Hydrosilanes, reduction with

In 2009, Beller (Scheme 45) [147] and Nagashima (Scheme 46) [148] independently reported an iron-catalyzed hydrosilane reduction of carboxamides to amines. Although inexpensive PMHS and TMDS as an H-Si source are usable, the yield of product considerably decreased when hydrosilane containing only one H-Si moiety or iron sources such as Fe(acac)2 and FeX2 (X = F, Cl) was used. In both thermal and photoassisted conditions, almost the same reactivities were observed upon using a combination of Fe catalyst with TMDS (Scheme 46). 

Reduction with Hydrosilanes in the Presence of Chiral Nucleophilic Activators I 319... 

Diastereoselective reduction of ketones. Trialky Isilanes can reduce aldehydes and ketones in the presence of a catalytic amount of tetrabutylammonium fluoride in HMPT cf. reductions with activated hydrosilanes, II, 554). Under these conditions esters and nitriles are not reduced. The reaction is stereoselective. Thus 2-methylcyclohcxanone is reduced selectively to cA-2-methylcyclohexanol the cvT-selectivity depends on the bulkiness of the hydrosilanc, being highest (95%) with triphenylsilane. 

Unactivated conjugated dienes also undergo reduction with a hydrosilane in situ to form aUylating nucleophiles. The double bond of the aUyl residue has a (Z)-configuration. 

Lithium aluminium hydride reduction, with inversion of the chlorosilane formed yielded the hydrosilane ([a]D + 20°). Assuming a neat inversion in the reduction step, it appeared that the formation and trapping of the silyl radical occurred with 65% retention. 

Mixtures of hydrosilanes and acid reduce oximes to hydroxylamines in good yields 4 3 an (e) -oxime gave erythro-l-phenyl-l-benzyloxyamino propan-2-ol in 99% selectivity (Scheme 186) the reduction with lithium aluminium hydride gave only an 82 18 ratio of the (E)- to (Z)-isomers. 

Reduction with hydrosilanes. Ketones undergo enantioselective reduction by PhjSiHj in the presence of [Rh(cod)Cl]2 and Ir(cod)Cl2. Ligands such as 11, " 73, 2-i 3 and 74 (for ketoesters) are effective. Note that it is possible to use isopropanol instead of... 

Pentacoordinated hydrosilanes react with excess aryl isocyanates to give isocyanurates. Carboxylic acids are directly reduced to aldehydes in a one-pot process through the thermal decomposition of pentacoordinated silyl carboxylates (eq 4). The aldehydes are extracted from the crude product mixture by distillation, or separated from the trisiloxane by column chromatography over FlorisU. The reaction is selective, since fluoro, nitro, cyano, methoxy and heteroaryl substituents do not react with the silane. The present method also permits the reduction of Q , -unsaturated acids. The efficiency follows the order (2) > (1) > (3). 

The 1,4-reduction of ot,p-unsaturated ketones with hydrosilane often competes with the 1,2-reduction. In the case of Wilkinson catalyst Rh(PPh3)3Cl, alkylsilane, EtsSiH, leads to the 1,4-reduction, whereas arylsilane, Ph2SiH2 leads to the 1,2-reduction [19]. The phebox-Rh catalysts proved to be superior for the 1,4-reduction (Scheme 13). The use of alkoxysilanes, such as (EtO)3SiH and (EtO)2 MeSiH, exclusively gave the conjugate reduction products 21 (1,4/1,2 = 100/0) with an excellent ee, up to 95% ee [18]. In contrast, the use of EtsSiH and Me2PhSiH caused a decrease in the selectivity to the 1,4-reduction. With the increase in... 

There are some valuable reviews concerning preparation, reactivity, and synthetic applications of silicon Lewis acids [8-12]. These reviews are based on reports published in the last century. This review mainly deals with more recent advances in silicon Lewis acid-promoted synthetic reactions carbon-carbon and carbon-heteroatom bond formation and reduction with hydrosilanes. After... 

