KENNETH CHARLES WESTAWAY and SYED FASAHAT ALI. Can. J. Chem. 57,1354Chem. 57, (1979. The nucleophilic substitution reactions of a series of 4-substituted phenylbenzyldimethylammonium ions with thiophenoxide ions at 0°C in N,N-dimethylformamide have been used to demonstrate how a change in the leaving group alters the structure of the SN2 transition state. Heavy atom (nitrogen) kinetic isotope effects, secondary a-deuterium kinetic isotope effects and Hammett p values provide qualitative descriptions of both the nucleophilea-carbon and a-carbon -leaving group bonds in the transition states of these reactions. The results indicate that changing to a better leaving group causes the bond between the a-carbon and the nucleophile to be much more fully formed while the bond to the leaving group is essentially unchanged. The results are discussed in the light of current theories of substituent effects on SN2 reactions and a possible explanation for the surprising results (i) that the greatest effect is in the bond more remote from the point of structural change and (ii) that more nucleophilic assistance is required to displace a better leaving group is given. KENNETH CHARLES WESTAWAY et SYED FASAHAT ALI. Can. J. Chem. 57,1354Chem. 57, (1979. On a utilise les reactions de substitutions nucleophiles d'une skrie d'ions X-4 phenylbenzyldimkthylammonium avec les ions thiophknolates a 0°C dans le N,N-dimkthylformamide pour dkmontrer I'influence d'un changement dans le nucleofuge sur la structure de I'ktat de transition SN2. Les effets isotopiques cinetiques des atomes lourds (azote), les effets isotopiques cinetiques secondaires des deutkrium en a et les valeurs p de Hammett fournissent des descriptions qualitatives des liaisons nuclkophile -carbone en alpha'et carbone en alpha groupe nuclkofuge dans les ktats de transition de ces reactions. Les r6sultats indiquent que si I'on utilise un meilleur nuclCofuge la liaison entre le carbone alpha et le nucleophile devient plus developpke alors que la liaison vers le groupe nuclkofuge est pratiquement inchangee. On discute des rksultats a la lumiere des thkoriques actuelles des effets de substituants sur les rkactions SN2 et I'on propose une explication plausible pour les resultats surprenants: (i) que I'effet est le plus grand dans la liaison la plus Bloignee du point de changement structural et (ii) qu'il faut plus d'aide de la part du nucleophile pour dtplacer un meilleur nuclkofuge.[Traduit par le journal] Introduction have been used in studies on a wide range of reac~l t h~~~h the mechanisms of the simple one-step tions in an effort to determine how the structure of (SJ) nucleophilic substitution reaction at saturated the reactants affects the structure of the S,2 transicarbon, eq. [I], have been investigated for over half tion states. In addition, several theoretical studies (18-23) have predicted the answer to this question.

A very large secondary a-deuterium kinetic isotope effect of 1.179 i 0.007 (1.086 i 0.003 per a-deuterium) has been observed for the SN2 reactlon of thlophenoxide ion with benzyldimethylphenylammonium ion in D M F at 0°C. This large isotope effect which is far outside the range reported for SN2 reactions, is attributed to the fact that the extraordinarily large steric crowding around the Ca-H bonds in the substrate is reduced in the SN2 transition state. The structure of the transition state is shown to be consistent with this hypothesis.KENNETH CHARLES WESTAWAY et SYED FASAHAT ALI. Can. J. Chem. 57, 1089(1979). Dans la reaction SN2 de l'ion thiophenolate avec l'ion benzyldimethylphenylamn~oi~ium dans le D M F a O°C, on a observe un effet isotopique cinttique secondaire du deuterium en alpha qui est tres grand 1.179 + 0.007 (1.086 i 0.003 par deuterium a). Cet effet isotopique important, qui est a l'exterieur des valeurs rapportees pour des reactions SN2, peut btre attribue au fait que l'encombrement sttrique qui est particulierement important au niveau des liaisons Ca-H dans le substrat est reduit dans l'etat de transition SN2. On demontre que la structure de I'etat de transition est en accord avec cette hypothese.[Traduit par le journal]

From the hesh fruits of Solanum albicaule KOTSCHY solamargine was isolated in 0.5 % yield. Three other glycoalkaloids also present in very small quantities have not been isolated so far in pure state. In view of the great importance which solasodine has gained in the commercial synthesis of pharmacologically active steroids, it appeared worthwhile to look for other sources of this compound. T o achieve this objective we recently studied the glycoalkaloidal constituents of Solanum albicaule.