Phenylsilane/cesium fluoride

Wacker-Tsuji oxidation of olefins (eq 23) and Pd-mediated homocoupling of aryl boronic acids (eq 24) were completed on the interior of PDMS thimbles. In each of these reactions, the catalyst was PdCl2 and a polar protic solvent was used on the interior of the thimbles. The products were fiuxed to the exterior of the thimbles by the addition of CH2CI2 or hexanes to the exterior. Isolated yields were 56-93%. 

In each of these reactions, the Pd was site isolated on the interior of the thimbles. To characterize the concentration of Pd in the products, the reactions were run to completion and the product was extracted to the exterior. The solvent on the exterior of the thimble was isolated and removed. The concentration of Pd in the product was found by ICP-MS. Between 0.225 and 0.002% of the Pd that was added to the interior of the thimble was found in the products on the exterior of the thimbles. In other words, 99.998% and no less than 99.775% of the Pd remained on the interior of the thimbles even after the product was extracted to the exterior. 

The reason for the low flux of PdCl2 was that it was soluble only in polar protic solvents. Its solubility in PDMS was negligible, so it did not flux through the walls of the thimbles. When phosphines were added to the reaction mixture to coordinate to PdC and render it less polar, the phosphines and Pd readily fluxed through the walls of the PDMS thimbles. This method demonstrates that catalysts that do not partition into PDMS will remain site isolated by it. 

Final Notes. Small molecules, organometallic catalysts, inorganic catalysts, and polymeric catalysts were all successfully site isolated at levels up to 99.998% using PDMS thimbles. When site isolating catalysts or reagents, the most important parameters to consider are the solubilities and rates of diffusion within the swollen PDMS matrix and the solubilities in the organic solvents on the interior and exterior of thimbles. 

The Mathematics ofDiffitsion Clarendon Press Oxford, 1970. 

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Related Reagents. (2-Dimethylaminomethylphenyl)phenyl-silane diphenylsilane-cesium fluoride phenylsilane-cesium fluoride triethylsilane triphenylsilane. 

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

Azomethine ylides. Reduction of oxazolium salts (1) with phenylsilane and cesium fluoride provides an unstable 4-oxazoline (2), which can react as an azomethine ylide with a dipolarophile such as DM AD to give a dihydropyrrole (3). ... 

The oxazolium salt (160), formed from the oxazole (159) by reaction with methyl triflate in acetonitrile, may be reductively ring opened by treatment with phenylsilane in the presence of cesium fluoride to give the azomethine ylide (161) (presumably this species is in tautomeric equilibrium with the corresponding oxazoline). The azomethine ylide can be trapped as an adduct with a suitable dipolarophile, such as dimethyl acetylenedicarboxylate (DMAD). In the case of this reagent the adduct (162) can be ox-... 

The authors found that phenylsilane/anhydrous cesium fluoride was the preferred reagent combination for this reaction sequence. Other silanes were effective, whereas tributyltin hydride afforded low yields (< 20%) and only in the presence of a fluoride source. Tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) and tetrabutylammonium fluoride (TBAF) destroyed the silane. Acetonitrile was the best solvent among the ethers and halogenated solvents evaluated." In a subsequent study " the authors converted a 2,5-disubstimted oxazole 15 (Rl = R3 = CH3, CeHs, or OC2H5, R2 = H) to the oxazolium salt followed by treatment with trimethylsilylcyanide (Scheme 1.290). The resulting intermediate 2-cyano-4-oxazoline 1107 spontaneously ring opened to the stabilized azomethine... 

Physical Data phenylsilane bp 120 °C d0.877gcm . Cesium fluoride mp 683 °C. 

Solubility phenylsilane sol THF, MeCN. Cesium fluoride sol H2O slightly sol THF, MeCN. 

Form Supplied in phenylsilane is a clear liquid cesium fluoride is a white solid both widely available. 

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