Acyl metal complex

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... 

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. 

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. 

All these reactions arc very significant in explaining the mechanisms of several important industrial processes see below. It is noteworthy that the mctal-carbon bond of acyl metal complexes presents some double bond (and, therefore, carbenoid) characters the C=0 stretclung frequencies arc lower than the normal ketonic frequency [23]. 

I. rf Acyl Metal Complexes by Oxidative Addition. 

S.8.2.9.2. (t )-(T-Alkyl-, -Aryl-, and -Acyl-Metal Complexes by Reaction with Metal-Complex Anions. 

Synthesis of aryl- or vinyl-metal compounds is best accomplished in two steps. Initially an acyl-metal complex can be formed by nucleophilic displacement of halide from an acyl halide viz. ... 

Reaction with acyl metallate complexes 5.8.2.8.5 C HgCIOPd C6H5C(0)PdCl Formation 5.S.2.9.3 C,H,CI02Ru j -C5H5Ru(CO)2C1 Formation S.8.2.8.3 Reaction with NaBPh4 5.8.2.11.1 C HsCljFelOjSn... 

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