REACTIONS PROCEEDING WITH CO INSERTION

Carbon monoxide, a common ligand in organometallic chemistry, is known to insert into palladium-carbon bonds readily. This feature of the metal is frequently utilized when palladium catalyzed reactions are run in the presence of CO. The products of such reactions, also known as carbonylative couplings, incorporate a carbonyl group between the coupling partners. 

The fate of the acyl palladium complex depends on the circumstances. In the presence of a suitable nucleophile (alcohol, amine) it is converted into the corresponding carboxylic acid derivative. The side product, a palladium hydride is converted to the active form of the catalyst in a reductive elimination step, resulting in the formation of an equimolar amount of acid, which is quenched by an added base (in most cases the excess of the nucleophile). 

If the reaction mixture contains a reagent that is capable of transmetalation, then the other coupling partner is transferred onto the palladium this way. The formed complex, after ensuring the cis alignment of the two moieties in a quick isomerisation step, undergoes reductive elimination to give the desired carbonyl compound and regenerate the active form of the catalyst. 

Formally copper catalyzed couplings are analogous to palladium and nickel catalyzed reactions. Carbon-carbon and carbon-heteroatom bonds can be formed in such transformations alike. From the mechanistic point of view there is a significant difference between nickel, palladium and copper catalyzed processes however. While in the former cases the catalyst usually oscillates between the 0 and +2 oxidation states, in copper mediated transformations the common oxidation numbers are +1, +2 and +3. 

1 The cross-coupling reactions were reviewed recently by several authors including  

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