Reactivity of Cyclic (Alkyl)(amino)carbenes (CAACs) and Bis(amino)cyclopropenylidenes (BACs) with Heteroallenes: Comparisons with their N ‐Heterocyclic Carbene (NHCs) Counterparts

Similarly to NHCs, CAAC a and BAC a react with CO 2 to give the corresponding betaines. Based on the carbonyl stretching frequencies of cis ‐[RhCl(CO) 2 (L)] complexes, the order of electron donor ability was predicted to be CAAC a ≈BAC a >NHCs. When the betaines ν asym (CO 2 ) values are used, the apparent ordering is BAC a >NHCs≈CAAC a that indicates a limitation for the use of IR spectroscopy in the ranking of ligand σ‐donating ability. Although all carbenes react with carbon disulfide to give the corresponding betaines, a second equivalent of CS 2 reacts with the BAC‐CS 2 leading to a bicyclic thieno[2,3‐diamino]‐1,3‐dithiole‐2‐thione, which results from a novel ring expansion process. Surprisingly, in contrast to NHCs, CAAC a does not react with carbodiimide, whereas BAC a exclusively gives a ring expanded product, analogous to that obtained with CS 2 . The intermediate amidinate can be trapped, using the lithium tetrafluoroborate adduct of BAC b as a carbene surrogate.