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 NR, R = C6H4OMe-4 (1a), CHMe2 (2a)] and [Pd(η2-dmfu)(P–N)2] with IC6H4CF3-4, ISnBu3 and PhC CSnBu3 have been studied under pseudo-first-order conditions. The oxidative addition of IC6H4CF3-4 yields [PdI(C6H4CF3-4)(P–N)] (1b or 2b). No reaction takes place with PhC CSnBu3 and also with ISnBu3 in the presence of an excess of PhC CSnBu3. In the presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhC 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 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.