Metal enolates determination

Metal enolates found varied application in chemical analysis. An outstanding group are certain lanthanide enolates used as shift reagents in NMR spectroscopy. The analytical methods discussed in Section IV are based on formation of a metal enolate for separation, detection, identification and determination of metal ions or the use of a metal enolate as ancillary reagent to improve analytical quality. Of special relevance in analytical chemistry are the metal /3-diketonates, M(dik) , derivatived from deprotonated /3-diketones (dikH),... 

Metal ynolates are not as easy to prepare in a similar fashion as metal enolates, because the intermediates may be labile monosubstituted ketenes. Several preparative methods for alkali metal ynolates have been reported, among which some have been used as intermediate steps in one-pot organic syntheses. Silyl ynolates have been prepared from lithium ynolates. There have been few reports on the other metal ynolates. Since there is no universal method to determine the yield of metal ynolates, the efficiency of preparation is estimated from the results of some of the following reactions. 

Alcohols, such as methanol and ethanol, lead to the sole formation of saturated alcohols from unsaturated ketones when the former are present in excess during the reduction. Mixtures of ketones and alcohols are generally formed when 1 equiv. of these proton donors is employed. These alcohols have an acidity comparable to that of saturated ketones, and when they are present, an equilibrium can be established between the initially formed metal enolate and the saturated ketone. The latter is then reduced to the saturated alcohol. Such reductions generally do not occur to a very significant extent when 1 equiv. of r-butyl alcohoP or some less acidic proton donor, such as triphenylcarbinol, is employed. The acidity of the ketone involved, as well as the solubility of the metal enolate in the reaction medium, are important in determining whether alcohols are formed. 

In recent years, investigations of the diastereoselectivity and enantioselectivity of alkylations of metal enolates of carboxylic acid derivatives have become one of the most active areas of research in synthetic organic chemistry. Intraannular, extraannular and chelate-enforced intraannular chirality transfer may be involved in determining the stereochemistry of these alkylations. 

The enantioface selective protonation of prochiral enol derivatives is a simple and attractive route for the preparation of optically active carbonyl derivatives. Reports of stoichiometric protonation of metal enolates by a chiral proton source at low temperature leads to optical yields from 20 to 85% ee and yeast esterase catalyzes the hydrolysis of 1-acetoxycycloalkenes with enantioselectivi-ties between 41 and 96% for enol protonation [17,18]. These reactions involve enolates under basic conditions. Hydrolysis of enol ethers under acidic conditions proceeds via a rate-determining carbon protonation and is catalyzed by carboxylic acids [19,20]. Raymond et al. [21] reasoned that a complementary... 

The stereochemistry of the reactions of chiral carbonyl compounds with nucleophiles has been a topic of considerable theoretical and synthetic interest since the pioneering study by Cram appeared in 1952. The available predictive models focus entirely on the conformational and stereoelectronic demands of the chiral carbonyl substrate, the implicit assumption being that the relative stabilities of the competing transition states are determined only by stereoelectronics and the minimization of nonbonded interactions between the substituents on the chiral center and the nucleophile. These models totally ignore the possibility, however, that the geometric requirements of the nucleophile may also have an effect on reaction diastereoselectivity. Considerable evidence is now available, particularly in the reactions of Type I (Z)-crotylboronates and Z(0)-metal enolates, that the stereochemistry of the nucleophile is indeed an important issue that must be considered when assessing reaction diastereoselectivity. 

Reactions involving Enols or Enolic Derivatives.—A review of the structure and reactivity of alkali-metal enolates includes some steroidal reactions. A study of the mechanism of isomerization of androst-5-en-17/3-ol-3-one to testosterone indicated that the acid-catalysed process proceeds through rate-determining enolization whereas the base-catalysed reaction proceeds through rate-determining protonation of an enolate ion. Bromination of the 4,4,6-trimethyl-A -3-oxo-compounds (111)—(113) gave the 2a-bromo-derivatives, each of which showed anomalous o.r.d. curves. Bromination at C-2 was favoured for the... 

In the mechanism of the Biginelli synthesis [265], the rate-determining step is the acid-catalyzed formation of an acylimine 35 from aldehyde and urea. By N-protonation (or metal-N-coordination), the imine 35 is activated (as an iminium ion) and intercepted by the P-ketoester (as enol or metal enolate) to give rise to an open-chain ureide 36, which subsequently cyclizes (via the cyclic ureide 37 and its dehydration) to afford the dihydropyrimidinone 33. Biginelli compounds of type 33 have been synthesized independently in multistep sequences [266]. 

Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

Substrate control This refers to the addition of an achiral enolate (or allyl metal reagent) to a chiral aldehyde (generally bearing a chiral center at the a-position). In this case, diastereoselectivity is determined by transition state preference according to Cram-Felkin-Ahn considerations.2... 

The mechanism and thermodynamics of transesterification of acetate-ester enolates in the gas phase have been investigated. The catalytic effect of alkali-metal t-butoxide clusters on the rate of ester interchange for several pairs of esters has been determined in non-polar and weakly polar solvents. Reactivities increase in the order (Li+ < Na+ < K+ < Rb+ < Cs+) with the fastest rates reaching lO catalytic... 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

THF and DME are slightly polar solvents which are moderately good cation solvators. Coordination to the metal cation involves the oxygen lone pairs. These solvents, because of their lower dielectric constants, are less effective at separating ion pairs and higher aggregates than are the polar aprotic solvents. The crystal structures of the lithium and potassium enolates of methyl /-butyl ketone have been determined by X-ray crystal-... 

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