Silane, tris reduction

Scheme 5.8.20 Tris(trimethylsilyl)silane mediated reduction...

In an effort to identify a more stereoselective route to dihydroagarofuran (15), trimethylsilylated alkyne 17 was utilized as a substrate for radical cyclization (Scheme 2). Treatment of 17 with a catalytic amount of AIBN and tri-n-butyltin hydride (1.25 equiv) furnishes a mixture of stereoisomeric vinyl silanes 18 (72% combined yield) along with an uncyclized reduction product (13% yield). The production of stereoisomeric vinyl silanes in this cyclization is inconsequential because both are converted to the same alkene 19 upon protodesiiyiation. Finally, a diastereoselective di-imide reduction of the double bond in 19 furnishes dihydroagaro-... 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

The silane, prepared by reduction of tris(2-propylthio)silylenium perchlorate with diisobutylaluminium hydride at —78°C, is subsequently isolated by high-vacuum distillation from the mixture. Heating must be very mild to prevent explosion. 

Dicyclohexyl Ether [Brpnsted Acid Promoted Reduction of a Ketone to a Symmetrical Ether].313 Cyclohexanone (3.92 g, 40 mmol) and tri(n-butyl) silane (1.78 g, 20 mmol) were placed in a round-bottomed flask. TFA (75 mmol) was added slowly over a one-hour period to the reaction mixture, which was held at —35°. After complete addition, the reaction flask was placed in a freezer at —15° for 70 hours. Direct distillation gave dicyclohexyl ether 2.91 g (16 mmol, 80%) bp 119-121718 Torr. 

The quininium and quinidinium fluoride catalysts, 10 (R=H etc., X=F) and 8 (R=H, X=F), were used for the asymmetric reduction of alkyl aryl ketones in conjunction with silanes.1751 One of the most efficient silanes proved to be tris(trimethylsiloxy)silane, which together with 8 (R=H, X=F) reduced acetophenone to give the alcohol 102 in almost quantitative yield with 78 % ee, as shown in Scheme 31. The... 

In this approach, the cis-disubstituted tetrahydrofuranone 75 [accessible with high diastereoselectivity via radical cyclization of the acylselenide 74 with tris(tri-methylsilyl) silane and triethylborane] was transformed into the corresponding enyne 76 by reduction and Wittig olefination. The subsequent electrophilic cyclization was carried out in analogy with the biomimetic synthesis of panacene (Scheme... 

Reduction of aryl alkyl ketones with moderate to good (ee 30-80%) entantio-selectivity has been achieved using trialkoxysilanes in the presence of chiral quininium fluorides (or hydroxides) [20]. Greater selectivities were noted (ee >65%) when tris(trimethylsiloxy)silane was used. 

Tris(trimethylsilyl)silane reacts with phosphine sulfides and phosphine selen-ides under free radical conditions to give the corresponding phosphines or, after treatment with BH3-THF, the corresponding phosphine-borane complex in good to excellent yields (Reaction 4.45) [82]. Stereochemical studies on P-chiral phosphine sulphides showed that these reductions proceed with retention of configuration. An example is given in Reaction (4.46). 

Optimization of the previously reported Mannich-type reaction of trimethyl (pent-2-en-3-yloxy)silane with the sulfone Is derived from phenyl acetaldehyde (Table 5, entry 11) led to the corresponding (3-amino ketone in a good yield with moderate diastereoselectivity (2 mol% Bi(0Tf)3-4H20, yield = 84%, 24v/24v syn/anti = 72 28) (Scheme 8). Reduction of the major diastereoisomer 24v with lithium tri-ferf-butoxyaluminohydride afforded 25 as the only one diastereoisomer. Further cyclization of the latter with NaH afforded 4-benzyl-6-ethyl-5-methyl-l,3-oxazinan-2-one 26. The relative configuration of the six-membered carbamate was established as cis-cis by NMR analysis. 

Coupling reactions have also been observed in the reduction of vinyl silanes with potassium in DME at low temperatures. Although reduction of trans-1,2-bis(trimethylsilyl)ethylene or l,l,2-tris(trimethylsilyl)-ethylene gives rise to radical anions detectable by ESR, the reduction of l,l-bis(trimethylsilyl)ethylene gives rise to an ESR spectrum attributed to the radical anion of l,3-bis(trimethylsilyl)-... 

Stereoselective reduction of -keto amides.1 a-Methyl-p-keto amides are reduced by this silane in combination with tris(diethylamino)sulfonium difluorotri-methylsilicate (10, 452-453) in DMPU to the anti-alcohol, but are reduced by this silane in TFA to the syn-alcohol. 

Reduction The asymmetric reduction of a series of aryl alkyl ketones with quaternary ammonium fluorides and silanes was reported by Drew and Lawrence [55]. In these reactions, the best catalysts (e.g., 6f) were from the qui ni ne/quinidine series in fact, a fluoride salt prepared from cinchonine gave no induction. The use of trimethoxysilane resulted in faster rates but lower enantioselectivites when compared with tris(trimethoxy)silane. It is interesting that, with the... 

Tris(trimethylsilyl)silane is found to be an efficient reducing agent for a variety of functional groups. In particular, the reduction of halides, chalcogen groups, thiono esters and isocyanides are the most common ones. The efficiency of these reactions is also supported by available kinetic data. The rate constants for the reaction of (TMS)3Si radicals with a variety of organic substrates are collected in Table 2. 

Sodium borohydride-Palladium chloride. Sodium borohydride-Rhodium(lII) chloride. Sodium borohydride-Tin(II) chloride. Sodium cyanoborohydride. Sodium 9-cyano-9-hydrido-9-borabicyclo[3.3.1]nonane. Sodium dithionite. Sodium hydride-Sodium t-amyl oxide-Zinc chloride. Sodium trimethoxyborohydride. Tetra-/i-butylammonium borohydride. Tetra-n-butylammonium cyanoborohydride. Tetra-n-butylammonium octahydrotriborate. Tri-n-butyltin hydride. Triethoxy silane. Triisobutylaluminum-Bis(N-methyl-salicyclaldimine)nickel. Zinc borohydride. REDUCTIVE CYCLIZATION Cobaloximc(I). 

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