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 (ΔH≠(31.24±0.59) kcal/mol; ΔS≠–(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 anti1-homophosphole syn- and anti-8b was prepared by H2O2 oxidation of anti-8b complexed by CuCl and subsequent reduction with Cl3SiH. The enantiomerization [(+)-anti-8b (–)-anti-8b] proceeds at 169.5°C 22 times faster than the inversion at the phosphorus atom (anti-8bsyn-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).