The widely-used organometallic-based reducing agents can be broadly classified as either ionic, such as lithium aluminum hydride and sodium borohydride, or free-radical such as tri-n-butyltin hydride. The mechanistic differences between these two classes of reducing agents very often complement one another in their ability to reduce organic substrates. Organosilanes have been found to possess the ability to serve as both ionic and free radical reducing agents. These reagents and their reaction by-products are safer and more easily handled and disposed of than other reagents. Their reductive abilities are accomplished by changes in the nature of the groups attached to silicon, which can modify the character of the Si-H bond in the silane. For example, the combination of triethylsilane and an acid has proven to be excellent for the reduction of substrates that can generate a “stable” carbenium ion intermediate. Examples of substrates that fall into this class are olefins, alcohols, esters, lactones, aldehydes, ketones, acetals, ketals, and other like materials. On the other hand, triphenylsilane and especially tris(trimethylsilyl)silane have proven to be free-radical reducing agents that can substitute for tri-n-butyltin hydride. The reductions with silanes can take place with acid catalysis in which the silane provides the hydride to a carbenium ion intermediate. This is often the situation in the reduction of carbonyls, ketals, acetals and similar species. The silane reductions can also be carried out with fluoride ion-catalysis which acts on the silicon moiety to generate a silane with more hydridic character.
Some of the key reductions possible with silanes are summarized in Table 1.
Silane Reduction of Organic Functional Groups
+ = good
++ = excellent
a. Reduces C=C of enones to saturated ketones.
b. Especially good for reduction of cyclic ketals and acetals.
|R2C=CR2 ➞ R2CH-CHR2||++||++a||+a|
|R-OH ➞ R-H||+||++||+|
|R-X ➞ R-H||+||+||++|
|RCHO ➞ RCH2OH||++||+|
|R2C=O ➞ R2CHOH||++||+||+||+|
|RCO2R' ➞ RCH2OH||++||++||+|
|RCOCI ➞ RCH2OH’||++|
|RR’C(OR’’)2 ➞ RR’CHOR’’||++||++b|
|RR’C=NHR’’ ➞ RR’CHNHR”||+||++||+|
|RCN ➞ RCH2NH2||++|
|RCH2NR2’ ➞ RCH2OH||+||+|
|R3P=O ➞ R3P:||+||+||++||+|
|ArNO2 ➞ ArNH2||++|
Hydridosilanes are readily produced on an industrial scale through the use of Grignard chemistry starting with trichlorosilane, methyldichlorosilane, and dimethylchlorosilane, among others, as key raw materials. Alternatively, the Si-X (X = primarily Cl or OR) bond can be reduced to Si-H.
The organosilanes are basically hydrocarbon-like in that they are stable to water, are, in general, flammable and are lipophilic. In contrast to hydrocarbons the low molecular weight silanes such as monosilane, methylsilane, and dichlorosilane are pyrophoric. The silanes will react with base or, more slowly, with acid to give the corresponding siloxane with the evolution of hydrogen gas. They show a strong, characteristic, carbonyl-like absorption in the infrared at about 2200 cm-1.1
The metallic nature of silicon and its low electronegativity relative to hydrogen—1.8 versus 2.1 on the Pauling scale—lead to polarization of the Si-H bond such that the hydrogen is hydridic in nature. This provides an ionic, hydridic reducing agent that is milder than the usual aluminum-, boron-, and other metal-based hydrides. Thus, triethylsilane, among others, has been used to provide the hydride in Lewis Acid-catalyzed reductions of various carbenium ion precursors. In addition the Si-H bond can be employed in various radical reductions wherein the silane provides the hydrogen radical.
Table 2 shows the Si-H bond strengths for various silanes. From this data the rather wide variation in the bond strengths from tris(trimethylsilyl)silane on the low-energy end to trifluorosilane on the high-energy end can be noted. This is yet another example of the extraordinary effect that groups attached to silicon can have on the chemistry of the silane and that these effects can go beyond the simple steric effects that have been so successfully applied with the silicon-based blocking agents.2-4
Bond Strengths of Various Hydridosilanes
|Compound||Product Code||Bond Strength |
|Bond Strength |
Although triethylsilane has been the most popular of the silicon-based reducing agents, in principal any Si-H-containing system can provide the hydride for many or most of these reductions. Considerations would include availability, economics, and silicon-containing by-products. The silicon-containing by-products are usually the silanol or disiloxane in the case of the trisubstituted silanes, or silicones in the case of the di- or monosubstituted silane reducing agents. Such considerations can result in greater ease of handling and purification of the final product.
Silicon-based reductions have been reviewed, though never in a comprehensive manner.6,8-13
Griller and Chatgilialoglu6 realized that the low bond energy of the Si-H bond in tris(trimethylsilyl)silane compared well with that of the Sn-H bond in tri-n-butyltin hydride (322 kJ mol-1; 77 kcal mol-1), and that this reagent should, therefore, be a viable alternative for radical reductions and one that would avoid the potential problems of working with toxic tin materials and trace tin-containing impurities in the final product. This proved to be the case and a number of radical reductions with tris(trimethylsilyl)silane have been reported and reviewed.14,15 Included among these are the reductions of organic halides,16-18 esters,19 xanthates,20 selenides,20 sulfides,20 thioethers,20 and isonitriles.20
As an example, the diphenylsilane reduction of thioesters to ethers has been recently reported.21
As pointed out above, the silanes provide a mild form of the hydride group and as such can be useful in various hydridic reductions. The general and, admittedly simplified, view of such reductions can be visualized as shown below where a carbenium ion is reduced by a silane. In this scenario the carbenium ion receives the hydride from the silane and the silane takes on the leaving group from the carbon center.
It has been shown that in the gas phase the reaction shown below is exothermic by
approximately 8 kcal/mol indicating that the trimethylsilicenium ion is, at least in the gas phase, more stable than the tert-butyl carbenium ion.22 Although the existence of free silicenium ions in solution do not exist under normal, “unbiased” conditions it can be assumed that the silicon center is free to take on considerable positive charge in its reactions. Reductions based on this premise include olefins, ketones, aldehydes, esters, organic halides, acid chlorides, acetals, ketals, alcohols as well as metal salts.