Negishi cross-coupling reaction catalyzed by an aliphatic, phosphine based pincer complex of palladium. biaryl formation via cationic pincer-type Pd ~(IV) intermediates

摘要

The aliphatic, phosphine-based pincer complex [(C_(10)H _(13)-1,3-(CH_2P(Cy_2)_2)Pd(Cl)] (1) is a highly active Negishi catalyst, enable to quantitatively couple various electronically activated, non-activated, deactivated, sterically hindered and functionalized aryl bromides with various diarylzinc reagents within short reaction times and low catalyst loadings. Experimental observations strongly indicate that a molecular mechanism is operative with initial chloride dissociation of 1 and formation of the cationic T-shaped 14e- complex [(C_(10)H_(13)-1,3-(CH_2P(C_6H _(11))_2)_2)Pd]~+ (B), which undergoes oxidative addition of an aryl bromide (Ar'Br) to yield the cationic, penta-coordinated aryl bromide pincer complexes of type [(C_(10)H _(13)-1,3-(CH_2P(Cy_2)_2)Pd(Br) (aryl')]~+ (C) with the metal center in the oxidation state of +IV and the aryl unit in cis position relative to the aliphatic pincer core. Subsequent transmetalation with Zn(aryl)_2 result in the cationic diaryl pincer complexes of type [(C_(10)H_(13)-1,3-(CH _2P(Cy_2)_2)Pd(aryl)(aryl')]~+ (D), which reductively eliminate the coupling products, thereby regenerating the catalyst. The neutral square planar aryl pincer complex - a possible key intermediate in the catalytic cycle - was found to be reversibly formed in the reaction mixture but is not involved in the catalytic mechanism. Similarly, palladium nanoparticles as the catalytically active form of 1 could have been excluded.