Stability negative hyperconjugation

P Fluonnation always strongly stabilizes carbamons both by induction and by negative (anionic) hyperconjugaQon, 7 The latter "no-bond resonance has been controversial, but its importance is now well established both theoretically [133, 134] and expenmentally [67] The X-ray crystal structures of salts 8 [fi5] and 9 [136] provide cogent evidence for negative hyperconjugation... 

On a thermodynamic level, the presence of fluorine atoms in the P position strongly stabilizes the anions (planar or not) either by inductive effect or by negative hyperconjugation (Figure 1.8). 

Given that fluorine has a destabilizing influence upon planar carbanions, geometry considerations favor the formation of pyramidal anions.69 71 The influence of /(-fluorination on carbanion stability has been described in a variety of ways. In early work the role of negative hyperconjugation was proposed to account for these effects.72... 

R = Me, Ph, t-Bu, CF3, and CC13) does not correlate with the Es parameters of the groups.424 This is probably because CF3 and CC13 have lower activation energies than predicted on the basis of Es due to negative hyperconjugation stabilizing the inversion transition state. 

If d-7t bonding or negative hyperconjugation were to play an important role in the stabilization of the conjugate bases of the dimethyl-9-flurorenylsulfonium ion, the benzyltriflones, or thiophenylnitromethane, one... 

As discussed above, 2-halotetrahydropyrans tend to adopt a conformation in which the halogen is located axially, and which is stabilized by negative hyperconjugation of the type nQ —> cr C-Hai- X-ray structural analyses have shown that in these... 

Raabe, G. Gais, H.-J. Fleischhauer, J. Ab initio study of the effect of fluorination upon the structure and configurational stability of a-sulfonyl carbanions the role of negative hyperconjugation. J. Am. Chem. Soc. 1996,... 

The enhanced stability of (3-fluoro carbanions (Figure 4.37) has been attributed to fluorine negative hyperconjugation (FNHC see Section IIIB). For instance, negative hyperconjugation (see Section 111) has been invoked to explain the enhanced reactivity (100-fold) [27] and the higher gas-phase acidity (by 5.4 pKa units) [91] of 4.38A over the... 

However, there is evidence against /-orbital overlap and the stabilizing effects have been attributed to other causes.In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and negative hyperconjugation play a minor role, if any. An ot silicon atom also stabilizes carbanions. ... 

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