N-o* interaction

The dominant interaction is the n—o interaction and since it is maximized in an anti arrangement, it is concluded that HN=NH will be expected to exist in the sterically crowded cis geometry. Of course, if the n—a interactions are weak they will not be able to reverse the trans over cis preference dictated by steric effects. 

Our qualitative analysis predicts an order of relative stability which is gauche > cis > trans and a gauche angle of 90°. The prediction of the relative stability of the con-formers is based on the realization that a 2p lone pair is a better donor than an sp2 lone pair and a n—o interaction is stronger for an anti than a syn orientation. 

Clearly, the preferred order of relative stability is gauche > anti > syn, since the n-o interaction is much stronger than the a—a interaction. Furthermore, anti 0nh-°nh is more favorable than syn aNH—aNH interaction. Ab initio calculations show that the preferred conformation of hydrazine is the gauche conformer with a dihedral angle of IOO0314. ... 

Possible Examples of Matrix Element Control of n—o Interactions... 

The sp2 hybridized carbanion 496 can also be viewed as an sp3 hybridized anion and can therefore look like 497 or 498. In 487, the electron pair is antiperiplanar to the two C —0 bonds of the dioxane ring, so that the carbanion orbital can be delocalized by an overlap with the antibonding orbitals of the two C —0 sigma bonds (n-o interaction). On that basis, carbanion 496 would be closer to 497 than 498, and the equatorial approach of the electrophile is thus readily understood. Banks has however given a different explanation based on the work of Klein (152, 153). 

The secondary electronic effects in the ester function are essentially similar to the anomeric effect discussed previously for the acetal function, involving an n-o interaction. The only difference is that the central carbon is trigonal (sp2 hybridized) in esters and tetrahedral (sp3 hybridized) in acetals. 

The carbonyl oxygen in both the Z (4) and the ( ) esters has an electron pair oriented anti peri planar to the C - OR bond, and an n-o interaction should therefore exist because this electron pair orbital can overlap with... 

Esters and Related Functions 57 Thus, primary electronic effects (n-n interaction) form the conjugated system of the ester function whereas secondary electronic effects (n-o interaction) are the result of the orientation of non-bonded electron pairs anti peri planar to the o C-0 bonds of the ester function. Clearly, the primary are energetically more important than the secondary electronic effects and this terminology is justified by the fact that these two effects have their origin in the same chemical principle, orientation in space of electron pairs with resultant electronic delocalization. 

The principle of microscopic reversibility predicts that the reverse process must follow the same path which is indeed stereoelectronically allowed the oxygen atom in T has two secondary electronic effects (n-o ) (one electron pair of the oxygen atom is anti peri planar to the C-N bond while the other is antiperiplanar to C —Y bond) and the nitrogen has one (the nitrogen electron pair is antiperiplanar to the C —Y bond). Thus, there are three secondary electronic effects (n-o ) in ] and by the ejection of Y to form 4, two of these (due to the two electron pairs antiperiplanar to the C—Y bond) have been transformed into primary electronic effects (n- ) in the product 4. The third secondary electronic effect remains a n-o interaction in the product. The ejection of Y can therefore take place with the help of the primary and one secondary electronic effects. 

On the other hand, cleavage of the C-N bond gives directly the stabilized amide ion 60. In the ion 60, one nitrogen electron pair (p-orbital) is delocalized through a n-x interaction (primary electronic effect) while the other is delocalized by an n-o interaction (secondary electronic effect indeed, the newly generated electron pair is anti peri planar to the C —0 a bond of the carbonyl function). Thus, both electron pairs of the nitrogen atom are delocalized. 

FIGURE 1.10 The anomeric effect, (a) The n-o interaction stabilizes the a anomer. (b) The P anomer experiences unfavorable dipole-dipole interaction that is reduced in the a anomer. (c) Greater electrostatic repulsion between the lone-pair electrons of the endocyclic oxygen and the electronegative anomeric substituent in the (1 anomer. 

Most common single bonds (C — O, C — N) have shielding properties that parallel those of the C — C bond. There appears to be a sign reversal, however, for the C — S bond. In all these heteroatomic cases, the geometry is more complex than that for the C — C bond. In some instances, a lone electron pair can have a special effect. In N-methylpiperidine (3-7), the axial lone pair shields the vicinal by an n—o interaction without any effect on Hgq. As a result, increases to about 1.0 ppm or more. 

G. N. Shustov, A. B. Zolotoy, and R. G. Kostyanovskii. Asymmetrical nonbridgehead nitrogen-30 geminal systems—21. The influence of vicinal n-o interaction of the pyramidal stability of tricoordinated nitrogen atom in the X-N-Y geminal system. Tetrahedron 38, 2319-2326 (1982). 

Figure 1.25 Molecular orbital description of secondary bonding and related interactions showing the n -> o interaction.

The anomeric effect is readily rationalized in PMO terms, using the hybrid model for the lone pairs (see p. 27 and Deslongchamps, 1983). In the g g form of dimethoxymethane [21], one lone pair on each oxygen atom is anti-periplanar to the cr c o orbital involving the other oxygen atom. Thus two favourable n-o interactions exist in this conformation. The generalized anomeric effect was rationalized in similar terms by David et al. (1973) who used the canonical model for the lone pairs. Calculations for the CHCl—O—C system predicted stabilization of conformer [23a] due to the n-cf c-ci interaction by 3.3kcalmol with respect to conformer [23b], a... 

The failure of n->o interactions and the electron-delocalization model to rationalize the reverse anomeric effect is argued by Sinnott (1988) to be the major flaw in this model. 

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