A New Catalyst System for the Heck Reaction of Unreactive Aryl Halides

A simple solution to the age-old problem of the Heck reaction of cheap but unreactive chloro- and bromoarenes [for example, reaction (1)] has been found in the catalyst [Pd(CH₃CN)₂Cl₂]⋅6 Ph₄PCl in the presence of N,N-dimethylglycine as additive. The reaction then proceeds with exceptionally high efficiency.     

Transition Metal Catalyzed Coupling Reactions under C−H Activation

C−C coupling by transition metal catalyzed C−H activation has developed into a diverse area of research. The applicable catalysts are manifold, and the variety of products obtained range from basic chemicals to pharmaceuticals and building blocks for carbon networks. One reaction, in which several C−C bonds are formed under C−H activation of a methyl group, is the conversion of ortho-iodoanisole according to Equation (1).     

A Surprising Solid-Phase Effect: Development of a Recyclable “Traceless” Linker System for Reactions on Solid Support

Triazenes as “traceless” linkers for solid-phase synthesis have been utilized for the attachment of arenes to a solid support and yield the corresponding products after various organometallic reactions (Heck reaction and asymmetric dihydroxylation, see the reaction scheme) and cycloadditions (Diels–Alder reaction). The triazene linker is distinguished by its accessibility, thermal robustness, and capability to undergo regeneration.     

Bond-Forming Initiation in Spontaneous Addition and Polymerization Reactions of Alkenes

Spontaneous addition and polymerization reactions of alkenes of different electron-densities result in a wide variety of small organic molecules and high polymers. Tetramethylenes are proposed as key intermediates, i.e. resonance hybrids of 1,4-diradical and zwitterionic limiting structures. Their character is determined by the substituents at the terminals: Zwitterionic character is favored by strong donors, such as alkoxy and dialkylamino groups, at the carbenium center and strong acceptor group such as diesters and cyano-esters, at the acceptor end, and aryl and vinyl groups as donors. Zwitterionic tetramethylenes initiate ionic homopolymerization, while diradical tetramethylenes initiate alternating copolymerization, thus providing an extremely sensitive technique for the detection and characterization of these intermediates. The effects of the donor and acceptor substituents can be arranged as an “Organic Chemist's Periodic Table”, wherein the areas of mechanistic change clearly emerge and which provides predictive capability.—This unifying concept of bondforming initiation is extended to spontaneous addition and polymerization reactions of heteroatom acceptor molecules and 7,7,8,8-tetrasubstituted quinodimethanes and of compounds possessing labile σ-bonds, such as halogens and peroxides. Radical-ion pairs, charge-transfer complexes and adventitious impurities are excluded as significant initiators.     

Metal Insertion into C−C Bonds in Solution

What kind of ligated metal center is necessary for insertion into the “hidden” C−C bond? How can one tune the metal center for C−C bond activation by variation of the steric and electronic properties of ligands? What are the possible mechanisms of C−C bond activation in various reaction systems? A systematic look at the available data on C−C bond activation in solution provides some answers to these questions.     

Formation of CC Bonds by Addition of Free Radicals to Alkenes

There are many reactions in which CC bonds are formed by addition of free radicals to alkenes. Information about the mechanism is important for the synthesis of specific target molecules. The rate of addition of alkyl radicals to alkenes is controlled by steric and polar effects. The stabilities of the educts and products are of only limited importance, since the transition states for these exothermic reactions occur very early on the reaction coordinate. Variations in reactivity and selectivity can be described using frontier orbital theory: for nucleophilic radicals the dominant interactions are those between SOMO's and LUMO's, and for electrophilic radicals those between SOMO's and HOMO's. The large differences in the steric effects of α - and β- substituents of alkenes can be explained by postulating an unsymmetrical transition state— the radical approaches one of the C atoms preferentially. Regioand stereoselectivities can be predicted and are determined, in general, by steric effects.     

Palladium-triazine aminoalcohol nanocomposite, its reactivity on Heck reaction

Several studies on dendrimer synthesis and reactivity have been carried out in order to control the size and functionality of compounds. From such studies, it has been suggested that these molecules may be used as ligands to synthesize potential homogeneous catalysts, firstly, in order to get the benefits of both homo- and heterogeneous catalysis (i.e. high activity and/or selectivity, good reproducibility, accessibility of the metal site, intermediaries detection, etc.); secondly, because, unlike other polymeric species, they can be readily recoverable after reaction. In this paper, following our interest in homogeneous catalysts, we would like to present our findings from studies on the synthesis and characterization of a prime molecule, triazine aminoalcohol, as starting or zero generation dendrimer and its interaction-reaction with palladium nanoparticles as well as our results on the reactivity on Heck type catalysis.     

CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism

The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product-like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity-selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.     

Carbon Dioxide as an Alternative C1 Synthetic Unit: Activation by Transition-Metal Complexes

The carbon dioxide molecule has been of limited importance as a synthetic unit in organic chemistry. When it is coordinated to transition metals, however, completely new possibilities arise; CO₂ can bond to metal complexes in a variety of ways and can enter into insertion and coupling reactions, or become catalytically attached to other substrates. The formation of C? C bonds between carbon dioxide and unsaturated hydrocarbons under conditions of homogeneous catalysis makes available new synthetic routes to industrially interesting organic compounds.     

Atom Economy—A Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way

Enhancing the efficiency of the synthesis of complex organic products constitutes one of the most exciting challenges to the synthetic chemist. Increasing the catalogue of reactions that are simple additions or that minimize waste production is the necessary first step. Transition metal complexes, which can be tunable both electronically and sterically by varying the metal and/or ligands, are a focal point for such invention. Except for catalytic hydrogenation, such methods have been rare in complex synthesis and virtually unknown for C? C bond formation until the advent of cross-coupling reactions. These complexes may orchestrate a variety of C? C bond-forming processes, important for creation of the basic skeleton of the organic structure. Their ability to insert into C? H bonds primes a number of different types of additions to relatively nonpolar π-electron systems. Besides imparting selectivity, they make feasible reactions that uncatalyzed were previously unknown. The ability of these complexes to preorganize π-electron systems serves as the basis both of simple additions usually accompanied by subsequent hydrogen shifts and of cycloadditions. The ability to generate “reactive” intermediates under mild conditions also provides prospects for new types of C? C bond-forming reactions. While the examples reveal a diverse array of successes, the opportunities for new invention are vast and largely untapped.     

Recent Progress in Application of Graphene Supported Metal Nanoparticles in C−C and C−X Coupling Reactions

The carbon‐carbon and carbon‐heteroatom bonds catalytic formation is among the most significant reactions in organic synthesis which extensively applied for synthesis of natural products, heterocycles, dendrimers, biologically active molecules and useful compounds. This review provides the latest advances in the preparation of graphene supported metal nanoparticles and their application in the catalytic formation of both carbon‐carbon (C−C) and carbon‐heteroatom (C−X) bonds including the Suzuki, Heck, Hiyama, Ullmann, Buchwald and Sonogashira coupling reactions. Numerous examples are given concerning the use of these catalysts in C−C and C−X coupling reactions along with the reliable and simple preparation methods of these catalysts, their characterization and catalytic properties and also the recycling possibilities.     

Bifunctional RuII‐Complex‐Catalysed Tandem C−C Bond Formation: Efficient and Atom Economical Strategy for the Utilisation of Alcohols as Alkylating Agents

Catalytic activities of a series of functional bipyridine‐based Ru^II complexes in β‐alkylation of secondary alcohols using primary alcohols were investigated. Bifunctional Ru^II complex ( 3 a ) bearing 6,6’‐dihydroxy‐2,2’‐bipyridine (6DHBP) ligand exhibited the highest catalytic activity for this reaction. Using significantly lower catalyst loading (0.1 mol %) dehydrogenative carbon?carbon bond formation between numerous aromatic, aliphatic and heteroatom substituted alcohols were achieved with high selectivity. Notably, for the synthesis of β‐alkylated secondary alcohols this protocol is a rare one‐pot strategy using a metal–ligand cooperative Ru^II system. Remarkably, complex 3 a demonstrated the highest reactivity compared to all the reported transition metal complexes in this reaction.     

Mechanistic Insights into Cu-Catalyzed Asymmetric Aldol Reactions: Chemical and Spectroscopic Evidence for a Metalloenolate Intermediate

In situ IR spectroscopy and transmetalation experiments confirm a postulated catalytic cycle. The metalloenolate 1 describes the active intermediate in the aldol reaction catalyzed by [CuF₂{(S)-tol-binap}] (see reaction scheme). (S)-tol-binap=(S)-(−)-2,2′-bis(di-p-tolylphosphanyl)-1,1′-binaphthyl.     

