Hyperconjugation vicinal

Hydrophilic (Section 19 5) Literally water loving a term applied to substances that are soluble in water usually be cause of their ability to form hydrogen bonds with water Hydrophobic (Section 19 5) Literally water hating a term applied to substances that are not soluble in water but are soluble in nonpolar hydrocarbon like media Hydroxylation (Section 15 5) Reaction or sequence of reac tions in which an alkene is converted to a vicinal diol Hyperconjugation (Section 4 10) Delocalization of a electrons... 

Interactions of vicinal bonds have been extensively studied and are well known as hyperconjugation, resonance, and others [89-91], The a bonds vicinal to a reacting 71 bond have been proposed to participate in organic reactions and to control the selectivity [92, 93], Recently, we noticed the importance of the participation of the a bonds geminal to a reacting % bond (Scheme 35) [94] and have made extensive applications [95-102], Here, we present an orbital phase theory for the geminal bond participation and make a brief review. 

The principle that maximum hyperconjugative stabilization results from orienting the best vicinal donors (3.161a) anti to the best vicinal acceptors (3.161b) has an immediate corollary. 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Fig. 3.64 dramatically ilustrates how the hyperconjugative preference for strong vicinal donor-acceptor interactions may overcome pyramidalization barriers and other expected constraints on a torsion-barrier profile. 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

As a general conclusion, we can say that vicinal hyperconjugative interactions in transition-metal species tend to be much stronger than those in main-group compounds. Torsional degrees of freedom are therefore much more strongly hindered in metallic species, and the notion of pure torsional motion of simple rigid-rotor form lacks physical relevance in this limit. 

The localized product-NBO view also makes clear the importance of the non-planarity of the TS in achieving favorable hyperconjugative stabilizations, because interactions of 7t-planar geometry. Indeed, one can see that favorable vicinal n-a or a-7r overlap should primarily involve one end (hybrid) of each NBO, oriented, if possible, in the anti conformation for maximum stabilization. This viewpoint allows one to recognize the importance of angular and orientational factors that would not be evident in a purely topological framework. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

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 effect of the hyperconjugation has been explored by evaluating the relative energy of the conformers removing all the hyperconjugation within the molecules or only the geminal or vicinal terms. The results have shown that the removal of the... 

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