Calculated linear free energy relationships in the course of the Suzuki–Miyaura coupling reaction

Concerning the Suzuki–Miyaura coupling reaction, we report detailed theoretical study of thermodynamic data (at the B3PW91/LANL2DZ level with polarization functions added and including dispersion force corrections) concerning each step of the catalytic cycle. In each case, we have observed a Hammett plotting of δE or δG versus σ_p of substituent of various reagents, which accounts for multiple-substituent effects. Therefore, these calculations predict the influence of the electron-donor/electron-acceptor character of each substituents on the thermodynamic requirements for reaction. Our major conclusion is that the transmetalation between a palladium hydroxo complex and an arylboronic acid is preferable to the transmetalation with a palladium bromo complex. Moreover, the alkaline hydrolysis of palladium bromo complex appears as a strong exothermic reaction. Concerning the reductive elimination step alternative processes have been investigated.     

Computational characterization of the role of the base in the Suzuki-Miyaura cross-coupling reaction

The role of the base in the transmetalation step of the Suzuki-Miyaura cross-coupling reaction is analyzed computationally by means of DFT calculations with the Becke3LYP functional. The model system studied consists of Pd(CH=CH2)(PH3)2Br as the starting catalyst complex, CH2=CHB(OH)2 as the organoboronic acid, and OH- as the base. The two main mechanistic proposals, consisting of the base attacking first either the palladium complex or the organoboronic acid, are evaluated through geometry optimization of the corresponding intermediates and transition states. Supplementary calculations are carried out on the uncatalyzed reaction and on a process where the starting complex is Pd(CH=CH2)(PH3)2(OH). These calculations, considered together with available experimental data, strongly suggest that the main mechanism of transmetalation in the catalytic cycle starts with the reaction of the base and the organoboronic acid.     

Rapid room temperature Buchwald-Hartwig and Suzuki-Miyaura couplings of heteroaromatic compounds employing low catalyst loadings

The use of second-generation [(NHC)Pd(R-allyl)Cl] complexes for Suzuki-Miyaura and Buchwald-Hartwig cross-coupling reactions involving heteroaromatic halides at room temperature is reported. The first examples of room temperature Suzuki-Miyaura cross-coupling of deactivated aryl chlorides with alkenyl boronic acids are also disclosed. Terminal substitution at the allyl moiety of the palladium complex facilitates its activation at room temperature leading to very active catalytic species enabling the present catalytic transformations to be performed rapidly using very mild reaction conditions. Catalyst loadings can be as low as 10 ppm for the Buchwald-Hartwig aryl amination and 50 ppm for the Suzuki-Miyaura reaction.     

Density functional studies on palladium-catalyzed Suzuki-Miyaura cross-coupling reactions assisted by N- or P-chelating ligands

DFT studies with the B3LYP functional have been carried out on the Suzuki-Miyaura cross-coupling reactions of phenyl chloride and phenylboronic acid catalyzed by palladium complexes with N- or P-chelating ligands. The full catalytic cycle, from the addition of reactants to the catalyst to the release of the cross-coupled product from the complexed intermediate, has been examined. The stages within the cycle, such as oxidative addition, transmetalation, and reductive elimination, were validated by linking the mechanistically relevant intermediates and transition states. Various derivatives of diimine, diphosphine, and diamine were considered as potential model ligands. The catalytic reaction employing diimine as the chelating ligand has been verified as the one with the most energetically feasible route.     

Synthesis, structure and catalytic activity of a bimacrocylic NHC palladium allyl complex

The preparation of a bimacrocyclic NHC palladium allyl complex 4 is described. The complex was obtained by transmetalation with allyl palladium chloride dimer from the NHC silver complex 2 in 85% yield. Complex 4 was fully characterized by spectroscopic methods and by single-crystal X-ray analysis. In a preliminary catalytic study, complex 4 showed high activity in the Suzuki-Miyaura cross-coupling of unactivated aryl chlorides and bromides with 1-naphthalene-boronic acid at low catalyst loading. Good results were also obtained in the Mizoroki-Heck reaction of aryl bromides with styrene, but a decrease in yield was observed when aryl chlorides were used.     

