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Synthesis of silyl enol ethers and related compounds
Various ionic liquids have been tested for the preparation of silyl enol ethers from aldehydes and ketones with (bistrimethylsilyl)acetamide (BSA). Yields have been strongly influenced by the nature of the cation of the ionic liquid.
M. Smietana, C. Mioskowski, Org. Lett., 2001, 3, 1037-1039.
A tethered ruthenium complex with a Ru-S bond catalyzes a dehydrogenative coupling between enolizable carbonyl compounds and equimolar amounts of triorganosilanes. The complex plays a dual role by activating the Si-H bond to release a silicon electrophile and by abstracting an α-proton from the intermediate silylcarboxonium ion, only liberating dihydrogen as the sole byproduct.
C. D. F. Königs, H. F. T. Klare, Y. Ohki, K. Tatsumi, M. Oestreich, Org. Lett., 2012, 14, 2842-2845.
N-Heterocyclic carbenes (NHCs) catalyze the silyl transfer from trialkylsilyl ketene acetals to ketones. A series of enolizable ketones as well as cyclohexanecarboxaldehyde were efficiently converted into the corresponding silyl enol ethers at 23°C in THF.
J. J. Song, Z. Tan, J. T. Reeves, D. R. Fandrick, N. K. Yee, C. H. Senanayake, Org. Lett., 2008, 10, 877-880.
An efficient, practical, robust method for the regio- and stereoselective preparation of ketene trimethylsilyl acetals derived from tert-butyl esters can be performed under convenient reaction conditions: LDA-TMSCl, 0-5 °C, and cyclopentyl methyl ether (CPME) as solvent. The stereocontrolled preparation of highly reactive β-ketoester-derived tert-butyl (1Z,3E)-1,3-bis(TMS)dienol ethers is also described.
T. Okabayashi, A. Iida, K. Takai, Y. Nawate, T. Misaki, Y. Tanabe, J. Org. Chem., 2007, 72, 8142-8145.
N,O-Silyl dienyl ketene acetals are useful reagents for highly enantioselective vinylogous aldol additions to various aldehydes in the presence of SiCl4 and the catalytic action of a chiral phosphoramide.
S. E. Denmark, J. R. Heemstra, Jr., J. Am. Chem. Soc., 2006, 128, 1038-1039.
1-Siloxy-1-alkenylcopper species were generated by 1,2-Csp2-to-O silyl migration of the copper enolates of acyltriphenylsilanes. The alkenylcopper species reacted with methyl, benzyl, allylic, tributylstannyl halides and in the presence of Pd(0) catalyst with aryl and alkenyl iodides to give geometrically pure (Z)-enol silyl ethers.
A. Tsubouchi, K. Onishi, T. Takeda, J. Am. Chem. Soc., 2006, 128, 14268-14269.
Palladium-catalyzed hydrosilylation of α,β-unsaturated ketones and cyclopropyl ketones with hydrosilanes gives (Z)-silyl enolates in good yields.
Y. Sumida, H. Yorimitsu, K. Oshima, J. Org. Chem., 2009, 74, 7986-7989.
A three-component nickel-catalyzed coupling of enals, alkynes, and silanes forms an enol silane and a trisubstituted alkene with >98:2 stereoselectivity. The reaction tolerates a broad range of functionality including aldehydes, ketones, esters, free hydroxyls, and basic secondary amines.
A. Herath, J. Montgomery, J. Am. Chem. Soc., 2008, 130, 8132-8133.
The use of an oxazaborolidinium ion catalyst enables a highly stereoselective synthesis of (Z)-silyl enol ethers from alkyl aryl ketones and trimethylsilyldiazomethane (TMSD). The conversion of cyclic ketones gives ring-expanded silyl enol ethers.
B. Chul, Kang, S. Y. Shim, D. H. Ryu, Org. Lett., 2014, 16, 2077-2079.
Lithium diisopropylamide (LDA) promotes virtually quantitative conversion of allylic ethers to (Z)-propenyl ethers with very high stereoselectivity in THF at room temperature. The reaction time for the conversion increases with more sterically hindered allylic ethers.
C. Su, P. G. Williard, Org. Lett., 2010, 12, 5378-5381.