Polymethylhydrosiloxane reduction

This procedure illustrates the bromopropylation of an alcohol by allylation and then hydroboration/bromination,7 a clean, selective procedure compared with other approaches via 3-substituted propyl bromide derivatives. Conversion to the tetraalkyl tin, then brominative cleavage, is the standard sequence for preparation of trialkyl tin derivatives.8 The standard lithium aluminum hydride (UAIH4) reduction of the tin bromide was not usable here because of contamination of the product by UAIH4 byproducts that were not easily separated, and the polymethylhydrosiloxane reduction method9 was not successful. However, use of NaBH4 in f, 2-dimethoxyethane was effective and convenient.10... 

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. 

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). 

The situation for the hydrosilylation of C = N functions with regard to ecology and economy is somewhat similar as for the hydride reduction, except that fewer effective catalytic systems have been developed [91]. Despite some recent progress with highly selective Ti-based [92] and Cu-based [93] catalysts using cheap polymethylhydrosiloxane as reducing agent, hydrosilylation will see its major applications in small-scale laboratory synthesis. 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

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... 

PMHS corresponding polymeric reagent (PMHS, polymethylhydrosiloxane), a substantial rate increase was observed over the monomeric model (complete reduction of acetophenone in less than 1 min with PMHS vs. only 60% conversion in 1 h with (EtO)2SiHMe) [55]. The related hydrosilylation of 86 by chiral PTC uses an interesting ep/iedra-derivedhalometallated catalyst 25 (Scheme 10.12) [56],... 

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 redox reaction between diphenylbenzidine and thiosulfate is of first order with respect to thiosulfate, diphenylbenzidine, and hydrogen ions.301 Zinc-diamine-catalysed reduction of various ketones with polymethylhydrosiloxane in protic conditions show moderate enantioselectivities. Probable mechanisms are proposed.302 Reduction of non-activated aryl and alkyl halides by a neutral ground-state organic molecule (13) afforded the corresponding indolines in excellent yields. A tentative mechanism has been suggested (Scheme 13).303... 

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-... 

Reductive cyclization of 1,6-diynes.2 Cyclization of these substrates to 1,2-dialkylidenecyclopentanes can be effected with 1 and a phosphine as catalyst and triethylsilane (10-fold excess) in place of polymethylhydrosiloxane. 

Palladium-catalyzed reduction of A -(/-butoxycarbonyl)indoles by polymethylhydrosiloxane gives readily N-(t-butoxycarbonyOindolines in good yields <2007S1509>. 

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. 

Racemic chiral enones are versatile substrates for DKR . Reduction of rac-80 with (5 )-[p-TolCuCl(BINAP)], polymethylhydrosiloxane and f-BuONa/f-BuOH in toluene, followed by TBAF, yields diastereoselectively (/f,R)-81 (equation 12). The chiral copper mediator preferentially reduces the (/f)-enantiomer of enone 80, whereas the unreacted (5)-80 efficiently racemizes via an enolate by deprotonation-reprotonation under the basic conditions of the reaction. 

Reductive TVansformations. The utility of 1 was first demonstrated in the enantioselective hydrosilylation of ketones. Uniformly high enantioselectivity, yield, and turnover were observed for aromatic (and some aliphatic) ketones when using the complex derived from RhCls (eq 1). Lower enantioselection is observed with t-Bu-pybox or i-Pr-pybox cobalt(I). The derived l Sn(OTf)2 complex gives alcohol products with up to 58% ee using methano-lic polymethylhydrosiloxane. A cationic ruthenium(III) catalyst diverts the usual reduction pathway to enolsilane formation, particularly when the nature of the silane is modified (eq 2). ... 

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>. 

Generation in situ. Organotin hydrides are generally prepared by reduction of an organotin halide or oxide with LiAlH4. Japanese chemists3 recently reported the preparation by reaction of organotin oxides with polymethylhydrosiloxane 4... 

Phthalic acid, 259 Phthalic anhydride, 104 a-Picoline, 161 a-Picoline N-oxide, 161 Picolinic acid, 16 Picramic acid, 271 Picric acid, 271 Pinacol reduction, 7 Pinosylvin, 31 Piperazines, 322 Piperidine, 33, 291 2-Piperidone, 194 Pivaldehyde, 105 Podophyllotoxin, 337 Podophyllotoxone, 337 Polonovski reaction, 308 Polyisoprenoids, 300-301 Polymethoxybenzophenones, 30—31 Polymethylhydrosiloxane, 294 Polyphosphate ester (PPE), 229-230 Polyphosphoric acid, 227, 231—232 Potassium, 232, 233 Potassium acetate, 96 Potassium amide, 232—233, 310 Potassium azodicarboxylate, 100 Potassium r-butoxide, 26, 45, 47, 77-78, 85, 133, 188, 212, 222, 225, 233-234, 236, 246... 

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. 

Direct Reduction of C=0 and C=N Bonds by Polymethylhydrosiloxane Promoted by Zn-Diamine Catalysts in Alcohol Solvents... 

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... 

Carboxylic acids, esters, amides, nitriles, nitro groups and most aromatic nuclei are not reduced under ionic hydrogenation conditions (133). An organosiloxane, polymethylhydrosiloxane [9004-73-3] (PMHS), is most economically favored for large-scale reductions. Polymethylhydrosiloxane is a versatile low cost hydride transfer reagent having a hydride equivalent weight of 60. Reactions are catalyzed by Pd or dibutyltinoxide. The choice of reaction conditions leads to chemoselective reduction, eg, allyl reductions in the presence of ketones and aldehydes (134—136). Esters are reduced to... 

Conjugate reduction. p,p-Disubstituted a,P-unsaturated esters undergo asymmetric reduction by polymethylhydrosiloxane in the presence of r-BuONa and a complex derived from CuCl and a slightly modified (i.e., p-tolyl) (5)-BINAP. 

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