Of the ortho effect in palladium/norbornene-catalyzed reactions: a theoretical investigation

Mechanistic questions concerning palladium and norbornene catalyzed aryl-aryl coupling reactions are treated in this paper: how aryl halides react with the intermediate palladacycles, formed by interaction of the two catalysts with an aryl halide, and what is the rational explanation of the "ortho effect" (caused by an ortho substituent in the starting aryl halide), which leads to aryl-aryl coupling with a second molecule of aryl halide rather than to aryl-norbornyl coupling. Two possible pathways have been proposed, one involving aryl halide oxidative addition to the palladacycle, the other passing through a palladium(II) transmetalation, also involving the palladacycle, as previously proposed by Cardenas and Echavarren. Our DFT calculations using M06 show that, in palladium-catalyzed reaction of aryl halides, not containing ortho substituents, and norbornene, the intermediate palladacycle formed has a good probability to undergo transmetalation, energetically favored over the oxidative addition leading to Pd(IV). The unselective sp(2)-sp(2) and sp(2)-sp(3) coupling, experimentally observed in this case, can be explained in the framework of the transmetalation pathway since the energetic difference between aryl attack onto the aryl or norbornyl carbon of the palladacycle intermediate is quite small. On the other hand, according to the experimentally observed "ortho effect", selective aryl-aryl coupling only occurs in the reactions of ortho-substituted metallacycles. The present work offers the first possible rationalization of this finding. These in situ formed palladacycles containing an ortho substituent could more easily undergo oxidative addition of an aryl halide rather than reductive elimination from the transmetalation intermediate as a result of a steric clash in the transition state of the latter. The now energetically accessible Pd(IV) intermediate, featuring a Y-distorted trigonal bipyramidal structure, can account for the reported selective aryl-aryl coupling through a reductive elimination which is easier than aryl-norbornyl coupling. Thus, the steric effect represents the main factor that dictates the energetic convenience of the system to follow the Pd(IV) or the transmetalation pathway. Ortho substituents cause a higher energy transition state for reductive elimination from the transmetalation intermediate than for oxidative addition to the metallacycle palladium(II) and the pathway based on the latter predominates.     

Triazole-based monophosphine ligands for palladium-catalyzed cross-coupling reactions of aryl chlorides

A variety of triazole-based monophosphines (ClickPhos) have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes. Their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides. Ligand 7i, which has a 2,6-dimethoxybenzene moiety, provided good results in Suzuki-Miyaura reaction to form hindered biaryls. A CAChe model for the Pd/7i complex shows that the likelihood of a Pd-arene interaction might be a rationale for its high catalytic reactivity.     

A Theoretical DFT‐Based and Experimental Study of the Transmetalation Step in Au/Pd‐Mediated Cross‐Coupling Reactions

In this work a combined theoretical and experimental investigation of the cross‐coupling reaction involving two metallic reaction centers, namely gold and palladium, is described. One metal center (Au) hereby is rather inert towards change in its oxidation state, whereas Pd undergoes oxidative insertion and reductive elimination steps. Detailed mechanistic and energetic studies of each individual step, with the focus on the key transmetalation step are presented and compared for different substrates and ligands on the catalytic Pd center. Different aryl halides (Cl, Br, I) and aryl triflates were investigated. Hereby the nature of the counteranion X turned out to be crucial. In the case of X=Cl and L=PMe₃ the oxidative addition is rate‐determining, whereas in the case of X=I the transmetalation step becomes rate‐determining in the Au/Pd‐cross‐coupling mechanism. A variety of Au–Pd transmetalation reaction scenarios are discussed in detail, favoring a transition state with short intermetallic Au–Pd contacts. Furthermore, without a halide counteranion the transmetalation from gold(I) to palladium(II) is highly endothermic, which confirms our experimental findings that the coupling does not occur with aryl triflates and similar weakly coordinating counteranions—a conclusion that is essential in designing new Au–Pd catalytic cycles. In combination with experimental work, this corrects a previous report in the literature claiming a successful coupling potentially catalytic in both metals with weakly coordinating counteranions.     

