Anions enol silyl ethers

The fluoride anion has a pronounced catalytic effect on the aldol reaction between enol silyl ethers and carbonyl compounds [13] This reacbon proceeds at low temperature under the influence of catalytic amounts (5-10 mol %) of tetra-butylammonium fluoride, giving the aldol silyl ethers in high yields (equation 11). 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

In a similar vein, various electron acceptors yielding anion radicals that undergo rapid unimolecular decomposition also facilitate the efficacy of Scheme 1 by effectively obviating the back-electron transfer. For example, the nitration of enol silyl ether with tetranitromethane (TNM) occurs rapidly (despite an unfavorable redox equilibrium)78 owing to the fast mesolytic fragmentation of the TNM anion radical79 (Scheme 15). 

The preparation of a-hydroxy carbonyl compounds has been accomplished by the oxidation of enolates using both oxygen6 and MoC PyHMPA-(MoOPh).7 Acyl anion equivalents offer another route to this useful class of compounds. The procedure presented here for the synthesis of 6-hydroxy-3,5,5-tr1methyl-2-cyclohexen-l-one illustrates the use of MCPBA oxidation of an enol silyl ether as a method for obtaining an a-hydroxy enone. The procedure is a scaleup of a published synthesis. ... 

Super anions. Unlike quaternary ammonium fluorides, 1 can be obtained anhydrous, li reacts with enol silyl ethers to form an unsolvated tris(diethylamino)-sulfonium (TAS) enolate in which there is negligible interaction hetween the ions.3 The naked enolate undergoes C-alkylation at —78 to — 30°. ... 

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

The combination of fluoride ions and enol silyl ethers provides a useful method for the generation of enolate anions [9]. Watts, Haswell and coworkers applied a borosilicate glass microreactor, having channel dimensions of 100 pm x 50 pm and equipped with an electrosmotic flow (EOF) pumping system, to the C-acylation of enolate anions, which leads to 1,3-diketones [10,11]. A THF solution of tetrabutyl-ammonium fluoride (TBAF) was placed in reservoir A, a THF solution of benzoyl... 

Titanium trichloride fimctions as an excellent reductive Nef alternative reagent. This aqueous reagent is very acidic, so that acid sensitive groups such as ketals and esters do not survive unless an acetate buffer is used. Systems prcme to acid-catalyzed rearrangements may then successfully undeigo the reaction (equation 10).Some veiy sensitive multifunctional compounds have been obtained using this modified Nef procedure (equation 11). A related process is the formation of 1,4-diketones via in situ generation of a nitronatc anion by the Lewis acid catalyzed addition of an enol silyl ether to a nitroalkene (equation 12). ... 

A wide range of nucleophilic substrates of different reactivity were trifluoromethylated with these reagents. The substrates include carbanions, activated aromatics, heteroaromatics, enol silyl ethers, enamines, phosphines, thiolate ions and iodide anions. " (Scheme 3.8) The least reactive substrates, such as triphenylphosphine, aniline and phenols, require the use of the most reactive dinitro derivative. Most of the reactions can be conveniently performed with the unsubstituted 5-trifluoromethyl dibenzothiophenium salt (35). The least reactive sulfonium salts are the acyclic sulfonium compounds which reacted only with the sodium thiolates.55,59... 

Reaction of the highly explosive fluorotrinitromethane with the anion of 2,4,6-trinitrotoluene, prepared with potassium hydroxide, gives the highly explosive a,2,4,6-tetranitrotoluene in 89% yield.609 Ketone enol silyl ethers and the equally highly explosive tetranitromethane react to give a-nitro ketones in low to very high yields (Eq. 188).610... 

Conversion of aldehydes into the corresponding enol silyl ethers can be accomplished with a number of enolization-silylation reagents KH/DME-TMS-Cl/EtsN, TMS triflate with amines or bis(tri-methylsilyl)acetamide ° among them. In the specific case of siloxyethylene, it has been prepared by the fragmentation of tetrahydrofuran with butyllithium and silylation of the resulting enolate anion (Scheme... 

Generation of enol silyl ethers from acyclic ketone precursors can be accomplished using the same kind of reagents. Depending on the reaction conditions, stereoselective formation of either the ( )- or the (Z)-isomer of the enol silyl ethers has been reported (Scheme 11). An in situ method of generating the enolate anion with lithium dialkylamides in the presence of trimethylchlorosilane leads to enhanced selection for the kinetically preferred enol silyl ether (e.g. 34a). Lithium r-octyl-r-butylamide (LOBA) is... 

Treating succinic anhydrides (64) with triethylamine, zinc chloride and trimethylchlorosilane in acetonitrile gives 2,5-bis(trimethylsiloxy)furans (65). ° The relative ease of silylation in these cases demonstrates one of the differences between enol silyl ether chemistry and classical enolate anion chemistry. It would have been quite difficult to generate the dianion of succinic anhydride. 

Aminohydroxylation. Enolate anions generated from enol silyl ethers (by CsF) undergo Ag-catalyzed a-aminohydroxylation with PhN=0. An enantioselective version is readily performed in the presence of the phosphite ligand 5. ... 

In this chapter, we present unique aspects of the rearrangement with a focus on (1) the production of carbanions under mild conditions, (2) the generation of synthetically valuable enol silyl ethers that are often the products, (3) the potential for incorporation of die Brook rearrangement in tandem and sequential anion relay processes, and (4) the utility of the rearrangement in generating chiral, configurationally stable carbanions. 

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