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

摘要

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 νasym(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 give 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.