including the recently published six-membered version. CAACs are even richer in electrons than NHCs and phosphines, but at the same time, due to the presence of a single pair of free electrons on nitrogen, CAACs are more accepting than NHCs.M. Melaimi, R. Jazzar, M. Soleilhavoup, G. Bertrand,, « Cyclic (Alkyl)(Amino)Carbenes (CAACs): Recent developments », Angew. Chem. Int. Ed., 2017, 56, p. 10046-10068 The electronic properties of CAACs stabilize highly reactive species, including organic and major group radicals, as well as paramagnetic metal species, such as gold complexes (0), which were completely unknown. CAACs have also allowed the isolation of bis(copper)acetylide complexes,L. Jin, D. R. Tolentino, M. Melaimi, G. Bertrand, « Isolation of Bis(copper) Key Intermediates in Cu-Catalyzed Azide–Alkyne “Click Reaction.” », Sci. Adv., 2015, 1, e1500304 which are key catalytic intermediates in the famous "Click Reaction", and which were supposed to be only transient species. He also used CAACs to prepare and isolate the first isoelectronic nucleophilic tricoordinated organoborane from amines.F. Dahcheh, D. Martin, D. W. Stephan, G. Bertrand, « Synthesis and Reactivity of a CAAC-Aminoborylene Adduct: A Hetero-Allene or an Organoboron Isoelectronic with Singlet Carbenes? », Angew. Chem. Int. Ed., 2014, 53, p. 13159R. Kinjo, B. Donnadieu, M. Ali Celik, G. Frenking, G. Bertrand, « Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines », Science, 2011, 333, p. 610–613 These recent developments appear paradoxical since they consist in using carbenes long considered as prototypic reactive intermediates to isolate otherwise unstable molecules. Among the large-scale applications already known of CAACs are their use as a ligand for transition metal catalysts. For example, in collaboration with Grubbs, Guy Bertrand has shown that ruthenium catalysts bearing a CAAC are extremely active in the ethenolysis of methyl oleate.V. M. Marx, A. H. Sullivan, M. Melaimi, S. C. Virgil, B. K. Keitz, D. S. Weinberger, G. Bertrand, R. H. Grubbs, « Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis », Angew. Chem. Int. Ed., 2015, 54, p. 1919
