Phosphonium salts, silyl-substituted

Phosphoniosilylotion. This combination reacts with acyclic or cyclic enones to give phosphonium salts, formed by addition of P(C6H,)3 to the p-position of the enone and silylation of the carbonyl group. The products can be converted into p-substituted enones by deprotonation (BuLi), a Wittig reaction, and hydrolysis. 

The trimethylsilylated ylides (1), easily generated from trimethyl chlorosilane and ylides, react with aldehydes 2 to form vi-nylsilanes 3 (2,3). The vinylphosphonium silanolates 4 are also formed. Compounds 3 are versatile reagents for further reactions (4). The ylide J (with R1 =H) reacts with aldehydes 2 to give the dienes j). The oxidation of with the adduct 6, from triphenyl-phosphite and ozone, gives access to a generaT synthesis of acyl-silanes (trimethylsilylketones) (2). The silylated ylides react to form phosphonium salts 7 with halogen compounds. The salts 7.can be desilylated by fluorine ions. The disubstituted ylides JO Tormed can be converted in statu nascendi with aldehydes V[ into the tris-substituted olefin J2 (2,3). In the case of R3-I, vinyl... 

Reactions. - The phosphonium salts (287) have been shown to undergo facile nucleophilic displacement of triphenylphosphine on treatment with a range of C, N and S-nucleophiles, giving a range of functional P-substituted silyl enol ethers. The 4-phosphonio-oxazolones (288) undergo dephospho-... 

Silyl-substituted phosphonium salts can only be stabilized by non-halogen substituted counter ions, e.g. Co(CO)4 (Scheme 6). Thus iodoethane gives the phosphonium salt from Me3SiPEt2, while condensing 1,4-dichlorobutane with the phosphacyclohexane 1 gives the spirophosphonium salt 2 (equation 4)10. The addition of Me3SiPH2 to vinyl phosphines has been used as a synthetic route to substituted phosphacycloalkanes, notably 3 (equation 5)11. 

However, a new route using silylated ylides (Scheme 2) provides pure compounds and hence trialkyl substituted alkenes. Ylides have been generated from the corresponding phosphonium salts by electrochemical reduction of fluoren-9-ylidenemethanes (15) to the dianions, which then act as bases. Generally the... 

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