Fluorine Hyperconjugation Holtz

Streitwieser and Holtz have discussed the reactivity patterns of the acyclic and the bicyclic molecules (4,28). They proposed largely on the basis of Bredt s rule that the rate constant for tris(trifluoromethyl)methane should be much larger than the rate constant for the bicyclic compound, X, if carbon-fluorine hyperconjugation were important. The observation that these compounds ex-... 

Roberts, Webb, and McElhill pointed out that the dipole moment of N,N-dimethyl-4-trifluoromethylaniline was appreciably greater than expected on the basis of the observed moments for N,N-dimethylaniline and trifluo-romethylbenzene (1). They attributed this result to carbon-fluorine hyperconjugation as shown in the contribution of structure XUIC to the hybrid. Holtz (3,4) and Sheppard (2,8) noted that the dipole moment of N,N-dimethyl-4-... 

The evidence for fluorine hyperconjugation derived from nmr measurements available in 1970 was not deflnitive (3,4). Taft and his associates had originally argued that the fluorine substituent chemical shifts (SCS) of p-fluorine atoms in molecules with substituents which are capable of resonance interactions, for example the nitro group, exhibited a special solvent dependence and shifted to lower field as the polar character of the solvent was increased (38,39). The fluorine substituent chemical shift for the aromatic fluorine atom in 4-fluo-robenzotrifluoride, indeed, exhibits a small downfleld shift from 4.95 in 3-methylpentane to 5.75 in nitromethane to 6.05 in 75% aqueous methanol. For comparison, the substituent chemical shift for the fluorine atom in 4-fluoroni-trobenzene exhibits a shift from 9.00 in 3-methylpentane to 10.55 in nitromethane to 11.20 in 75% aqueous methanol (39). Holtz concluded on these grounds that fluorine hyperconjugation, structure XVC, might contribute to the determination of the substituent chemical shift. However, Brownlee, Dayal, and Taft subsequently showed that the effects of dipolar aprotic solvents on the... 

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