Gem-Difluoroalkenes

Addition-elimination processes of a vinylic fluorine, from gem-difluoroalkenes." ... 

Displacement of fluorine from gem-difluoroalkenes has been developed as methodology for fluorine-free synthesis [108] (Scheme 49). 

Carbolithiation of a trifluoromethyl-substituted alkene 59 produces an unstable intermediate 60 which eliminates lithium fluoride to give gem-difluoroalkenes 61.35... 

Since a variety of 1-alkenylboron reagents including (E)- and (Z)-isomers are now available, their cross-coupling with 1-halo-l-alkenes affords various stereodefined alkadienes and trienes [64-66]. Many syntheses of alkadienes and trienes such as unsaturated fatty acid amides [76], alkenylsilanes [77], gem-difluoroalkenes [69, 78], cyclic alkenes [79], rran5-(C,o)-alIofamesene [80], trisporol B [81], and vinyl sulfides [82] are reported, by application of the reported Pd-catalyzed cross-coupling. The representative syntheses and reaction conditions ai e summarized in Table 2-2. 

The preferred sp3 hybridization for saturated fluorocarbons is a major driving force for the easy polymerization of TFE to PTEF with more exothermic enthalpy change (Scheme 1.34) [2]. The it-bond energy of tetrafluoroethylene is 52 kcal/mol, which is smaller than that of ethylene, 64-65 kcal/mol [3].The easy [2 + 2] cycloadditions of gem-difluoroalkenes (thermal concerted reactions of which are forbidden) proceed stepwise via biradical intermediates. The intermediacy of a biradical is supported by the fact that a 1 1 mixture of the cis- and trans-dideuteriocyclobutanes (7 and 8) was obtained in the [2 + 2] cycloaddition of TFE with cis- (4) and frans-dideuterioethylene (5) [4] (Scheme 1.34). 

The formation of C-P bonds by the radical addition of phosphines to exocyclic carbohydrate alkenes was originally reported by Whistler [109], who demonstrated that irradiation of alkene 171 in the presence of phenylphosphine afforded the phosphine oxide 172 (Scheme 29). Sinay [110] prepared the gem-difluoro-C-galactofuranoside 173 by phosphonyl radical addition to gem-difluoroalkene 174 during studies related to the preparation of potential inhibitors of the enzyme UDP-galactofuranose mutase. 

Mori T, Ichikawa J (2004) KCN-catalyzed C-C bond formation between imine and gem -difluoroalkene moieties a facile synthesis of 2,4-disubstituted 3-fluoroquinolines. Chem Lett 33 590-591... 

Ichikawa J (2005) Synthetic methods for the preparation of ring-fluorinated heterocycles via intramolecular vinylic substitution of gem-difluoroalkenes. In Soloshonok VA (ed) Fluorine-containing synthons, vol 911, ACS symposium series. American Chemical Society, Washington, DC, pp 262-275... 

A good entry point for this ehemistry would be the conversion of diazo compounds into gem-difluoroalkenes [168]... 

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