Neighbouring-group effects

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. 

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. 

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 ... 

Neighbouring group effects one single rate constant k0=ki=k2 three differei kQkj> 2... 

Exhaustive studies on well-defined systems are rather scarce (4) nevertheless 3 systems thoroughly analyzed by independant research groups are of outstanding interest a) the quaternization of polyvinylpyridines by alkyl halogenides (20-25) b) the chlorination of polyethylene (13,26-28) c) the basic or acid hydrolysis of PMMA (29-31). On the other hand, neighbouring groups effects have been quantitatively taken into account for the kinetic analysis of periodate oxidation of amylose (32,33). 

To conclude, kinetic measurements and structural analysis of the copolymers have allowed a quantitative and self-consistent description of the reaction of RCI Li species on PMMA taking into account PMMA chain reactivity through the simplified model of the nearest neighbouring group effects. Two main features are particularly relevant the definite influence of tacticity, and the independance of the reaction process on the total charge of the copolymer. In this sense, the R-C Li/PMMA systems are closed to the PMMA basic hydrolysis in presence of excess base. (29,31). 

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

A review of neighbouring-group effects in natural products includes steroidal examples. The 19-acetoxy-5a-bromo-6-oxo-steroids (228) fragment with base to give the 6-oxo-A -compounds (229) which are regarded as potentially useful intermediates in prostanoid synthesis.The preparation and properties of the 6-spirocyclopropane (230) have been reported. Attempts to cleave the cyclopropane symmetrically were unsuccessful although other cleavages are discussed. ... 

Weinges, K. The neighbouring group effect in the conversion of tetra-methyl-(- -)-catecbin into 2-chlorotetramethyl-(—)-isocatechin. Proc. chem. Soc. [London] 1964, 138. 

Reference 306 also deals with a neighbouring-group effect. 

The aminolysis of turns-1,2-bis(ethylsulfonyl)-1.2-dichlorocthcnc (336) by primary and secondary aliphatic amines in acetonitrile has been studied.305 Thermolysis in p-xylene solution of 4-azidothiazoles (337) displayed some interesting neighbouring-group effects,306 e.g. in (337) with R1 = Ph, R2 = (acceleration produced in parentheses) NO2 (19-fold), phenyliminomethyl (16-fold), formyl (4.5-fold), and acetyl (2.2-fold). Rate data for nucleophilic attack on phenyldimethylsulfonimn ions by... 

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