Reductions phosphine oxides, silylations

On the synthetic side, single diastereomers of P-keto phosphine oxides have been generated from intermolecular acylation of phosphine oxides using either chiral esters or chiral phosphine oxides. In most cases, reduction of the ketone products was not affected by the presence of extra chiral centres. Addition of metallated phosphine oxides to proline-derived ketoaminals provides a new route to optically active P-hydroxy phosphine oxides. The P-hydroxy phosphine oxide 97 has been prepared by the caesium fluoride mediated reaction of silyl-substituted phosphine oxide 98 and benzaldehyde." The synthesis of two (E)-(6-hydroxy-2-hexen-l-yl)diphenylphosphine oxides (99) has been reported. The Horner-Wittig reactions of these compounds with various carbonyl compounds... 

The standard mechanism (Scheme 40) includes oxidative addition, Pd-P bond formation, and P-C reductive elimination. When silyl- or stannylphosphines are used, [65] no base is required for Pd-P bond formation, which instead yields R3E-X (E - Si, Sn). Since these P-substrates are often made from secondary phosphines, and because of the toxicity of organotins, they are used less often. 

Five-coordinate Pt(TV) species with silyl ligands are poised to perform Si-C or Si-H reductive elimination from Pt(IV). Note that the microscopic reverse of the latter reaction, Si-H oxidative addition, was used to synthesize the first five-coordinate Pt(TV) complexes with silyl ligands (2a-c) [84]. Complex 6, which has Pt-Me groups and a Pt-SiMe3 group, was observed to react over time at room temperamre to form tetramethylsilane, the product of Si-C reductive elimination, and intractable Pt products [91]. The five-coordinate complex ( pypyr)Pt(H>2SiEt3, 7a, was found to react with HSiMeaEt to form product 7b. Study of this reaction showed that Si-H reductive eliminaticm from 7a was rate-determining and it occurred directly from the five-coordinate complex [97]. Reaction of 7a with phosphines at room temperature led to the formation of a Pt(II)H(PR3) complex and free silane, the product of Si-H reductive elimination. Complex 7a was observed to be an active catalyst for the hydrosilylation of ethylene, tert-butylethylene, and alkynes [97]. 

The proposed catalytic cycle for the above-described conjugate reduction is outlined in Scheme 17. Initial coordination of the nucleophilic Pd(0)-phosphine complex to the electron-deficient olefin to form complex I is a reversible process that occurs rapidly at room temperature. Oxidative addition of the sihcon hydride moiety to complex I would result in the hydrido olefin complex II. Migratory insertion of the hydride ligand into the electrophilic /S-carbon of the coordinated olefin can result in the palladium enolate intermediate in. Reductive elimination of the silicon moiety and the enolate completes the catalytic cycle and forms the silyl enol ether IV. The latter is prone to acid-catalyzed hydrolysis to produce the saturated ketone. "" ... 

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