Bearing a CF3 Group

As was the case for single fluorines and CF2 groups, branching near the CF3 group leads to increased shielding of the CF3 fluorines and thus more negative chemical shifts. The few examples available seem to bear that out. 

The trifluoromethylcycloalkane systems do provide, however, some examples of CF3 bound to a tertiary center, with 1-methyl-l-trifluoromethylcyclohexane, cyclopentane, and cyclobutane all absorbing at higher fields (-81, -78, and -80 ppm, respectively) than the 

As was the case for their fluorine spectra, the carbon NMR spectra of trifluoromethyl cyclohexanes show some slight differences depending on whether the CF3 group is equatorial or axial, with the axial CF3 appearing at slightly ( 1 ppm) higher field. 

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Synthesis of 16,16,16-trifluororetinal requires the constmction of a quaternary center bearing a CF3 group. For this, a Diels-Alder cycloaddition (cf. trifluoromethyl steroids) between a trifluoromethacrylate and a functionahzed diene has been conducted. The obtained adduct has been transformed further into the trifluoromethyl analogue of jS-cyclocitral (Figure 4.26). ... 

Many studies of separations, primarily by HPLC, have been reported with a view to establishing the mechanisms by which diastereomers separate (56-58). Although substantive criticism has been offered by way of demonstrated exceptions (45, 59), the picture as qualified by Pirkle (57) holds well for simple (otherwise functionally unsubstituted) amides and carbamates. Briefly, solution conformations of amides and carbamates are as shown in Figure 17 the central functional group acts to create a plane, and the asymmetric centers extend alkyl (aryl) residues to either side of that plane. An explanation based on a combination of steric bulk and hydrophobicity has been advanced to explain HPLC elution orders for carbamates. Elution order for GLC is inverted from that of HPLC in all instances studied, the least soluble (faster eluting) diastereomer by GLC is always the cisoid molecule. The notable exceptions are those compounds bearing a CF3 group at R. ... 

C. Aubert, J.-P. B gu and D. Bonnet-Delpon A Route to Aliphatic Cycles Bearing a CF3 Group ... 

Although several reviews have been published regarding the preparation of trifluoromethylated molecules, there seems to be none dealing specifically with the synthetic methods for the preparation of substances bearing a CF3 group at the multiple bond. This review especially concerns the stereoselective preparation of trifluoromethylated alkenes 1 and allenes 2 as shown in Rgure 26.1.Trifluoromethylated alkynes 3 as well as aromatics 4 are also included to some extent in this review. 

None of the three major resonance forms of the intermediate formed by attack at the meta position has a positive charge on the carbon bearing the —CF3 group... 

Multitrifluoromethyl-Substituted Alkenes. Spectral data for cis- and trans-l,l,l,4,4,4-hexafluoro-2-butene, and 2-(trifluoromethyl)-3,3,3-trifluoropropene are given in Scheme 5.42 as representative examples of alkenes bearing two CF3 groups. Note the significant shielding of the fluorines of the cis compound versus the trans compound, which has a chemical shift similar to that of 3,3,3-trifluoropropene (-67 ppm). 

Pyrrolidone anion as EGB generates (trifluoromethyl) malonic ester enolate from malonic ester bearing a CF3 moiety at the a-position (Eq. 26). Generally, it is difficult to generate the enolate anion with a-CFg group, because the enolate anion undergoes... 

Inspired by these results, Ko and coworkers subsequently obtained the directly related homoleptic, discrete complexes supported by biphenol-derived benzenesulfonate monoanionic ligands bearing a CF3 (57) or a OMe (58) substiment in para position of the sulfonate aromatic ring [74] (Fig. 17). The authors reported that in the presence of 2 equiv of BnOH, these complexes yielded potent binary catalyst systems for the ROP of CL (400 equiv, 25 °C) and TMC (200 equiv, 0-25 °C). The electron-withdrawing group in 57 allowed enhanced activity in comparison with 58 ... 

Another feature of 2-pyrone 87 is its ability to undergo Diels-Alder reactions with acetylenes. The cycloadducts decarboxylate spontaneously to form benzene rings bearing the CF3 group. The substitution pattern is determined by the regioselectivity of the [4-1-2] cycloaddition step. Thus, the reaction of 87 with l-(NJ -diethylamind)-l-propyne takes place at 0 °C to produce 92 as a single isomer. Less electron rich acetylenes require heating at 140-200 C. Treatment of 87 with acetylene at 200 C leads to 93, while with dimethyl acetylenedicarboxylate triester 94 is formed [37] (Scheme 28). 

An example of a C2F5 substituent bound to a carbon bearing one or more hydrogens is exemplified by the fluorine spectrum of 3,3,4,4,4-pentafluorobutene (Fig. 6.10). This spectrum exhibits a singlet for the CF3 group at -86.0 ppm, but a doublet at -118.2 ppm with three-bond H—F coupling of 7.9 Hz. 

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