Silylated phosphonium ylides

Isomerization of betaines 20k, 1 to silylated phosphorus ylides was also confirmed by their reaction with methanol-Ji, which occurs rather rapidly at room temperature to give Me2Si(OMe)2 and mixtures of a-deuterated phosphonium salts 72k, 1 (Scheme 33). It is most probable that, in this case, the reaction occurs mainly through the intermediate formation of a-silylated... 

In addition to the generation of phosphonium ylides from phosphonium salts by deprotonation with bases in some instances ylides may result from pyrolysis of phosphonium salts, especially silylated salts (equation 17). Similar fluoride ion induced desilylation (equation 18) of phosphonium salts proved to be a very useful alternative for the synthesis of ylides which are difficult to synthesize by the conventional salt method (as in the case of R, R = alkyl). - The most effective fluoride source is cesium fluoride and the reaction proceeds at room temperature. 

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

Some ylides 92 among them C-silylated ones have been synthesized in order to compare the stabilization influence on the carbanionic center of various C-substituents (I, SiMej, Ph3P ) [ 113]. It appears from this study that the stabilization due to the electron-withdrawing C-substituents (R or R ) is not so negligible by comparison with the hyperconjugative stabilization between the ylidic carbon and the phosphonium group [114]. This is particularly true for the iodine substituent. 

Silyl migrations readily occur in silylated ylides to give the ylides of optimum stability. Thus, deprotonation of the salts (21) and (23) gave the ylides (22) and (24), respectively. Intermolecular silyl transfers, from one ylide (or the corresponding phosphonium salt) to another, also lead to maximum stabilization. Silyl transfer does not occur in the product (26) from methylenetrimethylphosphorane and the chlorodisilane (25), pre-... 

The X-ray diffraction data for this compound are presented in Section 3. Betaines containing a hydrogen atom in the a-position to the phosphonium center and capable of reversible isomerization to silylated ylides are alkylated by ethyl bromide in a different ways. This reaction resulting in a complex mixture of products is considered below in Section 5.4. 

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... 

When activated by anionic catalysts [potassium fluoride, cesium fluoride, tctraalkylammonium fluorides, tris(dimethylamino)sulfonium difluorotrimethylsilicate, phosphazenium, hexa-methylpiperidinium and cobaltoccnium fluorides, tetrabuiylammonium difluoro(triphenyl-silyl)silicatc. the complex tetrakis(dimethylamino)ethene/pcrfluoropropene, ammonium (and phosphonium) periluorocyclobuianc ylides], trimethyl(perfluoroalkyl)silanes will generate C-Rp bonds from carbon-halogen bonds. 

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... 

Treatment of (2-trimethylsUylphenyl-)methyl phosphonium salts with NaHMDS provided the corresponding ylide that underwent anionic 1,4-silyl migration to form the respective phenyl trimethylsUyl ylides that on reaction with methyl iodide gave phenyl-(l-trimethylsUylethyl)phosphonium iodides (Scheme 74). ... 

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