Sulphoxides deoxygenation

Rajanikanth and Ravindranath44 have recently published a deoxygenation reaction for sulphoxides that uses metallic lithium in refluxing dimethoxyethane. Dialkyl and alkyl phenyl sulphoxides were reduced cleanly in yields around 70%, even if sterically hindered, but benzyl sulphoxides gave mixtures of products. For example, benzyl phenyl sulphoxide gave frans-stilbene (33%), benzyl phenyl sulphide (20%) and diphenyl disulphide (47%). These products can be rationalized by reaction pathways such as in equation (17) ... 

The deoxygenation of some penicillin and cephalosporin sulphoxides has been accomplished by the use of phosphorus pentasulphide.53 The reactive species responsible for deoxygenation was not identified. A particularly mild method for reducing sulphoxides to sulphides involves the reaction of the sulphoxide with 2-chloro-l,3,2-benzodioxaphosphole at room temperature.536 Yields in excess of 80% were reported. 

Selected examples of the deoxygenation of sulphoxides R SOR2 Added CHC1, % yield of sulphide... 

Nitrones and amine A-oxides are deoxygenated by a stoichiometric amount of benzyltriethylammonium tetrathiomolybdate to yield imines and amines, respectively (>80%) [7]. Nitro groups and sulphoxides are not reduced under these reaction conditions. 

Thiols and sulphides quench triplet carbonyl compounds. Evidence (including that from CIDNP studies) indicates that these reactions occur by a radical rather than an electron-transfer pathway (Cohen et al., 1979 Ver-meesch et al., 1978). It is interesting to note that sulphides will deoxygenate ketones producing sulphoxides, sulphones and presumably carbenes (Fox et al., 1979). Phosphines quench triplet carbonyl compounds (Davidson and Lambeth, 1969). They also deoxygenate carbonyl compounds to produce phosphine oxides and carbenes, and in this case, the reaction was proposed as occurring by an electron-transfer process (Fox, 1979). 

Deoxygenation of sulphoxides to sulphides has been achieved using sodium iodide/pyridine sulphur trioxide complexes in yields of excess of 80% (equation 14)188. 

In a study of the deoxygenation of sulphoxides it was found that the reaction is favoured by increasing the basicity of the tervalent phosphorus compound. With some phosphites a Michaelis-Arbuzov reaction was found to compete, e.g. in the reaction of the phosphite (27) with methyl... 

Tervalent phosphorus acid derivatives are normally liquids or low-melting solids which can be purified by distillation, or sometimes by recrystallization from a non-polar solvent. Most are oxidized in contact with the atmosphere, and many are easily hydrolyzed, so they must be kept under an inert atmosphere (N2 or Ar) during all manipulations. Flasks should be predried and solvents dried and deoxygenated before use. Tervalent phosphorus acid derivatives are, with few exceptions, thermally stable and can be kept indefinitely in ampoules under an inert gas (many halophosphines dissolve stopcock grease and should not be kept in stoppered flasks for prolonged periods). Inert solvents are hydrocarbons, ethers and, for most compounds, dichloromethane, ethyl acetate and tertiary amines. Aminophosphines react vigorously with tetrachloromethane and slowly with trichloromethane, and most tervalent phosphorus acid derivatives are oxidized by dimethyl sulphoxide and react with alcohols. 

Earlier, attention was drawn to a novel application of the oxidative desulphurization reaction which occurs between sulphoxides and phosphonothioic acid esters. When 1 mol of the thio acid is allowed to remain in contact with 2 mol of a racemic methyl alkyl sulphoxide for 2 weeks at room temperature, the unreacted sulphoxide is found to be optically active, a result which represents asymmetric deoxygenation of the sulphoxide. The chemical course of the reaction is consistent with Scheme 31. Since the appearance of the initial results, corrections have been made to the configurations of the phosphorus-containing substrates, and hence the stereochemical details and mechanistic arguments require reappraisal neverthless, the fact remains that (-t-)-sulphoxides are obtained by the use of the ( S)-(-)-acids, R (R O)P(S)OH , a feature which can be used to determine the chirality of other like acids. Dialkyl and alkyl aryl sulphoxides are also known to form diastereoisomeric complexes with (5)-(-)-/er butylphenylphosphinothioic acid, distinguishable by their NMR spectra, and through such complexation it is possible to assess the enantiomeric composition of the sulphoxide " ... 

Reagents for deoxygenating sulphoxides reported this year include... 

Deoxygenation of Sulphoxides, Selenoxides, and Telluroxides.— References and brief details from recent papers on this topic include ... 

Treatment of primary aromatic amines with Ph3PBr2 to produce the imino-phosphorane followed by addition of an alkyl iodide provides a convenient monoalkylation procedure.The reagent system BusP-CCU-KCN-lS-crown-6 effects a simple one-pot conversion of primary alcohols to nitriles. Finally, PI3 deoxygenates sulphoxides, selenoxides, aldehyde oximes, and nitro-alkanes, and deprotects acetals and ketals. ... 

The reactions of tertiary phosphines are frequently driven by formation of the thermo-dynamically-stable phosphoryl (P=0) bond. Deoxygenation reactions are common in phosphine chemistry many oxygen-containing compounds, for instance diacyl peroxides, peresters, alkyl hydroperoxides, ozonides, amine oxides and sulphoxides, react with phosphines ... 

New phosphorus-based deoxygenating agents include P2I4, which reduces epoxides to alkenes, besides dehydrating aldoximes to nitriles, and Ph3Pl2 and (Me2N)3P-l2-NaI which reduce sulphoxides to sulphides. ... 

C4H 03PSe M 217.063 Exists mainly as the OH-tautomer. Used for deoxygenation of sulphoxides. Free acid known in soln. only. 

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