Dialkyl phosphites hypophosphoric

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

The P-H bond is also weak enough to serve in thione-based radical chemistry. The first success was achieved with dialkyl phosphites [40a]. However, hypophosphorous acid and its salts proved to be even better [40b]. In a final paper [40c], the various phosphorus-based reagents were compared. The crystalline salt N-ethylpiperidine hypophosphite was very convenient and has been commercialized 140c]. The use of hypophosphorous acid has the advantages of nontoxicity, cheapness, and ease of removal from the organic reduction products. It already has several industrial applications. 

Barton, D. H. R., Jang, D. O. and Jaszberenyi, J. C. (1993) The invention of radical reactions. Part 32. Radical deoxygenations, dehalogenations, and deaminations with dialkyl phosphites and hypophosphorous acid as hydrogen sources. J. Org. Chem., 58, 6838-6842. 

Takamatsu, S., Katayama, S., Hirose, N., et al. (2001) Radical deoxygenation and dehalogena-tion of nucleoside derivatives with hypophosphorous acid and dialkyl phosphites. Tetrahedron Lett., 42, 7605-7608. 

Esters of pyrophosphorous add can be prepared by using sodium dialkyl phosphite and a dialkyl phosphorochloridite (5.321). If a phosphorochloridate ester is used, an ester of hypophosphoric acid is obtained (5.322). 

Pudovik, A.N., Moshkina, T.M., and Konovalova, I.V., New method of synthesis of esters of phosphonic and thiophosphonic acids. Part 31. Addition of phosphorous and hypophosphorous acids, dialkyl hydrogen phosphites, and esters of phosphonoacetic acid to esters of maleic acid, Zh. Obshch. Khim., 29, 3338, 1959 Chem. Abstr, 54, 152231, 1960. 

Oxidation of hypophosphorous acid with carbon tetrachloride and triethylamine at 90 °C in the presence of an alcohol gives dialkyl phosphates (Scheme 2). Monoalkyl phosphites can be converted into... 

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