Enolate complexes

The highest enantioselectivities in the base-catalyzed Michael additions have so far been obtained using achiral bases complexed to chiral crown ethers. The addition of methyl 2,3-dihydro-l-oxo-1//-indene-2-carboxylate (1) to 3-buten-2-one using 4 mol% of a [l,T-binaphthalcnc]-2,2 -diol derived optically active crown ether 3 in combination with potassium AY/-butoxide as the base illustrates this successful method 259 260 It is assumed that the actual Michael donor is the potassium enolate complex of 1 and crown ether 3. 

Divalent transition metal 3-keto-enolate complexes as Lewis acids. D. P. Graddon, Coord. Chem. Rev., 1969, 4,1-28(117). 

Deprotonation of Group 4 mono(pentamethylcyclopentadienyl) metal acet-amidinates can be achieved in high yield using sterically encumbered bases (Scheme 103) to provide anionic enolate complexes as purple powders. These can subsequently be allowed to react with electrophiles (e.g., PhCH2Cl, CH2CI2, Me2SiCl2) to produce several new classes of metal amidinates that are not accessible by conventional routes (Scheme 104). ° ... 

Graddon, D. P. (1968). Divalent transition metal j8-ketone-enolate complexes as Lewis acids. Coordination Chemistry Reviews, 4, 1-28. 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

Similar polymerization of MMA using enolate-zirconocene catalysts has also been found [223]. The mechanism of this catalytic reaction is related to the process described in Scheme XI because the cationic enolate complex is isolobal to that of the corresponding lanthanide complex. Recently, similar cationic... 

Murahashi and co-workers developed an aldol-type condensation between various activated nitriles and aldehydes or ketones, catalyzed by cyclopentadienylruthenium enolate complexes (Equation (30)).351,351a 351b... 

The EAN of iron in this complex is 34, but it may be a solvated ion. Treatment of the salt with water gives 2-butanone, which was presumed to have been formed via nucleophilic attack on the cation to give a TT-allyl alcohol complex. This complex was then assumed to rearrange via the tricarbonyl hydride to an enol complex, which collapses to the ketone ... 

In spite of the apparent difference between conjugate addition and carbocupra-tion reactions (Sect. 10.3.2), the similarities between the key organometallic features of the two reactions are now evident. In both reactions, inner sphere electron-transfer converts the stable C-Cu bond into an unstable C-Cu bond, and the cluster-opening generates a nucleophilic, tetracoordinated alkyl group. The difference is that the product of conjugate addition (PD) remains as a lithium enolate complexed with RCu (Scheme 10.5), while the initial product of carbocupration... 

INT2, Scheme 10.7) undergoes further reaction (Li/Cu transmetalation) and generates a new organocuprate compound. (Note however that this difference could become more subtle since the product of conjugate addition (PD) might behave more like an a-cuprio(I) ketone complexed with a lithium cation [52] than a lithium enolate complexed with copper(I)). In neither reaction was any evidence of radical intermediates (i.e., SET) found by theoretical calculations [79]. 

Although the pattern of this reaction is identical to that of the Morita-BayUs-Hilbnan reaction [6], we concluded on the basis of control experiments that the presence of a rhodium or ruthenium complex is crucial for smooth coupling. The possible intermediacy of the enolate complex in the above examples suggests a transition metal-cata-... 

Scheme 6.61 Mechanistic proposals of the 12-catalyzed asymmetric Michael addition of diethyl malonate to trans-P-nitrostyrene proposed by the Takemoto group (A, B, and C) and initial enolate complex (D) with the ammonium group as additional hydrogen-bond donor initiating an alternative mechanism suggested by Sods, Ptipai, and coworkers.

In several cases thietane derivatives have been used as ligands in organo-metallic compounds. Stable enolate complexes have been derived from tetra-methyl-3-thietanone and diiron nonacarbonyl. Both compounds were heated in n-hexane at 60°C in an argon atmosphere for longer periods of... 

The metallation should proceed via the formation of a chelated tetrahedral magnesium enolate complex, with a (Z)-geometry. The conformational rigidity would be enforced by chelation of both the imide enolate and bis(sulfonamide) ligand to the tetrahedral magnesium ion. 

Examination of the pH dependence of the polymerization process established that the enolate anion of the activator was the species responsible for initiation (46). Metal chelates of the dienones were also found to be effective. Chaberek believed that the semireduced dye radicals were active initiators in his system (47), in contradistinction to the work of Chen (17). Chaberek proposed that an excited dye-enolate complex was produced during the photochemistry and that this adduct reacted with monomer to yield a radical, capable of initiation, and a semireduced dye radical. Figure 4 shows the proposed mechanism. Several aspects of this mechanism should be modified in view of present knowledge. The semireduced radical (D ) has been conclusively shown to be a terminator (17) and not an initiator. 

Allyl esters of acetoacetates7-8-9-11 react with Pd° catalysts to generate initially a bisphosphine allylpal-ladium cation, with the 3-ketocarboxylate serving as counterion. Under the reaction conditions the (3-ketocarboxylate decarboxylates, yielding a -Tr-allylpalladium ketone enolate complex. The required nucleophile is thus formed in situ and is capable of Pd-mediated alkylation. A wide spectrum of reactions have been based on this chemistry which will be discussed in later sections. 

Alkylation Alkylation of the phenylindanone 31 with catalyst 3a by the Merck group demonstrates the reward that can accompany a careful and systematic study of a particular phase-transfer reaction (Scheme 10.3) [5d,5f,9,36], The numerous reaction variables were optimized and the kinetics and mechanism of the reaction were studied in detail. It has been proposed that the chiral induction step involves an ion-pair in which the enolate anion fits on top of the catalyst and is positioned by electrostatic and hydrogen-bonding effects as well as 71—71 stacking interactions between the aromatic rings in the catalyst and the enolate. The electrophile then preferentially approaches the ion-pair from the top (front) face, because the catalyst effectively shields the bottom-face approach. A crystal structure of the catalyst as well as calculations of the catalyst-enolate complex support this interpretation [9a,91]. Alkylations of related active methine compounds, such as 33 to 34 (Scheme 10.3), have also appeared [10,11]. 

These masked enolate complexes also react smoothly with carbonyl compounds, in reactions surprisingly reminiscent of the reaction of dimedone with formaldehyde (Fig. 5-13). The dinuclear products are of some interest as models for the dinuclear sites of some metalloproteins. 

Cycloheptatriene enol complexes, with chromium carbonyls,... 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

An example of an inhibitor being consumed is seen in the initiation of GTP with TMS cyanide catalyzed by TBA CN . Here the polymerization does not begin until all of the TMS cyanide has added to the MMA (Scheme 17). Hertler studied the system by NMR and showed that Me3SiCN complexes with the cyanide catalyst to the extent that polymerization will not take place [28] until the complex is gone. This complexation will certainly take place however, the cyanide catalyst should continue to add to monomer until it is also gone leaving the polymer enolate complex (Scheme 17a), which then initiates polymerization. 

Kimura, E. and Koike, T. (1998) Dynamic anion recognition by macrocyclic polyamines in neutral pH aqueous solution development from static anion complexes to an enolate complex, Chem. Commun. 15, 1495-1500. 

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