A structure-reactivity study of selected phosphorus compounds

Doctoral Thesis


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University of Cape Town

The X-ray crystal structure determinations of a series of ring substituted dimethyl-N-phenyl-phosphoramidates, (R)PhNHP(OCH₃)₂, (R = p-OMe, H, p-NO₂, o-Et and 2,6-diMe) have confirmed the preference of the phosphoramidates for dimer formation which was proposed earlier from infrared data. The formation of N - H - - - - O = P hydrogen bonded dimeric species is sensitive to steric effects and the presence of bulky substituents at the ortho positions of the benzene ring prevents dimer formation. The X-ray crystal structure determination of the carboxylic analogues of the phosphoric amides above, (R)PhNHC(O)R' (R = p-OMe, p-NO₂ and R' = Me or Ph), serves to indicate the differences in the bonding between the phosphoric and carboxylic amides. The geometry of the amide group in the carboxylic amides is subject to the influence of intermolecular hydrogen bonding effects while the phosphoric amide group in the dimer is less sensitive to other intermolecular interactions. In the phosphoric amides the distortion of the amide group from the favourable (planar) geometry arises principally from intramolecular steric repulsions. The bond lengths determined for the phosphoric amides indicate that the phosphoric amide group is less conjugated than the corresponding carboxylic amide group. The P = 0 bond order is insensitive to changes in substituent electron-releasing or -withdrawing ability, The solid state data indicate that the electron density on the nitrogen atom is not transferred to the phosphoryl group as readily as electron density is transferred from the nitrogen atom to the carbonyl group in carboxylic amides. This had previously been suggested from other spectroscopic studies on phosphoric and carboxylic amides. Therefore the approximate reciprocal dependence between the C = O and C - N bond lengths observed in carboxylic amides does not occur in the phosphoric amides. The changes in the electron-withdrawing or -releasing properties of the ring substituent in the phosphoric amides is confined to changes in the acidity of the amide hydrogen atom and is correlated with the N - H ---- 0 = P hydrogen bond length. The angular dependence of the approach of nucleophiles to quaternary phosphonium ions L₄P+ was investigated. The analysis of 36 structures of the general formula L₄P+X- indicates that the nucleophile generally adopts the "face" approach to the phosphonium centre. When the expected leaving group on the phosphorus atom has an acidic α-proton the nucleophile approaches adjacent to the leaving group as a result of a strong interaction between the α-proton and the nucleophile. This tendency had been found previously and is confirmed by this study. When the expected leaving group is of the form OR or NR and there is no possibility of interaction between the nucleophile and the leaving group the nucleophile approaches the phosphonium centre opposite the leaving group. The angular dependence of the approach of the nucleophiles to the phosphonium centres appears to be as a result of packing factors. The crystal structures of the hydrated and dehydrated forms of synthetic analcime indicate that the structures are essentially identical to structure of the natural form determined previously.

Bibliography: p. 366-374.