Solvolysis neighbouring group participation

There is some evidence for the formation of unstable benzazetidines from [2 + 2] cycloaddition of benzyne to imines (75BCJ1063). A novel formation of a benzazetidine is reported in the solvolysis of the exo iV-chloro compound (297). Neighbouring group participation by the benzene ring leads to the cation (298), which is intercepted by methanol to give the benzazetidine (299) (81CC1028). 

B. Solvolysis of Phosphoric Acid Derivatives.—Interest continues in neighbouring-group participation in the solvolysis of phosphate esters. As a potential model compound for investigating the mechanism of ribo-nuclease action, the phenyl hydrogen phosphate ester of c/j-3,4-tetrahydro-furandiol (24) has been the subject of a detailed study. Above (and probably also below) pH 4 hydrolysis gives solely the cyclic phosphate (25)... 

Other evidence for ion pair return is less relevant to solvolyses of simple secondary substrates. It is known that during solvolyses of certain optically active substrates racemization occurs more rapidly than solvolysis, but it is not known whether the ion-pair intermediates undergoing racemization are the same as those undergoing solvolysis (Hammett, 1970a). Such behaviour is usually observed in solvolyses where neighbouring group participation occurs and the intermediates are probably more stable than those from simple secondary solvolyses. As the stabilities of the intermediates increase, there appears to be a general trend towards formation of more dissociated species,4 and thus the relevance of these results is questionable. 

The influence of neighbouring group participation, by formation of bridged intermediates, causes (e.g. in the case of solvolysis of the primary brosylate) almost complete disappearance of secondary deuterium isotope effects, since the transition state resembles... 

The fact that solvolysis of (131) in 70 % aqueous acetone occurs ca. 120 times more rapidly than does the corresponding reaction of its saturated analogue, and that the product mixtures obtained from (131) and from (132) are closely similar, points to neighbouring-group participation by the bridgehead 71-bond in (131) and a common ionization intermediate (133) leading from (131) and (132). ... 

An easy conversion of naturally occurring quebrachitol (2-0-methyl-L-c/u>o-inositol) to mwco-inositol has been described, involving oxidative removal of the methyl group from the perbenzoyl derivative and inversion at C-2 by solvolysis of the corresponding tosylate with concomitant benzoate neighbouring-group participation. ... 

Solvolysis of trianisylvinyl bromide (229, X = Br) is only 1 7 times faster than that of a-bromo-p-methoxystyrene (Rappoport and Gal, 1968). The participation of the neighbouring /1-anisyl groups in the solvolysis, if any, is small, probably because the groups are held too far... 

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