Carbon-metal bonds acyl halides

While most of the chemistry discussed in this chapter has been developed in the past decade, several important methods have withstood the test of time and have made important contributions in areas such as natural product synthesis. Methods such as cuprate acylation and the addition of organolithiums to carboxylic acids have continued to enjoy widespread use in organic synthesis, whereas older methods including the reaction of organocadmium reagents with acid halides, once virtually the only method available for acylation, has not seen extensive utilization recently. In the following discussion, we shall be interested in cases where selective monoacylation of nonstabilized carbanion equivalents has been achieved. Especially of concern here are carbanion equivalents or more properly organometallics which possess no source of resonance stabilization other than the covalent carbon-metal bond. Other sources of carbanions that are intrinsically stabilized, such as enolates, will be covered in Chapter 3.6, Volume 2. 

Transfer of organic groups from tin to carbon electrophiles, e.g. alkyl halides and acyl hahdes, can occur in the presence of a transition metal (e.g. Pd) catalyst (equation 40). Reactivity sequences for elecfrophihc carbon-tin bond cleavages are generally allyl > phenyl > benzyl > vinyl > methyl > higher alkyl. The precise sequence is somewhat dependent on the solvent and electrophile. 

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. 

It is known that insertion of carbon monoxide to form an acyl complex is reversible, in which results depend on the pressure of carbon monoxide and temperature. If the above-mentioned mechanisms are correct, then acyl halides and aldehydes should be decarbonylated to form olefins provided that an acyl-palladium bond is formed by the oxidative addition of acyl halides or aldehydes to metallic palladium. This proved to be the case. When acyl halide was heated with a catalytic amount of metallic palladium or palladium chloride at 200°C. in a distilling flask, carbon monoxide and hydrogen halide were evolved rapidly, and olefin was collected in a good yield. This reaction is a new and useful preparative method of olefins. In the same way, aldehydes can be decarbonylated smoothly, but in this case, both olefin and the corresponding paraffin Were obtained. The latter probably arises by the hydrogenation of the olefin. Decarbonylation of certain aldehydes has been reported by several workers (3, 6), but no reasonable mechanism has been known. The mechanism of the palladium-catalyzed aldehyde formation discussed above gives clear explanation for the palladium catalyzed decarbonylation of aldehydes. 

Reductive elimination to form carbon-heteroatom bonds from acyl complexes occurs more readily than it does from alkyl or aryl complexes. This fast rate is consistent with the trends noted previously—namely, reductive elimination is faster from complexes containing groups that are sp hybridized, that are electrophilic, and that can coordinate to the metal center during tiie reductive elimination. Reductive elimination of acid halides has been proposed as a step in the Monsanto acetic acid process, and experimental evidence has been obtained for this type of reductive elinunation from acetylrhodium halide complexes generated in solution (Equation 8.66). - A closely related incorporation of KZO into add halides catalyzed by Wilkinson s catalyst provides further evidence for reversible addition and elimination of add halides involving Rh(I) and Rh(III), respectively. ... 

Cross Coupling. In cross-coupling reactions, an aryl, vinyl, or acyl halide or triflate undergoes a palladium-catalyzed Heck-type coupling to an aryl-, vinyl-, or alkyl-metal reagent (eq 14) to give a new carbon-carbon bond. ... 

The N2 hgand bound to a metal atom may also be susceptible to attack by electrophilic reagents other than acids. In fact, reactions of coordinated N2 or its protonated derivatives have been extensively studied with a range of organic compounds including alkyl and acyl halides, carboxyhc anhydrides, aldehydes, ketones, and activated aryl or vinyl halides to directly form the carbon-nitrogen bonds. 

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... 

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