Phosphines, Phosphine Oxides and Sulfides

Trichlorosilane is a very powerful reducing agent. On its own or mixed with mildly basic amines or phosphines it leads to products with retention of configuration in the reduction of acyclic phosphines (entry 1) whereas with more basic amines it affords inverted phosphines (entries 2 and 3) with the exception of phosphetane oxides (entry 4). Entry 3 shows that the famous ligand DiPAMP was prepared for the first time using this reagent. Hexachlorodisilane has 

Other reductive systems have been explored in much lower intensity (Table 1.4). 

Lithium aluminium hydride is a very powerful reducing agent for phosphine oxides but unfortunately produces racemic products (entries 1 and 2) due to pseudorotation of the pentacoordinated species formed upon addition of LiAlH4 to phosphine oxides. Imamoto and co-workers (entry 3) had the idea to introduce an alkylating reagent, methyl triflate, to stabilise the phosphorus centre prior to reduction with LiAlH4. With this system, phosphines with clean inversion of configuration were obtained (entries 3 and 4). It is likely 

Many phosphine sulfides are known but their use in P-stereogenic chemistry has been rather limited until recently. They are easily prepared by reaction of phosphines with elemental sulfur, a step which proceeds with retention of configuration. Hexachlorodisilane and Raney nickel have been formd to be stereoselective reagents for desulfurisation of phosphine sulfides. Some examples of reductions are collected in Table 1.5. 

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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