Palladium-carbene migratory insertion

The formation of 2-alkenyl-substituted furans was observed in the palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones. This reaction involved oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, P-hydride elimination, and catalyst regeneration (13JA13502). 

The reaction is initiated by the insertion of Pd(Il) complex into the C-H bond of the terminal alkyne to give palladium alkynyl intermediate. Then complexation with in situ generated diazo compound occurs to form a palladium carbene intermediate. Subsequently, Pd carbene migratory insertion into the Pd-alkynyl bond... 

A possible mechanism involving common oxidative addition, CO insertion, and decomposition of diazo compounds was proposed. The generated complex A undergoes a carbene migratory insertion into the palladium-acyl bond to form C-bound enolate B. Subsequently, (3-hydride elimination of complex B releases enone. For the equilibration between intermediate B with 0-bond enolate C, the latter is favored with the aid of strong electron-donating ligand PCys. Therefore, the transmetallation with EtsSiH and reductive elimination consequently take place to afford carbonyl compounds (Fig. 33). 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

Danoponlos et al. reported the crystal structure of a palladium pincer carbene complex that is the product of intramolecular 1,2-methyl migration from palladium to the carbene carbon atom, a process also referred to as a migratory insertion of the carbene into the Pd-methyl bond [434] (see Figure 3.150). The importance of this compound stems from the fact that it was the first unambiguous experimental evidence for this process actually to take place after it had been suspected for several years with the suspicion being backed by several theoretical calculations [447-449]. 

The second example is the reaction of a palladium pentafluorophenyl complex with a tungsten methoxy carbene. Transmetallation of the carbene to the Pd atom (a process demonstrated for analogous aminocarbenes) leads to the carbene pentafluorophenyl palladium intermediate and, by migratory insertion, the corresponding rf-a y complex (Scheme 56). ... 

It is obvious that the chirality will be eroded if the p-hydride elimination occurs at the chiral a-position of tosyUiydrazones. The selectivity of p-hydride elimination in this reaction can be interpreted as follows. Alkylpalladium complex, the intermediate generated after migratory insertion of palladium carbene, is favorable to afford 1,1-disubstituted chiral olefin (Fig. 18, path a) because of the less steric interactions in the transition state at p-hydride elimination. Apparently the alternative syn p-hydride elimination will lead to the erosion of chiral center. However, the latter pathway is not preferred because it leads to the eclipse of the bulky substituent with the methyl group of the substrate (Fig. 17, path b). 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

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