New Applications of 1,3,2-Diazaphospholenes in Catalysis

The constant urge to construct new molecules in an economical and sustainable fashion led to the development of numerous metal-catalyzed transformations. Organocatalysts consisting of abundant and more sustainable elements offer an elegant solution to overcome the drawbacks of metal-based catalysts, such as scarcity and toxicity. This thesis focuses on new catalytic applications of 1,3,2-diazaphospholenes (DAPs) as phosphorous-based organocatalysts. Over the past decade, P-hydrido variants of DAPs have emerged as powerful nucleophiles and weakly basic hydride donors in a variety of catalytic transformations. In radical mediated reactions, DAP-hydrides can serve as hydrogen atom transfer reagents and the corresponding DAP-radicals can facilitate halogen atom transfer processes. As one of the first applications of DAP-catalysis in radical transformations, we disclosed a DAP-radical induced intramolecular cyclization reaction. A light promoted reductive dehydrocoupling of DAP-hydride afforded dimeric DAP, in which the P-P bond can undergo homolytic cleavage to generate the key DAP-radical. Crucial for the development of the catalytic cycle was the facile regeneration of DAP-hydride from the emitted DAP-halides, which was achieved by a Lewis base assisted sigma-bond metathesis with pinacolborane. The strong nucleophilic nature of DAP-hydrides was exploited in a DAP-catalyzed aza-Michael induced ring closure reaction to afford aziridines. DAP-hydride readily adds to an alpha,beta-unsaturated carbonyl and subsequent intramolecular nucleophilic attack of the phosphorous enolate on a nitrogen atom leads to construction of a new C-N bond. We delivered a proof of concept of the feasibility to induce chirality via the phosphorous enolate. A mixture of pinacolborane and potassium carbonate ensured the pivotal DAP-hydride regeneration. Furthermore, a secondary alcohol derived xanthate was smoothly reduced by DAP-hydride. The optimized conditions of this DAP-catalyzed reduction gave access to an unusual alkoxy methanethiol. We could demonstrate that the essential DAP-hydride regeneration was realized by sigma-bond metathesis between DAP-thiols and phenylsilane. The susceptibility of DAPs to engage in radical processes was used to access a P-perfluorobutyl DAP, capable of transferring the carbon fragment onto quinolinium halides. Preliminary results suggested that this transformation could be rendered catalytic in DAPs. Our approach relies on the reaction of DAP-hydride with perfluoroalkyl iodides, presumably initiated by DAP-radicals.