Mechanistic study on the cross-coupling of alkynyl stannanes with aryl iodides catalyzed by eta~2-(dimethyl fumarate) palladium (0) complexes with iminophosphine ligands

摘要

The reactions of [Pd(eta~2-dmfu)(P-N)][dmfu = dimethyl fumarate; P-N = 2-(PPh_2)C_6H_4-1-CH = NR, R = C_6H_4OMe-4 (1a), CHMe_2(2a)] and [Pd(eta~2-dmfu)(P-N)_2] with IC_6H_4CF_3-4, ISnBu_3 and PhC ident to CSnBu_3 have been studied under pseudo-first-order conditions. The oxidative addition of IC_6H_4CF_3-4 yields [PdI(C_6H_4CF_3-4)(P-N)] (1b or 2b). No reactions takes place with PhC ident to CSnBu_3 and also with ISnBu_3 in the presence of an excess of PhC ident to CSnBu_3. In the presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhC ident to CSnBu_3 followed by fast reductive elimination to yield [Pd(eta~2-fn)(P-N)]. The same reaction sequence occurs for the system [PdI(C_6H_4CF_3-4)(P-N)]/P-N (1:1 molar ratio) to give [Pd(eta~2-fn)(P-N)_2]. The palladium (0) complexes are active catalysts in the cross-coupling of PhC ident to CSnBu_3 with aryl iodides ArI (Ar = C_6H_4CF_3-4, Ph). The catalytic efficiency depends on the complex:[Pd(eta~2-dmfu)(P-N)_2] > [Pd(eta~2-dmfu)(P-N)], and on the substituent R:C_6H_4OMe-4 > CHMe_2. 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(eta~2-dmfu)(P-N)] as catalyst, the oxidative addition is the rate-determining step, while for [Pd(eta~2-dmfu)(P-N)_2] the oxidative addition and the transmetalation steps occur at comparable rate.