O-Bonds, hyperconjugation

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 o- bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes. 

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-... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

It might be predicted that electrophilic attack would take place by a S l - type process, to give a /J-silyl carbocation on cleavage of the fi C—O bond. However, the relative orientations of the C—Si bond and the developing positive charge are such that hyperconjugative overlap is minimal (169). 

Neighboring group interactions with the vacant p orbital of the carbenium ion center can contribute to ion stabilization via charge delocalization. Such phenomena can involve atoms with unshared electron pairs (w-donors), C—H and C—C hyperconjugation, bent o-bonds (as in cyclopropylcarbenium ions), and 7t-electron systems (direct conjugative or allylic stabilization). Thus, trivalent carbenium ions can show... 

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 addition to a relatively small inductive effect, two modes of stabilization of positive charge by the P-R3M substituent have been suggested. The first involves the classical cation 7, where the positive charge is stabilized by hyperconjugation (G-p conjugation) between the C-M o bonding orbital and the vacant carbocation... 

Bond-formation towards the exo-face of dienophilic units such as those in 38 may also receive a degree of stereoelectronic assistance [106-108] as a result of antiperiplanar ct - (a) hyperconjugative donation from the strained crl -9 a and o4-4a carbon-carbon single bonds (Scheme 9) to the a-orbitals under construction on the exo-face (see Fig. 9). The corresponding situation for endo-face approach is less favorable, the endoxide (C-O) bonds are not able to assume such an antiperiplanar relationship. 

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