Winstein solvolysis scheme

The observation that the rate of loss of optical activity during the solvolysis of certain chiral substrates R — X exceeded the rate of acid production and the occurrence of a special salt effect led to the postulation of two distinct ion-pair intermediates [161, 162]. The basic Winstein solvolysis scheme is given by Eq. (2-20). 

According to the Winstein-Ingold solvolysis scheme (68), the observation of a rate decrease in the solvolysis of RX by either the formed or an added X- ( common ion rate depression", CIRD) serves as evidence for product formation from an intermediate free vinyl cation R+. The simplified solvolysis scheme, when ion pairs are neglected and both the solvent SOH and its conjugate base SO- may be present, is shown in Scheme II. 

The mechanism of ferrocenylmethyl benzoate solvolysis can be best understood in terms of the Winstein mechanism for solvolysis (Scheme 9.4.l). It can be assumed that, in the presence of lithium perchlorate, the solvent-separated ion pair reacts rapidly by the and kq route to give solvolysis products. This appears to be the major pathway. In this interpretation, the dissociation of the intimate ion pair (the k step) is mainly or entirely rate determining. 

Scheme 5 depicts Winstein s complete solvolysis mechanism.29 Ion-pair return can be from the intimate ion pair (ion-pair return or internal return), from the external ion pair (external ion-pair return), or from the free ions (external ion return). The term external return refers to the sum of external ion-pair return and external ion return. The special salt effect operates by diversion of the external.ion pair, probably through the mechanism shown in Equation 519, so that it can no longer... 

A detailed study of the specific rates of solvolysis of N,N,N, N -tetra-methyldiamidophosphorochloridate (80) (TMDAPC) with analysis in terms of the extended Grunwald-Winstein equation has been reported (Scheme 19). The stereochemistry of nucleophilic attack at tetracoordinate phosphorus was also discussed." The initial reaction of bis (2,4-dinitrophenyl) phosphate (BDNPP) (81) with hydroxylamine involves release of 1 mol 2,4-dinitrophen-oxide ion and formation of a phosphorylated hydroxylamine (82), which reacts readily with further NH2OH, giving the monoester (83). The intermediate (82) also breaks down by two other independent reactions one involves intramolecular displacement of aryloxide ion (83) and the other involves migration of the 2,4-dinitrophenyl group from O to N and formation of phosphorylated 2,4-dinitrophenylhydroxylamine (84) (Scheme 20)." ... 

The most detailed comparison of carbonium ions from diazonium ion and ester precursors is due to Diaz and Winstein (1966) who measured the relative rates of ion pair collapse and dissociation in the reaction of diphenyldiazomethane, DDM, with benzoic acids and in the solvolysis of benzhydryl benzoates (scheme 9). 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

Detailed kinetic investigations, primarily concerned with salt effects on solvolysis reactions, revealed inadequacies in each of the proposed mechanisms and led to the suggestion of a dual ion-pair scheme to explain the results. This work is most closely associated with Saul Winstein and his collaborators at the University of California, Los Angeles, who first proposed the involvement of two distinct ion-pair intermediates in solvolysis reactions. It has since been refined and elaborated on by others, and is the most generally accepted interpretation of nucleophilic substitution... 

The specific rates of solvolysis of benzenesulfonyl chloride and a series of 4-and 2,4,6-substituted derivatives (Scheme 25) have been very well correlated using the extended Grunwald-Winstein equation, with incorporation of A/r solvent nucle-ophilicity and Tq solvent ionizing power values. In two instances, it was shown that almost identical correlations were obtained after the incorporation of methyl groups into the 2- and 6-positions of the benzene ring. No evidence was found for a dissociative (5n1) mechanism or for a duality of mechanism. All of the results could be rationalised in terms of a concerted bimolecular displacement (5n2) mechanism, involving attack by solvent at sulfur. ... 

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