CRITICAL EXAMINATION CONCERNING THE MECHANISTIC POSTULATES IN SOLVOLYTIC DISPLACEMENT REACTIONS

定量分析离子中间体在溶剂分解过程中向起始物质的返回过程对研究其反应机理起着决定性的作用。利用同位素氧取代^<16>O/^<18>O引起的烷基磺酸酯^<13>C核磁共振信号中的分裂，定量确定了^<18>O在磺酸段三个氧内的置乱程度。在密封的核磁共振管中使用少量双标记起始戊酸酯，成功地完成了氧交换过程（返回过程）和整体溶剂溶解过程，然后进行了^<13>C核磁共振光谱。在新茶树烟酸酯的醇解过程中，在整个反应过程中均未检测到对起始烟酸酯的烷氧基氧的^<18> o -抢夺。2-（p- ome -苯基）乙基tosyate的乙酰分解得到了三个氧完全随机化的重新排列的tosyates。2-苯丙基和2-苯乙基苯甲酸酯的ace-裂解也产生了两种重新排列的多苯甲酸酯信号，并伴有不完全氧平衡。取代苯乙基苯甲酸酯的不同置乱行为可能与离子对的性质有关。在2-苯乙基tosylate的乙酰分解过程中，(1-^*C)- ots ^*和(2-^*C)- ots ^*的平行运行表明，在返回过程中，两个亚甲基碳对磺酰基氧的^<18> o的吸收是等效的。这提供了强有力的证据，证明^< 18>o乱序只发生在对称现象离子种形成之后。在甲磺酸苄酯的乙酰解过程中，检测到^<18> o -置乱，与S_N2机制不一致。以典型的k_c为底物的2-甲酰基甲酸乙酯的乙酰解也出现了离子对返回现象，表明有必要对溶剂解理论进行重新考察。可见，相对于质谱法，^<13>C核磁共振法一般适用于溶剂分解中还原过程的研究。少

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 … More 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