Metal enolates mechanisms

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). 

With conjugated enone substrates, the alkoxymetallation leads to the formation of a metal enolate that can undergo a facile protonation to accomplish the hydroalkoxylation. Following this mechanism, various /3-alkoxyketones were obtained in good yields by the addition of primary and secondary alcohols to methyl vinyl ketone under cationic Pd(n) catalysis.443 Similarly, [Rh(COD)(OMe)]2 was found to catalyze the hydroalkoxylation of both methyl vinyl ketone and phenyl vinyl ketone (Equation (121)).444... 

Several new methods for the asymmetric protonation of metal enolates have appeared however, the catalytic mechanisms are fundamentally the same as that described in Scheme 2 of the 1st edition. 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

Precatalyst 4(Sm) was utilized as a standard system [60]. The mechanism follows a coordination anionic polymerization via an eight-membered transition state (Scheme 3, see p. 985). Formation of a metal enolate turned out to be essential for the initiation of the MMA polymerization and was confirmed by the initiation activity of the enolate complex [(C5H4SiMe3)2Y(OCH=CH2)]2- The rate of polymerization is directed by steric factors depending on the metal (Sm > Y > Yb > Lu) and the auxiliary ligand (Cp > Cp ). Ethyl, isopropyl and f-butyl methacrylates are also stereospecifically polymerized, but the rate of poly-... 

Existing evidence indicates that C-alkylations of metal enolates with common electrophiles proceeds by an SN2-type mechanism that is, the highest occupied molecular orbital (HOMO) of the enolate attacks the lowest unoccupied molecular orbital (LUMO) of the alkylating agent. Scheme 16 illustrates the principle of stereoelectronic control, which states that the electrophile should approach in a plane perpendicular to the enolate to allow maintenance of maximum orbital overlap in the transition state (24) between the developing C—C bond and the ir-orbital of the carbonyl group. 

In contrast with the above Lewis acid-catalyzed asymmetric aldol reactions, chiral Pd and Pt cationic complexes have been found to catalyze the asymmetric process by a transmetalation mechanism involving a metal enolate intermediate (Section 10.2.1.3). 

Advances in the development of metal-catalyzed Mukaiyama aldol addition reactions have primarily relied on a mechanistic construct in which the role of the Lewis acidic metal complex is to activate the electrophilic partner towards addition by the enol silane. Alternate mechanisms that rely on metallation of enol silane to generate reactive enolates also serve as an important construct for the design of new catalytic aldol addition processes. In pioneering studies, Bergman and Heathcock documented that transition-metal enolates add to aldehydes and that the resulting metallated adducts undergo silylation by the enol silane leading to catalyst turnover. 

Asymmetric protonation of a metal enolate basically proceeds catalytically if a coexisting achiral acid A-H reacts with the deprotonated chiral acid A -M faster than with the metal enolate, a concept first described by Fehr et al. [44]. A hypothesis for the catalytic cycle is illustrated in Scheme 2. Reaction of the metal enolate with the chiral acid A -H produces (R)- or (S)-ketone and the deprotonated chiral acid A -M. The chiral acid A -H is then reproduced by proton transfer from the achiral acid A-H to A -M. Higher reactivity of A -M toward A-H than that of the metal enolate makes the catalytic cycle possible. When the achiral acid A-H protonates the enolate rapidly at low temperature, selective deprotonation of one enantiomer of the resulting ketone by the metallated chiral acid A -M is seen as an alternative possible mechanism. 

This hydride shift reaction mechanism is quite different from the base-catalyzed enolization mechanism proposed for phospho sugar isomerases such as triosephos-phate isomerase which generally do not require a metal ion for activity12261. 

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... 

The mechanism depicted in Scheme 3.3d may be the closest to reality for the reaction of an enolate with an alkyl halide, but this picture is dependent on the individual system under study. For our purposes, we can rationalize most enolate reactions by considering metal enolates as monomers (as in Scheme 3.3c), while realizing that the other coordination sites of the metal may be occupied by ligands that may be solvent molecules, additives such as HMPA, DMPU or TMEDA, ... 

Thus, one should expect similar behavior for transition metal enolates where there is significant covalent character to the M-O (or M-G) bond. This section will focus on polymerization of (meth)acrylate esters by group 4 metallocene (or the related group 3 and lanthanocene ") initiators where the mechanism of this process is analogous to the classical GTP process. Of course, the polymerization of (meth)acrylates by other transition metal complexes has been reported frequently in the literature however, in many cases the mechanisms of these processes are less well understood or involve free radical or other forms of initiation. Recent examples of other transition metal-mediated methyl methacrylate (MMA) polymerization processes that may proceed via a GTP or anionic mechanism are given. " "- " ... 

Type n aldolases are found predominantly in bacteria and fungi, and are Zn " -dependent enzymes (Scheme 2.182) [1378]. Their mechanism of action was recently affirmed to proceed through a metal-enolate [1379] an essential Zn " atom in the active site (coordinated by three nitrogen atoms of histidine residues [1380]) binds the donor via the hydroxyl and carbonyl groups. This facilitates pro-(/ )-proton abstraction from the donor (presumably by a glutamic acid residue acting as base), rendering an enolate, which launches a nucleophilic attack onto the aldehydic acceptor. 

B. SCOPE, LIMITATIONS, MECHANISM, AND APPLICATIONS OF THE PALLADIUM-CATALYZED a-ALLYLATION OF METAL ENOLATES AND RELATED DERIVATIVES... 

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]. 

Mukaiyama aldol reactions of silyl enol ethers are generally rationalized by a Lewis acid activation of the carbonyl group by in situ formation of a complex that reacts with the silyl enol ether or the silyl ketene acetal [99,167]. Transmetallation mechanisms according to which silicon is replaced under formation of a metal enolate have been discussed as well for catalytic versions of the reaction [168], in particular for late transition metals [169]. 

Mechanistic studies of enolate alkylation reactions have revealed the structural complexities of metal enolates and the dynamic intricacies of their reactions in solution (Figure 3.1) [28, 29]. Elegant, meticulous studies including the work of Arnett [30], Boche [31], Collum [32], Jackman [33], Seebach [29], Streitwieser [34], and Williard [28, 35] have provided insight into the mechanism of enolate alkylation as a multi-variable problem that is far from simple to deconvolute. It is thus clear that at the present level of understanding it is difficult to extrapolate the observations made in any one system into an overriding, generalized mechanistic construct. 

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

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... 

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

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