Synthesis and Conformational Properties of 3,8-Phosphonanedione 1-Oxides

Ozonolysis at −78 °C of 3-phospholene derivatives with cyclohexane or substituted cyclohexanes fused at the double bond provides a useful route to derivatives of the 3,8-phosphonanedione 1-oxide system. Synthesized were the 1-methyl, 1-phenyl, 1-hydroxy, 1-phenyl-cis-5,6-dimethyl, 1-phenyl-trans-5,6-dibromo, and 1-phenyl-5,6-epoxy derivatives. Opening of the 9,10 bond of a phenanthrene fused to a 3-phospholene provided a dibenzo[d,f]phosphonanedione derivative. 1-Methyl-3,8-phosphonanedione 1-oxide, shown by X-ray analysis to exist in a twist chair-chair form in the solid state, undergoes rapid interconversion of conformers at room temperature, giving an averaged ¹³C NMR spectrum. The interconversion is halted at −97 °C, where signals for two conformers are obtained (Tc = −84 °C, approximate ΔG^‡ = 9.6 kcal/mol). 1-Phenyl-5,6-dibromo-3,8-phosphonanedione 1-oxide, which X-ray analysis also showed to be in a twist chair-chair form in the solid state, however, showed nonequivalence of comparable ring carbons, implying the existence of a strongly biased equilibrium or a high barrier to ring inversion. The dibenzo[d,f]phosphonanedione derivative has marked rigidity, and the ¹³C NMR spectrum reveals that comparable ring carbons are non-equivalent at room-temperature. The other phosphonanedione derivatives gave ¹³C NMR spectra showing equivalence of comparable ring carbons, through either conformational interconversion or adoption of a symmetrical conformation. © 1982, American Chemical Society. All rights reserved.

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Author Andrew T. McPhail Chemistry