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Silicon-Based Reducing Agents

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Silicon-Based Reducing Agents Gelest, Inc. Silicon-Based Reducing Agents GeraldGerald L. LarsonLarson ViceVice President, President, Research Research && Development Development Materials for the reduction of: Aldehydes Ketones Acetals Ketals Esters Lactones Thioesters Enamines Imines Acids Amides Halides Olefi ns Metal Halides Supplement to the Gelest Catalog, “Silicon, Germanium & Tin Compounds, Metal Alkoxides and Metal Diketonates” which is available on request. 1 AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 127 Gelest, Inc. SILICON-BASED REDUCING AGENTS Introduction The widely used organometallic-based reducing agents can be broadly classifi ed 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 a 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 olefi ns, 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. Additionally, the silane reductions can also be carried out with fl uoride ion catalysis to generate a silane with more hydridic character. Some of the key reductions possible with silanes are summarized in Table 1. AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 128 2 Gelest, Inc. General Considerations 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, fl ammable 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 several silanes. From these data the rather wide variation in the Si-H bond strengths from tris(trimethylsilyl)silane on the low-energy end to trifl uorosilane 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 TABLE 2 BOND STRENGTHS OF VARIOUS HYDRIDOSILANES Compound Product Code Bond Strength Reference kJ mol-1 kcal mol-1 F3Si-H SIT8373.0 419 100 5 Et3Si-H SIT8330.0 398 95 6 Me3Si-H SIT8570.0 398 95 7 H3Si-H SIS6950.0 384 92 6 Cl3Si-H SIT8155.0 382 91 5 PhMeHSi-H SIP6742.0 382 91 6 Me3SiSiMe2-H not offered 378 90 6 PhH2Si-H SIP6750.0 377 90 6 (MeS)3Si-H not offered 366 87 6 H3SiSiH2-H SID4594.0 361 86 5 (Me3Si)3Si-H SIT8724.0 351 84 6 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 purifi cation of the fi nal product. Silicon-based reductions have been reviewed, though never in a comprehensive manner.6,8-13 A comprehensive review has been accepted for publication.14 AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 3 129 Gelest, Inc. Silicon-Based Radical Reductions 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 kJmol-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 fi nal product. This proved to be the case, and a number of radical reductions with tris(trimethylsilyl)silane have been reported and reviewed.15,16 Included among these are the reductions of organic halides, 17-19 esters,20 xanthates, selenides, sulfi des, thioethers, and isonitriles.21 As an example of a free radical reduction, the diphenylsilane reduction of thioesters to ethers has been recently reported.22 This reaction uses the catalytic triphenyltin hydride as the actual reducing agent. S 1) Ph2SiH2 O Ph3SnH cat. O Et3B/rt/1h 2) AIBN/85-100 °C 60% Ionic Reductions with Silanes – General Considerations As pointed out above the silanes provide a mild form of the hydride and as such can be useful in various hydridic reductions. The general and, admittedly simplifi ed, 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. R3C-X + R3Si-H R3C-H + R3Si-X 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.23 Although the existence of free silicenium ions do not exist in solution 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 those of olefi ns, ketones, aldehydes, esters, organic halides, acid chlorides, acetals, ketals, alcohols as well as metal salts. + + (CH3)3C + (CH3)3Si-H (CH3)3C-H + (CH3)3Si Silane Reduction of Alcohols to Alkanes The general equation for the silane reduction of alcohols to alkanes is illustrated below. The reaction proceeds best when the alcohol can lead to a stabilized carbenium ion. Thus, secondary and tertiary aliphatic alcohols and benzylic alcohols are readily reduced. Trialkyl substituted silanes are more reactive than dialkylsilanes and di- or triarylsilanes. Typical and highly effective conditions for these reductions are treatment of the alcohol with the silane and trifl uoroacetic acid in dichloromethane. Triethylsilane is often the silane of choice due to its ease of handling and high reactivity.23,24 Acid R3C-OH + R3Si-H R C-H + R Si-OH and/or R Si-O-SiR catalyst 3 3 3 3 The reduction of secondary alcohols with a silane and a protic acid does not occur. These reductions require the use of a strong Lewis acid such as boron trifl uoride or aluminum chloride.25,26 AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 130 4 Gelest, Inc. Primary aliphatic alcohols are not reduced with silanes.27 Benzylic alcohols, on the other hand, are reduced under rather mild conditions to the corresponding toluene derivative.28 OH Me t t Bu Bu t t R3Si-H Bu Bu CH2Cl2/TFA tBu tBu The selective reduction of a benzylic alcohol in the presence of benzyl ethers, a tetrahydrofuran and an acetal has been reported.29 OEt OEt BnO BnO O O Et3Si-H/TFA OBn HO OBn CH2Cl2/ rt MeO MeO 76% Primary alcohols can be reduced to the alkane when the reaction is catalyzed by the very strong Lews acid, tris(pentafl uorophenyl)borane. The reaction requires two equivalents of the silane as the fi rst equivalent serves to silylate the alcohol.
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