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 CO2 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) approximate to BAC(a) > NHCs. When the betaines nu(asym)(CO2) values are used, the apparent ordering is BAC(a) > NHCs approximate to CAAC(a) that indicates a limitation for the use of IR spectroscopy in the ranking of ligand sigma-donating ability. Although all carbenes react with carbon disulfide to give the corresponding betaines, a second equivalent of CS2 reacts with the BAC-CS2 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 CS2. The intermediate amidinate can be trapped, using the lithium tetrafluoroborate adduct of BAC(b) as a carbene surrogate.