(Phosphanyl)phosphaketenes as building blocks for novel phosphorus heterocycles

Although BH₃ simply coordinates the endocyclic P of (phospholidino)phosphaketene 1 , the bulkier B(C₆F₅)₃ gives rise to a zwitterionic diphosphirenium, which is a novel type of 2π-electron aromatic system as shown by the calculated NICS values. While the reaction of 1 with Na[PCO(dioxane) ] is unselective, the same reaction with the sterically bulky (phospholidino)phosphaketene 1 [Ar** = 2,6-bis[di(4-tert-butylphenyl)methyl]-4-methylphenyl selectively affords a sodium bridged dimer containing a hitherto unknown λ³,λ⁵,λ³-triphosphete core. The latter formally results from "P^-" addition to a 1,3-P/C-dipole. Similarly, adamantyl isonitrile adds to 1 giving a 4-membered phosphacycle. In contrast to 1, the phosphaketene derived from the electrophilic diazaphospholidine-4,5-dione is unstable and reacts with a second molecule of Na[PCO(dioxane) ] to afford a 1,3,4-oxadiphospholonide derivative.