Ethers, silyl enol from enolate anions

The reactions of nitroalkenes (42) with various enols (43b) (vinyl ethers, silyl, and acyl enolates, ketene acetals) have been studied in most detail (110, 111, 125—154). As a mle, these reactions proceed smoothly to give the corresponding nitronates (35f) in yields from high to moderate. As in the reactions with enamines, the formation of compounds (44b) is attributed to the ambident character of the anionic centers in zwitterionic intermediates analogous to those shown in Scheme 3.43. 

Addition of a silylating reagent such as Me3SiCl to the reaction mixture traps the enolate anions and produces two silyl enol ethers in a ratio which reflects the ratio of the enolate anions. Thus if 2-methylcyclohexanone is added to the hindered base LDA at -78 °C and the mixture stirred for 1 hour at -78 °C and quenched with MeySiCl, then the major product is the silyl enol ether derived from the kinetic enolate. In contrast, heating 2-methylcyclohexanone, triethylamine, and Me3SiCl at 130 °C for 90 hours... 

The enol acetates, in turn, can be prepared by treatment of the parent ketone with an appropriate reagent. Such treatment generally gives a mixture of the two enol acetates in which one or the other predominates, depending on the reagent. The mixtures are easily separable.An alternate procedure involves conversion of a silyl enol ether (see 12-17) or a dialkylboron enol ether (an enol borinate, see p. 645) to the corresponding enolate anion. If the less hindered enolate anion is desired (e.g., 126), it can be prepared directly from the ketone by treatment with LDA in THE or 1,2-dimethox-yethane (DME) at... 

When a silyl enol ether is the trimethylsilyl derivative (Me3Si-0-C=C), treatment with methyllithium will regenerate the hthium enolate anion and the volatile trimethylsilane (Me3SiH). The enolate anion can be used in the usual reactions. In a similar reaction, a trimethylsilyl enol ether was treated with Cp2Zr (from Cp2ZrCl2/2 BuLiArHE/-78°C), and subsequent quenching with D2O led to incorporation of deuterium at the vinyl carbon (C=C-D). ... 

Alkylation or acylation of ketones, sulfides, and amines. This reagent generally reacts with alcohols or carboxylic acids to form 2,2,2-trifluoroethyl ethers or esters in satisfactory yields, except in the case of alcohols prone to dehydration. The reaction of these ethers provides a simple synthesis of unsymmetrical sulfides (equation I). A similar reaction can be used for preparation of secondary amines or amides (equation II). Enolate anions (generated from silyl enol ethers with KF) can be alkylated or acylated with a or b (equation III). Use of Grignard reagents in this type of coupling results in mediocre yields. 

Bridged bicyclic ketones 7 are obtained by the Cope rearrangement of silyl enol ethers or lithium enolate anions derived from ent/o-vinylcyclopropanes 4. i 11 The corresponding exo-isomers of 5 require flash-vacuum pyrolysis at temperatures in the range of 250-500°C to... 

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... 

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. ... 

We urge you to try to write a mechanism for the above process. You may find it useful to think of silyl groups as fat protons. Start off with silylating the carbonyl oxygen and proceed from there. Silyl enol ethers function as less reactive analogs of enolate anions. Thus, with titanium tetrachloride as a catalyst, silyl enol ethers undergo aldol condensations with a variety of carbonyl compounds. This is the Mukaiyama aldol condensation ... 

The proposed catalytic cycle [42] is analogous to that shown in Scheme 5.6, except for the additional release of an enolate anion due to the fluoride-induced desilylation. Oxidative addition of allyl carbonates leads to the formation of the allyl complex 78, COj, and an alkoxide RO . The fluoride source and the alkoxide RO are capable of liberating an enolate anion by desilylation. This explains why substoichiometric amounts of Bu4NPhgSiF2 are sufficient to maintain the catalytic cycle that is displayed in Scheme 5.25 for the allylation of 2-methylcyclohexanone through the silyl enol ether. The carbon-carbon bond-forming step is assumed to occur by a collapse of the ion pair 79 consisting of the cationic allylpalladium complex and the enolate anion. Aside from these ionic species, covalently bound palladium enolates were also discussed. 

A large proportion of Davis s work has been involved in the elucidation of the transition state employed in the transfer of the oxygen from the oxaziridine to the olefin substrate. Davis favoured the planar transition state and was at the time supported by theoretical calculations however, more recent calculations favour the spiro transition state [6]. Davis has also described the asymmetric oxidation of enolate anions by chiral oxaziridines, which led to a-hydroxyketones with enanti-oselectivities of up to 95% ee [56, 57], Silyl enol ethers have also been reported to give epoxides when treated with oxaziridines, but the instability of these compounds is too great to allow isolation [37,58,59], To date, only Davis has reported successful isolation of a-silyloxy epoxides [60],... 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

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. 

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