Addition, acyl, metals

By varying the metal complex anion and RX, a wide variety of metal compounds containing transition metal-carbon cr-bonds have been prepared directly by this reaction. In addition, acyl metal derivatives prepared by this method may be decarbonylated to give alkyl and aryl metal complexes. 

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... 

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. 

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. 

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. 

Formation of five-membered oxymetallocycles from cis addition of metal acyl fragments to alkynes has been observed with numerous CpW(CO)-(RC=CR)[C(0)R j reagents (197-199). Analogous products have been... 

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. 

A common route to alkoxy(l-alkynyl)carbene complexes 1 involves addition of a 1-lithio alkyne to a metal carbonyl to give an acyl metallate, which on alkylation affords an alkoxy(l-alkynyl)carbene complex (vide supra). Several transformations of acyl metallates with electrophiles other than alkylation agents have been reported. 

I. rf Acyl Metal Complexes by Oxidative Addition. 

Properly placed donor atoms in aldehyde ligands can stabilize the acyl metal hydrides that result from oxidative addition of aldehyde C—H bonds. In all reported examples, the donor atom (which can be nitrogen, oxygen or phosphorous) must be located so that a five-membered chelate is formed upon oxidative addition. Stabilization... 

Moreover, aryl-oxazoles, -imidazoles [17], or-thiazoles [18], anhydrides [19], and imides [20] are accessible via intramolecular Heck-type carbonylations. In addition to typical acid derivatives, aldehydes [21], ketones [22], aroyl cyanides, aroyl acetylenes, and their derivatives [23] could be synthesized via nucleophilic attack of the acyl metal complex with the corresponding hydrogen or carbon nucleophiles. Even anionic metal complexes like [Co(CO)4] can act as nucleophiles and lead to aroylcobalt complexes as products [24]. 

On the synthetic side, single diastereomers of P-keto phosphine oxides have been generated from intermolecular acylation of phosphine oxides using either chiral esters or chiral phosphine oxides. In most cases, reduction of the ketone products was not affected by the presence of extra chiral centres. Addition of metallated phosphine oxides to proline-derived ketoaminals provides a new route to optically active P-hydroxy phosphine oxides. The P-hydroxy phosphine oxide 97 has been prepared by the caesium fluoride mediated reaction of silyl-substituted phosphine oxide 98 and benzaldehyde." The synthesis of two (E)-(6-hydroxy-2-hexen-l-yl)diphenylphosphine oxides (99) has been reported. The Horner-Wittig reactions of these compounds with various carbonyl compounds... 

The nucleophilic acylation of Michael acceptors can be accomplished by a number of stoichiometric methods, i.e. via the addition of metalated a-aminon-itriles [ 60,61 ] or under neutral conditions with formaldehyde-SAMP-hydrazone... 

Both methods can also be used in intramolecular ring closure reactions to form cyclic ketones. Similar, but not identical reaction mechanisms are assumed. The first reaction resembles hydroformylation and requires carbon monoxide insertion and an additional metal acyl alkene insertion step, while in the second reaction the carbon monoxide unit is already present in the substrate. This reaction starts with an oxidative addition to the aldehyde C-H bond, forming an acyl metal hydride, which then undergoes alkene insertion and reductive elimination. 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

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