Carbodicarbenes

This chapter deals with four different classes of two-coordinated carbon compounds. Carbenes were introduced into contemporary synthetic chemistry with the syntheses of the first stable carbenes by Bertrand and by Arduengo, who isolated the first stable N-heterocyclic carbene (NHC). The first carbone which was synthesized in 1961 by Ramirez is the carbodiphosphorane C(PPh3)2. The very large value for the second proton affinities (PAs) of the carbodicarbenes suggests that doubly protonated species should be stable species. The foregoing theoretical and experimental results reveal that π-donor substituents may influence the chemical reactivity of allenes such that they behave like carbodicarbenes. There are further compounds which are not genuine carbodicarbenes but which are related to them. Theoretical and experimental research on carbodicarbenes is still in an infant stage, but the results which have been reported thus far let us expect that the field will greatly expand in the coming years.