Bonding hyperconjugation

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Hyperconjugation (Sections 6.6, 6.9) An interaction that results from overlap of a vacant p orbital on one atom with a neighboring C-H a bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes. 

Two other types of 7r-interaction between boron and carbon centers that merit consideration are bond hyperconjugation (12a and 12b) and homoallylic delocalization (13a, 13b, and 13c, n = 1,2,.. . ), respectively. 

Scheme 1 Valence-bond- and MO-description of p-C-Si-a-bond hyperconjugation...

The 29Si-NMR chemical shift of 12 (8 = 66.34 ppm) is considerably deshielded as compared to the progenitor alkene 13 (8 = 0.14 ppm), which is in accord with substantial P-Si-C-bond hyperconjugative delocalization of positive charge to silicon. 

The relative importance of through-bond (hyperconjugative) and through-space (homoconjugative) interactions between the heteroatom and the double bond in 2,5-dihydro-furan, -thiophene and -pyrrole has been the subject of a CNDO/S study (76ZN(A)215). This analysis concluded that the proportion of through-space interaction increased from 19% in the dihydrofuran and 20% for dihydrothiophene to 83% for the dihydropyrrole (cf. Section 2.3.3.9). 

The proton NMR spectrum of the 1-adamantyl cation 49 in SbF5 solution at 25°C consists of resonances at 81 H 5.40, 4.52, and 2.67 with peak areas of 3 6 6 (Figure 3.8a). The 13C NMR spectrum (Fig. 3.8/ ) shows the y-carbons more deshielded than the (3-carbon atoms, indicating strong C—C bond hyperconjugation with the empty p orbital. The bridgehead 1-adamantyl cation 49 can also be prepared... 

Cubylcarboxonium ions have been also studied by Prakash, Olah, and co-workers.579,580 The parent cation 281 prepared under superacid conditions was stable at low temperature but decomposed to cubylacylium cation 282 as a result of further protonation and dehydration [Eq. (3.71)]. In addition to cation 281, di- and tetra-carboxonium ions and the corresponding protonated methyl esters were also observed as long-lived species stable under superacidic conditions. Experimental evidence and theoretical data indicated that the strained cubyl system effectively stabilizes the carbocationic centers through C—C bond hyperconjugation (283). On the basis of 13C data, three conformers of protonated dimethyl cubane-l,4-dicarboxylate (284-286) could be identified. 

In the ethane case, however, the AIM analysis helps in understanding the overlap of the bonds and the location of the electrons as derived from the density picture, but it does not tell us anything about the origin of the rotational barrier. For that, we need methods that quantitatively give us energies that can be associated with the effects of donor-acceptor bonding (hyperconjugation) and electron-electron repulsion (Pauli repulsion) as noted above. 

If the donor-free syn adduct (48) is generated by adding a Lewis acid, then it can rapidly isomerize to the anti adduct (50) at 25 °C. Available evidence indicates that the rotational barrier about the C bond in (48) and (50) is very small. A possible explanation is that pir-pir bonding with the 3p-orbital on aluminum lowers the C=C double bond character. Furthermore, o--bond hyperconjugation in the transition state for rotation (49) reduces its energy and hence the barrier to rotation. That the facile isomeriza-... 

Most carbocations are high in energy, but some are higher in energy than others. There are four ways that carbocations can be stabilized interaction of the empty C(p) orbital with a nonbonding lone pair, interaction with a 1t bond, interaction with a cr bond (hyperconjugation), and by being part of an aromatic system. Hybridization also affects the stability of carbocations. 

Figure 4.5 Carbocation resonance forms and the pi-type orbital overlap that those resonance forms represent. Top Resonance with a lone pair is best. Middle Resonance with a pi bond is next. Bottom sigma bond hyperconjugation is poorest the H, pi bond resonance form is minor.

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