Ketene chemistry after 100 years: Ready for a new century

The existence of ketenes was established 100 years ago, and in recent years the pace of discovery in the study of these reactive intermediates has accelerated - extremely unstable ketenes such as difluoroketene have been directly observed for the first time, while a variety of bisketenes stabilized by silyl substituents have been isolated as long-lived compounds at room temperature. New methods for generating ketenes for applications in synthesis have been devised, and these have been applied in the preparation of many valuable targets. Asymmetric synthesis by nucleophilic additions and by [2+2] cycloadditions to ketenes to form beta-lactams and beta-lactones has been achieved with high yields and high stereoselectivities. The free-radical chemistry of ketenes is receiving increasing levels of attention, and new rearrangement reactions of ketenes have been developed. These advances create new opportunities to utilize the unique structures and high reactivity of ketenes for even greater progress in the future. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)