The metal-acyl bond

In the majority of metal-ac complexes, the ketonic CO stretching frequency is rather lower, 1650 cm i [53,201], than that formd for organic ketones ( 1725 cm i)- This be due either to some double boding of the metal with the -orbitals of the 0=0 group or there may be some direct interaction of the metal with the 0=0 group. 

In either case it would be expected that the carbonyl group would be unusually susceptible to electrophilic attack. In agreement, the complexes 3r-CsH5Fe(CO)LMe are readily protonated with hydrogen chloride forming cations for which representations such as 7.28 or 7.29 are possible [202]. 

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The second insertion in the productive cycle of Fig. 14 would involve the chelated acyl complex (5). Again it might have been difficult to convert this to the olefin complex (6) in the nickel case as the strong chelate Ni-oxygen bond has to be weakened. However, for nickel it seems that (5) is replaced with the five-coordinated acyl complex 5a by uptake of one additional CO. However, 5a is not amenable for ethylene uptake as a first step in the insertion of ethylene into the metal-acyl bond since ethylene will have to replace the more strongly bound CO (>10 kcal mol ). It is thus Ukely that the CO/ethylene polymerization cycle is blocked by a species such as 5a or the four-coordinated chelate (5) of Fig. 14. 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - - ... 

Intramolecular insertions of alkynes have also been observed, and one example of a well-characterized system is shown in Equation 9.74. Kinetic studies of this insertion process have shown that exchange of the alkyne for a coordinated phosphine occurs and that the product is formed by a migratory insertion of the coordinated alkyne into the metal-acyl bond. ... 

In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

Strong bases, for example hydroxide or methoxide, react with metal acyl carbonyls with cleavage of the metal-acyl bond. On the other hand, weak bases, for example triphenylphosphine, iodide, or cyclohexylamine, react with metal acyl carbonyls with metal-carbonyl bond cleavage, e.g. 

BH3 acts both as a reducing agent for the acyl carbonyl and as a promoting agent for subsequent CO insertion into the metal-alkyl bond. As yet the process has been carried as far as C Hg, with Mn(CO)5(CH3), CO, and I B THF as reactants. 

The important difference between the insertion mechanism (2.2) and the migration mechanism (2.3) is the following. In the insertion mechanism carbon monoxide inserts into the metal methyl bond and the acyl bond formed takes... 

A further method for preparing acyl complexes consists in the treatment of alkyl complexes containing at least one carbonyl ligand with a strong ligand [44,105,106], Thereby 1,1-insertion of the carbonyl group into the metal-carbon bond can... 

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. 

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. 

Metal-alkyl bonds can be carbonylated and metal-acyl bonds can be decarbonylated32). The main mechanism suggested for the carbonylation reaction is alkyl migration from the metal to a terminal CO group with entrance of a ligand L, and vice versa for decarbonylation32). 

The putative mechanism involves coordination and activation of the lactide by the metal complex (1, Fig. 2). The lactide, once activated, is subsequently attacked by the metal alkoxide group (another way to view this is that lactide inserts into the metal alkoxide bond) (2, Fig. 2). The putative intermediate then undergoes ring opening of the lactide, by an acyl bond cleavage, and a new metal alkoxide bond is... 

The interaction of the filled p-n orbital of the ligand with the appropriate empty d orbital of the metal, explains the observed double-bond character of the metal-carbon bond. The great facility with which the acyl-oxygen is protonated in such complexes is additional evidence for this statement. 

The stabilities of the metal-carbon bond formed from oxidative additions are as varied as their mechanistic pathways. Metal-carbon bond strengths increase going down a triad in an isostructural series of complexes. Alkyl migration to CO ligands on the metal to form acyl derivatives is more facile in first-row transition metals because of their lower metal-carbon bond energies. The thermal stability of alkyls vs. acyls does not follow any pattern, except that the availability of a sixth coordination site in ML (acyl) complexes favors the alkyl carbonyl isomer. The corresponding acyl, which can be made by running the reaction of the alkyl or aryl halide in CO (at 1-3 atm), is more stable by... 

The CO insertion [reaction (g)] into the metal-carbon bond of the nickel(II) complex produces an unstable acyl intermediate, which undergoes a rearrangement to a nickel(O) complex, presumably via reductive elimination of the acyl group from nickel to nitrogen. ... 

Alkaline hydrolysis yields the products PhMeCHC(0)C(0)0 and [Co(CO)4] . The mechanism of reactions (k)-(m) is consistent with the failure to observe a second carbon monoxide insertion to the same metal-acyl bond, as pointed out above (11.3.1.). 

If a group, such as (C(0)0CH2CH2CH2CH=CH2), was used, the olefin was conforma-tionally constrained to bind perpendicular to the Pd—C bond and insertion-cyclization did not occur. Alkene insertion reactions into metal-acyl bonds usually occur more rapidly than insertions into metal-alkyl bonds. 

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