Neighbouring groups

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). 

Sulphoxides with -carboxylic acid or amide groups are converted, in nearly quantitative yields, to the sulphone whilst other acids and amides did not show oxidation. In the cases where oxidation did occur, the acid group was converted to the acid chloride (equation 31) whilst the amide was converted to the nitrile (equation 32). These results indicate that neighbouring-group participation in the case of carboxylic acids and amides occurs only when a five-membered intermediate is formed. In the case of hydroxyl groups, four-, five-or six-membered intermediates are favourable. 

Recruitment of neighbouring group II neurons. Sjrnchronous discharge... 

They also compared compounds which might be expected to give strong neighbouring group effects should a carbonium ion intermediate be formed, viz. 

Similar kinetics are exhibited by the reduction of vicinal dihalides by Cr(II) , which proceeds 10-40 times faster than that of the allylic halides. Such activation by a second halogen atom suggests a neighbouring group effect, viz. 

The dehalogenation of the a-haloalkyl radical is a fast step which can take place by several possible routes . Dibromides are reduced much faster than dichlorides and rra j-l,2-dibromocylohexane is reduced 100 times faster than the m-isomer. This accords with neighbouring group assistance which bromine seems particularly capable of offering (see subsection 6.4.10). 

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)... 

As a increases, the average distance between ionized groups decreases so that these neighbouring groups begin to have an effect. When a exceeds 0-3, individual water spheres begin to overlap and eventually coalesce into a cylindrical form. With further increases in a, a second outer cylindrical sheath of water appears in which water molecules are oriented by the cooperative effect of two or more carboxyl groups. 

In the case of partly modified polymers the spectra B, C and D show more complicated structures which can be presumably due to significant neighbouring-groups effects between phenyl carbonate groups and modified or unmodified groups linked to the next aliphatic methine carbon atoms. 

Hesse691 has shown that the unimolecular chemistry of cation radicals of di- or polyfunctionalized alkanes is strongly dependent upon the interaction of the functional groups, mainly via neighbouring group participation in the transition states and the... 

Another example of oxygen as a neighbouring group occurs in the hydrolysis of the 2-bromopropanoate anion (39) at low [eOH], which is also found to proceed with retention of configuration (40). The rate is found to be independent of [eOH], and the reaction is believed to proceed ... 

N can act as a neighbouring group in similar circumstances, e.g. the hydrolysis of Me2NCH2CH2Cl, but the rate is markedly slower,... 

Such species with a bridging phenyl group are known as phenonium ions. The neighbouring group effect is even more pronounced with an OH rather than an OMe substituent in the p-position. Solvolysis is found to occur % 106 times more rapidly under comparable conditions, and matters can be so arranged as to make possible the isolation of a bridged intermediate (5), albeit not now a carbocation ... 

This neighbouring group participation by bromine (cf. p. 93) does not of course prove that addition to alkenes proceeds via cyclic bromonium ions, but it does mean that such species are no longer merely ad hoc assumptions, and to that extent are correspondingly more plausible as intermediates. 

In organic chemistry, the retarding influence of neighbouring groups on reactions between molecules. 

Ester hydrolysis, neighbouring group participation by carbonyl groups in, 28, 171... 

Neighbouring group participation by carbonyl groups in ester hydrolysis, 28, 171 Nitration, nitrosation, and halogenation, diffusion control and pre-association in, 16, 1 Nitrosation, mechanisms, 19, 381... 

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