Alkenes continued trifluoroacetic acid

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

Another resin-capture approach has been pubhshed in relation to the synthesis of tetrasubstituted ethylenes via Suzuki coupling reactions (Scheme 20) [42, 53]. A 25-member hbrary was synthesized using five alkynes, five aryl halides, and a polymer-bound aryl iodide. The alkynes 55 were converted into bis(boryl)alkenes 56 in solution, and the crude intermediates were used in Suzuki reactions with an excess of aryl halide. When all of the bis(boryl)alkene 56 had been consumed, the aryl iodide resin 59 was added to the reaction mixture and the reaction continued on the solid support. Side products such as 58, arising from a double Suzuki reaction, remained in solution and could be washed away. Compounds 60 were cleaved from the polymer using trifluoroacetic acid and products 61 were obtained in > 90% purity. 

New methods of introducing double bonds into sugar rings continue to appear, and this year 3-methyl-2-selenoxo-l,3-benzothiazole applied to epoxides is advocated. In dichloromethane and in the presence of trifluoroacetic acid it gives the alkenes in high yield.Otherwise, potassium iodide, zinc, and phosphorus pentoxide in DMF at 90 °C can be used effectively, methyl 2,3-... 

---