Enolates equivalents

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]... 

Akylsilanes are more reactive than vinylskanes in Friedel-Crafts reactions, as shown in the selective acylation of 2,3-disilylalkenes. The akylsilanes, a-skyloxyakyltrialkylsilanes, have been used as enolate equivalents in the preparation of 1,4-diketones (178). The mild reaction conditions required for these reactions tolerate many other functional groups, providing valuable synthetic routes. 

The Darzens condensation reaction has been used with a wide variety of enolate equivalents that have been covered elsewhere. A recent application of this important reaction was appljed toward the asymmetric synthesis of aziridine phosphonates by Davis and coworkers.In this application, a THF solution of sulfinimine 34 (0.37 mmol, >98% ee) and iodophosphonate 35 (0.74 mmol) was treated with LiHMDS (0.74 mmol) at -78 °C to give aziridine 36 in 75% yield. Treatment of 36 with MeMgBr removed the sulfinyl group to provide aziridine 37 in 72% yield. 

Disconnection of the 1,3-dlketone obviously comes next but it will be better to add an activating group to control the reaction. One possibility is to acylate specific enol equivalent (56) with acid chloride (57). 

Specific enol equivalents will be needed for both synthons (61) and (66), Since (61) is to give a double bond but (66) is to give an alcohol, the logical choices are a Wittig reagent - actually (67) - for (61) and a Reformatsky reagent for (66). The ester to aldehyde conversion (65 63) Is easiest by over-reduction and re-... 

Tiic synthesis of enone (34) requires an aldol condensation between acetone and KCHO this may not give a good yield as RCHO may prefer to condense with Itself if it has enolisable protons. The alternative disconnection (33b) avoids this problem as we can use acetoacetatc for the synthon (34) and a specific enol equivalent for (35),... 

This is a l,4 diketone and disconnection of the central bond separates the two rings. We require a specific enol equivalent lor (4) - they used activated ketone (6) - and a reagent for unnatural synthon (5) -they used a-chloroketone (7). 

One published synthesis uses a propargyl bromide for (21) and an activating group to provide a specific enol equivalent (22),... 

Scheme 12.46 Further apphcations of ketenes as enolate equivalents...

Further studies by Bode and co-workers have shown that enolate formation from a-chloroaldehydes and subsequent reaction with 4-oxo-enoates or unsaturated a-ketoesters 232 generates dihydropyranones 233 in excellent diastereo- and enantio-selectivities, and with impressively low catalyst loadings [90], This work has been extended to the generation of enolate equivalents from bisulfite adducts of a-haloaldehydes 234 under aqueous conditions (Scheme 12.50) [91]. 

Scheldt and co-workers have also accessed enolate equivalents from enals to furnish cyclopentanes 236 asymmetrically. Formation of the enolate equivalent from enals 235 with the NHC, followed by an intramolecular Michael reaction and 0-acylation, gives the lactone products 236, which are readily opened by either alcohols or amines to generate functionalised cyclopentane derivatives 237 in excellent ee. 

Reaction conditions that involve other enolate derivatives as nucleophiles have been developed, including boron enolates and enolates with titanium, tin, or zirconium as the metal. These systems are discussed in detail in the sections that follow, and in Section 2.1.2.5, we discuss reactions that involve covalent enolate equivalents, particularly silyl enol ethers. Scheme 2.1 illustrates some of the procedures that have been developed. A variety of carbon nucleophiles are represented in Scheme 2.1, including lithium and boron enolates, as well as titanium and tin derivatives, but in... 

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. 

The Mukaiyama aldol reaction refers to Lewis acid-catalyzed aldol addition reactions of silyl enol ethers, silyl ketene acetals, and similar enolate equivalents,48 Silyl enol ethers are not sufficiently nucleophilic to react directly with aldehydes or ketones. However, Lewis acids cause reaction to occur by coordination at the carbonyl oxygen, activating the carbonyl group to nucleophilic attack. 

The Mukaiyama aldol reaction can provide access to a variety of (3-hydroxy carbonyl compounds and use of acetals as reactants can provide (3-alkoxy derivatives. The issues of stereoselectivity are the same as those in the aldol addition reaction, but the tendency toward acyclic rather than cyclic TSs reduces the influence of the E- or Z-configuration of the enolate equivalent on the stereoselectivity. 

