Oxidation of Phosphines to Phosphine Oxides

CATALYTIC OXIDATION OF PHOSPHINES TO PHOSPHINE OXIDES 8.1. Complexes of Ni, Pd and Pt as catalysts 

A large number of transition metal complexes have been found to catalyze the oxidation of organic phosphines to phosphine oxides. Wilke, Schott and Heimbach [85] showed that complexes of the type [M(PPh3)4] catalyze the oxidation of triphenylphosphine to triphenylphosphine oxide in toluene solution, equation (59). 

When M is Ni reaction occurs at temperatures above — 35 °C whereas the corresponding Pd and Pt complexes show catalytic activity when warmed at temperatures above 90 °C. Using [Ni(PPh3)4], about 50 moles of phosphine were oxidized per mole of nickel complex while for each mole of the palladium complex used, over 500 moles of triphenylphosphine were converted to triphenylphosphine oxide. 

The oxidation of triphenylphosphine catalyzed by [Pt(PPh3)4] has been extensively studied [87,88,117,118] by Halpem and co-workers who have elucidated many aspects of the reaction mechanism. By the use of optical absorption spectroscopy it has been shown that the predominant species present in a benzene solution of [Pt(PPh3)4] is [Pt(PPh3)3], equation (60). 

Subsequent dissociation to [Pt(PPh3)2] and triphenylphosphine or recombination with triphenylphosphine occur to only a minor extent over a rather wide range of phosphine concentrations. 

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The dioxygen adduct so formed is highly reactive and will catalyse the oxidation of phosphine to phosphine oxide and isocyanide to isocyanate (77). But clearly a survey of these reactions is not within the scope of this review and is provided elsewhere (7, 77). 

NR = nonreactive toward hydrocarbons PO = oxidation of phosphines to phosphine oxides MF — peroxometallacyclic adduct formation with cyanoalkenes NSE — nonstereoselective epoxidation SE=stereoselective epoxidation AE = asymmetric epoxidation HA- hydroxylation of alkanes HB=hydroxylation of arenes OA = oxidation of alcohols to carbonyl compounds K = ketonization of Lermina 1 alkenes SO oxidation of S02 to coordinated S04 MO = metallaozonide formation with carbonyl compounds I = oxidation of isocyanides to isocyanates. 

A large number of Group VIII metal-dioxygen complexes catalyze the oxidation of phosphine to phosphine oxide or isocyanides to isocyanates by molecular oxygen.6,12,56,140,141 146,184 However, their use as catalysts for the oxidation of alkenes generally leads to the same products as those obtained from free radical chain autoxidations.184,196-198... 

If the Rh-catalyst is stabilized by a monodentate phosphane ligand, it can be destroyed because the lifetime of the oxidation of phosphine to phosphine oxide is less than 1 h. Yet, if the catalyst is stabilized by a bidentate ligand, and the catalyst has a lifetime of 2-3 h, then it can convert into different species that have no catalytic activity. However, such a synthetic approach would suffer as a result of a short lifetime and a low TON. 

NR s not reactive toward hydrocarbons S = stereoselective epoxidation E = epoxidation HA = hydroxylation of alkanes OA = oxidation of alcohols to carbonyl compounds PO oxidation of phosphines to phosphine oxides OC = oxidative cleavage of alkenes K= ketonization of alkenes DO = hydroxylation of alkenes to diols AO al1ylic oxidation of alkenes. 

Both sulphur trioxide and sulphuryl chloride fluoride oxidize phosphines (and triphenylarsine) to the corresponding oxides in excellent yields under mild conditions. The effect of various iron compounds as catalysts for the oxidation of phosphine to phosphine oxide have been investigated triphenylphosphine oxide is formed quantitatively at 30—70 °C in oxygen at one atmosphere. 

Although this system appears to possess all of the attributes necessary for the catalytic oxidation of phosphine to phosphine oxide, the catalytic activity of these cobalt complexes was not reported. 

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