Synthesis and characterisation of hindered organophosphorus compounds

Ph.D.The main objective of the research described in this dissertation was the preparation and characterisation of hindered organophosphorus compounds. For this purpose, ionic and free radical mechanisms were applied in the synthesis of selected hindered organophosphorus compounds, some with interesting spectral properties. A brief background of the element phosphorus and the early development of organophosphorus chemistry is provided. The early development of the chemical nerve agents derived from phosphorus, their toxicity and illicit manufacture by terrorist groups is discussed. The vital role played by the organisation for the prohibition of chemical weapons in enforcing the prohibition of the development, production, acquisition, stockpiling and the use of chemical nerve agents and their destruction by the state party is highlighted. The methodologies such as Michaelis Arbuzov, Michaelis Becker, Perkow Pudovik, Abramov and radical protocol reactions used to synthesise phosphorus containing compounds, are reviewed. In the present research project, diphenylphosphonous chloride and phenylphosphonous dichloride reagents were used in nucleophilic substitution reactions with bulky alcohols. This resulted in the synthesis and characterisation by 1H, 13C, 31P NMR and GCMS spectroscopy of the compounds shown in Scheme 0.1, where R’ was derived from 2,2dimethylpropanol, 3methylbutanol, 1,2dimethylpropanol, 3,3dimethyl2butanol, 1,1dimethylethanol and 2methyl2butanol. (R)xPCl(3x) R'OH Et3N (R)xP(OR')(3x) H2O2 or S8 (R)xP(OR)(3x) Y R=Aryl, and Y=O, S The importance of using activated alcohols in the form of metal alkoxides in the cases of 1,1dimethylethanol and 2methyl2butanol for successful reactions was demonstrated. The influence of steric hindrance on the reactivity of these ionic reactions was studied by substituting diphenylphosphonous chloride, or phenylphosphosphonous dichloride, with tertbutylphosphonous dichloride or ditertbutylphosphonous chloride. This resulted in no nucleophilic substitution reaction taking place between tertbutylphosphonous dichloride and hindered alcohols in the presence of triethylamine. For successful reactions, the use of excess activated hindered alcohols in the form of either the lithium or potassium alkoxide was required. It was found that replacing both of the chlorine atoms in tertbutylphosphonous dichloride with hindered alcohols like 1,1dimethylethanol and 2methyl2butanol was sluggish, and in the present study this could not be realised. It was demonstrated that ditert butylphosphonous chloride is resistant to react with activated hindered alcohols due to enhanced steric hindrance in the organophosphorus reagent. The use of freeradical mechanisms in the phosphoruscarbon (PC) bond forming reaction is briefly reviewed. The importance of finding a nontoxic replacement of organotin reagents in radical protocols is also highlighted. The scope of the present work was limited to the reaction of phosphonylcentered radicals generated by the triethylboraneoxygen system with various alkenes ranging from less electron rich to more electron rich alkenes, including those containing a free hydroxy moiety. The reaction of diphenyl thiophosphite or diphenyl phosphite (Scheme 0.2) in the presence of triethylborane under aerobic conditions with enol ether alkenes afforded the expected antiMarkovnikov products