Preparation, of phosphine oxides

Copley et al.91 characterized OPPh3 H20 as (OPPh3)2 H2Os and it is not clear in subsequent work that this or its implications have been appreciated. A new mild and promising method using S03 or S02 FC1 as the oxidizing agent for the preparation of phosphine oxides has been reported by Olah et al.9i... 

The synthesis of chiral phosphines from resolved phosphonium salts or phosphine oxides is an intrinsically limited approach. The groups attached to phosphorus must be present prior to resolution and, furthermore, the preparation of phosphine oxides and phosphines from phosphonium salts by chemical or electrochemical cleavage reactions requires that one of the groups bonded to phosphorus be substantially easier to cleave than the other three. 

A direct approach for the preparation of phosphine oxides in reasonable yield involves the treatment of elemental phosphorus (either white or red) with primary haloalkanes in a basic (KOH) medium of water/dioxane with the presence of a phase-transfer catalyst. ... 

Details have appeared of the preparation of phosphine oxides from phosphonic or phosphinic esters using hydride sources and alkyl halides. The method is claimed to be convenient and give good yields, and is illustrated by the preparation of benzyldibutylphosphine oxide (5). 

Two key breakthroughs occurred in 1967-1968. The first one was the development of a new, relatively flexible route for the preparation of phosphine oxides, based on the separation of asymmetrically substituted menthylpho-sphinates. This method, described in Chapter 2, allowed the preparation of... 

SCHEME 4.202 Preparation of phosphine oxides in water using nickel catalysts [338]. 

Most common is the preparation of alkyl phosphonic acid esters (phospho-nates) 4 (Z,Z = OR) from phosphorous acid esters (phosphites) 1 (Z,Z = OR). The preparation of phosphinic acid esters (Z = R, Z = OR) from phosphonous acid esters, as well as phosphine oxides (Z,Z = R) from phosphinous acid esters is also possible. 

Recently, Fossum et al. prepared several phosphine-oxide-containing monomers (Scheme 6.23).163 These monomers were used to prepare hyperbranched polymers in a typical aromatic nucleophilic substitution. However, only oligomers with M lower than 2500 g/mol were obtained. These results did not surprise us, since our previous work demonstrated that the para-hydroxyl group of the phosphonyl group is not very reactive and would require higher reaction temperatures.11... 

This year s literature has been characterized by an increasing number of papers devoted to theoretical studies of the bonding in phosphine oxides and related compounds, and these are discussed in Section 1. The chemical aspects of phosphine oxides have not shown any major new developments over the past year, and, once again, these have been sub-divided into sections on the preparation and on the reactions of phosphine oxides. 

B. By Hydrolysis Reactions.—Details have appeared of the synthesis of dibenzophosphorin oxides (15) from 5-alkyldibenzophospholes, by reaction with methyl propiolate in the presence of water, and of confirmatory syntheses from phosphinic acid chlorides, as shown below. Evidence for the suggested mechanism of the ring-expansion reaction is presented. The hydrolysis of enamine phosphine oxides is an efficient, although somewhat indirect, method for the preparation of j8-ketoalkylphosphine oxides (16) [see Section 3(iii), for the preparation of enamine oxides]. Reasonable yields (48—66%) of trialkylphosphine oxides (17) have been obtained by the alkaline hydrolysis of the products from the pyrolysis at 220 °C of red phosphorus with alkyl halides, in the presence of iodine. 

The anaerobic oxidation of phosphines to their oxides by hydroxide ion has been shown to involve the liberation of hydrogen, possibly from the intermediate (21). These oxidations were studied with water-soluble phosphines, since solubility was found to be the main factor controlling the rate of oxidation. The preparation, and detailed n.m.r. spectrum, of PP-dimethyl-P P -diphenyldiphosphine disulphide (22) is a relatively rare example of a study of a mixed disulphide. Many examples of routine oxidation of phosphines to their oxides have appeared. These include the preparation of polyhalogenoarylphosphine oxides using dichromate... 

Two contrasting conclusions have been reported in the reactions of lithium aluminium hydride in THF with phosphine oxides and phosphine sulphides respectively. The secondary oxide, phenyl-a-phenylethylphos-phine oxide (42), has been found to be racemized very rapidly by lithium aluminium hydride, and this observation casts some doubt on earlier reports of the preparation of optically active secondary oxides by reduction of menthyl phosphinates with this reagent. A similar study of the treatment of (/ )-(+ )-methyl-n-propylphenylphosphine sulphide (43) with lithium aluminium hydride has revealed no racemization. These results have been rationalized on the basis of the preferred site of attack of hydride on the complexed intermediate (44), which, in the case of phosphine oxides (X = O), is at phosphorus, and in the case of the sulphides (X = S), is at sulphur. Such behaviour is comparable to that observed during the reduction of phosphine oxides and sulphides with hexachlorodisilane. ... 

