REACTIVITY OF CYCLOPENTENYL-ANION-ANALOGOUS HETEROCYCLES - HOMOPHOSPHOLES - SYNTHESIS, 1,5-ELECTROCYCLIZATION, AND INVERSION AT THE PHOSPHORUS ATOM

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 (DELTAH(not-equal) = (31.24 +/- 0.59) kcal/mol; DELTAS* = -(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 --> (-)anti-8b] proceeds at 169.5-degrees-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-degrees 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 reversible anti-8b) and the dihydrophosphole [(-)-23 reversible (+)-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).