UBC Theses and Dissertations
Methylation of fluorocyclophosphonitriles Ranganathan, T.N
The main part of this thesis is concerned with the study of the reactions of fluorocyclophosphonitriles, (NPF₂)₃₋₅, with methyllithium, which exhibit a new orientation pattern, and which appear to involve inductive interactions of the substituent (the CH₃ group) with a homomorphic π-system. Nuclear magnetic resonance spectroscopy has been the principal tool for deducing the structures of the isomers isolated by gas liquid chromatography from reactions of the fluorocyclophosphonitriles with methyl-lithium. ⁵JCH₃F and ⁴JFF have been observed for the first time in phosphonitrilic derivatives. Although the assignments of the frequencies in the infrared spectra of the methylfluoro- and methylcyclophosphonitriles are inadequate for a complete vibrational analysis, the pattern of frequencies found for trans-1,5-N₄P₄F₆Me₂, shows that it is not centro-symmetrical and neither is the derivative 1,1,5,5-N₄P₄F₄Me₄. In the crystal the latter compound has a saddle shape, and it is likely that the flexibility is caused by the interconversion of the tub to saddle forms, which can take place by bond torsional movements only, without angular deformation. The most important aspect of the work is the fresh evidence which is provided on the nature and extent of delocalisation within effect of the substituents is seen in the significant alternation of bond lengths in 1, l-N₄P₄F₆Me₂. Since the molecular framework is nearly planar, the conditions for the application of simple Huckel theory are satisfied, and the effect of a π-inductive perturbation at P has therefore been estimated through the calculation of bond-atom polarisabilities πr₅,t = [formula omitted] for a delocalised π-system based on an 8-membered ring. The close correspondence in pattern of the observed deviations in the individual bond lengths from the mean and the bond-atom polarisabilities seems very direct evidence for electronic delocalisation in phosphonitrilic molecules, and suggests that simple Huckel methods can provide a useful guide to their chemistry. The predominantly geminal substitution observed in the reaction of (NPF₂)₄ with LiMe has been well explained by an inductive effect of the methyl substituent on the π-system of the phosphonitrilic ring. The atom-atom polarisabilities for the three ring systems investigated have been calculated using simple Huckel theory, which show that, from this cause alone, a second nucleophilic substitution should take place principally at P₁, as found. Also the atom-atom polarisabilities explain the observed geminal and antipodal substitution, an orientation pattern which has not been recognised previously. This is the first case in which cyclic delocalisation has been shown to have a decisive effect on phosphonitrilic chemistry.
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