Metal acyls

Similar ester carboxylate group containing polymeric titanate esters are obtained by the reaction of titanoxanes with carboxyUc acids (26), by reaction of a tetraalkyl titanate with a carboxyUc acid and 1—2 moles of water (27), or by reacting a polymeric metal acylate with a higher boiling carboxyUc acid and removing the lower boiling carboxyUc acid by distillation (28). 

I.3.4.2.5.2. Other Transition-Metal-Acyl Complexes 1.3.42.5.2.1. Chiral Cobalt-Acyl Complexes... 

One of the properties of transition metal acyl complexes is their ability to lose CO, usually on heating or photolysis. This so-called decarbonylation often represents a special case of the reverse of the CO insertion in Eq. (8), where L = CO. 

This is the most extensively, as well as intensively, studied type of the decarbonylation. Nevertheless, kinetic data are scarce. Conversion of metal acyls to their respective alkyls has been investigated for two systems RCOMn(CO), 37, 50, 51) and XCH2COIr(PPh3)2Cl2 (X = Ph or a substituted Ph group) 160). 

The ultraviolet light-induced decarbonylation has been successfully effected in several types of transition metal acyl. It is particularly important when applied to those systems which do not eliminate CO under thermal conditions, e.g., CpFe(CO)2COR (141). Most of the work has been of a synthetic rather than mechanistic variety, however. 

Yu. A. Ol dekop and N. A. Maier, Synthesis of Organometallic Compounds by Decarboxylation of Metal Acylates. Science and Technology, Minsk, 1976. 

In such a sequence the first complex incorporating the elements H, C, and O is a metal formyl species in Section II,A we describe the preparation and properties of such complexes. In Section II,B, stoichiometric reductions of both metal carbonyl and metal acyl species are presented and in Section II,C, homogeneous CO/H2 conversion catalysts are discussed. 

Further reduction could then yield either a metal acyl species, 37, or a bridged alkyl species, 38. 

The depressed reactivity of the CO bond in metal carbonyls relative to organic carbonyls is not apparent in the case of BH3 and A1H3. For example, Masters and coworkers have observed that I B THF reduces metal acyl compounds to the corresponding alkyls, eq. 16. Although no mechanistic studies have been reported, it... 

Investigations in our laboratory by Rebecca Stimson have demonstrated that it is possible to combine the borane reduction of a metal acyl with the Lewis acid promoted CO insertion reaction which has been discussed earlier in this paper (29). In this reaction, which is presumed to proceed by equation 17, the... 

The purpose of this article is to review recent results on the carbonylation chemistry of actinide-to-carbon sigma bonds, bearing in mind the unique properties of 5f-organometallics cited above. We focus our attention on the properties of bis(pentamethylcyclopentadienyl) actinide acyls. Just as transition metal acyls (A) occupy a pivotal role in classical carbonylation chemistry, it will be seen that many of the unusual... 

In important recent work, Shriver has demonstrated that electrophiles can promote the migration of alkyl groups to coordinated CO. Lewis acid adducts of metal acyl complexes are isolated [37]. Thus it is possible that electrophilic species might also facilitate the generation of catalyst-bound formyls. 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

The potential participation of an alternative route, involving a binuclear elimination reaction between a metal-acyl and a metal-hydride has also been probed [73]. In Rh-catalysed cydohexene hydroformylation, both [Rh4(CO)i2] and [Rh(C(0)R)(C0)4] are observed by HP IR at steady state, the duster species being a potential source of [HRh(CO)4] by reaction with syn-gas. The kinetic data for aldehyde formation indicated no statistically significant contribution from binudear elimination, with hydrogenolysis of the acyl complex dominant. For a mixed Rh-Mn system. 

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

Loss of selectivity during the active catalyst cycle can occur, for example, at the level of the metal methyl, leading to CH4 or the metal acyl, leading to C2 byproducts. 

The individual reaction steps of both Rh or Ir and Mel catalysed MeOH or MeOAc carbonylation oxidative addition of Mel to the metal center, migratory insertion to generate a metal acyl species and elimination from the metal acyl to generate the... 

In Rh catalysed systems, where the metal acyl species also clearly contained iodide, a further possibility was introduced, compared with the mechanism postulated by BASF for their Co systems, that elimination of Acl could occur. The earliest publications from Monsanto which described the proposed mechanism noted that they could not distinguish between a final step involving (i) reductive elimination of Acl followed by hydrolysis of Acl (Eq. (32)), (Eq. (33)) and (ii) hydrolysis at the metal center followed by some other initially unspecified mechanism of recycling HI to Mel (Eq. (34)) [3]. 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

Formation of metal acyl or carbonyl complexes from 1-alkynes in the presence of water is often assumed to proceed via attack on an intermediate vinylidene complex to give a hydroxycarbene complex (Equation 1.24) ... 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

The synthesis of chiral cyclobutanone 3 involving asymmetric induction in the a-alkylation of the chiral cyclic transition metal acyl enolate derived from 1 has been reported. The resulting aldol product 2 can be demetalated to the cyclobutanone with 100% ee.24... 

In the process of carbonyl insertion the 1,1 migratory insertion of the coordinated CO ligand into the metal-carbon bond results in the formation of a metal-acyl complex (Figure 1-7). This process, as nearly all elementary steps discussed so far, is reversible, but even when using atmospheric CO pressure the equilibrium is mostly shifted towards insertion. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Comparisons to methods for transition metal acyl synthesis are instructive. Although many transition metal alkyls can be readily carbonylated to transition metal acyls, the carbonylation of transition metal hydrides to transition metal formyls has not been observed [Eq. (2)] (4, 5). As will be seen, many transition metal formyls are thermodynamically unstable with respect to transition metal hydrides and CO. Thus approach A-i (Scheme 1) is not preparatively useful. (See, however, Addendum, p. 34.)... 

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