Acyl–Carbene and Methyl–Carbene Coupling via Migratory Insertion in Palladium Complexes

Abstract

The migratory insertion reaction of a carbene into a palladium−acyl bond has been observed both for monoaminocarbenes and for methoxycarbenes. The acyl derivative [PdCl(COMe){C(NEt 2 )-Ph}(PPh 3 )] undergoes an acyl−carbene coupling, leading to the enolate-type complex [PdCl{C(COMe)(NEt 2 )Ph}(PPh 3 )] 2 (2). This complex decomposes either by reductive elimination to give the iminium salt or by protonation of the enolate to give a ketoamine. In a similar fashion, the reaction of [Pd 2 (μ-Cl) 2 (COMe) 2 (SMe 2 ) 2 ] with [W(CO) 5 {C(OMe)Ph}] leads to an undetected palladium enolate that, after protonation, is stabilized by coordination to palladium ([PdCl 2 {(OH)MeCCPh(OMe}(SMe 2 )], 8). The reaction of [Pd 2 (μ-Cl) 2 Me 2 (SMe 2 ) 2 ] with [W(CO) 5 {C(OMe)-Ph}] leads to the migratory insertion of the carbene into the Pd− methyl group to give an alkyl palladium complex. The transfer of CO from tungsten, followed by insertion into the Pd−Me group, also occurs. This leads to the formation of a Pd−COMe group, which also undergoes migratory insertion of the carbene fragment. These results support that migratory insertion is a key C−C coupling step, as proposed for the new Pd-catalyzed transformations that use carbene precursors.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

“…The difference between this carbene coupling reaction and the traditional coupling reaction (Scheme ) is the supposed metal carbene migratory insertion process (from 47 to 48 ), although no direct experimental evidence supports this mechanism in the seminal work. Experimental evidence on palladium carbene migratory insertion has been obtained with some heteroatom-stabilized carbene complex, such as some palladium NHC complexes − or palladium carbene complexes obtained from transmetalation with classic Fischer carbene complexes. − However, the migratory insertion process of metal carbene complexes that do not have a heteroatom atom adjacent to the carbene center is very difficult to be confirmed experimentally . This is because metal carbene complexes that lack stability from the heteroatoms are generally too reactive to be isolated, although nonheteroatom stabilized palladium carbene complexes with some distinctive ligands can also be synthesized and characterized .…”

Reactive Palladium Carbenes: Migratory Insertion and Other Carbene–Hydrocarbyl Coupling Reactions on Well‐Defined Systems

“…The migratory insertion reaction is not restricted to a specific Pd–R group in [PdR{carbene}XL] complexes. As it has been discussed above, aryl and acyl groups experience the process, but the reaction also takes place for R = Me …”

“…The monitorization of the decomposition of complexes [PdR{C(NEt 2 )Ph}X(PPh 3 )] {R = C(O)Me, C 6 F 5 ; X = Cl, Br; Scheme and Scheme } showed that the migratory insertion is first order in palladium and independent of the concentration of phosphine . This indicates that in these cis ‐carbene–R complexes the reaction is intramolecular and does not need of previous ligand dissociation to occur.…”

“…Both complexes involved in the migratory insertion sequence ( B to C , Scheme ) were characterized in the reaction shown in Scheme . Complex 40 evolves by migratory insertion of the monoaminocarbene into Pd–acyl bond and the alkyl derivative 41 was detected and characterized in solution . Besides a facile protonation of the enolate group, this complex also undergoes a decomposition reaction to the iminium salt.…”

“…The carbonyl transfer is faster than the carbene transfer for monoaminocarbenes, and for R = Me, Ph only the acyl palladium carbene is observed [Equation ] . However the rate of carbene transfer vs. acyl formation is closer for the methoxy carbene and the product of migratory insertion of the carbene into the Pd–Me bond ( 42 , path b, Scheme ) could be observed and characterized …”