Phosphonium salts lithium aluminum hydride

The reduction of optically active phosphonium salts by lithium aluminum hydride, which probably does involve 70 as an intermediate, affords racemic phosphines, presumably by pseudorotation in 70 before it decomposes 63). 

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... 

An additional approach toward the preparation of tertiary phosphines is by the reduction of more highly coordinated phosphorus species, particularly phosphine oxides [0=PR3] and phosphine sulfides [S=PR3] (see Section 5.2), but also phosphonium salts [ILtP+X"] and quasiphosphonium salts [R3P-YR +X ] (see Sections 4.2 and 4.4). Numerous reducing agents have been used to accomplish these conversions, including hexachlorodisilane [CbSi-SiCft], trichlorosilane [HSiCft], phenylsilane [PhSiHs], and lithium aluminum hydride [LiAlH4]. 

The second key building block, phosphonium salt 172, was prepared as outlined in Scheme 3.39. Iodide 163 was reacted in a copper(I)-catalyzed alkylation with the bisbromo-magnesium salt 164 to give triyne 165 in 90% yield. Reduction of the propargylic alcohol 165 to allylic alcohol 166 was achieved with lithium aluminum hydride (LAH), and then a... 

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