Transition metal-acyl complexes

Transition metal-ar l complexes, M—COR, may be formed eitiier from an ar l halide and metal anions (compare alkyls, see section A), or, by carbonylation of some metal carbonyl-alkyl, M- r-R complexes. These carbonylation reactions are frequently reversible. Indeed, with some alkyl cobalt tetracarbonyl complexes infrared and kinetic studies 

In general, acyl conqilexes M—COR, where Rn = allyl, are thermally more stable than when R, = alkyL 

The carboi latifflii of pentacarbonylmai uiese alkyls and arylsf 

The carbonylation of methylmanganese pentacarbonyl and the decar-bonylation of acetyhnanganese pentacarbonyl have been studied in detail. The thermal decomposition of acetyhnanganese pentacarbonyl labelled with lO in the acetyl CO did not give appreciable radioactivity in the gas phase. This observation suggests the reaction scheme  

Similarly, acetyhnanganese pentacarbonyl formed by treatment of methyl-pentacarbonyl with did not have activity in the acetyl CO group, i.e. 

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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. 

It is evident that transition metal formyl complexes possess a unique and rich chemistry. There are numerous surprising contrasts to reactions of transition metal acyl complexes. Since synthetic routes to formyl com-... 

Acyl anions (RC(=0)M) are unstable, and quickly dimerize at temperatures >-100 °C (Section 5.4.7). These intermediates are best generated by reaction of organolithium compounds or cuprates with carbon monoxide at -110 °C and should be trapped immediately by an electrophile [344—347]. Metalated formic acid esters (R0C(=0)M) have been generated as intermediates by treatment of alcoholates with carbon monoxide, and can either be protonated to yield formic acid esters, or left to rearrange to carboxylates (R0C(=0)M —> RC02M) (Scheme 5.38) [348]. Related intermediates are presumably also formed by treatment of alcohols with formamide acetals (Scheme 5.38) [349]. More stable than acyl lithium compounds are acyl silanes or transition metal acyl complexes, which can also be used to perform nucleophilic acylations [350],... 

C(O)CHCH) and 27.5 Hz (C(O)CHCH) shifts in the resonances for the matallacyclic ring H resonances can be observed upon the addition of 0.1 M Li[CF3S03] to a THF-d8 solution of 2c. Similarly, alkyl cations and H+ are known to attack the O atom of the metallacyclic ring carbonyl of 2b and 2c, and the product formed from the methylation of 2b has been crystallographically characterized (73). Furthermore, alkali cations, through an interaction similar to that proposed in equation (2), are known to inhibit the decarbonylation of transition metal acyl complexes (26). 

Intermediate (iv), M(CO)(COR)L2 is represented by Table 5, which tabulates complexes of the type Tr P) CO) COR), But as can be seen, none of the known structures involves Co, Rh, or Ir. Even if we turn to a less restricted class, namely 7>(F)(COR) compounds (Table 6), we find a suprisingly small tabulation in view of the interest in transition-metal acyl complexes. Although no d 4-coordinate complexes of the Co triad are found, several d complexes of the triad occur, compound 44 being perhaps the closest (but distant) analogue of intermediate (iv). 

For a comprehensive review on aldol additions of Davies-Liebeskind eno-lates and other transition-metal acyl complexes, see McCallum, J.S. and Liebeskind, L.S, (1996) in Houben-Weyl, Stereoselective Synthesis, vol. E21b (eds G. Helmchen, R.W. Hoffmann, J. Mulzer, and E. Schaumann), Thieme, Stuttgart, pp. 1667-1712. 

Transition metal acyl complexes are implicated in many catalytic reactions of carbon monoxide. Alkylation of a coordinated carbonyl group initially results in the formation of an acyl ligand, which then may rearrange,72 Scheme 11.22 ... 

A 1 2 mixture of l-methyl-3-ethylimidazolium chloride and aluminum trichloride, an ionic liquid that melts below room temperature, has been recommended recently as solvent and catalyst for Friedel-Crafts alkylation and acylation reactions of aromatics (Boon et al., 1986), and as solvent for UV/Vis- and IR-spectroscopic investigations of transition metal halide complexes (Appleby et al., 1986). The corresponding 1-methyl-3-ethylimidazolium tetrachloroborate (as well as -butylpyridinium tetrachlo-roborate) represent new molten salt solvent systems, stable and liquid at room temperature (Williams et al., 1986). 

