Polymethylhydrosiloxane PMHS

Ben2yl azide 1855 and N-benzyloxycarbonylbenzylamine 1859 are both transformed by the cheap polymethylhydrosiloxane (PMHS) 1856, in the presence of (B0C)20 and Pd/C, into 92-94% N-BOC-benzylamine 1857 and the polymer 1858 [81]. (Scheme 12.22). Aromatic and aliphatic amine oxides are readily reduced by 1856/Pd/C into their corresponding amines. Thus, e.g., pyridine-N-oxide 860 and quinohne-N-oxide 877 give pyridine and quinohne in 90 and 92% yield, respectively. Analogously, benzyldimethylamine-N-oxide is converted in 88% yield into free benzyldimethylamine [82]. 

Beller and coworkers reported hydrosilylation reactions of organic carbonyl compounds such as ketones and aldehydes catalyzed by Fe(OAc)2 with phosphorus ligands (Scheme 21). In case of aldehydes as starting materials, the Fe(OAc)2/PCy3 with polymethylhydrosiloxane (PMHS) as an H-Si compound produced the corresponding primary alcohols in good to excellent yields under mild conditions [67]. Use of other phosphorus ligands, for instance, PPhs, bis(diphenylphosphino) methane (dppm), and bis(diphenylphosphino)ethane (dppe) decreased the catalytic activity. It should be noted that frans-cinnamaldehyde was converted into the desired alcohol exclusively and 1,4-reduction products were not observed. 

In 2005, Riant et al. reported the synthesis of a new air-stable S/N-chelating zinc catalyst, depicted in Scheme 10.50, which was fully characterised by all spectroscopic methods. This complex, prepared from the corresponding ferrocene oxazoline, was applied to the enantioselective hydrosilylation of ketones in the presence of polymethylhydrosiloxane, PMHS, providing modest enan-tioselectivities (<55% ee). ... 

Chiu et al. developed a catalytic reductive aldol cyclization of alkyne-diones such as 115 and 117 using [Ph3PCuH]6 (10mol%) as catalyst and polymethylhydrosiloxane PMHS (200 mol %) as terminal reductant. The... 

Recently, the silane-mediated reductive cyclization of activated alkynes with tethered ketones using Stryker s reagent as a catalyst was reported.112,90b Alkynyl ketone substrate 84a was treated with a catalytic amount of Stryker s reagent in the presence of polymethylhydrosiloxane (PMHS) to afford the cA-fused hydrindane 84b as a single diastereomer. This method is applicable to both five- and six-membered ring formation, but often suffers from competitive over-reduction of the reaction products (Scheme 59). 

Direct reduction of an aldehyde or ketone to the corresponding ether could potentially telescope two reactions, reduction and protection, into one step. S. Chandrasekhar of the Indian Institute of Chemical Technology, Hyderabad, reports (Tetrahedron Lett. 2004,45,5497) that in the present of polymethylhydrosiloxane (PMHS) and catalytic B(C6F,), TMS ethers of alcohols will convert aldehydes to the corresponding dialkyl ethers. The reaction works well for both saturated and benzylic alcohols. This may prove to be a useful alternative to Williamson synthesis for the preparation of complex ethers. 

Polymethylhydrosiloxane (PMHS) has been reported to be a more selective reducing agent when coupled with Pd° catalysts than R3SnH, permitting, for example, the reduction of allylic acetates in the presence of enones and acyl halides (equation 104).300... 

Reduction with LiAlH(OBu )3293 or LAH292 also gives selective hydride addition to the less substituted allyl end (equations 317 and 318). In contrast, formate reductions selectively deliver hydride to the more substituted allyl terminus (equations 319 and 320).302-303 Si—H-mediated reduction, conveniently performed with polymethylhydrosiloxane (PMHS), demonstrates no clear pattern of regioselectivity (equation 321).320 LiHBEt3 delivers hydride regioselectivity to the less substituted allyl terminus (equation 322)289-291... 

