Other Silicon Hydrides

Among other silicon hydrides reported are n-butylsilane, diethylsilane, tri-isopentylsilane, tricyclopentylsilane, triphenylsilane, tri-sec-butylsilane, di-tert-butylsilane, di-ferf-butylmethylsilane, tri-tert-butylsilane,204 phenylsilane, dieth-ylmethylsilane,202 diphenylsilane,134,208,209 dichloroethylsilane,192 PMHS,77 and polyethylhydrosiloxane.207... 

A related reduction utilizing other silicon hydrides has been investigated by Benkeser. Aryl aldehydes and ketones reacted with tiichlorosilane-tri-n-propylamine to give an organosilicon derivative (59 Scheme 17), which on alkaline hydrolysis underwent silicon-hydrogen exchange, and consequently yielded the corresponding hydrocarbons. 

Active platinum catalyst. A catalyst regarded as more active than the unsupported catalyst of Brown and Brown is made by reduction of chloroplatinic acid with tribenzylsilane or other silicone hydride."... 

The review is divided into sections according to the type of metal hydride for convenience in discussing the information systematically. At one extreme, kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. At the other extreme, kinetic studies of tin hydrides are limited to a few stannanes, but the rate constants of reactions of a wide range of radical types with the archetypal tin hydride, tributylstannane, are available. Kinetic isotope effects for the various hydrides are collected in a short section, and this is followed by a section that compares the kinetics of reactions of silicon, germanium, and tin hydrides. 

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-... 

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. 

Trialkyltin hydrides represent an important class of reagents in organic chemistry because of their utility in radical reactions. However, problems of toxicity and the difficulty of product purification made trialkyltin hydrides less than ideal reagents. Several workup procedures and structurally modified trialkyltin hydrides have been developed to facilitate the separation of tin residues from the reaction mixture. Tris(trimethylsilyl)silicon hydride has also been synthesized and is often used successfully in radical reactions. However, its reactivity is different from that of trialkyltin hydrides in a number of important respects. Other tin hydride surrogates are also available. ... 

A number of other thiocarbonyl derivatives have been used [222] (sec below), as well as various hydrogen atom sources (tin and silicon hydrides,... 

The silicon hydrides are prepd either by acid decompn of Mg silicide or by redn of SiCl4 with LiAlH4 (Refs 1-4, 12, 14,15, 16 26). Disilane and other higher hydrides have been conveniently synthesized by subjecting SiH4 to a silent electrical discharge (Ref 26, p 175). Prepn of individual silanes of ordn interest are given below... 

Higher silicon hydrides with Ge, e.g. Si2H5GeH3 and other compounds produced in these pyrolysis reactions, have also been obtained by hydrolysis of Mg or Ca germanide-silicides446, 475, 636 and have been separated by gas chromatography446, 447. P and As derivatives produced by pyrolysis also be obtained in coupling reactions ... 

Hydrosilylation is by far the most important route for obtaining monomers and other precursors to fluorinated polysiloxanes. Hydrosylilation80 is the addition of silicon hydride moiety across an unsaturated linkage using transition metal complexes of platinum or rhodium such as Speier s catalyst, hexachloroplatinic acid in isopro-... 

Thicker sections can be cross-linked by hydrosilylation addition. This is the same chemistry used to produce fluorosilicone monomers with the vinyl functionality present on silicon. The catalyst reaction occurs between a vinyl group and silicone hydride.66 The advantage of this system is that it does not produce volatile by-products. On the other hand, the disadvantage is that it is available only as a two-part system.67 Recently, however, one-part, platinum-catalyzed products have been developed.66 The reaction is very rapid and at room temperature it is completed in 10 to 30 minutes. It is accelerated with increasing temperature and at 150°C (302°F) it is completed within a few seconds. This makes the compounds ideal for fast automated injection molding operations.68 One-part systems use the chemical complexing of the catalyst, which is activated at elevated temperatures, or its encapsulation into an impermeable shell, which is solid at room temperature and melts at elevated temperatures68... 

The principle of reacting a silicon hydride HSiR3 with a polynuclear carbonyl (or carbonyl cyclopentadienyl derivative) was successfully applied to other transition metals such as Mo 37 Mn40,137 81,207 Re135> 137> 207),... 

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