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Organometallic crown ethers have also been synthesized.96-98 Recently, a crown-cation group was shown to interact with an appended transition metal acyl ligand (29)." Complexes of this type have potential applications in Lewis acid-accelerated alkyl migration to coordinated carbonyls. 

The hydrocarboxylation can take place by insertion of the alkene into a metal-hydride bond followed by CO insertion and finally reaction of the acyl complex with solvent as illustrated in equation (36). Alternatively, a transition metal-carboxylate complex can be generated initially. Insertion of the alkene into the metal-carbon bond of this carboxylate complex followed by cleavage of the metal-carbon bond by solvent completes the addition, as shown in equation (37). Both sequences provide the same product. 

Although a variety of new preparative routes has been developed in recent years (for reviews see refs 1 -10), the transformation of the metal-carbonyl carbon bond of a metal-carbonyl complex into a metal-carbene carbon bond is still the most useful and versatile method for preparing transition-metal carbene complexes. The addition of a carbanion to the carbon atom of a carbonyl ligand yields an anionic acyl complex that subsequently can be reacted with an electrophile to give a neutral carbene complex. Thus, the syntheses of anionic acyl and neutral carbene complexes are closely related, for almost all the carbene complexes considered in this section acyl complexes are precursors, although most have not been isolated and characterized. The syntheses of acyl complexes via CO insertion (for reviews see refs. 11, 12) or by reaction of metal carbonyl anions with acyl halides is outside the scope of this section. 

Carbon monoxide can be inserted into some metal—C bonds to give metal acyl complexes [M]C(0)R. However, early transition, lantanide and actinide metals usually afford [M](k -0=CR) complexes. When R is an aUcyl group (CR R"R" ), a 1,2-shift of some of its substituents can give rise to an enolato complex (equation 5). 

Alexander and Wojcicki have used to show that when an acyl transition metal carbonyl complex is decarbonylated, abstraction of the carbonyl ligand occurs according to Equation 6 ... 

The transformation of an alkylmetal carbonyl complex into a metal-acyl complex is one of the most common types of migratory insertion reactions (Equation 9.3). Examples of CO insertion into metal-alkyl complexes are known for all of the transition elements. This reaction class has been the subject of review articles. These reactions occur by a family of diverse, delicately balanced reaction pathways the dominant mechanism depends on the reaction conditions, especially the solvent. Although these pathways are now imderstood in considerable detail, the precise identities of the intermediates in some of these reaction pathways are unknown. 

In many cases, CO insertion reactions have been shown to be thermodynamically unfavorable. In these cases, de-insertion of CO occurs. For example, acetyhnetal complexes do not imdergo insertion to form stable a-dicarbonyl compounds. Instead, a-dicarbonyl compoimds de-insert CO to form metal-acyl complexes and free CO (Equation 9.27)P Likewise, the carbonylation of perfluoroalkyl transition metal complexes is usually uphill thermodynamically. Decarbonylation of perfluoroacetyl complexes is one method to prepare perfluoroalkyl complexes (Equation 9.28). ... 

In marked contrast to the reductive elimination of all l, acyl and aryl-azolium salts, examples of mechanistically well-understood reductive eliminations of 2-haloazolium and azolium [i.e. C-H reductive elimination) salts from transition metal centres are much rarer, despite the ubiquity of transition metal-NHC complexes containing either coordinated halides or hydrides (or both). This may be due, in part, to the ease with which a 2-haloazolium or azolium salt can oxidatively add to a low valent transition metal centre, meaning that even if a 2-haloazolium or azolium salt were formed during a reaction via reductive elimination, a further oxidative addition reaction can rapidly re-form the starting complex. Furthermore, when an azolium salt is the decomposition product of a reaction, it is often not possible to discern whether this has been formed by a genuine NHC-hydride reductive elimination, or... 

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