Enolization metal catalysis

The metal catalysis method has been used for the preparation of simple enols, for example, by isomerization of allylic alcohols. These enols are stable enough for isolation (see p. 75), but slowly tautomerize to the aldehyde or ketone, with half-lives ranging from 40-50 min to several days. ... 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

The metal catalysis method has been used for the preparation of simple enols, by isomerization of allylic alcohols, e.g.,71a... 

Studies of catalytic asymmetric Mukaiyama aldol reactions were initiated in the early 1990s. Until recently, however, there have been few reports of direct catalytic asymmetric aldol reactions [1]. Several groups have reported metallic and non-metallic catalysts for direct aldol reactions. In general, a metallic catalysis involves a synergistic function of the Bronsted basic and the Lewis acidic moieties in the catalyst (Scheme 2). The Bronsted basic moiety abstracts an a-pro-ton of the ketone to generate an enolate (6), and the Lewis acidic moiety activates the aldehyde (3). 

Imidoiodanes, ArINTs (Section 2.1.12.4), can be used for various amidations under transition metal catalysis (Section 3.1.21) [584-586] or under metal-free conditions [587,588], In particular, o-alkoxyphenyliminoiodane 518 readily reacts with silyl enol ethers 517 in the presence of BFs-etherate to give products of a-tosylamination 519 in good yields (Scheme 3.205) [588], Furthermore, reagent 518 in the presence of catalytic amounts of iodine readily reacts with adamantane to give the product of tosylami-nation (520) in excellent yield under very mild conditions. For comparison, PhINTs reacts with adamantane and iodine (0.2 equiv) in dichloromethane at room temperature in 2 h to afford 1-tosylaminoadamantane 520 in only 63% yield [589],... 

Alternatively, the iminium-activation strategy has also been apphed to the Mukaiyama-Michael reaction, which involves the use of silyl enol ethers as nucleophiles. In this context, imidazolidinone 50a was identified as an excellent chiral catalyst for the enantioselective conjugate addition of silyloxyfuran to a,p-unsaturated aldehydes, providing a direct and efficient route to the y-butenolide architecture (Scheme 3.15). This is a clear example of the chemical complementarity between organocatalysis and transition-metal catalysis, with the latter usually furnishing the 1,2-addition product (Mukaiyama aldol) while the former proceeds via 1,4-addition when ambident electrophiles such as a,p-unsaturated aldehydes are employed. This reaction needed the incorporation of 2,4-dinitrobenzoic acid (DNBA) as a Bronsted acid co-catalyst assisting the formation of the intermediate iminium ion, and also two equivalents of water had to be included as additive for the reaction to proceed to completion, which... 

The first use of rare earth metals in the aldol reaction began in the case of cerium enolate (198). Subsequently, Kagan and Kobayashi groups reported systematically the use of rare earth metalscatalyzed for the Mukaiyama aldol-type reaction of silyl enol ethers with aqueous formaldehyde solution (199,200). The efficiency of rare earth metals in a Mukaiyama aldol reaction of 1-trimethylsiloxycyclohexene with benzaldehyde was examined in aqueous THF (Scheme 52). Of the rare earth metal trifiates screened, catalytic efficiency was increased in the order of Yb (91%) > Gd (89%) > Lu (88%) > Nd (83%) > Dy (73%) > Er (52%) > Ho(47%) > Sm (46%) > Eu (34%) > Tm (20%) > La (8%) > Y (trace) (201,202). For different aldol or aldol-type reactions, every rare earth metal occupied its special position in the aldol reaction with distinctive catalytic activity. There were several reviews concerning the rare earth metals catalyzed aldol reactions (203,204). New progress in this context will be discussed herein according to rare earth metals catalysis especially for the past 10 years. 

The aldol reaction is an important carbon-carbon bond-forming method for constructing p-hydroxy carbonyl compounds in which new stereogenic centers are created. Especially, regio- and stereoselective aldol reactions are the most useful for organic synthesis of complex molecular skeletons [11-15]. From a viewpoint of atom economy, an aldol reaction via direct formation of an enolate with a catalytic amoimt of base is highly desired, and high Brpnsted basicity of the alkaline-earth metal compounds is suitable for this purpose. In recent researches on chiral alkaline-earth metal catalysis, direct-type asymmetric aldol and related reactions have been developed. 

Various methods to convert enol derivatives directly into olefins via metal catalysis have been reported. Enol trifluoromethanesulphonates react well with lithium dialkylcuprates stereoselectively to give coupled products (Scheme 18). ... 

Asymmetric Metal Catalysis The first asymmetric ct-sulfenylation using prochiral substrates under a chiral catalyst was reported by Mukaiyama and co-workers in 1986 (Scheme 46.45). In this pioneering study, the reaction between tin(II) enolates of ketones or 3-acyl-oxazolidin-2-ones and thiosulfonates 394 proceeded with modest-to-high enantioselectivities in the presence of chiral diamine 396. 

In this important area of silyl enol ether chemistry, similar transformations have been realized by transition metal catalysis for example, silyl enol ethers carrying suitable olefinic side chains have been cyclized in the presence of palladium(II) complexes. " ... 

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

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