Photochemical reactions of phosphinic azides : rearrangements and nitrene intermediates

• Main Author: Westlake, Sally
• Published: University of Leicester 1981
• Subjects: 541; Physical chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.291499

The photolysis of diarylphosphinic, alkylphenylphosphinic, and dialkylphosphinic azides in a variety of solvents was studied. Photolysis took place using a low pressure mercury discharge lamp fitted into a specially designed reaction vessel at 254nm. Some small scale reactions were performed using a Rayonet reactor. Photolysis in methanol gave products resulting from the trapping, by the solvent, of monomeric metaphosphonamidates, formed by the loss of nitrogen and migration of one of the phosphorus substituents to the site of loss. Products resulting from nitrene intermediates were also observed. Photolysis reactions were also performed using benzene, dimethylsulphoxide, and dimethylsulphide as the solvent in an attempt to trap any nitrene intermediates produced. Benzene proved to be very unsatisfactory as a nitrene trap for these particular azides so it was used mainly as an inert diluent for the other solvents. Diphenylphosphinic and di-t-butylphosphinic azides were photolysed in dime thylsulphoxide and mixtures of dimethylsulphoxide, methanol, and benzene. In both cases the phosphinic and phosphonic acids were observed as products. The products were thought to be due to the attack of the oxygen of dimethylsulphoxide on the azide in the former case and attack on the metaphosphonamidate in the latter. Photolysis reactions in dimethylsulphide gave products resulting from the attack of the sulphur lone pair on the s-nitrene intermediates and from t-nitrene abstraction of a hydrogen from one of the dimethylsulphide methyl groups followed by combination of the two radicals produced. In mixtures of dimethylsulphide and methanol products due to rearrangement were also observed. The solvolytic reactions of the phosphinic azides with methanol were studied. In some of the photolysis reactions solvolysis was shown to be a serious competing reaction. A total of nine azides were synthesised of which five were new compounds. Many of the products found in the photolysis reaction mixtures were also new compounds and authentic samples of most of these compounds were synthesised in order to assist their identification and characterization. Trimethylammonium-N-diphenylphosphinylimine (ylid) was synthesized and photolysed under similar conditions to the phosphinic azides. This ylid is believed to be a nitrene precursor and its reactions were studied in order to compare the resulting products with those obtained from phosphinic azides. The ylid did not give products of rearrangement under any conditions.