Phosphonium ylids, oxidation

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. 

Several aromatic acyl silanes and the fascinating but unstable pink carbonyl bis(trimethylsilane) have been prepared in reasonably good yields by the oxidation of phosphonium ylids (Scheme 17)13. 

There are many examples of Wittig-type reactions in the literature, in which the behavior of phosphonium ylids, phosphine oxide ylids, and phosphonate ylids is significantly different. 

Phosphonium ylids undergo a most important reaction with aldehydes and ketones, known as the Wittig reaction (3.111). The products of this reaction are olefinic compounds and phosphine oxides. 

Under the action of water, most phosphonium ylids wiU undergo hydrolysis to hydrocarbon and phosphine oxide in the same way as phosphonium salts (6.102, 6.371), which are most certainly formed as intermediates. 

The monomer Cl3P=NH is a member of the class of compounds with general formulae R3P=NR, known variously as monophosphazenes, phosphine imides, phosphinimines or iminophosphoranes. They are isoelectronic with phosphonium ylids and with phosphine oxides, all of which can be represented by the alternative ionic formulae which can be compared with phosphoranylidene phos-phoranes (6.454) and (7.440). 

Monophosphazenes are more acidic than phosphonium ylids. Amino triphenylphosphonium bromide is converted by triethylamine (7.475), but the corresponding phosphonium ylid requires metal alkyls (7.476). Phosphine oxides can be obtained from monophosphazenes by the action of carbon dioxide, sulphur dioxide, alkyl isocyanates or ketones ((7.477) through (7.480)). 

Over twenty years ago, Wittig found that alkyl substituted phosphonium salts could be deprotonated by strong bases to yield phosphonium ylids which in turn could react with aldehydes and ketones to yield olefins and the corresponding phosphine oxide [1]. The sequence is formulated in equations 1.1-1.3 for the reaction of methyl-triphenylphosphonium halide with cyclohexanone in the presence of base to give me thylenecyclohexane. 

Durch Einelektronen-Oxidation werden aus Phosphonium-Salzen die entsprechenden Ylide erhalten, die mit Ketonen in einer Wittig-Reaktion zu Olefinen (bis zu 95% Aus-beute) umgesetzt werden konnen6. Zuweilen treten neben dem Ylid auch die Spaltpro-dukte auf ... 

Alkyl phosphonium halides are decomposed by strong heating to give tertiary phosphines (6.32), whereas phosphonium hydroxides easily give phosphine oxide and hydrocarbon (6.109). The action of heat on triphenyl ((-alkoxycarbonyl alkyl) phosphonium salts is to produce ylids (6.391) (Section 6.10). 

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