Triethylsilane1

Et3SiH

[617-86-7]  · C6H16Si  · Triethylsilane  · (MW 116.31)

(mild reducing agent for many functional groups)

Physical Data: mp -156.9 °C; bp 107.7 °C; d 0.7309 g cm-3.

Solubility: insol H2O; sol hydrocarbons, halocarbons, ethers.

Form Supplied in: colorless liquid; widely available.

Purification: simple distillation, if needed.

Handling, Storage, and Precautions: triethylsilane is physically very similar to comparable hydrocarbons. It is a flammable, but not pyrophoric, liquid. As with all organosilicon hydrides, it is capable of releasing hydrogen gas upon storage, particularly in the presence of acids, bases, or fluoride-releasing salts. Proper precautions should be taken to vent possible hydrogen buildup when opening vessels in which triethylsilane is stored.

Introduction.

Triethylsilane serves as an exemplar for organosilicon hydride behavior as a mild reducing agent. It is frequently chosen as a synthetic reagent because of its availability, convenient physical properties, and economy relative to other organosilicon hydrides which might otherwise be suitable for effecting specific chemical transformations.

Hydrosilylations.

Addition of triethylsilane across multiple bonds occurs under the influence of a large number of metal catalysts.2 Terminal alkynes undergo hydrosilylations easily with triethylsilane in the presence of platinum,3 rhodium,3a,4 ruthenium,5 osmium,6 or iridium4 catalysts. For example, phenylacetylene can form three possible isomeric hydrosilylation products with triethylsilane; the (Z)-b-, the (E)-b-, and the a-products (eq 1). The (Z)-b-isomer is formed exclusively or preferentially with ruthenium5 and some rhodium4 catalysts, whereas the (E)-b-isomer is the major product formed with platinum3 or iridium4 catalysts. In the presence of a catalyst and carbon monoxide, terminal alkynes undergo silylcarbonylation reactions with triethylsilane to give (Z)- and (E)-b-silylacrylaldehydes.7 Phenylacetylene gives an 82% yield of a mixture of the (Z)- and (E)-isomers in a 10:1 ratio when 0.3 mol % of Dirhodium(II) Tetrakis(perfluorobutyrate) catalyst is used under atmospheric pressure at 0 °C in dichloromethane (eq 2).7d Terminal alkenes react with triethylsilane in the presence of this catalyst to form either normal anti-Markovnikov hydrosilylation products or allyl- or vinylsilanes, depending on whether the alkene is added to the silane or vice versa.8 A mixture of 1-hexene and triethylsilane in the presence of 2 mol % of an iridium catalyst ([IrCl(CO)3]n) reacts under 50 atm of carbon monoxide to give a 50% yield of a mixture of the (Z)- and (E)-enol silyl ether isomers in a 1:2 ratio (eq 3).9 Hydrolysis yields the derived acylsilane quantitatively.9

A number of metal complexes catalyze the hydrosilylation of various carbonyl compounds by triethylsilane.10 Stereoselectivity is observed in the hydrosilylation of ketones11 as in the reactions of 4-t-butylcyclohexanone and triethylsilane catalyzed by ruthenium,12 chromium,13 and rhodium12,14 metal complexes (eq 4). Triethylsilane and Chlorotris(triphenylphosphine)rhodium(I) catalyst effect the regioselective 1,4-hydrosilylation of a,b-unsaturated ketones and aldehydes.15,16 Reduction of mesityl oxide in this manner results in a 95% yield of product that consists of 1,4- and 1,2-hydrosilylation isomers in a 99:1 ratio (eq 5). This is an exact complement to the use of phenylsilane, where the ratio of respective isomers is reversed to 1:99.16

Silane Alcoholysis.

Triethylsilane reacts with alcohols in the presence of metal catalysts to give triethylsilyl ethers.17 The use of dirhodium(II) perfluorobutyrate as a catalyst enables regioselective formation of monosilyl ethers from diols (eq 6).17a

Formation of Singlet Oxygen.

Triethylsilane reacts with ozone at -78 °C in inert solvents to form triethylsilyl hydrotrioxide, which decomposes at slightly elevated temperatures to produce triethylsilanol and Singlet Oxygen. This is a convenient way to generate this species for use in organic synthesis.18

Reduction of Acyl Derivatives to Aldehydes.

