Hyperconjugation geminal

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. 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

As noted above (note 111), geminal NBO delocalizations may be associated with alternative resonance structures related to (3.181). Hence, this structure might better be considered to depict geminal hyperconjugative corrections to the classical single-bonded Lewis structure. 

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... 

The effects of withdrawal or donation of electrons through the o bonds (induction) can be augmented or decreased by ir effects such as hyperconjugation. Pople and Bothner-By found that lone pairs of electrons can donate electrons and make J more positive, whereas the TT orbitals of double or triple bonds can withdraw electrons and make J less positive. The aforementioned large increase in the geminal coupling of imines or formaldehyde compared with that of ethene results from reinforcement of the effects of ct withdrawal and tt donation, as illustrated in structure 4-18. The effect of it withdrawal occurs for carbonyl, nitrile, and aromatic groups, as the values for acetone (—14.9 Hz), acetonitrile (—16.9 Hz),... 

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.

O Hagan and coworkers recently described an interesting facially polarized all-cis 1,2,3,4,5,6-hexafluorocyclohexane. NBO analysis of this molecule revealed strong interactions between fluorine lone pairs and o orbitals associated with the geminal C-C and C-H bonds. Subsequently, Juaristi and Notario expanded the theoretical analysis to fluorinated trioxanes and trithianes and confirmed that fluorine lone pairs are good donors in a variety of n - hyperconjugative interactions, where Y=H, C, O, 8. ... 

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