多壁碳纳米管负载钯(II)催化剂催化Suzuki-Miyaura反应的研究

多壁碳纳米管(MWNTs)负载钯(II)催化剂以乙醇/水(V/V=1∶1)混合溶液为溶剂,室温条件下,高效催化卤代芳烃与苯硼酸的Suzuki-Miyaura偶联反应。该催化剂绿色环保,能够在循环使用5次后仍有较好的稳定性。     

Evidence for the intermediacy of π-allylic metal complexes in the reaction of allylic halides with grignard reagent in the presence of transition metal halides

The reactions of allyl bromide and crotyl chloride with Grignard reagent catalyzed by π-allyl and crotyl metal complexes of nickel, cobalt, and iron, and the stoichiometric reaction of the complexes with the Grignard reagent have been examined. The similarity in catalytic behaviour of the complex and the corresponding metallic halide affords further evidence in support of the previous proposal that the π-allylic metal intermediate plays an important role in the catalytic reaction. The stoichiometric reaction suggests that the dependence of distribution of product in the catalytic process on the type of both allylic halide and metal is attributable to the facility of ligand exchange between the π-allylic complex and Grignard reagent.     

Iron pincer complex and its graphene oxide composite as catalysts for Suzuki coupling reaction

We report the synthesis of Fe-NCN pincer complex as homogenous catalyst and its composite by immobilizing the complex on amino functionalized graphene oxide as a heterogeneous catalyst for Suzuki coupling reactions. Both the complex and the composite were employed in catalyzing the Suzuki-Miyaura cross-coupling reaction between the aryl halide and phenyl boronic acid in acetonitrile solvent media with Cs₂CO₃ as a base. Effect of substitution over aryl halide was also investigated. Immobilization of the pincer complex had advantageous recovery and reuse of the catalyst as compared to its homogenous analog with no significant decrease in the catalytic efficiency.     

Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions

The electron-poor palladium(0) complex L₃Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L₂PdR]⁻, as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R′X (or related S_N2-type reactions) followed by further transmetalation. Gas-phase fragmentation of the resulting heteroleptic palladate(II) complexes results in the reductive elimination of the cross-coupling products RR′. This reaction sequence corresponds to a catalytic cycle, in which the order of the elementary steps of transmetalation and oxidative addition is switched relative to that of palladium-catalyzed cross-coupling reactions proceeding via neutral intermediates. An attractive feature of the palladate-based catalytic system is its ability to mediate challenging alkyl–alkyl coupling reactions. However, the poor stability of the phosphine ligand L against decomposition reactions has so far prevented its successful use in practical applications.     

Computational study of the transmetalation process in the Suzuki-Miyaura cross-coupling of aryls

The transmetalation step of the Suzuki-Miyaura cross-coupling between aryl groups is analyzed by means of DFT calculations with the Becke3LYP functional. The halide considered is Ph-Br, and the organoboronic acid is Ph-B(OH)₂. The model catalyst is Pd(PH₃)₂, and the base, OH⁻. The transmetalation is considered to start from the Pd(Ph)(PH₃)₂Br complex, the product of the oxidative addition. The results are compared with those of a previous study on the analogous reaction with vinyl groups, and it is shown that the reaction mechanism is very similar.     

Bis-diimidazolylidine complexes of nickel: investigations into nickel catalyzed coupling reactions

Air and moisture stable homoleptic bis(diimidazolylidine)nickel(II) complexes, ([(diNHC)(2)Ni](2+)) 3a,b and their corresponding silver(I) 4a,b and palladium(II) 5a,b complexes were synthesized and characterized by NMR and single crystal X-ray analysis. The catalytic potential of complex 3a was assessed in Mizoroki-Heck and Suzuki-Miyaura coupling reactions. In the Suzuki-Miyaura coupling reaction, nickel precatalyst 3a was active for the coupling of aryl chlorides as well as aryl fluorides. The analogously synthesized Pd(II) complexes resulted in formation of (diNHC)PdCl(2) species which were not active for the coupling of aryl fluorides. For the Mizoroki-Heck reaction, it was found that aryl iodides could be activated in the absence of nickel or palladium precatalysts when using Na(2)CO(3) or NEt(3) as base while aryl iodides and aryl bromides could be activated in the Suzuki-Miyaura reaction sans precatalyst when K(3)PO(4) was used as base.     

