Delocalized nucleophile

Thus the changes of the stereochemistry always depend on electronic factors the more delocalized nucleophilic anions react mainly with inversion, the ones with more concentrated charge with retention. 

As we just argued, for reactions of electrophiles and nucleophiles/increases as the nucleophile becomes more delocalized. Thus, the series of delocalized nucleophiles, in Fig. 6.10, is more selective to changes (of any kind) that affect the gap, G, compared with the series of localized nucleophiles. This would be general for other processes as well delocalization of the single electrons in the R states of the diagram results in higher/values, and vice versa. Such trends abound in electrophile—nucleophile combinations they were analyzed also for radical addition to olefins (40), and are likely to be a general feature of reactivity. 

A The silver ion reacts rapidly with the chloride whilst the acetate anion is a relatively weak delocalized nucleophile. Consequently, the primary carbocation may rearrange to the more stable tertiary carbocation or eliminate a proton prior to attack by the nucleophile. 

Standard electrode potentials of alkyl halides and nucleophiles have been used to pi edict die rates of SKT alternatives to Ss2 substitution mechanisms 134.49.50.08.601. A reaction much faster than predicted on the basis of Marcus theory is a likelv S 2 process, while similarity between prediction and rate, favors SET. Despite some disagreement as to the proper treatment rtf a dissociative electron transfer 48.7(). this approach has proven useful in helping to classify reactions as probably SN2 or SET in mechanism (most successfully with large, delocalized nucleophiles). 

In a related study, l,3-dihydro-l,3-azaborine 82 was synthesized and proved to be a stable stmctural motif with considerable aromatic character (2011JA2152). By X-ray crystallography, 82 was shown to have significant electron delocalization. Nucleophilic substitution occurs at the B-atom and EAS reactions occur at the C-atom across the ring from the boron. 

Fig. 2 Reaction of a free radical with a nucleophile. (A) Localized nucleophilic radical (e.g. McjC-) and delocalized nucleophile (e.g. Me2C=NO 2) E increases with LUMO energy (SOMO constant). (B) Delocalized electrophilic radical (e.g. PhCOCHj ) with localized nucleophile E decreases as HOMO energy increases (SOMO and LUMO constant)...

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

There exist a number of d -synthons, which are stabilized by the delocalization of the electron pair into orbitals of hetero atoms, although the nucleophilic centre remains at the carbon atom. From nitroalkanes anions may be formed in aqueous solutions (e.g. CHjNOj pK, = 10.2). Nitromethane and -ethane anions are particularly useful in synthesis. The cyanide anion is also a classical d -synthon (HCN pK = 9.1). 

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

Surprisingly carbonyl-substituted carbanions of phosphonates, in which the negative charge is delocalized over two oxygen atoms, are much more nucleophilic than the corresponding phosphoranes. This effea has first been observed by Homer, and has often been utilized in the synthesis of acylated olefins (R.D. Clark, 1975). 

Arylamines contain two functional groups the amine group and the aromatic ring they are difunctional compounds The reactivity of the amine group is affected by its aryl substituent and the reactivity of the ring is affected by its amine substituent The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamme nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution... 

Kinetic data on acetate displacement from C-3 using a number of sulfur and nitrogen nucleophiles in aqueous solution at near neutral pH demonstrate that the reaction proceeds by an 5 1 mechanism (B-72MI51004). The intermediate in this reaction is depicted as a dipolar allylic carbonium ion (9) with significant charge delocalization. Of particular significance in this regard is the observation that the free carboxylate at C-4 is required since... 

NMR and, 3, 951 aromaticity, 3, 945 delocalization energy, 3, 959 deprotonation, 3, 972 disulfones reactions, 3, 970 double bond character, 3, 945 electronic energy levels, 3, 946 electrophilic reactions, 3, 965 electrophilic substitution, 3, 960 half-wave potential, 3, 968 NMR, 3, 952 H NMR, 3, 951 nucleophilic reactions, 3, 969 oxidation, 3, 967 oxides... 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

The other C=N systems included in Scheme 8.2 are more stable to aqueous hydrolysis than are the imines. For many of these compounds, the equilibrium constants for formation are high, even in aqueous solution. The additional stability can be attributed to the participation of the atom adjacent to the nitrogen in delocalized bonding. This resonance interaction tends to increase electron density at the sp carbon and reduces its reactivity toward nucleophiles. 

In order to predict the structure of the product, you must identify the factors that will tend to favor selective ketal formation. Consider selective carbonyl protonation first. Obtain energies and atomic charges, and display electrostatic potential maps of the alternative protonated ketones (protonated ketone A, protonated ketone B). Identify the more stable isomer. Compare geometries and draw whatever Lewis structures are needed to account for your data. Why is one isomer more stable than the other Is the more stable isomer also that in which the positive charge is better delocalized Will the more stable isomer undergo nucleophilic attack more or less easily than the other Explain. 

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