Additions of Ketenes and Keteniminiums to Olefins

These reactions constitute easy access to the cyclobutanone ring (Fig. 1), found in a number of natural products and a useful building block to 5-membered carbocyclic and heterocyclic compounds.  

Because of limitations, such as their easy polymerization, ketenes can be replaced by keteniminiums, less prone to follow undesired evolutions and of broader versatility for intramolecular applications. In both cases, the reaction must be effected in the presence of the co-reagent. Even with this precaution, the yields are not always satisfactory, and sonochemical improvements were sought. 

The two more important reactions to generate ketenes are the dehalogenation of a-chloroacyl chlorides with zinc and the base-mediated dehydrochlorination of acyl chlorides. Both methods were explored sonochemically. 

In the preparation of dichloroketene from a-chloroacetyl chloride, which normally requires an activated zinc-copper couple, non-activated commercial zinc can be employed under sonication,. which probably plays its promoting 

Olah et al. used sonication to prepare stable, highly crowded ketenes (Eq. 1) by dehydrochlorination of acyl chlorides with an organic strong base. o However, no mention was made of the possible advantage of sonication in the subsequent cycloaddition step. 

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