Synthesis and Molecular Structure of Mesocyclic 3,7-Dimethyl-1,1,5,5-tetraphenyl-1λ⁵,5λ⁵-diphosphocin, a 1,5-Bis(λ⁵-phospha)cyclooctatetraene

Cyclization of 1,1-bis [(diphenylphosphino)methyl]ethene (1) with 3-chloro-2-(chloromethyl)-1-propene results in the formation of the 3,7-dimethylene-1,1,5,5-tetraphenyl-1,5-diphosphocanium dichloride (2). The cyclic bis(ylide) was obtained upon double deprotonation with strong base and undergoes a double exo-endo proton-transfer rearrangement to give the title compound 3,7-dimethyl-1,1,5,5-tetraphenyl-1λ⁵,5λ⁵-diphosphocin (3). Both 2 and 3 have been characterized on the basis of ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectra, mass spectrometry, and elemental analysis. Orange-yellow crystals of composition 3·0.5Et₂O grown from diethyl ether at −25 °C have been studied by X-ray crystallography: space group C2/c, a = 25.003 (4) Å, b = 16.773 (1) Å, c = 15.614 (2) Å, β = 116.781 (6)°, V = 5845.74 ų, Z = 8. The mesocycle shows a puckered tublike conformation with two conjugatively isolated methallylic fragments connected via the two λ⁵-phosphorus atoms, which act as barriers against full π-electron delocalization, owing to the nodal planes of their d functions.