The quest for unligated oxaphosphiranes with phosphorus in different coordination numbers.

After more than 160 years of oxirane (epoxide) chemistry, the first derivative of a phosphorus analogue, namely oxaphosphirane, has been synthesized. Reactions with borane sulfane, a peroxy compound and elemental sulfur, as well as tetra-chloro-ortho-benzoquinone (TOB) reveal a significant destabilization upon increasing the coordination number at phosphorus from σ3 to σ4, thus somehow supporting previous reports on such fleeting species. Theoretical studies provide insight into ring strain energy, ring/ring interconversion and the oxidation pathways.