| Research Abstract |
Quantitative analysis of the return process from ionic intermediates to the starting material in solvolysis plays a decisive role in the investigation of its reaction mechanism.
The splitting, in the ^<13>C NMR signals of alkyl sulfonate esters, induced by isotopic oxygen substitution ^<16>O/^<18>O, was utilized to determine quantitatively the extent of ^<18>O scrambling within three oxygens in the sulfonate moiety. Oxygen exchange process (return process) as well as overall solvolysis process was successfully followed by ^<13>C NMR spectroscopy using a small amount of doubly labeled starting tosylate in a sealed NMR tube. In the acetolysis of neophyl tosylate, no ^<18>O-scrambling at the alkoxy oxygen of the starting tosylate was detected at all for whole range of reaction. The acetolysis of 2-(p-OMe-phenyl)ethyl tosyate gave the rearranged tosylates with complete randomization of the three oxygens. The ace- tolyses of 2-phenylpropyl and 2-phenylethyl tosylates also gave the rearranged
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tosylates of two signals, accompanying the incomplete oxygen equilibration. The different scrambling behaviors for substituted phenylethyl tosylates, may be attributable to the nature of the ion-pairs. In the acetolysis of 2-phenylethyl tosylate, parallel runs of (1-^*C)-OTs^* and (2-^*C)-OTs^* indicated the equivalence of two methylene carbons for ^<18>O-incorporation from sulfonyl oxygens in the return process. This provides strong evidence that the ^<18>O scrambling occurs only after the formation of symmetrical phenonium-ion species. In the acetolysis of benzyl tosylate, ^<18>O-scrambling was detected, being inconsistent with S_N2 mechanism.
The acetolysis of 2-adamantyl tosylate referred to a typical k_c substrate also showed the occurrence of ion-pair return, suggesting the necessity of re-examination of solvolysis theories. It is apparent that the ^<13>C NMR method superior to the mass spectroscopic method can generally be applicable to the investigation of the return process in solvolysis. Less
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