Mizoroki–Heck reactions catalyzed by palladium dichloro-bis(aminophosphine) complexes under mild reaction conditions : the importance of ligand composition on the catalytic activity

Dichloro-bis(aminophosphine) complexes of palladium with the general formula [(P{(NC5H10)3−n(C6H11)n})2Pd(Cl)2] (where n=0-2) are easily accessible, cheap and air stable, highly active and universally applicable C–C cross-coupling catalysts, which exhibit an excellent functional group tolerance. The ligand composition of amine-substituted phosphines (controlled by the number of P–N bonds) was found to effectively determine their catalytic activity in the Heck reaction, for which nanoparticles were demonstrated to be their catalytically active form. While dichloro{bis[1,1′,1′′-(phosphinetriyl)tripiperidine]}palladium (1), the least stable complex (towards protons) within the series of [(P{(NC5H10)3−n(C6H11)n})2Pd(Cl)2] (where n=0-3), is a highly active Heck catalyst at 100°C and, hence, a rare example of an effective and versatile Heck catalyst that efficiently operates under mild reaction conditions (100 °C or below), a significant successive drop in activity was noticed for dichloro-bis(1,1′-(cyclohexylphosphinediyl)dipiperidine)palladium (2, with n=1), dichloro-bis(1-(dicyclohexylphosphinyl)piperidine)palladium (3, with n=2) and dichloro-bis(tricyclohexylphosphine)palladium (4, with n=3), of which the latter is essentially inactive (at least under the reaction conditions applied). This trend was explained by the successively increasing complex stability and its ensuing retarding effect on the (water-induced) generation of palladium nanoparticles thereof. This interpretation was experimentally confirmed (initial reductions of 1-4 into palladium(0) complexes of the type [Pd(P{(NC5H10)3−n(C6H11)n})2] (where n=0-3) were excluded to be the reason for the activity difference observed as well as molecular (Pd0/PdII) mechanisms were excluded to be operative) and thus demonstrates that the catalytic activity of dichloro-bis(aminophosphine) complexes of palladium can – in reactions where nanoparticles are involved – effectively be controlled by the number of P–N bonds in the ligand system

Mizoroki-Heck cross-coupling reactions catalyzed by dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium under mild reaction conditions

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC5H10)3-n(C6H11)n})2Pd(Cl)2] (where n = 0-2), belong to a new family of easy accessible, very cheap, and air stable, but highly active and universally applicable C-C cross-coupling catalysts with an excellent functional group tolerance. Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC5H10)3)2Pd(Cl)2] (1), the least stable complex within this series towards protons; e.g. in the form of water, allows an eased nanoparticle formation and hence, proved to be the most active Heck catalyst within this series at 100°C and is a very rare example of an effective and versatile catalyst system that efficiently operates under mild reaction conditions. Rapid and complete catalyst degradation under work-up conditions into phosphonates, piperidinium salts and other, palladium-containing decomposition products assure an easy separation of the coupling products from catalyst and ligands. The facile, cheap, and rapid synthesis of 1,1',1"-(phosphinetriyl)tripiperidine and 1 respectively, the simple and convenient use as well as its excellent catalytic performance in the Heck reaction at 100°C make 1 to one of the most attractive and greenest Heck catalysts available. We provide here the visualized protocols for the ligand and catalyst syntheses as well as the reaction protocol for Heck reactions performed at 10 mmol scale at 100°C and show that this catalyst is suitable for its use in organic syntheses

Negishi cross-coupling reaction catalyzed by an aliphatic, phosphine based pincer complex of palladium biaryl formation via cationic pincer-type PdIV intermediates

The aliphatic, phosphine-based pincer complex [(C(10)H(13)-1,3-(CH(2)P(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(2)P(C(6)H(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(2)P(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(2)P(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

New catalysts for ROMP

The mechanism of the rhenium complex-catalyzed ring-opening metathesis polymn. of norbornene was investigated using Re complexes of type Re+(NO)2(PR3)2 (R = iPr, Cy) as catalysts. The polymer outcome showed preference of Z configuration in the polymer backbone and high mol. wts. of polymers