Ylidic Four‐Membered Rings with Four π‐Electrons—Another Exciting Chapter in Phosphorus Chemistry

In contrast to the antiaromatic and nonelusive mono- and oligophosphetes featuring only two-coordinate phosphorus atoms, unsaturated, four-membered phosphorus heterocycles with at least one η5, σ4-phosphorus atom such as triphosphete 1, which was prepared by Karsch et al., are surprisingly stable compounds. These systems can undergo ring expansion to give five-membered heterocycles such as 2 (according to Bertrand et al.), react to form complexes, or undergo addition and substitution reactions.