The combination of polymethylhydrosiloxane (PMHS) and catalytic amounts of TBAF (1 mol%) and Cp2Ti(OC6H4Cl-4)2 (2 mol%) provides an efficient and convenient method for the partial reduction of lactones 187 to lactols 189 via hydrosilylation and subsequent hydrolysis of the resulting O-silyllactols 188 (equation 76)184. 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF with high efficiency [20]. The asymmetric version of this process has been developed by Lawrence and... 

For the preparation of capillary columns for gas chromatography, the fused silica column wall is deactivated using polysiloxanes, and modified with a suitable stationary phase. Hetem19 discussed the use of polymethylhydrosiloxanes (PMHS) for deactivation and subsequent coating with a polymerized C18-type silane for stationary phase formation. Stationary phases used in packed GC are analogous to HPLC. 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF, at high efficiency [9]. The asymmetric version of this process has been developed by Lawrence and coworkers using chiral ammonium fluoride 7c prepared via the method of Shioiri [10]. The reduction of acetophenone was performed with trimethoxysilane (1.5 equiv.) and 7c (10 mol%) in THF at room temperature, yielding phenethyl alcohol quantitatively with 51% ee (R) (Scheme 4.6). A slightly higher enantioselectivity was observed in the reduction of propiophenone. When tris(trimethylsiloxy)silane was used as a hydride source, the enantioselectivity was increased, though a pro-... 

The in situ generation of tin hydrides from inexpensive starting materials, polymethylhydrosiloxane (PMHS TMSO-(SillMeO) -TMS), can be applied to reducing the use of tin compounds (Equation (15)).57... 

Asymmetric reduction of a, /I-unsaturated esters, lactones or lactames can be effected with copper-hydride catalysts and chiral phosphanes such as various BINAP related compounds in excellent yields and enantioselectivities (equation 23). As the hydrosilane component, polymethylhydrosiloxane (PMHS) is frequently used for this reaction. 

Polymethylhydrosiloxane (PMHS), (CH3)3SiO(CH3HSiO) Si(CH])3, where n is 35. PMHS is an easily handled, chemically inert liquid it is made by Dow Corning and supplied by Aldrich. See 3,294. 

Isoxazolines have also been converted into 1,3-amino alcohols by polymethylhydrosiloxane (PMHS)-Pd(OH)2/C. When the reduction was performed in the presence of (B0C)20, WBOC protected compounds were directly achieved (BOC = f-butoxycarbonyl). For example, WBOG-y-amino alcohols 132 were synthesized from the corresponding isoxazolines 131 in one step and 78-88% yield (Equation 15) <2004SL1303>. 

The use of polymethylhydrosiloxane (PMHS) as a reducing agent for organic compounds... 

Pd-C, cyclohexene, 1 M HCl, EtOH, 83% yield. Ammonium formate and polymethylhydrosiloxane (PMHS) can also be used as a source of hydrogen. 

Pd/C, HCO2NH4, EtOH, AcOH, >82% yield. Polymethylhydrosiloxane (PMHS) can be used as a hydrogen source as well. ... 

Comparable yield in 4 can be obtained only through reduction of 1 with stoichiometric amoimts of polymethylhydrosiloxane (PMHS) in the presence of catalytic amount of an active zinc compound [9], However this process requires basic hydrolysis of the siloxane formed and a separation step. 

Polymethylhydrosiloxane (PMHS), a safe and inexpensive polymer co-product of the silicon industry, is an efficient alternative reducing agent for C=0 and C=N bonds when associated with catalysts (1). Mimoun et al. recently reported a new system based on zinc hydride catalysts which enables the chemoselective reduction of unfunctionalized and a,/ -unsaturated- aldehydes, ketones and esters (2). Because gummy silicon residues, which are usually associated with silane reductions, do not form, this PMHS system is attractive for synthetic / industrial purposes. Nevertheless, in contrast to tin-catalyzed reductions of ketones with PMHS... 

In 1967 Hayashi and eoworkers used polymethylhydrosiloxane (PMHS) to prepare organotin hydrides from the eorresponding tin oxide derivatives (Eqs. 10 and 11)... 

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