Hyperconjugation cations

Figure 7.9 Positive hyperconjugation cation/n -acceptor interacting with a a-donor.

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]

The 2-methyl-2-butyl cation provides the opportunity to explore the effect of C—C hyperconjugation. At the 6-31G /MP4 level of calculation, little energy difference is found between structures C and D which differ in alignment of CH3 or H with the empty p orbital. ... 

FIGURE 4.16 Hyperconjugation in ethyl cation. Ethyl cation is stabilized by delocalization of the electrons in the C—H bonds of the methyl group into the vacant 2p orbital of the positively charged carbon. 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

The vinyl cation (Fig. 40 and III.13) in which the empty p orbital on the positive center is coplanar with the terminal methylene group, is clearly another candidate for hyperconjugative donation. The in-plane orbital readily overlaps with the odd p orbital, with formation of two delocalized orbitals and an energy gain... 

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

There are several structural types of delocalization, summarized in Table 5.1. The stabilization of the dimethylalkylidine cation (3) is an example of double hyperconjugation. ... 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

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. 

Scheme 7.7, ORTEP adapted from reference 32), with [BCCgFj) or [CBnHgBrg] as the counterions. The X-ray structure clearly shows evidence of an sp hybridized center with a C -C -C angle of 178.8° and an abnormally short C -C double bond distance of 1.22 A (compared to 1.32 A, Table 7.1), and a nearly normal C -CA (sp -sp) distance of 1.45 A. The most striking feature of 8, however, is the very long C -Si distance of 1.97 A (compared to 1.87 A, Table 7.1), which is attributed to hyperconjugation, as shown in the scheme. Elongation of the bonds a- to the cation center is a characteristic of hyperconjugation, also observed in the structure of the adamantyl cation. ...

The 1.8 kcal mol 1 less favorable change in Gibbs free energy for the addition of water to [18+] to give [18]-OH in 50/50 (v/v) trifluoroethanol/water (p/CR = -11.3)104 than for addition of water to Me-[6+] in the same solvent (pATR = -12.6)13 shows that the former carbocation is stabilized relative to the alcohol. This stabilization may be the result of the smaller entropic price paid to restrict the / —CH bonds in the five-membered ring at [18+] to conformations that are favorable for hyperconjugation with the cationic carbon. 

The tert-butyl cation structure (7) with Cs symmetry is better suited for hyperconjugation than the C3h form and is thus energetically slightly favored.29 The energy surface for methyl-group rotation is however very flat. 

The 2-butyl cation is the smallest secondary cation that can be stabilized either by C-C or C-H hyperconjugation. Experimental results give evidence for two equilibrating isomers.33 MP2/6-311G(d,p) calculations show that the symmetrically hydrido-bridged structure 11 is marginally more stable than the partially methyl-bridged structure 10.34 35... 

The 2-methyl-2-butyl cation (12) is the smallest tertiary carbocation structurally suitable for stabilization through C-C hyperconjugation. 

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