Tertiary Alkyl Fluorides

The proton spectrum consists of a doublet at S 1.38 (3/FH=21), whereas the carbon spectrum exhibits a doublet at 28.7 ppm with 2/fc = 25Hz along with a much weaker doublet at 94.1 ppm ( = 162 Hz). 

A comment about the carbon NMR spectrum of f-butyl fluoride is appropriate. Because of the signal weakness of carbons such as the tertiary carbon of f-butyl fluoride, which bear fluorine, but no hydrogens, many published tabulations of13 C spectra of compounds that contain such structural features fail to report these crucial signals. They can easily be missed, especially if you do not know what you are looking for. Even with relatively concentrated samples, it is usually necessary to run such spectra appropriately in order to accumulate sufficient FT data to see these weak signals. 

---

Tertiary alkyl fluorides exhibit an additional downfield shift of about +25 ppm, which is also very sensitive to branching as seen in Scheme 3.6. The fluorine spectrum for /-butyl fluoride is shown in Fig. 3.8. The signal at -131 ppm is split into 10 peaks with a three-bond H—F coupling constant of 21 Hz. 

The 5 (or 6)HF Et3N system reacts with a variety of branched alkenes to give tertiary alkyl fluorides. Hydrofluorination of dienes proceeds at the more reactive C = C bond selectively. Thus, ethyl geranate (1) and ethyl nerolate (2) react with 5HF EtaN at the noncon-... 

In contrast to the primary alcohols, tertiary alcohols eliminate water smoothly at 0°C in the presence of a 2-10 molar excess of hydrogen fluoride followed by polymerization.259 Reduced polymerization and satisfying yields of tertiary alkyl fluorides are achieved only at low temperatures (— 50 C). The reactivity of secondary aliphatic alcohols is thus interpreted to be between the other two types. High yields of alkyl fluorides are difficult to obtain because of the competing reactions and the effect of temperature on the equilibrium. 

Summarizing, it can be stated that hydrogen fluoride/pyridine and hydrogen fluoride/amine mixtures are useful for the synthesis of simple secondary and tertiary alkyl fluorides, but probably arc not suitable for fluorination reactions in complex multifunctional structures due to the highly acidic properties of these reagents. 

The 5 (or 6) hydrogen fluoride/triethylamine complex can also be used as a mild and selective hydrofluorination agent for preparing tertiary alkyl fluorides. ... 

Alkylation of pseudoephedrine a-fluoropropionamide can be used to prepare enantiomerically enriched tertiary alkyl fluoride centers (eq 27). In contrast to the alkylation of pseudoephedrine a-fluoroacetamide, alkylation of pseudoephedrine a-fluoropropionamide proceeds with high diastereoselectivity when LDA in used as the base in the reaction and low diastereoselectivity when LHMDS is used. In these reactions, deprotonation of pseudoephedrine a-fluoropropionamide with LDA, proposed to occur under kinetic control, is believed to form the corresponding... 

Alkyl fluorides are relatively stable and have been rarely used as alkylation agents in alkylation chemistry compared with other alkyl halides. Maruoka et al., however, found that MesAl efficiently catalyzes the alkylation of KSA with tertiary alkyl fluorides (Scheme 10.94) [255]. Under the same conditions alkylation with the corresponding chloride does not proceed. The strong activation of tertiary alkyl fluorides by McjAl originates from fhe exceedingly high affinity of aluminum for the fluorine atom (Section 10.2.1.1). 

Oshima and coworkers have reported that treatment of tertiary alkyl fluorides with diphenylphosphine in the presence of a stoichiometric amount of BF3 OEt2 gives rise to the corresponding tert-alkyldiphenylphosphine tetrafluoroborate salts, which can be trapped by elemental sulfur to give the phosphine sulfides (144) (Equation 86) [88]. Remarkably, this reaction works despite the presence of the highly coordinating Lewis basic phosphine. 

---