Reduction with silicon hydrides

Abstract Gold and silver nanoparticles were obtained by in situ reduction with silicon hydride groups grafted to the mesoporous MCM-41 silica surface. Nickel-, eobalt-, and iron-containing silicas were synthesized by chemisorption of appropriate metal aeetylacetonates with following reduction in the acetylene atmosphere. Such metal-containing MCM-41 matrices have been applied for preparation of carbon nanostructures at pyrolytie deeomposition of acetylene. From transmission electron microscopy (TEM) data a lot of carbon nanotubes were formed, namely tubes with external diameter of 10-35 nm for Ni-, 42-84 nm for Co-, and 14—24 nm for Fe-eontaining silieas. In the metal absence on the silica surface low yield of nanotubes (up to 2%) was detected. 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Enantioselective 1,4-reduction of enones can be done using a copper-BINAP catalyst in conjunction with silicon hydride donors.158 Polymethylhydrosilane (PMHS) is one reductants that is used. 

The reduction of ketones with silicon hydrides has been occasionally performed by radical chemistry for a synthetic purpose. The radical adduct is stabilized by the a-silyloxyl substituent and for RsSi (R = alkyl and/or phenyl) the hydrogen abstraction from the parent silane is much slower than a primary alkyl radical (cf. Chapter 3). On the other hand, (TMS)3SiH undergoes synthetically useful addition to the carbonyl group and the reactions with dialkyl ketones afford yields > 70% under standard experimental conditions, i.e., AIBN, 80-85 °C [45,51]. Reaction (5.25) shows as an example the reduction of 4-tcrt-butyl-... 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

As an example, the propagation steps for the reductive alkylation of alkenes are shown in Scheme 7.1. For an efficient chain process, it is important (i) that the RjSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene, and (ii) that the alkyl radical reacts faster with the alkene (to form the adduct radical) than with the silicon hydride. In other words, the intermediates must be disciplined, a term introduced by D. H. R. Barton to indicate the control of radical reactivity [5]. Therefore, a synthetic plan must include the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and that the concentration of silicon hydride often serves as the variable by which the product distribution can be influenced. 

Direct preparation of azo compounds in good yields is accomplished by treatment of nitro compounds with lithium aluminum hydride [576], with magnesium aluminum hydride [577], with sodium bis(2-methoxy ethoxy)aluminum hydride [575], with silicon in alcoholic alkali [331] or with zinc in strongly alkaline medium [578], Hydrazobenzene was obtained by controlled hydrogenation of nitrobenzene in alkaline medium (yield 80%) [572] and by reduction with sodium bis 2-methoxyethoxy)alumium hydride (yield 37%) [544],... 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

In recent investigations Schott and Herrmann561 563 attempted to prepare silicon hydride by reduction of tribromosilane with magnesium ... 

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