Mechanistic study on the cross-coupling of alkynyl stannanes with aryl iodides catalyzed by η2-(dimethyl fumarate)palladium(0) complexes with iminophosphine ligands

The reactions of [Pd(η²-dmfu)(P–N)] [dmfu = dimethyl fumarate; P–N = 2-(PPh₂)C₆H₄-1-CH&z.dbd;NR, R = C₆H₄OMe-4 (<compoundref>1a</compoundref>), CHMe₂ (<compoundref>2a</compoundref>)] and [Pd(η²-dmfu)(P–N)₂] with IC₆H₄CF₃-4, ISnBu₃ and PhC&z.tbd;CSnBu₃ have been studied under pseudo-first-order conditions. The oxidative addition of IC₆H₄CF₃-4 yields [PdI(C₆H₄CF₃-4)(P–N)] (<compoundref>1b</compoundref> or <compoundref>2b</compoundref>). No reaction takes place with PhC&z.tbd;CSnBu₃ and also with ISnBu₃ in the presence of an excess of PhC&z.tbd;CSnBu₃. In the presence of fumaronitrile (fn), <compoundref>1b</compoundref> and <compoundref>2b</compoundref> undergo transmetalation by PhC&z.tbd;CSnBu₃ followed by fast reductive elimination to yield [Pd(η²-fn)(P–N)]. The same reaction sequence occurs for the system [PdI(C₆H₄CF₃-4)(P–N)]/P–N (1 ∶ 1 molar ratio) to give [Pd(η²-fn)(P–N)₂]. The palladium(0) complexes are active catalysts in the cross-coupling of PhC&z.tbd;CSnBu₃ with aryl iodides ArI (Ar = C₆H₄CF₃-4, Ph). The catalytic efficiency depends on the complex: [Pd(η²-dmfu)(P–N)₂] > [Pd(η²-dmfu)(P–N)], and on the substituent R: C₆H₄OMe-4 > CHMe₂. The reactivity and spectroscopic data suggest a catalytic cycle involving initial oxidative addition of ArI to a palladium(0) species, followed by transmetalation of the product and by fast reductive elimination to regenerate the starting palladium(0) compound. For [Pd(η²-dmfu)(P–N)] as catalyst, the oxidative addition is the rate-determining step, while for [Pd(η²-dmfu)(P–N)₂] the oxidative addition and the transmetalation steps occur at comparable rate.