Phosphonium salts using hydrides

The overall sequence of three steps may be called the Wittig reaction, or only the final step. Phosphonium salts are also prepared by addition of phosphines to Michael alkenes (hke 15-8) and in other ways. The phosphonium salts are most often converted to the ylids by treatment with a strong base such as butyllithium, sodium amide, sodium hydride, or a sodium alkoxide, though weaker bases can be used if... 

While the mechanism in the absence of Eti or HI is still a matter of conjecture, it is unlikely that a hydride mechanism was operable since, whereas we could possibly envision an imidazolium salt donating a hydrogen via carbene formation, there is no corresponding viable source of hydride when using pyridinium and phosphonium salts which are also effective solvents for the process. Therefore, by process of elimination, it was more likely that the process was operating via a nucleophilic process. 

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]. 

A great variety of bases has been used to generate ylides from the corresponding phosphonium salts various nitrogen bases, alkoxides, alkali metal hydrides, carbanionic bases, alkali metal hydroxides and carbonates, ethylene oxides, basic ylides and others. ... 

Other carbon bases that have been used successfully to convert phosphonium salts into ylides include sodium methylsulfinate and the corresponding potassium compound (prepared from alkali metal hydride and DMSO), tritylsodium, sodium acetylide and other strongly basic ylides. 

The reaction of all-/ra/i5-/ -ionylideneacetaldehyde with the phosphonium salt (79), using sodium hydride as base, gave a mixture of furanones as shown. ... 

Phosphonium ylides (alkyhdene phosphoranes) can be prepared by a number of methods, but in practice they are usually obtained by action of a base on (alkyl)triphenylphosphonium salts, which are themselves readily available from an alkyl halide and triphenylphosphine. The phosphonium salt can usually be isolated and crystallized, but the phosphonium ylide is generally prepared in solution and used without isolation. Formation of the phosphonium ylide is reversible, and the reaction conditions and the strength of the base required depend entirely on the nature of the ylide. A common procedure is to add a stoichiometric amount of a solution of n-butyllithium to a solution or suspension of the phosphonium salt in ether or THF, followed, after an appropriate interval, by the carbonyl compound. Other bases, such as sodium hydride or sodium or potassium alkoxides, in solution in the corresponding alcohol or in dimethylformamide, are used commonly. 

The other exception to the general mechanism occurs when an a, p-unsaturated acid chloride is used as the substrate. The presence of a jS-hydrogen would allow the possibility of jS-hydride elimination to occur, but the product would have a terminal carbon-carbon triple bond, which has not been observed previously. Apparently, reductive elimination of R-Cl is also too slow, and as a result phosphonium salts are the major product (Equation 10) ... 

For the second part of the side-chain, hexanal is first extended by three carbon atoms via a Wittig reaction. The acetal is hydrolysed and then, in a sequence of several reaction steps converted into a phosphonium salt, which is used in another, so-called Li-salt-lfee , Wittig reaction, leading to the expected high (Z)-selectivity. Finally, the ester is reduced to the corresponding aldehyde with diisobutylaluminium hydride the overall yield ofthe (Z,Z)-dienaldehyde, based on hexanal, amormts to 50 %. 

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