Control by electronic effects

The exclusive formation of 227 indicates that the orientation of the addition is controlled by electronic effects, the azide terminus attacking the more nucleophilic carbon. 

In the addition to unsymmetrical acetylenes the orientation is controlled by electronic effects. Thus, while in the reaction of phenyl azide with phenylacetylene the two isomeric diphenyltriazoles 252 and 253 were obtained in almost equal amounts , methyl propiolate yielded mainly l-phenyl-4-carbomethoxy-1,2,3-triazole (254) and only minor amounts of the isomeric 255 °. Dimethylaminoacety-lene yielded l-phenyl-5-dimethylamino-1,2,3-triazole (256) as the sole product... 

N. Okamura, M. Kawai, Y. 1998 J. Am. Chem. Soc. 120, 1186-1192 NAD(P)+-NAD(P)H Models. 88. Stereoselection without steric effect but controlled by electronic effect of a carbonyl group Syn/Anti reactivity ratio, kinetic isotope effect, and an electron-transfer complex as a reaction intermediate. 

The experimental data indicate that the interaction between CO and glassy metals is controlled by electronic effects. Alloying Ni with Zr changes the local electronic structure at the Ni site and increases the electron density in the antibonding 27r molecular orbital of the adsorbed CO. This occurs through back-donation from the Ni 3d band and weakens the C—O bond. Thus, the potential barrier for CO dissociation is lowered. On Cu50ZrSo, CO dissociates at all exposures and only Zr is oxidized (35). 

The Rubottom oxidation has found widespread application in organic synthesis. A few recent examples of the use of this methodology for the construction of complex molecules are described below. As noted above, the stereoselectivity in these reactions is usually controlled by steric effects, which dictate the face-selectivity of the epoxidation step. The chemoselectivity is generally controlled by electronic effects, as the electrophilic oxidants react more rapidly with the electron-rich enol ether than with other double bonds in the substrate. 

The reaction of amines (74) and (75) with A-chlorosuccinimide has been studied under conditions of kinetic control. The ratios of syn- and anti-N-chloro-amines are substantially influenced by the nature of substituents in the aromatic rings. It has reasonably been concluded that the product ratios are partly controlled by electronic effects, but their nature has not been clearly established. 

Vinyl (or cyclopropyl) silyl ethers have also been used to generate carbon-centered radicals by treatment with CAN. 3,6-Dihydroxyphthalate esters are produced by dimerization when bisenolsilylated 1,3-diketones are treated with CAN. An elegant example is the three-component condensation of cyclopropyl silyl ether, cyclopentenone, and methyl vinyl ether (eq IS). Oxidation of cyclopropyl silyl ether gives the 8-ester radical, which undergoes tandem radical addition processes apparently controlled by electronic effects. Subsequent oxidation and trapping affords the 2,3-disubstituted cyclopen-tanone in an excellent stereoselectivity. Other substrates include tertiary aminocyclopropanes, iV-(silylmethyl)amides, and VUV-dialkylanilines. For example, CAN-mediated oxidation of IVUV-dialkylanilines in water affords the coupling products -tetraalkylbenzidines. ... 

The reaction mechanism is proposed after the isolation and structural confirmation of the activated alkene insertion species nickelacyclopentanes 6a from the reaction of nickel-carboryne with 2-vinylpyridine (Scheme 7.5). The sequential insertion of alkene and alkyne with excellent regioselectivity control by electronic effect results in the formation of 5. Alkyne inserts regioselectively into the Ni-Caikyi bond of the nickelacyclopentane, whereas the Ni-Cgage bond remains intact. Reductive elimination yields the final products 5. In fact, treatment of 6a with 3-hexyne affords the expected dihydrobenzocarborane... 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the Inductive effect for the attack at ortho- and para-positions and hence makes the deactivation less for ortho- and para-attack. Reactivity Is thus controlled by the stronger Inductive effect and orientation Is controlled by resonance effect. 

The conductance is proportional to the number of carriers in the neck (as was shown above) and to the mobility of these carriers. Thus, unless one makes the unusual assumption that the bulk properties of the neck are very different indeed from those of the grain, or that the bulk electron mobility varies widely with time at a constant low temperature, the conductivity must be controlled by surface effects. 

Phenyl azide undergoes stereospecific addition to (Z)- and ( )-methyl-styrene as shown in Scheme 28.2 7,144 Although the reaction is sensitive to steric effects, as evidenced by the failure of phenyl azide to add to tetra-methylethylene,40 the orientation is controlled by electronic rather than steric factors.27,34,40... 

A study of the intramolecular alkoxymercuration of ( )-5-arylpent-4-en-l-ols (70) has indicated that the regioselectivity is closely related to the Hammett constants of the para-substituents on the benzene ring. Large solvent effects on the regioselectivity were also observed (Scheme 17 and Tables 2 and 3). By contrast, the related oxymercuration of -methylstyrene is 100% a-sclcctivc. This comparison shows that the regioselectivity of the intermolecular reaction is controlled by electronic factors, whereas the cyclization is governed by a delicate balance of steric and electronic effects.65... 

Thus, the band positions of the irradiated semiconductor are key thermodynamic variables for in the control of the observed redox chemistry resulting from photo-catalyzed single-electron transfer (charge trapping) across the semiconductor-electrolyte interface. Whether oxygenation, rearrangement, isomerization, or other consequences follow this initial electron transfer seems to be controlled by surface effects on the relative rates of reaction. 

Radicals are species with at least one unpaired electron, which, in contrast to organic anions and cations, react easily with themselves in bond-forming reactions. In the liquid phase, most of these reactions occur with diffusion-controlled rates. Radical-radical reactions can be slowed only if radicals are stabilized by electronic effects (stable radicals) or shielded by steric effects (persistent radicals). However, these effects are not strong enough to prevent diffusion-controlled recombination of, for example, benzyl radicals or tert-butyl radicals.1 Only in extreme cases are the radical or di-tert-butylmethyl radical recombination rates low.2 While the recombination rates of the triphenyl-methyl radical is reduced due to both steric and radical stabilizing effects, the steric effect alone slows the recombination of the di-/t>/-/-butyl methyl radical. Since neither of the radicals have C-H bonds (I to the radical centre, disproportionation reactions, in which the hydrogen atom is transferred, cannot occur. 

The stereoselectivity in the cycloadditions of imines 176 and 179 is only modest, and is probably controlled more by steric than by electronic effects [63 b, 65]. Examples of the kinetic stereochemical outcome of cycloaddition with chloral- and fluoral-derived N-sulfonyl imines and cyclic dienes are shown in Eq. (37)-(39) [65]. 

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