Metal-acyl bonds

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. 

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. 

S. t) Alkyl Complexes by Insertion of Monoolefins Into tr-Alkyl-, o-Aryl-, (T-VInyl-, and tr-Acyl>Metal Bonds. 

Water and alcohols cleave some acyl-metal bonds to give carboxylic acid derivatives and a metal hydride, another example of an inverse cleavage. This reaction has been studied with acylcobalt carbonyl derivatives such as... 

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. 

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. 

For coordination initiators, the metal coordinates with the carhonyl oxygen followed hy insertion of an alkoxy (or other anionic fragment depending on the initiator) into the acyl-oxygen bond. 

Attempts to bring about acylation reactions of bisbenzenechromium and bistoluenechromium have thus far failed, the ring-metal bond being readily cleaved under the reaction conditions involved (25). On the other hand, benzenechro-mium tricarbonyl has recently been acylated to produce acetophenonechromium tricarbonyl (XV), the ring-metal bond in this case remaining intact (15, 96). 

Although attempts to acylate bisbenzenechromium under Friedel-Crafts conditions have thus far resulted in cleavage of the ring-metal bond, the successful metalation of this 7r-arene complex has been recently reported, using amylsodium (10). The resulting dimetalated product has been characterized as a dicarb-methoxy derivative, although the position of the two substituents has not yet been determined. 

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. 

Migratory insertion is the principal way of building up the chain of a ligand before elimination. The group to be inserted must be unsaturated in order to accommodate the additional bonds and common examples include carbon monoxide, alkenes, and alkynes producing metal-acyl, metal-alkyl, and metal-alkenyl complexes, respectively. In each case the insertion is driven by additional external ligands, which may be an increased pressure of carbon monoxide in the case of carbonylation or simply excess phosphine for alkene and alkyne insertions. In principle, the chain extension process can be repeated indefinitely to produce polymers by Ziegler-Natta polymerization, which is described in Chapter 52. 

Scheme 9.1 Preparation of the methylmanganese and -rhenium complexes 2a,b from the pentacarbonyl metallates la,b and their conversion into the acyl metal derivatives 3 (L = CO, PR3, NH2R) via migratory insertion of a carbonyl ligand into the M—CH3 bond (a M = Mn b M = Re)...

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