| Research Abstract |
The splitting in the C-13 NMR signals, induced by isotopic oxygen substitution O-16/O-18, was utilized successfully to determine quantitatively the extent of O-18 scrambling within three oxygens in the sulfonate moiety. This method was applied to the investigation of return process in the solvolysis of various arylalkyl tosylates. The solvent dependency of (1-F) value in adamantyl solvolyses shows in good accordance with that of alpha-t-butylbenzyl one. Accordingly the ion-pair return scarcely affects the result of the solvent effect analysis of alpha-t-butyl benzyl solvolysis based on 2-adamantyl Y_<OTs>.
Despite the nucleophilically limiting nature of aryl-assisted and benzylic solvolyses, the solvent effect on these solvolyses failed to give a single linear correlation with the Winstein-Grunwald Equation, using the adamantyl Y_<OTs> but showed characteristic dispersion for various binary aqueous solvent series. Developing charge at the reaction center in the transition state of benzy
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lic k_c carbocationic process is highly delocalized through effective pi-overlapping with aryl group, and this may cause a significant loss of the highly oriented specific solvation at the reaction center in comparison with that for aliphatic localized 2-adamantyl cation. The solvent polarity scale Y_<DELTA> based on p-methoxyneophyl solvolysis combined with the Y_<OTs> has shown a wide applicability to the solvolyses of benzylic arylalkyl tosylates involving varying degree of pi-delocalization interaction with the incipient carbocation charge ; log (k/k_o) = aY_<OTs> + bY_<DELTA>. The equation can be applied to various series of aryl-assisted solvolyses in an extremely high precision, suggesting a continuous spectrum of k_c-k_<DELTA> mechanisms. Benzylic k_c solvolyses can also be treated with linear combination of Y_<OTs> and Y_<DELTA>. Application of revised equation, aY_<OTs> and bY_<DELTA> including cN_<OTs> term has also been extended to the nucleophilic solvolyses of alkyl and arylalkyl tosylates, with limited success, indicating no precise additivity for the nucleophilic solvent involvement. Less
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