Nucleophile effects

Olefinic, unsaturated hydrocarbons have CH groups adjacent to C=C double bonds. The C=C vibrations do not show up directly except as weaker contributions in the combination region, but the nucleophilic effect of the C=C bond tends to shift nearby CH groups to significantly higher wavenumber, and this is indeed seen in hydrocarbon spectra (Figure 5.32). 

In general, the reaction of unsaturated 5(4//)-oxazolones 497 with nitrogen nucleophiles effects ring opening to give the corresponding unsaturated acylamino amides 498 (Scheme 7.158). Depending on the nucleophile, for example, amines, hydrazines, oximes, and so on, the products obtained can be cyclized and this process allows the synthesis of a wide variety of new heterocyclic compounds. 

Efforts to cause the carbon nucleophile available at C-2 (carbohydrate numbering) of the osulose derivative 66 to displace the methoxy group with allylic rearrangement and with consequent formation of a tricyclic product by use of Pd(0) catalysts [34] were unsuccessful, but the intended reaction proceeds "smoothly when tin(IV) chloride is used together with acetic anhydride in dichloromethane. Clearly, the Lewis acid activates the allylic ether group, and the C-2 nucleophile effects its displacement. Concurrently, acetolysis of the benzylidene ring occurs and the product isolated is the cu-decalin analogue 67 [33],... 

The effect of electrons in the unsaturation in the ultimate and penultimate monomer groups on the ionicity of the catalysts has been shown by Wilke (133). His work shows that, when the catalyst contains triphenylphosphine, the added increasing nucleophilic effect of the double bonds converts the catalyst to an ethylene incorporating specie. After incorporation of the ethylene, cyclization and reduction of the metal occurs to produce cyclodecadiene and cydohexene. This effect is analogous to that shown earlier (88) of a diene system complexing with the catalyst modifying the ionicity to favor ethylene incorporation. 

Palladium (II) compounds coordinate to alkenes to form 71-complexes. Roughly speaking, the decrease of alkene electron density caused by coordination to an electrophilic Pd(II) compound enables an attack by nucleophiles on the coordinated alkenes. The attack of a nucleophile with concomitant formation of a carbon-palladium e-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. Unlike the products of mercuration which are stable and isolable, palladation product 1 is usually unstable and undergoes rapid decomposition. Palladation is followed by two reactions. The elimination of H-Pd-X from 1 to form the vinyl compounds 2 is one path, resulting in nucleophilic substitution of the alkene. Displacement of the Pd in 1 by an other nucleophile effects nucleophilic addition of the alkene to give 3. Depending on the reactants and conditions, either nucleophilic substitution of the alkene or the nucleophilic addition to the alkene takes place [4,5]. 

Nucleophile Effective HOMO El (eV) Electrophile Effective LUMO E (eV)... 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

Several attempts to make the thiol acrylate of T by treating f with acryoylchloride or acryoyl anhydride failed to give the desired product. This material is probably unstable due to the nucleophilic effect of the nearby oxygen on nitrogen. Presumably a mixture of readily hydrolyzed 0- and N-substituted derivatives was formed. Thus, an attempt was made to form the thio-hemiacetal with formaldehyde. Unexpectedly, two moles of formaldehyde condensed to give hemiacetal, 22 which was then converted to acrylate 23 (Scheme V). 

A comparison of the rates of solvolysis of the tert-butyldimethylsulfonium ion and the 1-adamantyldimethylsulfonium ion presents strong evidence that the solvent dependence of the tert-butyldimethylsulfonium ion solvolysis rates is governed primarily by solvent nucleophilicity effects. Leaving-group contributions based upon 1-adamantyldimethylsulfonium ion solvolyses are better incorporated into the establishment of the solvent nucleophilicity scale based upon triethyloxonium ion solvolysis. Alternative solvent nucleophilicity scales based upon the solvolysis of S-methylbenzo-thiophenium ions are discussed. Analyses of the extent of nucleophilic participation by the solvent in the solvolyses of methyldiphenyl-sulfonium and benzhydryldimethylsulfonium ion will be presented. The relative nucleophilicities of various anionic and neutral nucleophiles toward the triethyloxonium ion in ethanol have been determined. 

Chloroindole)RuCp as well as (nitroindole)RuCp react with a variety of O, S, N, and C-nucleophiles to give substitution products [91]. One example of an enolate nucleophile effecting an intramolecular cyclization is shown in Eq. (22)... 

The anions of malonate esters, cyclopentadiene, p-keto esters, ketones, aldehydes, a-nitroacetate esters, Meldrum s acid, diethylaminophosphonate Schiff bases, p-diketones, /3-sulfonyl ketones and esters, andpolyketides represent the wide variety of carbon nucleophiles effective in this reaction. Generation of the stabilized anions normally is... 

The nucleophilicity of the phosphorus center in the catalyst may affect catalytic activity. Thus, catalyst CP29 was designed to test the nucleophilicity effect by exchanging the phenyl groups in catalyst CP17 with methyl groups. The catalyst CP29 was then examined in the aza-MBH reaction of... 

Solvolysis reactions in media of low nucleophilicity are characterized by increased tendencies toward carbonium ion rearrangements and increased racemiza-tion when optically active substrates are employed. We have seen examples of extensive rearrangements in our discussion of carbonium ions generated in superacid media, in which the observed ion was quite often the most stable possible ion of a particular system. A later section of this chapter deals with the stereochemistry of nucleophilic substitution reactions, and examples of solvent nucleophilicity effects on stereochemistry will be encountered there. 

The first step of the Sfjl mechanism involves the formation of ions. Since polar solvents can solvate ions, the rate of Sjj 1 processes is enhanced by polar solvents. On the other hand, solvation of nucleophiles ties up their unshared electron pairs. Therefore, Sfj2 reactions, whose rates depend on nucleophile effectiveness, are usually retarded by polar protic solvents. Polar but aprotic solvents [examples are acetone, dimethyl sulfoxide, (CH3)2S=0, or dimethylformamide, (CH3)2NCHO] solvate cations preferentially. These solvents accelerate 8 2 reactions because, by solvating the cation (say, K in K CN), they leave the anion more naked or unsolvated, thus improving its nucleophilicity. 

PA-polymethyl methacrylate graft copolymers were the products of the polymerization of methyl methacrylate on polyacetylene doped by Na [107]. Polystyrene, polyisoprene, and cis-1,4-polybutadiene were used as polymer carriers [108,109]. Acetylene was polymerized with Ti(OBu)4-AlEt3 in a toluene solution of the polymer carrier. The authors considered the formation of graft copolymers to be the result of the nucleophilic effect of a growing PA chain on the electrophilic sites in the polymer carrier. 

Zinc ions and other ions in metalloprotems could be consider as having a dual role. The first is an orientation or template effect while the second is a concentration (of the nucleophile) effect at the site of reaction. 

Unfortunately such cases are not common. Most frequently one has to use for this purpose compounds showing a strong nucleophilic effect. 

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