Hydrosilylation reduction

The following sections provide an overview on the state of the art for the enantioselective hydrogenation (including transfer hydrogenation) of various classes of C = N groups, together with a short, critical assessment of the presently known catalytic systems. Only selective (ee >80%) or otherwise interesting catalysts are included and, furthermore, other reduction methods for C = N functions (hydride reduction, hydrosilylation) are only covered summarily. 

Reductive hydrosilylations of ketones using chiral C2-symmetric bis(oxazolines)... 

In contrast with the common activity of oxo-rhenium compounds in oxidation catalysis (see above), the Re -dioxo-complex [Re02l(PPh3)2] catalyzes the reductive hydrosilylation of aldehydes and ketones to give silyl ethers (reaction... 

Diisopropylchlorosilane was used in the stereoselective reduction of /Miydroxyketones to 1,3-diols60. In this stepwise process the first step is the silylation of the hydroxyl group and then a Lewis acid catalyzed intramolecular reduction (hydrosilylation) of the carbonyl to form the dioxasilacycles 22, cleavage of which provides the diol (equation 52). 

Organosilanes as Reducing Agents. The two principal categories of reductive chemistry of hydridosilanes are hydrosilylation and ionic reduction. Hydrosilylation is the catalyzed addition of a hydridosilane to a multiply bonded system. This chemistry is a principal technology in silicon—carbon bond formation. Ionic reduction by silanes, a class of chemistry more properly considered within the context of organic synthesis, is the subject of detailed reviews (132). 

The Rh-complex of the chiral diphosphine ligand, SyPhePhos (Scheme 7.10.), was used as catalyst for the reductive hydrosilylation of the C=N bond of the imine precursor of the antidepressant, Pyrazidol, with an ee of 73% (Scheme 7.11). 

Keywords Transfer hydrogenation Organocatalysis Hantzsch ester Trichlorosilane Phosphoric acid Aminocatalysis Thioureas Lewis bases Reduction Hydrosilylation... 

Reduction of esters and amides via catalytic hydrosilylation eliminates the requirement of stoichiometric amount of metal hydride reagents. In the case of ester reduction, hydrosilylation is operationally simpler than using DIBALH (rt vs. —78°C), which frequently overreduces the substrates. 

For the reductive hydrosilylation of a ketone by SiH4 to take place, a free energy barrier of about 14 kcal/mol (for the more realistic ItBu, IMes, and IPr models) with respect to the isolated reactants has to be overcome. Comparable energy barriers are obtained for the two successive steps of the catalytic cycle, not allowing to identify a rate-limiting step, if there should be one. 

Reaction (31) shows an example of hydrosilylation of ketones, i.e., reduction of 4-ferf-butyl-cyclohexanone affordir mainly the tmns isomer, indicating that the axial H-abstraction is favored7... 

Chen et al. utUized a direct chemical reaction with a given solution (wet treatment) to modify the surface of the silicone rubber. The presence of a layer of PEO on a biomaterial surface is accompanied by reductions in protein adsorption, and cell and bacterial adhesion. In order to obtain a PEO layer on top of the silicone rabber surface, the surface was firstly modihed by incorporating an Si-H bond using (MeHSiO) , and followed by PEO grafting to the surface using a platinum-catalyzed hydrosilylation reaction. These PEO-modified surfaces were demonstrated by fibrinogen adsorption both from buffer and plasma, as well as albumin adsorption from buffer. Reductions in protein adsorption of as much as 90% were noted on these surfaces. 

With alkyl- and arylsilanes, concurrent isomerization and hydrosilylation occur, but the rates of both processes fall away rapidly due to some reduction to the metal such deactivation is temperature-dependent, and high yields of adduct are obtained with these silanes when additions are carried out slowly at ambient temperature. 

Hydrosilylation by Ziegler-type catalyst systems [e.g., Ni(acac)2/AlEt3] has been examined for the reaction of 1-octene with EtjSiH in benzene 178). Complications include competing isomerization and reduction to metal however, 1,3-dienes or terminal acetylenes are readily hydrosilylated withRC i CH, the major product is CH2 CR. CRiCHSiXj. 

Lowering the reaction temperature led to a significant increase in stereoselectivity. The catalytic runs performed at - 60 °C gave the best results with acetophenone being hydrosilylated with 90% ee and 92% yield in the presence of 57c. Similar enantioselectivities (88-91%) were obtained in the reduction... 

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

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. 

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. 

In all of these cases, paUadium-catalyzed hydrosilylation proceeds via hydropalla-dation followed by reductive elimination of alkyl- and silyl group from the palladium. In the reaction of o-aUylstyrene (24) with trichlorosilane, which gives hydrosilylation products on the styrene double bond 25 and cycUzed product 26, the hy-dropalladation process is supported by the absence of side products which would result from the intermediate of the silylpaUadation process (Scheme 3-11) [37]. 

A 1990 patent reported the quantitative cydization of 1,5-hexadiene 66 to 67 with Cp 2LuCH3 (Cp = C5Mc5 anion) in the presence of PhSiH3 (Scheme 14) [39]. This result was followed by reports that Cp 2NdCH(SiMe3)2 reductively cyclized both 66 and 1,6-heptadiene (68) under similar conditions [40] and that Cp 2SmCH(SiMe3)2 also served as a precatalyst for the cydization of 1,5-hexadiene [41]. In the case of the neodymium and samarium catalysts competitive alkene hydrosilylation was observed. 

The reductive coupling of of dienes containing amine groups in the backbones allows for the production of alkaloid skeletons in relatively few steps [36,46,47]. Epilupinine 80 was formed in 51% yield after oxidation by treatment of the tertiary amine 81 with PhMeSiEh in the presence of catalytic 70 [46]. Notably, none of the trans isomer was observed in the product mixture (Eq. 11). The Cp fuMcTIIF was found to catalyze cyclization of unsubstituted allyl amine 82 to provide 83. This reaction proceeded in shorter time and with increased yield relative to the same reaction with 70 (Eq. 12) [47]. Substitution of either alkene prevented cyclization, possibly due to competitive intramolecular stabilization of the metal by nitrogen preventing coordination of the substituted olefin, and resulted in hydrosilylation of the less substituted olefin. 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

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