[79-38-9] · C2ClF3 · Chlorotrifluoroethylene · (MW 116.47)
(addition reactions; cycloadditions; trifluorovinylations)
Physical Data: mp -158 °C; bp -28.4 °C.
Solubility: sol organic solvents (ether, THF).
Form Supplied in: flammable gas, inhibited with 1% Bu3N.
Handling, Storage, and Precautions: inhalation toxicity LC50: 1000 ppm; flammable; use in a fume hood.
Chlorotrifluoroethylene is a starting material for the preparation of fluorinated molecules, via addition or addition-elimination reactions, the regioselectivity of which is not always, but can be, excellent. Halogenation according to eq 1 is a good example.1 Hydrobromination under UV irradiation also gives a unique pentahaloethane (eq 2).2
Lewis acid catalyzed addition of CCl4 proceeds via addition-substitution (eq 3).3
The nucleophilic addition of alcohols,4 thiols,5 and primary and secondary amines6 in the presence of base occurs readily; the regioselectivity corresponds to a better stabilization of the negative charge by -CFCl, than by -CF2 (eqs 4 and 5). With diethanolamine,7 further substitution occurs (eq 6).
Various organometallic reagents8-11 follow the addition-elimination pattern (eqs 7-9), as do lithium amides (eq 10).
Thiolates and alcoholates follow the same scheme. Allylates represent an interesting case since the initially formed allyl fluorovinyl ether undergoes a fluorine accelerated Claisen rearrangement, even at -35 °C (eq 11).12
Insertion of difluorocarbene generates chloropentafluorocyclopropane (eq 12).13
Various cycloadditions are observed at high temperatures (pressure vessel) with alkenes, alkynes, and conjugated dienes ([2 + 2] addition prevails largely over [4 + 2]) (eqs 13-15).14-16
A useful synthon for the nucleophilic introduction of the trifluorovinyl moiety is obtained by chlorine/lithium exchange (eq 16).17
The vinyllithium species must be handled at low temperatures. It reacts with aldehydes or ketones to give trifluorovinylcarbinols, which are a source of a-fluoro-a-ethylenic acids (by acidic treatment). They are also a source of a,b-difluoro-a-ethylenic alcohols if a lithium reagent is added, in a second step, to the reaction mixture (eq 17).18
The analogous zinc derivative19 is much more thermally stable, and can be vinylated, arylated, or acylated by a palladium catalyzed reaction with vinyl or aryl halides, or with acyl chlorides (eq 18).18
The analogs CF2=CFBr and CF2=CFI have been used in several similar reactions, but they are more expensive. However, when treated with Zinc powder, they deliver the corresponding zinc derivative.20
Raymond Sauvêtre & Jean F. Normant
Université Pierre et Marie Curie, Paris, France