Stereochemical Control by the Enolate or Enolate Equivalent. The facial selectivity of aldol addition reactions can also be controlled by stereogenic centers in the nucleophile. A stereocenter can be located at any of the adjacent positions on an enolate or enolate equivalent. The configuration of the substituent can influence the direction of approach of the aldehyde. 

Enantioselective Catalysis of the Aldol Addition Reaction. There are also several catalysts that can effect enantioselective aldol addition. The reactions generally involve enolate equivalents, such as silyl enol ethers, that are unreactive toward the carbonyl component alone, but can react when activated by a Lewis acid. The tryptophan-based oxazaborolidinone 15 has proven to be a useful catalyst.148... 

Methyl 1-phenylthiovinyl ketones can also be used as enones in kinetically controlled Robinson annulation reactions, as illustrated by Entry 6. Entry 7 shows a annulation using silyl enol ether as the enolate equivalent. These reactions are called Mukaiyama-Michael reactions (see Section 2.6.3). 

Addition of enols, enolates, or enolate equivalents to imines or iminium ions provides an important route to (3-amino ketones. 

Acyliminium ions are sufficiently electrophilic to react with enolate equivalents such as silyl enol ethers207 and isopropenyl acetate.208... 

The scope of the conjugate addition reaction can be further expanded by use of Lewis acids in conjunction with enolate equivalents, especially silyl enol ethers and silyl ketene acetals. The adduct is stabilized by a new bond to the Lewis acid and products are formed from the adduct. 

As in the case of aldol addition, the scope of conjugate addition reactions can be extended by the use of techniques for regio- and stereospecific preparation of enolates and enolate equivalents. If the reaction is carried out with a stoichiometrically formed enolate in the absence of a proton source, the initial product is the enolate of the adduct. The replacement of a tt bond by a ct bond ensures a favorable AH. 

Conditions for effecting conjugate addition of neutral enolate equivalents such as silyl enol ethers in the presence of Lewis acids have been developed and are called Mukaiyama-Michael reactions. Trimethylsilyl enol ethers can be caused to react with electrophilic alkenes by use of TiCl4. These reactions proceed rapidly even at -78° C.308... 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

Palladium-Catalyzed Arylation of Enolates. Very substantial progress has been made in the use of Pd-catalyzed cross coupling for arylation of enolates and enolate equivalents. This reaction provides an important method for arylation of enolates, which is normally a difficult transformation to accomplish.171 A number of phosphine ligands have been found to promote these reactions. Bulky trialkyl phosphines such as /n. v-(/-butyl)phosphinc with a catalytic amount of Pd(OAc)2 results in phenylation of the enolates of aromatic ketones and diethyl malonate.172... 

Conditions for arylation of enolate equivalents have also been developed. In the presence of ZnF2, silyl enol ethers, silyl ketene acetals, and similar compounds react. For example, the TMS derivatives of /V-acyl oxazolidinones can be arylated. 

In Section 8.2.3.2, we discussed arylation of enolates and enolate equivalents using palladium catalysts. Related palladium-phosphine combinations are very effective catalysts for aromatic nucleophilic substitution reactions. For example, conversion of aryl iodides to nitriles can be done under mild conditions with Pd(PPh3)4 as a catalyst. 

The focus of Chapters 1 and 2 is enolates and related carbon nucleophiles such as silyl enol ethers, enamines, and imine anions, which can be referred to as enolate equivalents. 

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

In Section 3.5 on alkene isomerization, it was mentioned that Li and co-workers reported a RuCl2(PPh3)3-catalyzed shuffling of functional groups of allylic alcohols in water (Eq. 3.35).140 Since the reaction proceeds through an enol intermediate, allyl alcohols can thus be considered as enol equivalents.203 This has been developed into an aldol-type reaction by reacting allyl alcohols with aldehyde (Scheme 3.11).204 The presence of In(OAc)3 promoted the aldol reaction with a-vinylbenzyl alcohol and aldehyde.205... 

The aldol reactions introduced thus far have been performed under basic conditions where enolate species are involved as the reactive intermediate. In contrast to the commonly accepted carbon-anion chemistry, Mukaiyama developed another practical method in which enol species can be used as the key intermediates. He is the first chemist to successfully demonstrate that acid-catalyzed aldol reactions using Lewis acid (such as TiCU) and silyl enol ether as a stable enol equivalent can work as well.17 Furthermore, he developed the boron tri-fluoromethane sulfonate (triflate)-mediated aldol reactions via the formation of formyl enol ethers. 

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