Reactivity of Bis(amino)cyclopropenylidenes (BACs) and Cyclic(alkyl)(amino)carbenes (CAACs):
Carbenes have played an important role as reactive intermediates in synthetic organic chemistry for many years. The isolation of stable versions of these important carbon species has resulted in an explosion of research and applications not only as ancillary ligands for transition metal (TM) based catalysts, but also as small molecule activators, and organocatalysts in their own right. One aspect of my work is exploration of the reactivity of bis(diisopropylamino) cyclopropenylidene (BAC) and cyclic(alkyl)(amino) carbenes (CAACs), first isolated by our group. Due to their unique steric and electronic properties, these species are perhaps the most distinct representatives of the carbene ligand set and as such allow the isolation of a variety of catalytically active TM complexes unobtainable by other routes and frequently different from their cyclic diamino carbene (NHCs) counterparts.
Inspired by the novel reactivity of these new carbenes in TM chemistry, I have also explored the activation of heteroallenes using these species. Similarly to NHCs, CAACs and BACs react with carbon dioxide and carbon disulfide and to give the corresponding zwitterionic betaines. However, unlike NHCs, a second equivalent of CS2 reacts with the BAC-CS2 adduct leading to a bicyclic thieno[2,3-diamino]-1,3-dithiole-2-thione, which results from a novel ring expansion process. Surprisingly, and also in contrast to NHCs, CAAC does not react with carbodiimide, whereas BAC exclusively gives a ring expanded product, analogous to that obtained with CS2. The intermediate amidinate can be trapped, using a lithium tetrafluoroborate adduct of BAC as a carbene surrogate.
In related work, I demonstrate that depending on their electronic and steric properties, stable singlet carbenes can react with white phosphorus at room temperature to yield P4, P3, P2, and even P1 fragments that are stabilized by the carbene moiety. Therefore, stable singlet carbenes can achieve the tasks that transition metals do with P4, namely activation, aggregation, and importantly, fragmentation. The next challenge is to use the resulting adducts, to prepare useful organophosphorus derivatives. This would avoid the use of Cl2 gas, which is important to meet the growing demand in phosphorus derivatives using environmentally friendly processes.