Current Organic Chemistry - Volume 15, Issue 18, 2011

The importance of the palladium in many fields of the chemistry is undeniable: development of catalysts for new efficient processes, synthesis of new promising materials for hydrogen storage, application of metal complexes for organic syntheses, improvement of electronic and optical devices based on liquid crystals, research on new anticancer drugs, new outstanding structures defying the bonding interpretation, among others. All these subjects constitute the central body of a huge number of contributions centered on the palladium chemistry. More precisely, it has been during the last five years that we have witnessed to an impressive development of all fields mentioned: the rate of publication and the wide scope of subjects covered suggested that this is now, probably, the moment to make an up-to-date on the palladium chemistry. This is the main purpose of this special volume. Fourteen authors, specialists in different fields of the chemistry, have contributed to this volume, each one with a different topic (some of them related), and each one with a different perspective. One of the fields best covered is the use of Pd in different catalytic processes. The field of the synthesis and applications in catalysis of the palladium nanoparticles if the subject of the comprehensive contribution of Prof. Gómez. The ligands have an outstanding role in the Pd-catalyzed processes. Therefore, Prof. Fairlamb has devoted one chapter to the use of π-acidic molecules, while Prof. Hierso's chapter deals with the use of polyphosphines. The latter has an added-value, as is the characterization of “through-space” couplings between phosphorus nuclei. The use of elusives and extremely reactives arynes as starting materials is analyzed in-depth in the contribution of Prof. Larock. The useful arylation of carbonyls is reviewed by Prof. Martin, and the synthesis of the industrially relevant polythiophenes (showing a wide range of applications) is revised by Prof. Navarro. Among some of the most relevant ligands, the N-heterocyclic carbenes (NHCs) hold a prominent place, and three different perspectives are presented here. The synthesis of hemilabile NHCs is reviewed by Prof. Chen, and some of the most recent applications of these ligands are collected and presented here by Prof. Poyatos. On the other hand, abnormal NHCs, which represent an exciting class of ligands, are covered by Prof. Albrecht. The field of the C-H bond activation and its application to the metal-mediated organic synthesis has been the most extensively developed topic. Therefore, the formation of new C-C bonds through oxidative coupling has been the subject of the contribution of Prof. Jiao, while the article of Prof. Hii deals with the formation of new C-Heteroatom bonds. Prof. Szabo has devoted his contribution to the synthesis of pincer complexes and their application in the functionalization processes, while Prof. Dunina has reported a very personal account of the synthesis of chiral cyclopalladated derivatives, and our own contribution closes this volume reviewing the most interesting aspects of the orthopalladation as a synthetic tool....

Palladium represents one of the most exploited catalysts in organic synthesis, mainly applied in versatile methods for the formation of C-C bonds, but also in selective double bonds hydrogenation and oxidation processes, founding numerous industrial applications in particular in fine chemicals production. In the last decade, the use of metallic nanoparticles in catalysis has exploded (ca. 5,200 contributions in the period 2000-March 2010) and specifically ca. 40% of the reported works corresponds to palladium applications. Despite this widespread synthetic use of palladium nanoparticles (PdNP), less than 200 contributions analyse the mechanism of the catalyzed process taking into account the involvement of PdNP. The present review deals with the applications of PdNP in organic synthesis, starting from well-defined materials as catalytic precursors which represent less than 400 reported contributions. The discussion is organized by the type of organic transformation, mainly focused on C-X bond formation, hydrogenation and oxidation reactions.

Recent advances in Pd-catalysed cross-couplings have exploited subtle changes (ligands, additives, reagent choice) that can influence both catalyst activity, selectivity and longevity. In the last few decades it has been noted that the addition of alkenes (olefins) as ligands for Pd can have a dramatic effect on these reactions. More recently, chemists have focussed on designing alkene-containing ligands to take advantage of their benefits. This review summarises the fundamentals of alkene ligands in Pd-catalysed cross-couplings. The development of alkene and mixed ‘hemilabile’ phosphine-alkene ligands is further highlighted.

