11) and endo-5,6-dimethylidene-2-norbornyl brosylate (12) yielded exo-5,6-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)E-3/s at 25 deg C, ΔH(excit.)=23.6+/-0.7 kcal/mol ΔS(excit.)=0.7+/-2 cal/mol*K) and 12 (k_t(endo)=(1.9+/-0.08)E-9/s at 25 deg C, ΔH(excit.)=27+/-1kcal/mol, ΔS(excit.)=-8+/-2.5 cal/mol*K)were measured and compared with the polarimetric rateconstants (k_a/k_t(exo)=6.8at 25 deg C, k_a/k_t(endo)=1.0 at 121 deg C) of the buffered acetolyses of the optically active brosylates (+)-11 and (+)-12.Neither a common-ion (KOBs) nor a special ion effect (LiClO4) on k_t(exo) 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: 37.5percent), exo-(7syn-D)-5,6-dimethylidene-2-norbornyl (27: 37.5percent) and <(5anti-D)-3-methilidene-2-nortricycyl>methyl acetates (28:25percent).The acetolysis f endo(2exo-D)-5,6-dimethylidene-2-norbornyl brosylate(25) yielded (2endo-D)(29:54percent), exo-(1-D)-5,6-dimethylidene-2-norbornyl (30:36percent) and ,(6-D)-3-methylidene-2-nortricyclyl>methyl acetates (31:10percent).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 involking anchimerically assisdted ionization of the exo brosylate 11 to symmetrical ion-pairs cyclopropyl carbinyl cation intermediates) which undergo internal (and probably also internal) return.Acetolyses of the endo-brosylate 12 is not anchimetrically 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.