Reaktivität von Cyclopentenyl‐Anion‐analogen Heterocyclen: Homophosphole — Synthese, 1,5‐Elektrocyclisierung, Inversion am Phosphor‐Atom

Reactivity of Cyclopentenyl‐Anion‐Analogous Heterocycles: Homophospholes — Synthesis, 1,5‐Electrocyclization, and Inversion at the Phosphorus Atom *[1] The key step of a novel homophosphole synthesis is the reaction of phospholes 3a‐d with diazomethane in the presence of water leading to the oxidized 1,3‐dipolar cycloadducts anti ‐ 7a‐d . The 1,5‐electrocyclization of homophosphole was observed by the temperature‐dependent rate of racemization of optically active anti ‐ 8b (Δ H ≠ (31.24±0.59) kcal/mol; Δ S ≠ –(2.32±1.47) cal/mol K). In order to determine the activation parameters for the inversion at the phosphorus atom a (60:40) mixture of syn ‐ and anti 1‐homophosphole syn ‐ and anti ‐ 8b was prepared by H 2 O 2 oxidation of anti ‐ 8b complexed by CuCl and subsequent reduction with Cl 3 SiH. The enantiomerization [(+)‐ anti ‐ 8b → (–)‐ anti ‐ 8b ] proceeds at 169.5°C 22 times faster than the inversion at the phosphorus atom ( anti ‐ 8b → syn ‐ 8b ). This shows unambiguously that the electrocyclic ring opening does not require a planar configuration at the phosphorus atom and can start from the nonplanar anti configuration due to a dihedral angle of ca. 0° between the orbital of the lone pair at the phosphorus atom and the cyclopropane Walsh orbitals favourable for a strong electronic interaction in this configuration. From the comparison of the activation parameters for the inversion at the phosphorus atom in the homophosphole ( syn ‐ 8b ⇆ anti ‐ 8b ) and the dihydrophosphole [(–)‐ 23 ⇆ (+)‐ 23 ] one can extrapolate, that the homoaromatic resonance stabilization in the planar homophosphole is small (ca. —2 kcal/mol) contrary to the surprisingly large aromatic stabilization in the planar phosphole (ca. —20 kcal/mol).