Efficient Heck vinylation of aryl halides catalyzed by a new air-stable palladium-tetraphosphine complex

Controlling Reactivity and Selectivity in the Mizoroki-Heck Reaction: High Throughput Evaluation of 1,5-Diaza-3,7-diphosphacyclooctane Ligands

We report a high throughput evaluation of the Mizoroki-Heck reaction of diverse olefin coupling partners. Comparison of different ligands revealed the 1,5-diaza-3,7-diphosphacyclooctane (P2N2) scaffold to be more broadly applicable than common "gold standard" ligands, demonstrating that this family of readily accessible diphosphines has unrecognized potential in organic synthesis. In particular, two structurally related P2N2 ligands were identified to enable the regiodivergent arylation of styrenes. By simply altering the phosphorus substituent from a phenyl to tert-butyl group, both the linear and branched Mizoroki-Heck products can be obtained in high regioisomeric ratios. Experimental and computational mechanistic studies were performed to further probe the origin of selectivity, which suggests that both ligands coordinate to the metal in a similar manner but that rigid positioning of the phosphorus substituent forces contact with the incoming olefin in a π-π interaction (for P-Ph ligands) or with steric clash (for P-tBu ligands), dictating the regiocontrol.

Well-defined palladium(0) complexes bearing N-heterocyclic carbene and phosphine moieties: efficient catalytic applications in the Mizoroki-Heck reaction and direct C-H functionalization

Two series of well-defined palladium(0) complexes with phosphine-functionalized N-heterocyclic carbene ligands were prepared. These complexes featured six- and seven-membered chelate rings in the two series. Among the seven-membered chelate complexes, those featuring the PCy2 moiety exhibited observable fluxional behavior on the NMR time scale, corresponding to the interchange between two sets of conformations. Most of these novel complexes were successfully structurally characterized by single-crystal X-ray diffraction studies. These two series of palladium(0) complexes were tested for their potential catalytic applications in two mechanistically distinct reactions, namely, Mizoroki-Heck coupling and direct C-H functionalization reactions. One of the six-membered chelate complexes was found to be an efficient pre-catalyst for mediating the coupling reactions between aryl chlorides and alkenes. The palladium(0) complex could also be effectively applied in the direct C-H functionalization reactions of aryl bromides with 1,2-dimethylimidazole.

A palladium-nanoparticle and silicon-nanowire-array hybrid: a platform for catalytic heterogeneous reactions

We report the development of a silicon nanowire array-stabilized palladium nanoparticle catalyst, SiNA-Pd. Its use in the palladium-catalyzed Mizoroki-Heck reaction, the hydrogenation of an alkene, the hydrogenolysis of nitrobenzene, the hydrosilylation of an α,β-unsaturated ketone, and the C-H bond functionalization reactions of thiophenes and indoles achieved a quantitative production with high reusability. The catalytic activity reached several hundred-mol ppb of palladium, reaching a TON of 2 000 000.

Polystyrene-Immobilised Phenanthroline Palladium(II) Complex: A Highly Efficient and Recyclable Catalyst for Oxidative Heck Reactions

Oxidative Heck coupling reactions of arylboronic acids with olefins were carried out in the presence of the polystyrene-immobilised phenanthroline palladium(II) complex using air as a co-oxidant to afford the Heck coupling products in high yields under base-free conditions. This heterogeneous palladium catalyst can be recovered by simple filtration and used for several times without loss of activity or selectivity.

Palladium-catalyzed heck coupling reaction of aryl bromides in aqueous media using tetrahydropyrimidinium salts as carbene ligands

An efficient and stereoselective catalytic system for the Heck cross coupling reaction using novel 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (1, LHX) and Pd(OAc)₂ loading has been reported. The palladium complexes derived from the salts 1a-f prepared in situ exhibit good catalytic activity in the Heck coupling reaction of aryl bromides under mild conditions.

Piperazine-Derived Palladium Complexes Immobilised on a Merrifield Resin as a Catalyst for the Heck Reaction

A palladium catalyst supported on a Merrifield resin was prepared by a simple procedure and exhibited high activity and stability for the phosphine-free Heck reaction of activated and non-activated aryl or heteroaryl substrates with olefins to afford substituted cinnamic acids, stilbenes and their hetero-aromatic relatives.

Modular Synthesis of a New Family of Tripodal Ligands, all-cis-1,2,3-Tris(diphenylphosphinomethyl)cyclopropane and Relatives

Bisbenzyl-protected all-cis-1,2,3-tris(hydroxymethyl)cyclopropane has been obtained in three steps from commercially available cis-2-butene-1,4-diol (75% overall yield) and used to prepare a number of all-cis-1,2,3-trisubstituted cyclopropane derivatives. Among them, the new tripodal ligand all-cis-tris(diphenylphosphinomethyl)cyclopropane (TriCyp-PPP) has been demonstrated to furnish-with pi-allylpalladium chloride dimer-a highly active catalyst for the allylation of diethyl methylmalonate enolate with allyl acetate.

The Heck–Mizoroki cross-coupling reaction: a mechanistic perspective

The Heck-Mizoroki cross-coupling reaction is an important part of the synthetic chemist's toolbox, and it has been applied to a huge variety of different substrates. In contrast, the mechanism of the process is much less studied, and consequently less understood. There have been numerous studies reported over recent years, both experimental and theoretical, aimed at uncovering the inner working of this palladium-mediated coupling process. This perspective aims to review and compare these works and to provide an up-to-date view of this reaction.

Exploiting Noninnocent (E,E)-Dibenzylideneacetone (dba) Effects in Palladium(0)-Mediated Cross-Coupling Reactions: Modulation of the Electronic Properties of dba Affects Catalyst Activity and Stability in Ligand and Ligand-Free Reaction Systems

The reactivity of palladium(0) complexes, [Pd(0) (2)(dba-n,n'-Z)(3)] (n,n'-Z=4,4'-F; 4,4'-CF(3); 4,4'-H; 4,4'-MeO) and [Pd(0)(dba-n,n'-Z)(2)] (n,n'-Z=4,4'-CF(3); 4,4'-H; 3,3',5,5'-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dba-n,n'-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L(n)Pd(0)-eta(2)-dba interaction (in dpi-pi* synergic bonding, n=1 or 2), which ultimately increases the global concentration of catalytically active L(n)Pd(0) available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n'-Z ligand stabilizes the Pd(0)-eta(2)-dba interaction, reducing catalytic activity. The specific type of dba-n,n'-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L(n)Pd(0) concentration (and the associated catalytic activity) and catalyst longevity. Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dba-n,n'-Z ligands in Heck arylation is more convoluted and dependent on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n'-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n'-Z with amine and/or base effectively lessens its interference in the catalytic cycle.