569680-54-2Relevant academic research and scientific papers
Reactivity of Cyclopentenyl-Anion-Analogous Heterocycles: Homophospholes - Synthesis, 1,5-Electrocyclization, and Inversion at the Phosphorus Atom
Klaerner, Frank-Gerrit,Oebels, Dirk,Sheldrick, William S.
, p. 473 - 484 (2007/10/02)
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(excit.) = (31.24 +/- 0.59) kcal/mol; ΔS(excit.) = -(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-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> proceeds at 169.5 deg 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 deg 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> 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). Key Words: Homophospholes / Phospholes, 4,5-dihydro, optically active / 1,5-Electrocyclization / Phosphomethine ylides / Inversion at the phosphorus atom
