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

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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(NEt2)Ph}(PPh3)] undergoes an acyl–carbene coupling, leading to the enolate-type complex [PdCl{C(COMe)(NEt2)Ph}(PPh3)]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 [Pd2(μ-Cl)2(COMe)2(SMe2)2] with [W(CO)5{C(OMe)Ph}] leads to an undetected palladium enolate that, after protonation, is stabilized by coordination to palladium ([PdCl2{(OH)MeC═CPh(OMe}(SMe2)], 8). The reaction of [Pd2(μ-Cl)2Me2(SMe2)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.

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