Ketenes

Ketene is a class of highly reactive organic compounds that feature the cumulated double bond system R1R2C=C=O, with the central carbon bearing two substituents R1 and R2. The parent, ketene (H2C=C=O), is a volatile species typically generated and consumed in situ due to its reactivity. Substituted ketenes vary in physical state from gases to liquids or solids, depending on the nature of the R groups.

The ketene moiety is highly electrophilic at the central carbon, making ketenes versatile intermediates in organic synthesis. They react readily with nucleophiles such as water, alcohols, and amines to form carboxyl-containing derivatives. They also participate in cycloadditions, most notably with alkenes and with imines, to give beta-lactones and beta-lactams, respectively.

Common reactions of ketenes include hydrolysis to yield carboxylic acids corresponding to the substituents on the ketene carbon, ester formation with alcohols, and amide formation with amines. In cycloadditions, the [2+2] process between a ketene and an alkene affords a beta-lactone, while reaction with an imine provides a beta-lactam. These reactions are widely exploited for bond construction in complex molecule synthesis.

Preparation and handling: ketenes are typically generated in situ from acyl chlorides by dehydrohalogenation or dechlorination, often in the presence of base or suitable reagents, and used immediately to avoid rapid oligomerization or polymerization. Stabilized derivatives, such as silyl ketenes, can be isolated under certain conditions.

Because of their high reactivity and broad reactivity with nucleophiles and dipolar partners, ketenes serve as important reactive intermediates in organic synthesis and pharmaceutical chemistry.