Synthesis and reactions of carbenes

This chapter reviews the synthesis and reactions of carbenes. The best evidence for the existence of carbenes comes from a group of structures which contain a carbene but are stable compounds. The most important of these are known as the ‘N-heterocyclic carbenes’—the carbene is incorporated into a five-membered ring and stabilized by the presence of two adjacent electron-donating nitrogen atoms and the bulky N-substituents. These stable carbenes are very much the exception: most carbenes are too reactive to be isolated. Ultimately, carbenes can be formed from many other reactive intermediates, such as carbocations, carbanions, and diazoalkanes. The chapter then looks at how they can react to give yet further reactive intermediates such as ylids.