Palladium supported on modified magnetic nanoparticles: a phosphine‐free and heterogeneous catalyst for Suzuki and Stille reactions

An efficient magnetic nanoparticle‐supported palladium (Fe₃O₄/SiO₂‐PAP‐Pd) catalyst is reported for the Suzuki cross‐coupling and Stille reactions. This method provides a novel and much improved modification of the Suzuki and Stille coupling reactions in terms of phosphine‐free catalyst, short reaction time, clean reaction and small quantity of catalyst. Another important feature of this method is that the catalyst can be easily recovered from the reaction mixture and reused with no loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Nickel‐Catalyzed Reductive Coupling of Aldehydes with Alkynes Mediated by Alcohol†

A nickel‐catalyzed reductive coupling of aldehydes with alkynes using 1‐phenylethanol as reducing agent has been developed. The key achievement of this work is that we demonstrate environmentally benign 1‐phenylethanol can serve as a viable alternative reducing agent to Et₃B, ZnEt₂ and R₃SiH for the nickel‐catalyzed reductive coupling reaction of aldehyde and alkynes.     

Anodic and Cathodic CC-Bond Formation

Electrolysis allows the reactivity of a substrate to be changed, or its polarity to be reversed (“redox umpolung”). The carbon skeleton and the functional groups of a synthetic building block can thus be utilized more economically, and at the same time the number of reaction steps in multistage syntheses can be reduced. The tools necessary for an electrolytic process are a cell, a power source, electrodes, and an electrolyte, the latter being chosen in accordance with the reduction or oxidation potential of the substrate. A series of electroanalytical methods provides information on the electrode reaction mechanisms. At the anode, arenes, phenolic ethers, and electron-rich olefins dimerize via intermediate radical cations. In the Kolbe electrolysis, carboxylic acid anions decarboxylate to form radicals which can couple to form e.g. long chain alkene derivatives having pheromone activity, or add to ole-fins. At the cathode, activated olefins hydrodimerize via radical anions or, in the presence of appropriate reagents, can be acylated, alkylated, and carboxylated. Pinacols, crossed hydrodimers, and cyclic and arylated compounds are accessible via the cathodically produced radicals, while the formation of strained small rings or the reductive addition of halides to carbonyl compounds takes place through intermediate carbanions.     

Domino‐Heck Reactions of Carba‐ and Oxabicyclic,Unsaturated Dicarboximides: Synthesis of Aryl‐Substituted,Bridged Perhydroisoindole Derivatives

The C? C coupling of the two bicyclic, unsaturated dicarboximides 5 and 6 with aryl and heteroaryl halides gave, under reductive Heck conditions, the C‐aryl‐N‐phenyl‐substituted oxabicyclic imides 7a – c and 8a – c (Scheme 3). Domino‐Heck C? C coupling reactions of 5, 6 , and 1b with aryl or heteroaryl iodides and phenyl‐ or (trimethylsilyl)acetylene also proved feasible giving 8, 9 , and 10a – c , respectively (Scheme 4). Reduction of 1b with LiAlH₄ (→ 11 ) followed by Heck arylation and reduction of 5 with NaBH₄ (→ 13 ) followed by Heck arylation open a new access to the bridged perhydroisoindole derivatives 12a , b and 14a , b with prospective pharmaceutical activity (Schemes 5 and 6).     

Cyclopropenylmethyl Cation: A Concealed Intermediate in Gold(I)‐Catalyzed Reactions

The last years have witnessed many gold‐catalyzed reactions of alkynes. One of the most prominent species in the reaction of two alkyne units is the vinyl‐substituted gold vinylidene intermediate. Here, we were able to show that the reaction of a haloacetylene and an alkyne proceeds via a hitherto overlooked intermediate, namely the cyclopropenylmethyl cation. The existence and relative stability of this concealed intermediate is verified by quantum chemical calculations and ¹³C‐labeling experiments. A comparison between the cyclopropenylmethyl cation and the well‐known vinylidene intermediate reveals that the latter is more stable only for smaller cycles. However, this stability reverses in larger cycles. In the case of the smallest representative of both species, the vinylidene cation is the transition state en route to the cyclopropenylmethyl cation. The discovery of this intermediate should help to get a deeper understanding for gold‐catalyzed carbon–carbon bond‐forming reactions of alkynes. Furthermore, since enynes can be formed from the cyclopropenylmethyl cation, the inclusion of this intermediate should enable the development of new synthetic methods for the construction of larger cyclic halogenated and non‐halogenated conjugated enyne systems.     

Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide

The synthetic potential and the mechanistic aspects of inorganic insertion reactions of carbon monoxide, especially into metal-carbon σ-bonds, are considered. Reactions of this type are encountered among most 3d, 4d, and 5d elements. In one case it has been demonstrated that the CO insertion proceeds by alkyl migration; this is likely to be a general feature of all such reactions. If an alkyl migration takes place, then insertion of CO into the M? C bond is governed kinetically by the cleavage of that bond. CO abstraction from RCO? M bonds most commonly proceeds by rate-determining vacation of a coordination position. Both CO insertion and abstraction are usually highly stereospecific.