Synthesis and structural analysis of [alpha]-aminophosphonic acids and their phosphonate ester derivatives
The aims of this project fall into two main categories. The primary objective was the preparation and characterisation of novel [alpha]-aminophosphonic acids and their phosphonate ester derivatives. Compounds of these types are of interest because of their potential for biological activity, particularly in the area of agricultural chemistry. The second objective was to investigate the conformation of selected examples of these types of molecules in solution. This was achieved by considering the proton-proton and proton-phosphorus coupling constants. These coupling constants were obtained by the full analysis of the 1H NMR spectra. The strategy adopted for the preparation of the phosphonic acids started from the synthesis of imine precursors. The imines were prepared by condensing aminodiphenylmethane or [alpha]-methylbenzylamine with a wide variety of aldehydes and ketones, using very mild reaction conditions. The imine precursors were then reacted with diethyl (or dimethyl) phosphite to give diethyl (or dimethyl) 1-substituted-l-(diphenylmethylamino)methanephosphonates, which were isolated and characterised. The phosphonate esters prepared from the reactions of imines derived from [alpha]-methylbenzylamine were not isolated. A series of the phosphonate esters [diethyl 1-phenyl-l-(diphenylmethylamino)methanephosphonate, diethyl l-(l’-naphthyl)-l-(diphenylmethylamino)methanephosphonate and diethyl l-(1’-pyrenyl)-l-(diphenylmethylamino)methanephosphonate] were selected for full 1H NMR spectral analysis. The effects of the aromatic ring currents exerted by the extended aromatic substituents were investigated. The complex aromatic regions of the 1H NMR spectra were not analysed. The X-ray crystal structures of selected diethyl 1-substituted-1- (diphenylmethylamino)methanephosphonates, where the substituents are: phenyl, 1’-naphthyl, 9'-anthryl, I'-pyrenyl, 2'-pyrrolyl and piperonyl, were determined and are discussed. The phosphonate esters were subjected to acid-hydrolysis to yield the [alpha]-aminophosphonic acids, including l-N-phenylethylamino-l-(2'-thienyl)phosphonic acid which was prepared from the imine derived from [alpha]-methylbenzylamine. Full 1H NMR spectral analysis was carried out on [alpha]-aminopropanephosphonic acid at three different pH values. The results of these analyses are discussed and compared with the results of the NMR spectral analysis of [alpha]-aminopropanephosphinic acid.
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