Hydrophosphination of Strained Rings
The uncatalyzed, ambient temperature hydrophosphination of unactivated alkynes was discovered accidentally several years ago, using phosphine boranes. The exceptional nucleophilic potency of phosphineborane anions has since been expanded to include reactivity with allenes, carbodiimides and now strained rings. Carbocyclic bicyclobutanes are highly strained systems (ca. 65 kcal/mol strain energy), that are readily prepared. These carbocycles have been found for the first time to react with phosphineborane nucleophiles, often in minutes at ambient temperature, generating a new tetra-substituted carbon atom in a cyclobutane ring. In the case of bicyclobutane nitriles, the cyano functionality then provides a useful chemical handle for further derivitization. We have now extended this chemistry to include azabicyclobutanes, generating an array of novel phosphine-substituted azetidines. The presence of the amine center then leads to the production of optically pure adducts via classical resolution methods. The development of the hydrophosphination chemistry, details of the interesting spectroscopy, calorimetry and solid state properties of these strained systems, and initial applications will be discussed.