Trifluoromethyl group substituent effects

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

An oxygen bound to a trifluoromethyl group has much less effect upon its chemical shift than a chlorine substituent. Thus, the fluorines of trifluoromethyl ethers (—58 ppm) are not as deshielded as those of CF3C1 (—28ppm). Those of CF3 sulfides and selenides are deshielded... 

In contrast, 13C nmr spectra of ring-substituted /J-bromocumyl cations [2] can be unambiguously interpreted in terms of open /1-bromocarbocations, since the ring substituent effects on the chemical shifts are similar to those on the corresponding non-brominated cations [1], even for the electron-attracting p-trifluoromethyl group (Olah et al, 1972). 

Bis [(trifluoromethyl)thio] acetaldehyde (83a) has been prepared from an enam-ine precursor (84), although refluxing in aqueous ethanolic HCl is required to effect this reaction.The aldehyde is less stable than its enol tautomer (83b), and many reactions typical of aldehydes fail. For example, addition of aqueous silver nitrate immediately yields the silver salt of (83b), rather than giving precipitation of (elemental) silver. The (trifluoromethyl)thio substituent has pseudohalogenic character and, together with the hydroxy group, stabilizes the alkene tautomer in the manner of a push-pull alkene. The enol-aldehyde equilibrium mixture in acetonitrile shows an apparent of 2.6 when titrated with aqueous hydroxide. 

The first problem was resolved when it was shown that the Es values for symmetric groups are a linear function of van der Waals radii. The latter have long been held to be an effective measure of atomic size. The second and third problems were solved by Charton, who proposed the use of the van der Waals radius as a steric parameter and developed a method for the calculation of group van der Waals radii for tetracoordinate symmetric top substituents MZ3, such as the methyl and trifluoromethyl groups. In later work the hydrogen atom was chosen as the reference substituent and the steric parameter V was defined in equation 28 ... 

This bibliographical survey shows the wide range of syntheses of the fluorinated difunctional compounds, whatever the position of the fluorinated substituent in the molecule. However, major efforts were developed, for example, to protect a group from hydrolysis in the case of esters protected by gera-trifluoromethyl groups. Actually, because of the electroattractive effects of the difluoromethyl groups in a position about the functions, the fluorinated polyesters and polyurethanes exhibit weak points which affect the applications of such polymers. 

Trifluoromethyl is a wonderful group with numerous substituent effect surprises84. Within the context of discussions of positive halogen , it is well-established85 that solution-phase nucleophilic reactions of trifluoromethyl iodide are consistent with the presence of formally positive iodine, e.g. equation 53. ]... 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

The stabilizing effect of the secondary orbital interactions of the trifluoromethyl substituent with dienes is exemplified in the cydoaddition reaction of a-(trifluorornethyI)styrene (7) with Danishefsky s diene 4. A phenyl substituent is known to stabilize the c t/o-phenyl transition state. Furthermore, the steric hindrance of a trifluoromethyl group should favor its exo approach. However, cycloadducts 8 are obtained in a 1 1 mixture of tran.sicis stereoisomers which are converted into the cyclohexenone 9. Under the same thermal conditions, st-mcthyl-styrene is unreactive. ... 

---