Hyperconjugation rotational barrier

In comparison with previous plots of this section, the no-crco anomeric interaction of Fig. 3.65 can be seen to be a rather typical example of hyperconjugative donor-acceptor interactions. Consequently, there seems to be no valid reason to invoke a special effect for the conformational preferences of sugars, obscuring their essential conformity with a unified donor-acceptor picture of ethane-like rotation barriers. 

The strikingly different characteristics of transition-metal hyperconjugative interactions are particularly apparent in their influence on internal rotation barriers. To illustrate, let us first consider ethane-like Os2H6, whose optimized staggered and eclipsed conformations (displaying conspicuous deviations from those of ethane) are shown in Fig. 4.81. 

Schaefer and coworkers20 have used long-range NMR coupling constants to investigate rotational barriers about the C(sp2)—C(sp3) bonds in benzyl compounds. The barrier for benzylsilane was found to be 1.77 kcalmol-1, compared to 1.2 kcalmol-1 for ethylbenzene. The increased barrier for benzylsilane is attributed to increased stabilization of the stable conformer, in which the C—Si bond lies in a plane perpendicular to the benzene plane, by a hyperconjugative interaction between the C—Si bond and the jr-system. 

The 1 -(para-methoxyphenyl )-2-(triisopropylsilyl)vinyl cation 131 has also been generated, characterized, and studied323,333 to elucidate the importance of a-7t aryl and (3-0 hyperconjugative stabilization. A comparison was made with the para-methoxyphenylvinyl cation 124, which has a higher demand for a-aryl 7t-stabilization. The para carbon of cation 131 is 7 ppm more shielded compared to that of cation 124, indicating that the (3-silyl stabilization effect is operative. The experimentally determined rotation barrier of the para-methoxy substituent for 131 (<8 kcal mol-1) and 124 (9.0 kcal mol-1) shows a less significant double-bond character of the MeO-C(4)... 

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. 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Calculations have shown that the rotational barrier of the C-O bond in methanol (1.1 kcal/mol) is significantly lower than the corresponding rotational barrier of methyl hypofluorite (MeOF, 3.7 kcal/mol) or methyl hypochlorite (MeOCl, 3.5 kcal/mol), in which a strong [Pg.19]

In toluene (LXX), like in a number of analogs, the preferred conformation shows eclipsing between a hydrogen atom and the w-system, as represented in diagram LXX. This conformation minimizes a- and w-electron loss to the 7r-system (hyperconjugation) and therefore allows maximal one-electron interaction [80]. Toluene is characterized by an extremely low rotation barrier (a few cal/mol) due to the fact that this barrier is sixfold, and the latter is always found to be extremely small. Indeed, an equivalent conformation is found after every 60° rotation, and no real relief of conformational strain is obtained after a rotation of only 30°. 

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-... 

The temperature-dependence of the ESR spectrum of the 2-stannylethyl radical shows that it is most stable in the conformation 3-8, in which the C-Sn bond eclipses the axis of the singly occupied 2p orbital. The rotational barrier about the C -Cp bond, which is probably a reasonable measure of the C-Sn hyperconjugation, is ca. 8 kJ mol-1.12-14... 

Considerably higher values (by more than lOkJ/mol) of the rotational barriers for heteroatom derivatives of methanol, HCHj—OX (X = F, Cl, O", OH, and OHj), than for methanol itself were rationalized by Wu and Houk (334) in terms of the hyperconjugation and rt-type orbital... 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

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