Internal return, solvolysis tosylates

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

Winstein (1957c). They argued that during solvolysis of neophyl substrates [46, X = Cl, Br] internal return from the proposed phenonium ion-pair intermediate [47] would yield the tertiary derivative [48] (Fig. 13) and this would solvolyse rapidly. Thus, the titrimetric rate constants should correspond to the ionization rate constant k j (Fig. 2). Later neophyl tosylate [46, X = OTs] and its p-methoxy derivative were used (Smith et al., 1961 Diaz et al., 1968b Yamataka et al., 1973), and recently Schadt et al. (1976) defined as a scale of solvent ionizing power for tosylates, designated F2-AdOTs or Yqt, using eqn (5) but based on 2-adamantyl tosylate [6] instead of t-butyl chloride. A correlation of the rates of solvolysis of neophyl tosylate with F0 Xs (Fig. 14) is satisfactory (correlation coefficient... 

The large amounts of five- and six-membered cyclic tosylates, products of internal return (49% yield in one case 12a of Table 3) points also to a highly selective intermediate, although the reason for such a remarkable selectivity towards external nucleophiles is not clear. On the other hand, acetolysis of 6-phenyl-5-hexynyl brosylate, which is apparently anchimerically assisted by the triple bond (4>bs./ caic. = 1 6) yields only the five-membered ring product in addition to open-chain solvolysis products (13 of Table 3). The effect of the phenyl group in orienting the oyclization reaction would indicate that intermediate species like 43 may become important when R =Ph. 

The recovered tosylates on the other hand contained more D at C(9) than the isolated acetates (approximately twice as much in the recovered tosylate from the solvolysis of [395] compared with that recovered from the solvolysis of the mixture). These results indicated stereospecific rearrangement via internal return of the tosylate ion pair. 

Lee has shown that in the acetolysis of — [1 — C]-cyclopentenyl-ethyl tosylate and 2-exo-[4 — C]-norbornyl tosylate k5/k3 is < 100 and <200, respectively. The distribution of the tag indicates that on solvolysis of exo-norbomyl brosylate the ion pair is internally returned. 

The reduction of ketone 559 by LiAlH results mainly in the endo alcohol 556. The thermodynamic equilibrium of exo and endo alcohols leads mainly to exo alcohol 560. Hence, the formation of endo alcohol 556 upon solvolysis of tosylate 558 cannot be due to steric factors — the endo-side attack is sterically less favourable than the exo-side one the resulting alcohol 556 is not a thermodynamically controllable product. Consequently, the data obtained cannot be used to assume a classical structure of the intermediate cation 561. At the same time the participation of the C —C bond and the intermediate non-classical ion are quite compatible with these facts. The acetolysis of the optically active tosylate 558 is accompanied by complete racemization, the rate of the latter being 3 times as high as that of acid elimination (internal return). The completeness of racemization shows the reaction to proceed via a symmetrical trishomocyclopropenyl ion or rapid equilibration of unsymmetrical cations 561 or to be accompanied by a 1,3-hydride shift from to C . 

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