Reduction of phosphine oxides

An oxygen transfer from HMPA-O to silicon, which is known from the reduction of phosphine oxides with chlorosilanes, is not observed. On the contrary, the liberated HMPA can be recovered quantitatively (eq. 17, 18). 

Fig. 3. Proposed mechanism for stereoselective reduction of phosphine oxides...

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... 

Reduction of phosphine oxides under mild conditions is also... 

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... 

However, a very common strategy in phosphine synthesis continues to be a final stage reduction of phosphine oxides with silane reagents, of which trichlorosilane is the most popular. Among a considerable range of new monophosphines obtained by trichlorosilane reduction, usually in the final step of the synthesis, are the phosphorus core conjugated triaryl-dendrimer unit... 

A detailed account of the reduction of phosphine oxides by phenylsilane (25) has been published. Tim reagent is commercially available, gives 85—96%... 

Reduction of phosphine oxides. Chemoselective reduction of phosphine oxides in the presence of sulfoxides are observed with AlH, in refluxing THE... 

Diaryl ketones are directly reduced to hydrocarbons with EtiSiH-BFi OEt2. In the reduction of phosphine oxides" (i-PrO)4Ti is used as the Lewis acid catalyst. 

Preparation of Phosphines by Reduction. - The reduction of phosphine oxides in the final stage of phosphine synthesis remains a common strategy, with silane reagents often being used. Trichlorosilane has found application in the synthesis of the axially chiral systems (83), 73 (84), 7 (85), 75 the chiral... 

A few papers have reported the use of various aluminium-based reducing agents in phosphine synthesis. Lithium aluminium hydride reduction of phosphine oxide precursors provides a route to new 2-phosphinomethyl-1/7-pyrroles and diisobutylaluminium hydride (DIBAL) has been found to be an excellent reagent for the reduction of phosphinites, phosphinates and chlorophosphines. 2-Chloroethylphosphine (CICH2CH2PH2) has been prepared for the first time by a chemoselective reduction of diethyl 2-chloroethylphosphonate with dichloroalane (HAICI2), prepared in situ from LiAlH4 and AlCls. A patent has described the use of metallic aluminium... 

Those compounds were synthesised by alkaline hydrolysis or cathodic reduction of phosphonium salts or by reduction of phosphine oxides with silanes. In both cases, the optically pure precursor had to be prepared by chemical resolution (see Chapter 2, Section 2.2), a step that is time-consuming, inefficient and very dependent on the structure of the compound being resolved. 

Table 1.3 Selected examples of successful reduction of phosphine oxides with silanes.

Table 1.4 Examples of reduction of phosphine oxides with non-silane based reagents.

The stringent conditions required for the reduction of phosphine oxides are incompatible with many functional groups and often cause severe racemisation of the phosphine. Those problems have been largely circumvented by switching to phosphine boranes, as discussed in the next section. 

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