Aldehydes, halogenation ketenes

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

While there are no examples of the formation of fluorinated unsaturated aldehydes by de-halogenation, dehalogenation is the preferred method for the synthesis of reactive fluorinated ketenes. Examples of dehalogenations giving unsaturated fluorinated ketones and fluorinated ketenes are listed in Table 9. 

Insertion Reactions into Element-Halogen Bonds 11.6.4. Insertions of Aldehydes, Ketones, and Ketenes... 

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