Ketenes, fluorine-containing

Several routes to this class of compounds have been reported, such as (a) crossed Claisen condensation reactions (50-53) (b) acylation of the anion derived from ethyl fluoroacetate (54) or self-condensation of the anion derived from ethyl bromofluoroacetate (55) (c) electrophilic fluorination of the anion of p-ketoesters (56,57) (d) acylation-hydrolysis of fluoroolefins (58) and (e) acylation of fluorine-containing ketene silyl acetals (Easdon, J.C., University of Iowa, unpublished data). The limitations associated with these methods and the success achieved in the alkylation-hydrolysis of a-fluoro phosphorus ylides prompted us to examine acylation-hydrolysis of these a-fluoro ylides as a general route to 2-fluoro-3-oxoesters. 

Konno reported a detailed study of the reaction of fluorine-containing propargyl acetates under Nicholas conditions with a variety of nucleophiles. Allylstannanes and allylsilanes provided moderate yields of the desired products, while enamines, silyl enol ethers, and silyl ketene acetals furnished the target compounds with excellent efficiency. In one example, cobalt-alkyne complex 25 reacts with silyl ketene acetal 26 in the presence of trimethylsilyl triflate to yield, after cobalt decomplexation, ester 27. ... 

A similar reaction was used for preparation of the remote stereocenter in 85, an intermediate in Paterson s synthesis of aurisides A and B. The asymmetric catalysis of the aldol reaction with fluoral has also been investigated by Mikami for highly enantioselective syntheses of fluorine-containing aldols. The reaction of the ketene trimethylsilyl acetal 86 with fluoral (87) in the presence of the BfNOL-catalyst 67 yielded the aldol adduct 88 in moderate yield (56%) and high enantioselectivity (90% ee). 

Very little is known about the formation of the less volatile products. Taking into consideration only the gaseous products, it can be seen that the hypothetical ratio, methyl groups/CO molecules, is over 2. This indicates that some of the less volatile products containing CO groups escaped detection. Biacetyl could be detected, in rather small quantities, using a fluorinated cyclic ether medium . In aqueous solution, a considerable amount of acetic acid is formed, probably the result of the hydrolysis of the ketene . ... 

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