Acylpalladium derivatives

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

All of the examples of trapping of acylpalladium species with enolates discussed above as part of the Type III Ac-Pd process involve trapping with O-enolates. As discussed earlier, however, acylpalladium derivatives can also be trapped with C-enolates (Eqs. 4 and 5 in Scheme 11), and this trapping with C-enolates has since been exploited for terminating acyclic carbopalla-dation process [135] (Scheme 48). However, this process does not appear to have been used for terminating cyclic acylpalladation processes. 

Negishi E, Makabe H (2002) Reactions of Acylpalladium Derivatives with Enolates and Related Amphiphilic Reagents. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 2455... 

Tsuji, J. Other reactions of acylpalladium derivatives palladium-catalyzed decarbonylation of acyl halides and aldehydes. Handbook of Organopaiiadium Chemistry for Organic Synthesis 2002, 2, 2643-2653. 

Inversion of configuration observed with allyl and benzyl derivatives has been deduced from the results sununarized in Scheme 4. In the case of allyl derivatives, a very reasonable and seemingly unmistakable assumption that nucleophilic attack on 7r-allyl ligands takes place on the side opposite to Pd was made. On the other hand, an equally plausible assumption that migratory CO insertion and methanolysis of acylpalladium derivatives both proceed with retention. ... 

First, coordinatively unsaturated active palladium catalyst, PdL2, is produced via dissociation of the ligands, which then reacts with acyl halide to give the acylpalladium intermediate. Since deinsertion of CO of the acylpalladium derivatives may occur simul-taneously, the next step, transmetallation (so-called metathesis), is the most crucial for the efficiency of the overall reaction. A variety of organometallic compounds, such as boron, aluminum, copper, zinc, mercury, silicon, tin, lead, zirconium, and bismuth, are used as the partner in this coupling reaction without loss of CO. In this section, the important features of the cross-coupling reactions of a variety of organometallic compounds with acyl halides and related electrophiles are discussed. 

Trapping of acylpalladium derivatives via hydrogenolysis must be a critical step in the classical Rosenmund reductiont of acyl halides to give aldehydes. Aldehydes can now be... 

VI.2.1 Reactions of Acylpalladium Derivatives with Oxygen, Nitrogen, and Other Group 15,16, and 17 Atom Nucleophiles... 

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