Preparation of Phosphines by Reduction.- A procedure for the reduction of phosphine oxides using trichlorosilane in inert... 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Furthermore, even the ligand, necessary to stabilize the catalyst, can reduce Pd(II) to Pd(0) complexes and formation of phosphine oxides [62-64], In the preparation of [Pd(AcO)2(dppp)], from Pd(AcO)2 and dppp in MeOH, phosphine oxides have been found to form together with methyl acetate and palladium metal [65]. The reaction can be schematized as follows ... 

Hydroxides and Acids— Insoluble Substances"— Indicators—Preparation of Basic Oxides—Properties of the Basic Oxides and Hydroxides— Sulphides— The Solubility-Product — Basic Oxides and Hydroxides of Complex Groups Alcohols, Aldehydes, Ethers and Sulphines, Amines and Phosphines. 

In a paper edited in 1953, concerned with the preparation of the stereoisomeric forms of pentaphenylphosphorus, Wittig and GeiBler described the reaction of methylene-triphenylphosphorane 1 and benzophenone 2, forming 1,1-diphenylethylene 3 and triphenylphosphine oxide 4 (Scheme 1). Soon afterwards, it could be demonstrated that alkylidenephosphoranes (phosphine alkylenes, phosphorus ylides) generally react with carbonyl compounds such as aldehydes and ketones to give alkenes with the formation of phosphine oxide 1,2). 

Preparation of alkyldiphenylphosphine oxides. General procedure from phospho-nium salts. Triphenyl phosphine is heated under reflux with an excess of alkyl halide. The precipitated phosphonium salt is filtered off, washed well with ether, and then heated with 30 per cent w/w aqueous sodium hydroxide (c. 4 ml/g) until all the benzene has distilled out. The mixture is cooled and extracted with dichloromethane, and the extracts are dried (magnesium sulphate) and evaporated to dryness. In this way ethyldiphenylphosphine oxide is obtained from triphenyl phosphine (65.6 g, 0.25 mol) and iodoethane (42.9 g, 0.275 mol) in dry toluene (250 ml) to give first the phosphonium salt (102.4 g, 97.9%) after 3.5 hours, from which the phosphine oxide is obtained as needles (53.2 g, 92.5%), m.p. 123-124 °C (from ethyl acetate) p.m.r. 5 (CDC13, TMS) 1.9-13 (m, 10H, Ph2PO), 2.3 (m, 2H, CH2) and 1.2 (dt, 3H, JHm, = 7 Hz, JMeP = 17 Hz, Me). 

Horner-Wittig modification Alternatively, phosphine oxide reacts with aldehydes in the presence of a base (sodium amide, sodium hydride or potassium t-butoxide) to give an alkene. The phosphine oxide can be prepared by the thermal decomposition of alkyl-triphenylphosphonium hydroxide. Deprotonation of phosphine oxide with a base followed by addition to aldehyde yields salt of (3-hydroxy phosphineoxide, which undergoes further syn-elimination of the anion Ph2P02. The lithium salt of (3-hydroxy phosphineoxide can be isolated, but Na and K salt of (3-hydroxy phosphine oxide undergoes in situ elimination to give alkene (Scheme 4.26). 

Tertiary phosphine oxides are also produced as significant by-products in several of the reactions of phosphines that have been noted previously, including the Wittig olefination and the conversions of alcohols to haloalkanes with triphenylphosphine as an adjunct reagent. The tertiary phosphine oxides produced in such reactions present a problem in chemical economics, as they themselves possess little chemical utility. The phosphine may be regenerated, but several steps are required, as previously noted with preparations of phosphines by reduction (see Section 3.2). 

FIG. 21-3. The preparation of phosphine, PH3. When a bubble of gaseous phosphine strikes the air it ignites spontaneously, and usually forms a smoke ring, consisting of phosphorus oxides. 

Keglevich et al. have reported a series of papers on the mechanistic aspects of what they term inverse Wittig reactions , i.e. the preparation of phosphoranes from the [2-f2] cycloadditions of phosphine oxides and acetylenedicar-boxylates, an example of which is given in Scheme 1. A raft of spectroscopic and structural evidence, coupled with theoretical calaculations, indicate that these reactions proceed via oxaphosphetane intermediates (16). ... 

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