N-Borane-Substituted Cyclic Phosphine Imides (BCPIs)

Phosphine imides are ubiquitous nucleophiles/Lewis bases in modern organic chemistry. The introduction of unexplored substituents on the phosphine imidoyl nitrogen and/or phosphorus atoms should facilitate the discovery of unprecedented utility for phosphine imides. Herein, we have designed and prepared a novel class of phosphine imides known as N-borane-substituted cyclic phosphine imides (BCPIs). Experimental and theoretical analyses of the electronic structure of BCPIs demonstrate the existence of substantial negative hyperconjugation between the nitrogen and the phosphorus atoms. Given a characteristic nucleophilic/Lewis basic reactivity of BCPIs, we represent the first experimental demonstration that a 5-oxazaphosphetane species is a key intermediate in the transformation of CO2 using phosphine imides. Moreover, although it has been previously considered unlikely, the spontaneous heterolysis of a BCl bond in a BCPI-coordinated chloroborane has been directly observed, suggesting that such process is a plausible key step in the Lewis acid-promoted generation of borenium species from chloroboranes. These results thus provide evidence of two species that have been missing in contemporary organic chemistry.