Phosphine oxides protonation

A method for determining the basicities of phosphoryl compounds has been decribed which is based on 31P n.m.r. chemical shift measurement and a two phase system consisting of an organic solvent and 12H sulphuric acid.245 The gas-phase protonation of aliphatic phosphine oxides have been determined by cyclotron resonance. There was a good correlation of pK4 with Kabachnik constants and HO... 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

Likewise, a systematic use of such anisotropy effects has been used to establish the conformation of small rings from the proton shielding in the corresponding allenic phosphine oxides,<1967,153tol56 1968,46 1969 164 to 166) for example in 170. 

FIGURE 14. Intramolecular hydrogen bonds the carbonyl, sulfoxide and phosphine oxide group as acceptor for the hydroperoxide proton ... 

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

In media of low proton availability (MeCN, dmf, hmpa)762, ylide formation was found and products derived from the radical cleavage of the phosphonium ion have been observed. The initial cation would interfere in the reaction process as an acid. A competition can exist between the one-electron pathway (dimerization, disproportionation of R ) and the two-electron pathway (ylide formation, Hofmann degradation, phosphine oxide formation) (Table 24). 

A striking difference to pyridine is the lack of basicity and nucleophilicity for phosphabenzene and arsabenzene, which are protonated at the a- and y-carbon atoms (this reaction is responsible for deuteration). Neither alkyl halides nor trialkyloxo-nium salts can alkylate phosphabenzene, therefore there will be no discussion of quaternary salts for these pnictogena-hetarenes. Whereas pyridine can be oxidized to the zwitterionic N-oxide, phosphabenzene affords non-aromatic -oxidation products 35 and 36 with tetracoordinated P(V) phosphorus atoms, similar to phosphin-oxides and phosphonic acids, respectively. 

Fig. 11.10. Wittig-Horner synthesis of alkenes with stereogenic C=C double bond via condensation of lithiated phosphine oxides B <-> B with aldehydes. In the two-step reaction the initially formed addition products syn- and anti-D are protonated and the resulting alcohols syn- and anti-C are isolated and separated the suitable diastere-omer is then stereoselectively converted into the desired alkene isomer. The one-step reaction leads to the same alkene in a cis,trans mixture.

H-CIDNP and ESR techniques. A laser flash photolysis and time-resolved ESR study of the formation of phosphinoyl radicals from benzoyldiphenyl-phosphine oxide and 257 has appeared. The addition of dialkyIphosphoryl radicals to a fullerene system has also been studied. Interest in adducts of phosphine oxides with proton donors, notably phenols and other solvent guests ° has continued. 

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