Acyl metalate

From a general point of view, the tautomeric studies can be divided into 12 areas (Figure 20) depending on the migrating entity (proton or other groups, alkyl, acyl, metals. ..), the physical state of the study (solid, solution or gas phase) and the thermodynamic (equilibrium constants) or the kinetic (isomerization rates) approach. 

I.3.4.2.5. Chiral Enolates of Acyl-Metal Complexes J. S. McCallum and L. S. Liebeskind I.3.4.2.5.I. Chiral Iron-Acyl Complexes... 

Reductive Elimination from Acyl-Metal-NHC Complexes... 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Another possible termination step that has been utilized for the cycloetherification of alkynols involves CO insertion and esterification of the resulting acyl metal with an exogenous alcohol. This process has typically employed MeOH as solvent and a stoichiometric oxidant since the catalyst is turned over in a reduced form. Following this mechanistic motif, a variety of alkynols have been cyclized under Pd(n) catalysis to five- and six-membered oxacycles with incorporation of methyl esters into the products.294,327-329 For the formation of five-membered ring products, this reaction has been carried out in both exo- and endo-mode to provide 1- and 2-substituted... 

When Wacker-type reactions are performed under a CO atmosphere, the (3-H elimination pathway can be suppressed in favor of CO insertion and subsequent nucleophilic cleavage of the acyl metal species.399 This alkoxycarbonylation process has found widespread utility, particularly in the synthesis of five- and six-membered oxacyclic natural products. For example, the THF core of tetronomycin was prepared by the Pd-catalyzed alkoxycarbonylation of 4-alkenol derivatives (Equations (117) and (118)), where stereocontrol was achieved by utilizing either the directing ability of a free hydroxyl or the conformational bias imposed by a bulky silyl ether.420 Additional examples making... 

To replace the aforementioned acyl-main group and acyl-transition metal complexes, the natural course of events was to search for a stable and easy-to-handle acyl-metal complex that reacts as an unmasked acyl anion donor. Thus, the salient features of acylzirconocene chlorides as unmasked acyl anion donors remained to be explored. In the following, mostly carbon—carbon bond-forming reactions with carbon electrophiles using acylzirconocene chlorides as acyl group donors are described. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

Recently proof has been reported for a heterometallic bimolecular formation of aldehyde from a manganese hydride and acylrhodium species [2], Phosphine free, rhodium carbonyl species show the same kinetics as the cobalt system, i.e. the hydrogenolysis of the acyl-metal bond is rate-determining. Addition of hydridomanganese pentacarbonyl led to an increase of the rate of the hydroformylation reaction. The second termination reaction that takes place according to the kinetics under the reaction conditions (10-60 bar, 25 °C) is reaction (3). The direct reaction with H2 takes place as well, but it is slower on a molar basis than the manganese hydride reaction. 

Alkylation of Enolates of Chiral Transition Acyl-Metal Complexes... 

I.I.I.3.4.2. Alkylation of Other Acyl-Metal Complexes Chiral Cobait-Acyl Complexes... 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

Direct cleavage of the acyl-metal bonds 88 with alcohols and amines gives esters 89 and amides. This corresponds to the last and key step of the carbonylation process. 

They have still not been achieved, however, for silicon-metal systems (entries 34,35,43, 44,54) even in reactions at high pressure (entry 54). Moreover, one example is known of the reverse process, in which CO is extruded from a sila-acyl metal derivative (cf. Section II,G,4,c) (entry 43) ... 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

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