Ionically tagged benzimidazole palladium(II) complex: preparation and catalytic application in cross-coupling reactions

An imidazolium chloride tagged palladium(II) complex has been conveniently prepared and structurally analyzed. It is active toward cross-coupling of arylboronic acid with aryl halide and benzoyl chloride, giving moderate to high yield of the desired biaryls and aryl ketones, respectively. The present phosphine-free (N-N)Pd(II) complex could be efficiently recycled at least four times with minor decrease of activity in the aqueous Suzuki-Miyaura coupling reaction.     

多元醇修饰Pd-氮杂环卡宾复合物的制备及其对Suzuki-Miyaura反应的催化行为

提高催化剂在反应体系中的分散性,是有效提高催化剂活性的重要手段。本文利用Ullmann反应在大位阻氮杂环卡宾配体(NHC配体)上修饰丙三醇基团,以此来提高催化剂的分散性,进而促进其催化效率的提升。并通过这种大位阻NHC配体与乙酰丙酮钯的络合反应,制备高活性的钯复合物催化剂,进一步以Suzuki-Miyaura偶联反应为模型反应,评价该催化剂的活性与稳定性。结果表明,该催化剂在多种溶剂中表现出较好的分散性,针对溴苯甲醚和苯硼酸的偶联反应,在不同催化条件下,均表现出良好的催化活性,其中转化数(TON)值最高可达到99000。而且该催化剂针对溴苯与苯硼酸的反应体系中,具有广泛的反应底物适用性。与传统的四-(三苯基膦)钯相比较,该催化剂在空气气氛中仍可保持高效的催化行为,表现出较好的稳定性。     

胺作为配体在钯催化偶联反应中应用

对近期我们研究小组及其它研究小组在利用胺作为钯催化偶联反应的配体研究进展进行了总结. 钯/胺作为催化体系主要应用的偶联反应包括: Suzuki-Miyaura交叉反应, Sonogashira交叉反应, Stille交叉反应, Hiyama交叉反应和Heck反应. 研究结果表明胺可以作为价廉和高效的配体促进钯催化交叉偶联反应.     

Development of catalytic asymmetric reactions via chiral palladium enolates

This article describes the generation of chiral palladium enolates and their application to several kinds of catalytic asymmetric reactions. Two methods to generate chiral enolates were developed using novel cationic palladium complexes 1 and 2 . In these processes, water or a hydroxo ligand on palladium metal plays an important role as a nucleophile to promote the transmetallation or as a Brønsted base to abstract an acidic α‐proton of the carbonyl group. These enolates showed sufficient reactivity with various electrophiles. Using a chiral Pd enolate as a key intermediate, highly enantioselective reactions such as catalytic aldol reactions, Mannich‐type reactions, Michael reactions, and fluorination reactions were developed. The unique structures of the palladium enolate complexes were elucidated and reaction mechanisms are proposed. © 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 4: 231–242; 2004: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20017     

碳负载钯催化剂对Heck反应的催化性能

研究了钯碳催化剂对芳基卤和取代芳基卤与丙烯酸和苯乙烯的Heck芳基化反应的催化性能.结果表明:在反应温度为80℃、反应时间为8h、四丁基溴化铵(TBABr)作为溶剂和三丁胺作为碱的条件下,钯碳催化剂对不同取代芳基卤与丙烯酸和苯乙烯的Heck芳基化反应具有良好的催化性能,产物收率在80%以上.     

Suzuki-Miyaura reaction in water,catalyzed by palladium nanoparticles stabilized by Pluronic F68 triblock copolymer

Palladium nanoparticles stabilized by Pluronic F68 triblock copolymer effectively catalyzed Suzuki-Miyaura reaction in water. The reactions with water-soluble aryl iodides and aryl bromides containing electron-withdrawing or electron-donating substituent occurred at room temperature. The catalytic efficiency was found to depend on the size of palladium nanoparticles and their morphology.