Neighboring-group-effect

One obvious area in which anchimeric effects might materialize is in chelation reactions this is the underlying reason that chelation is dominated by the first step (Sec. 4.4.1). We have seen it operate in the formation of Pt(II) chelates (Sec. 4.7.6). The accompanying values of AS are less negative than is usual for the associative substitution in Pt(II), consistent with the intramolecular mechanism. The rate constant for the ring-closure reaction (6.1) 

Since the bridged complex reacts without formation of the diaqua intermediate (this can be separately prepared and shown to be much less reactive than would be required), the anchimeric effect of the adjacent coordinated water is established. See also Ref. 8. 

Hydrophobic attractive interactions between a polymer and a small-molecule reactant often occur in reactions carried out in aqueous media [Imanishi, 1979 Morawetz, 1975 Overber-ger and Guterl, 1978, 1979]. Acid catalysis of ester hydrolysis by polysulfonic acids is most 

Overberger and coworkers carried out an interesting study on the hydrolysis of various 3-nitro-4-acyloxybenzoic acid substrates (V) catalyzed by imidazole (VI) and poly[4(5)-vinylimidazole] (VII) in ethanol-water mixtures [Overberger et al., 1973]. 

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The preparation of polyacrylamides and postpolymeri2ation reactions on polyacrylamides are usually conducted ia water. Reactions on the amide groups of polyacrylamides are often more compHcated than reactions of simple amides because of neighboring group effects. Reaction rates, for example, can differ considerably. 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

Figure 25.6 Mechanism of the Koenigs-Knorr reaclion, showing the neighboring-group effect of a nearby acetate.

The participation shown by the nearby acetate group in the Koenigs-Knorr reaction is referred to as a neighboring-group effect and is a common occurrence... 

El reaction and, 392 E2 reaction and, 386 S l reaction and, 373-374 Sjxj2 reaction and, 362-363 Kishner, N. L. 715 Knoevenagel reaction, 913 Knowles, William S., 734, 1027 Kodel, structure of, 1222 Koenigs-Knorr reaction, 990 mechanism of, 990 neighboring-group effect in, 990-991... 

Other groups such as esters, silylethers, and imides are also successfully incorporated through ADMET depolymerization with 14 (Fig. 8.21).49 For an ester functionality, at least two methylene spacer units must be present between die olefin site and die functional group in order to achieve depolymerization. This is due to die negative neighboring group effect, a deactivation of the catalyst by coordination of the functionality heteroatoms to die catalyst.50 By physically... 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

Negative neighboring group effect, 456 Network formation, 13 Networks. See also Epoxy-phenol networks Phenohc networks phenolic-based, 376 polyester-based, 58-60 Neutral hydrolysis, 564-565... 

A cyclic complex derivable by addition of positive bromine to a double bond has already been discussed as an example of the neighboring group effect. The same intermediate, except perhaps for the nature of the bonds to bromine, is formed in the addition of bromine to olefins and is responsible for the stereochemistry of the addition reaction and the nature of the by-products.232... 

The reader will no doubt have realized that the y-hetero rearrangement is well-known, being none other than the familiar neighboring group effect. The a-hetero rearrangement is known for all of the elements mentioned but boron, aluminum, and silicon. Perhaps a suitably constituted boron compound would rearrange to a zwitterionic product as follows ... 

A reaction that offers a closer parallel to the neighboring group effect of carbonium ion chemistry is a possible concerted mechanism for the Michael rearrangement ... 

Keywords Acyclic diene metathesis polymerization, ADMET, Condensation polymerization Functionalized polymers, Negative neighboring group effect, Branched polyethylene... 

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