A quest for stable phosphonyl radicals: limitations and possibilities of carbocyclic backbones and bulky substituents

Although phosphonyl radicals play an important role as transient species in many chemical transformations, such as photoinitiated polymerisation reactions, permanently stable phosphonyl radicals are yet to be discovered. In this computational study, we aim at a conceptual understanding of the electronic effects influencing the stabilities of phosphonyl radicals through computing radical stabilisation energies (RSEs) for a large set of phosphonyl radicals with carbocyclic backbones. The studied radicals exhibit ring sizes varying from 3- to 7-membered with full saturation or different grades of unsaturation adjacent to the P-centre in an endo or exocyclic fashion. To gain deeper insight into the stabilisation effects and delocalisation, the geometrical aspects, electronic structures, and spin distributions of the radicals were scrutinised. The five-membered, fully unsaturated ring (phospholyl oxide), which has a planar structure, offers the most substantial electronic stabilisation. By embedding this ring into a more extended pi-system, the possibility of gaining further stabilisation was also explored. To screen the effect of steric congestion on the stabilities of previously selected radicals toward dimerisation, a large number of bulky substituents with different sizes and shapes were systematically investigated. Our results outline that stable phosphonyl radicals seem accessible, provided that the electronic stabilisation effects are supplemented by well-designed bulky substituents. Exploring the electronic and steric stabilisation effects reveals that carbocyclic phosphonyl radicals with remarkably delocalised spin distributions and appropriately selected bulky substituents are suitable for synthetic purposes.