Aroyl chlorides and bromides give modest yields of aryl aldehydes when refluxed in diethyl ether with triethylsilane and Aluminum Chloride.19 Better yields of both alkyl and aryl aldehydes are obtained from mixtures of acyl chlorides or bromides and triethylsilane by using a small amount of 10% Palladium on Carbon catalyst (eq 7).20 This same combination of triethylsilane and catalyst can effect the reduction of ethyl thiol esters to aldehydes, even in sensitive polyfunctional compounds (eq 8).21

Radical Chain Reductions.

Triethylsilane can replace toxic and difficult to remove organotin reagents for synthetic reductions under radical chain conditions. Although it is not as reactive as Tri-n-butylstannane,22 careful choice of initiator, solvent, and additives leads to effective reductions of alkyl halides,23,24 alkyl sulfides,23 and alcohol derivatives such as O-alkyl S-methyl dithiocarbonate (xanthate) and thionocarbonate esters.22,23,25,26 Portionwise addition of 0.6 equiv of Dibenzoyl Peroxide to a refluxing triethylsilane solution of O-cholestan-3b-yl O-(4-fluorophenyl) thionocarbonate gives a 93% yield of cholestane (eq 9).22 The same method converts bis-xanthates of vic-diols into alkenes (eq 10).22 Addition of a small amount of thiol such as t-dodecanethiol to serve as a polarity reversal catalyst24 with strong radical initiators in nonaromatic solvents also gives good results.23,25 Treatment of ethyl 4-bromobutanoate with four equiv of triethylsilane, two equiv of dilauroyl peroxide (DLP), and 2 mol % of t-dodecanethiol in refluxing cyclohexane for 1 hour yields ethyl butanoate in 97% yield (eq 11).23

Ionic Hydrogenations and Reductive Substitutions.

The polar nature of the Si-H bond enables triethylsilane to act as a hydride donor to electron-deficient centers. Combined with Brønsted or Lewis acids this forms the basis for many useful synthetic transformations.27 Use of Trifluoromethanesulfonic Acid (triflic acid) at low temperatures enables even simple alkenes to be reduced to alkanes in high yields (eq 12).28 Boron Trifluoride monohydrate is effective in promoting the reduction of polycyclic aromatic compounds (eq 13).29 Combined with thiols, it enables sulfides to be prepared directly from aldehydes and ketones (eq 14).30 Combinations of triethylsilane with either Trifluoroacetic Acid/ammonium fluoride or Pyridinium Poly(hydrogen fluoride) (PPHF) are effective for the reductions of alkenes, alcohols, and ketones (eq 15).31 Immobilized strong acids such as iron- or copper-exchanged Montmorillonite K1032 or the superacid Nafion-H33 facilitate reductions of aldehydes and ketones32 or of acetals33 by increasing the ease of product separation (eq 16). Boron trifluoride and triethylsilane are an effective combination for the reduction of alcohols, aldehydes, ketones (eq 17),34 and epoxides.35 Boron Trifluoride Etherate sometimes may be substituted for the free gas.36

Triethylsilane in 3M ethereal Lithium Perchlorate solution effects the reduction of secondary allylic alcohols and acetates (eq 18).37 The combination of triethylsilane and Titanium(IV) Chloride is a particularly effective reagent pair for the selective reduction of acetals.38 Treatment of (±)-frontalin with this pair gives an 82% yield of tetrahydropyran products with a cis:trans ratio of 99:1 (eq 19).38b This exactly complements the 1:99 product ratio of the same products obtained with Diisobutylaluminum Hydride.38b

Triethylsilane and trityl salts39 or Trimethylsilyl Trifluoromethanesulfonate40 are effective for the reduction of various ketones and acetals, as are combinations of Chlorotrimethylsilane and indium(III) chloride41 and Tin(II) Bromide and Acetyl Bromide.42 Isophthaldehyde undergoes reductive polycondensation to a polyether when treated with triethylsilane and Triphenylmethyl Perchlorate.43

Triethylsilane reduces nitrilium ions to aldimines,44 diazonium ions to hydrocarbons,45 and aids in the deprotection of amino acids.46 With aluminum halides, it reduces alkyl halides to hydrocarbons.47

Related Reagents.

Phenylsilane-Cesium Fluoride; Tri-n-butylstannane; Tricarbonylchloroiridium-Diethyl(methyl)silane-Carbon Monoxide; Triethylsilane-Trifluoroacetic Acid.


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James L. Fry

The University of Toledo, OH, USA



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