Syntheses and Molecular Structures of an Isoelectronic Series of (2‐Hetero‐)1,3‐Diphosphabenzenes

Syntheses, NMR spectra, and X‐ray molecular structures of three (2‐hetero‐)1,3‐diphosphabenzenes are reported which constitute an isoelectronic triade of compounds with BH₂, CH, and N bridging units between the two phosphorus atoms. The remaining three carbon atoms of the rings are part of a 2‐methallylic system. The 2‐borataheterocycle 3 has been prepared by the reaction of CH₂ = C(CH₂)PPh₂)₂ (1) with Me₂SBH₂Br to give an intermediate 2 still containing the exocyclic olefinic bond. Treatment of 2 with a base, deprotonation of one ring CH₂ group, and proton migration from the other one to the olefinic CH₂ group afford the 5‐methyl‐1λ⁵‐phospha‐3‐phosphonia‐2‐boratabenzene 3. The PBP unit is tilted by 144.0(2)° against the planar C₃P₂ part of the ring. A heterocyclic precursor 4 for the carbon analog is generated from 1 and CH₂I₂, which on reaction with a strong base yields the diphosphabenzene 5 with a fully planar ring skeleton. For the synthesis of the aza analog 7, CH₂=C(CH₂Cl)₂ is treated with HN(PPh₂)₂ to give the precursor 6, which can be deprotonatad at nitrogen and carbon with a base. In the crystals of 7, two independent molecules of very similar structure are present. The molecules show planar heterocycles.