Among the most common high-resolution nuclear magnetic resonance parameters (NMR parameters), nuclear spin-spin coupling provides decisive data for organic, organometallic, biological and coordination compounds characterization. This electron-mediated coupling characterized by J constant is very classically thought as transmitted by unambiguously covalently bonded atoms. Yet, experimental and theoretical NMR studies have highlighted since the sixties the existence of scalar J spin couplings operating via clearly nonbonded interactions, these couplings are often called “through-space” internuclear spin-spin couplings (TS couplings). We discovered that the through-space spin coupling was not only a phenomenon detectable in fluorinated constrained organic molecules, but also in organometallic and coordinating compounds, and especially in palladium and group 10 coordination complexes of constrained polyphosphine ligands. The present article aims at revealing the TS contribution of indirect nuclear spin-spin in ferrocenyl polyphosphine and their palladium coordination complexes. Theoretical and fundamental aspects are introduced in continuous relationship with experimental spectroscopic and structural data. The role of lone-pairs in nonbonded spin-spin coupling is disclosed and correlated with through-space distance dependence of J constant intensity within palladium and nickel halide complexes. Some important consequences in structural characterization of the compounds in solution are discussed. The spatial proximity of phosphorus atoms — recognized from TS spin couplings — is at the origin of new reactivity and mechanistic understanding in C-C palladium-catalyzed cross-coupling, which are quoted herein. Future trends in the modelling of this fascinating phenomenon, towards a better understanding, are briefly evoked.

This review aims at providing synthetic organic chemists with enough background for research in the palladium-catalyzed chemistry of highly reactive aryne species obtained in situ by the attack of a fluoride ion on (o-trimethylsilyl)aryl triflates under mild reaction conditions. The chemistry generally involves oxidative addition of Pd(0) to the aryne or the organic halide or triflate and subsequent insertion of the aryne, alkyne, alkene or carbon monoxide, followed by various organic transformations to give substituted arenes, bicyclic products, heterocycles and polycyclic aromatic hydrocarbons. Palladium-catalyzed aryne annulations involving arynes, alkynes, aryl or vinylic halides and triflates, multicomponent couplings, and σ-bond insertions are the main focus, but other reactions, which do not fall under these categories, will be discussed along with an overview of the mechanistic pathways involved.

The palladium-catalyzed α-arylation of carbonyl-type compounds is a relatively emerging field within the cross-coupling arena. In recent years, these processes have experienced an exponential growth, particularly in the development of enantioselective variants of these reactions. Beyond any reasonable doubt, palladium-catalyzed α-arylation reactions represent a straightforward alternative to the classical synthesis of α-aryl carbonyl-type compounds that oftentimes involves time-consuming and economically inefficient steps. This review summarizes the last developments in the last 5 years, hence providing a general overview of the current state of the art for palladium-catalyzed α-arylation processes.

The synthesis of well-defined macromolecules using polymerization processes is a challenging and important task in organic chemistry. Palladium-catalyzed cross-coupling reactions represent an important tool in the chemist's armory, combining high efficiency, low catalyst loadings, high tolerance to a variety of functional groups and solvents and moderate reaction temperatures (25-100 °C). The present review is an account of the recent literature involving Pd-catalyzed, cross-coupling based polymerizations for the synthesis of polythiophenes, as well as the use of this and related compounds in polymer and material science.

Palladium complexes containing hemilabile N-heterocyclic carbene (NHC) ligands have become a promising family of complexes due to an excellent balance between stability and reactivity that enables them to catalyze a wide range of organic reactions. This review will focus on the palladium complexes of hemilabile NHC ligands, especially their synthesis and catalytic applications in organic transformations. This article is organized in the following categories according to the additional donating atoms: i) Synthesis and catalytic applications of Pd complexes containing bidentate CE (E = N, O, S, or P) ligands, ii) Synthesis and catalytic applications of Pd complexes containing tridentate CE2 (E = N, O, S, or P) ligands, iii) Synthesis and catalytic applications of Pd complexes containing tridentate C2N ligands, iv) Synthesis and catalytic applications of Pd complexes containing tridentate CNO type ligands, and (V) Synthesis of Pd complexes containing tetradentate C2N2 ligands.

