Lone electron pairs, hyperconjugation

The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. 

The reason for the higher basicity and nucleophilicity toward silicon ( silicophilicity ) of D2 compared to D3 and D4 is a different orbital interaction pattern involving the lone electron pair of oxygen, due to the difference in structures of their ring skeletons. Contribution from the hyperconjugation effect no—Kj sio in polysiloxane significantly reduces the electron density on oxygen. This effect is less important in D2 because the no— o siSi interaction is much weaker [8]. 

Scheme 2.35 Left, repulsive interaction between the fluorine lone electron pairs and the. T-orbita on the sp hybridized carbon, release of repulsive strain on the sp carbon. Right stabilization of the geminal difluoromethylene group by negative hyperconjugation.

It must be added that 2-phosphino-l,3-dithianes 44 provide a very interesting example of noncompetitive hyperconjugative anomeric interactions in a system where both heteroatoms Y = S and X = P (Scheme 8) possess lone electron pairs. In the equatorial conformer the endo interactions cannot operate, while in the axial one the exo effect is not manifested. 

A lack of manifestation of the exo anomeric effect in the S—C—P( ) system (see Section II. E) can be easily interpreted in terms of predominant steric over np-a g hyperconjugative interactions. This is in contrast to X—CHj—H2P( ) systems, for which the antiperiplanar arrangement of the C—X [X = F (267), Cl (141)] bond and phosphorus lone electron pair is preferred, perhaps due to much more effective np-[Pg.327]

Lone electron pair is not involved in hyperconjugative interaction... 

Recently, the conformational equilibria of several 2-polar-substituted tetrahydrothiop5rran 5-oxides was examined [29], It was reasoned that the reduced capacity of sulfur in sulfoxides to act as lone electron pair donor should be reflected in a substantial reduction of the ns - hyperconjugative... 

If an electron pair is present on metal, its direction should be such that a negative hyperconjugation of this lone pair with the a (Si-X) antibonding... 

An unshared electron pair can usually only assist nucleophilic displacements if this lone pair and the antibonding (cr ) orbital between carbon and the leaving group are either antiperiplanar or synperiplanar. If such a conformation cannot be achieved, a-heteroatoms can even reduce the rate of Sn2 or SnI reactions. Scheme 4.31 shows some examples of conformationally constrained, tertiary tosy-lates for which hyperconjugation between the heteroatom lone pairs and the antibonding C-OTs orbital is not possible [129]. Thus, although a-alkoxyalkyl sulfonates... 

In enamines, only one lone pair can interact with the vinyl moiety and, generally, this pair is engaged in the interaction with the it-systan. This means that E-/Z-isomerism in these systems does not involve negative hyperconjugation but rather relies on a more subtle balance of electronic (neutral hyperconjugation, electrostatics) and steric effects. 

Anomeric effects Hyperconjugative electron transfer into a C-H orbitals can lead to increased shielding and decreased chemical shift of protons antiperiplanar to unshared electron pairs. This effect in tris-amines is illustrated in Figure 12.22. In the acyclic compound, where free rotation is possible, the methyl resonance appears at 3.02 ppm. When lone pairs are forced to be syn-periplanar, the relative inefficiency of anomeric effects allows... 

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