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

Abstract

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≈BAC_a>NHCs. When the betaines ν_asym(CO₂) 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 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.