N-heterocyclic carbene (NHC) ligands have become ubiquitous ligands in the preparation of metal complexes with new catalytic applications. Mainly due to their applications in C-C bond formation reactions, a plethora of novel palladium-NHC complexes has been described, and a large number of review articles describing their chemistry have been published. In an attempt to provide a new vision of the topic, this article will focus the attention on the development of new palladium complexes with NHC ligands, paying special attention to their applications in catalytic processes other than the classical C-C coupling reactions. This article is divided in the following sections: i) design of reusable Pd-NHC complexes, ii) latest advances in the use of Pd-NHC complexes in homogenous catalysis and, iii) other applications of Pd-NHC complexes.

Developments in palladium chemistry have been spurred predominantly by the outstanding application potential of this metal in catalysis. The quest for new ligands in order to modulate the catalytic activity and selectivity of the palladium center has been greatly stimulated by the discovery of N-heterocyclic carbenes as formally neutral, strongly donating, and covalently binding ligands. Abnormal variations of N-heterocyclic carbenes, even though known (yet not recognized) for 30 years, have received very little attention until recently. In parts this may have been due to the fact that the free abnormal carbene ligand is much less stable than the normal carbene analogues. In the last decade, significant progress has been made in abnormal carbene palladium chemistry and reliable synthetic routes as well as promising catalytic applications have been developed. As a consequence, these types of complexes have gradually transformed from laboratory curiosities to unique formally neutral ligands with exceptional donor ability. Here, the advances in abnormal carbene palladium chemistry are summarized. In an attempt to stimulate the entry of newcomers in this fascinating field of research, elementary aspects of synthesis are discussed as well as progress in characterization of the complexes. Most recent (catalytic) applications may highlight the potential of this rapidly growing area of palladium chemistry.

The heterofunctionalisation of C-H, C=C and C≡C bonds is an important type of transformation that converts readily accessible precursors to valuable intermediates in organic synthesis, particularly when the heteroatom = N or O. The progress in this area of homogeneous catalysis by palladium is described in the current review, covering the period between 2005-early 2010. Asymmetric reactions remain a key challenge in this area, which is highlighted.

This review presents the development of Palladium-catalyzed oxidative Heck reactions of organometallic compounds (OMCs) with alkenes. The mechanism is discussed with many experimental studies. OMCs derived from Group III to Group VII are shown to be efficient substrates for the oxidative Heck reaction.

This review describes the usefulness of palladium pincer complexes in catalytic transformations of various electrophiles. We discuss two basic approaches to the functionalization of electrophiles: (i) palladium pincer complex catalyzed additions of allylic and other substrates to electrophiles, such as sulfonyl imines, aldehydes and related reagents; and (ii) pincer complex catalyzed generation of allylmetal reagents (such as boronates and other species) followed by a direct or palladium-catalyzed functionalization of the electrophiles under one-pot conditions. The study is focused on the reactivity and selectivity aspects of pincer complex catalysis. We show, for example, that allylations of imines and aldehydes are easier to perform with pincer complexes bearing π-acceptor phosphine ligands, while generation of organometallic species can be efficiently achieved by σ-donor, selenium or nitrogen containing pincer ligands. We present several examples of chiral pincer complexes in asymmetric catalysis. The high stability and well-defined stoichiometry of pincercomplexes allow a rational design of asymmetric catalytic reactions for carbon-carbon and carbon-heteroatom bond formations. Briefly, we have also reviewed the new emerging field of pincer complexes in Pd(II)/Pd(IV)-based catalytic cycles. These processes allow redox reactions involving pincer complex catalysts without altering of the typical pincer complex topology.

The author's personal account summarizes results of the study of optically active cyclopalladated compounds (CPCs) during the last 25 years. The review describes a vast collection of structurally and stereochemically different types of CPCs, new routes to them, analysis of chirality transfer and some examples of their application.

The orthopalladated complexes are among the most representative Pd(II) compounds. They display a wide prospect of applications, but they are mainly known by their use as intermediates in metal-mediated organic synthesis. In this review we describe how palladacycles are used to build up new molecules through regioselective formation of C-halogen, C-O, C-N, C-P, C-S and C-C bonds, either under catalytic or stoichiometric conditions. This methodology is based on the selective incorporation of the Pd into the organic skeleton to be modified and subsequent reactivity of the Pd-C bond toward different substrates. In many cases this strategy is alternative or even competitive, improving standard organic methods. The most recent achievements of the last five years will be covered here, presenting only the most impressive results.