THERMAL DEGRADATION OF DIMERS OF PHOSPHOLES |
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Authors: | Louis D Quin Kenneth C Caster |
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Institution: | Paul M. Gross Chemical Laboratory , Duke University , Durham, North Carolina, 27706 |
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Abstract: | Abstract The pathway followed by dimers of P-methylphospholes in their thermal degradation in solution is strongly dependent on the concentration. At low concentrations (around 0.04 M) in n-decane or toluene as solvent, decomposition is extensive after 17 h in the range 120–130°C. The major product is the P-methylphosphole from de-dimerization. At concentrations above 1.0 M, decomposition is faster and intermolecular interactions are more important. These interactions dominate over de-dimerization and lead to the loss of the bridging P in the 7-phosphanorbornene moiety to yield the cis-3a,7a-dihydrophosphindole system. From the dimer of 1,3-dimethylphosphole, isomeric dihydrophosphindoles are formed, which differ in location of the methyl group on the 6-membered ring and in configuration at phosphorus. These seem to result from attack by a phosphine group as a nucleophile on the bridging P, followed by bond rearrangements in the resulting phosphoranide ion. Decomposition was faster in the presence of tri-n-butylphosphine, which apparently acted as a catalyst through a similar mechanism. By using appropriate conditions each dihydrophosphindole isomer (1,3,5- or 1,3,7-trimethyl) could be made to predominate and this allowed their isolation. Characterization of the isomers and of some derivatives by 31P, 1H, and 13C NMR techniques was performed. Most of the thermolysis products were highly complex mixtures and many minor products remain unidentified. The dimer of 1-phenyl-3-methyl-phosphole did not decompose to the phosphole, but the product composition was dependent on concentration. |
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