The combination of hydrosilanes with silicon Lewis acids is valuable for reduction of ethers. For instance, trityl-protected alcohols and carbohydrates can be deprotected efficiently and selectively by the Et3SiOTf-catalyzed reduction with EtsSiH [154]. 

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

A catalytic mechanism, which is supported by deuterium-labeling experiments in the corresponding Ru-catalyzed procedure [146], is shown in Scheme 47. Accordingly, the reactive Fe-hydride species is formed in situ by the reaction of the iron precatalyst with hydrosilane. Hydrosilylation of the carboxyl group affords the 0-silyl-A,0-acetal a, which is converted into the iminium intermediate b. Reduction of b by a second Fe-hydride species finally generates the corresponding amine and disiloxane. 

Another method for reductive dimerization has been developed in hy-drosilylation. NiCl2-SEt2 is an effective catalyst in silylative dimerization of aromatic aldehydes with a hydrosilane (Scheme 12) [40]. A catalytic thiolate-bridged diruthenium complex [Cp RuCl(/ 2-SPrI)2RuCp ][OTf] also induces the conversion to 1,2-diaryl-1,2-disiloxyethane [41]. A dinuclear (siloxyben-zyl)ruthenium complex is considered to be formed, and the homolytic Ru - C bond fission leads to the siloxybenzyl radicals, which couple to the coupling product 14. 

One of the fundamental operations in organic synthesis remains the stereoselective reduction of carbonyl groups1241. In a process related to that reported by Hosomi et u/.[25], using hydrosilanes as the stoichiometric oxidant and amino acid anions as the catalytic source of chirality, a variety of ketones were reduced in good to excellent yield and with good stereoselectivity1261. This process reduces the amount of chiral catalyst needed and utilizes catalysts from the chiral pool that can be used directly in their commercially available form. 

Reduction of a., -unsaturated carbonyl compounds. Hydrosilanes, particularly (QH,)2SiH2, in the presence of Pd(0), and a Lewis acid, particularly ZnCl2, can effect selective conjugate reduction of unsaturated ketones, aldehydes, and carboxylic acid derivatives. Chloroform is the solvent of choice. In addition, 1 equiv. of water is required. Experiments with D,0 and (C6H,),SiD2 indicate that... 

In sharp contrast to the unique pattern for the incorporation of carbon monoxide into the 1,6-diyne 63, aldehyde 77 was obtained as the sole product in the rhodium-catalyzed reaction of 1,6-enyne 76 with a molar equivalent of Me2PhSiH under CO (Scheme 6.15, mode 1) [22]. This result can be explained by the stepwise insertion of the acetylenic and vinylic moieties into the Rh-Si bond, the formyl group being generated by the reductive elimination to afford 77. The fact that a formyl group can be introduced to the ole-finic moiety of 76 under mild conditions should be stressed, since enoxysilanes are isolated in the rhodium-catalyzed silylformylation of simple alkenes under forcing conditions. The 1,6-enyne 76 is used as a typical model for Pauson-Khand reactions (Scheme 6.15, mode 2) [23], whereas formation of the corresponding product was completely suppressed in the presence of a hydrosilane. The selective formation of 79 in the absence of CO (Scheme 6.15, mode 3) supports the stepwise insertion of the acetylenic and olefmic moieties in the same molecules into the Rh-Si bond. 

Another reaction is reductive cyclization. 1,6-Diynes and 1.6-enynes undergo reductive cyclization using hydrosilanes as a hydrogen source in AcOH. The 1,6-diynes 91 and 95 are converted into the 1,2-dialkylidenecyclopentane derivatives (1,3-dienes) 94 and 96. Triethylsilane is used as a hydrogen donor for the reaction[48]. The reaction involves the formation of a vinylpalladium bond in 92 via the insertion of an alkyne into the Pd—H bond, followed by the alkyne insertion to give 93, which is hydrogenolyzed with Si—H to give the 1,3-dienes 94 and 96. 

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