Allan J. Canty

Abstract: Palladium is one of the most widely studied elements in organometallic chemistry, partly owing to the important role of palladium complexes in organic synthesis and catalysis. Since the initial report of PdIMe3(bpy), a rich and diverse organopalladium(IV) chemistry has evolved, including reaction systems that are ideal for studying topics of contemporary interest, e.g., mechanisms of oxidative addition and reductive elimination reactions, selectivity in reductive elimination, and studies of alkyl group exchange between metal centers. An account of each of these topics is presented here, followed by an assessment of some of the possible roles of Pd(IV) in organic synthesis and catalysis. Palladium(IV) chemistry has developed rapidly since 1986 to include the isolation of alkyl-, benzyl-, (eta-l-allyl)-, and arylpalladium complexes. It is providing new comparisons of structure, solution dynamics, and reactivity among the nickel triad elements and is providing new systems for mechanistic studies.

Abstract: The most attractive and fundamental interaction between metal centers and organic molecules that could lead to new functionalization at carbon is direct activation of the C-H bonds of hydrocarbons In particular, transition metal complexes have been used not only in pioneering studies of the synthesis or detection of complexes resulting from C-H activation, but also in cyclometalation chemistry In most of the reactions reviewed here intramolecular assistance by a nucleophile plays a major role in the mechanism and/or in the stabilization of products, and thus they are relevant also to the development of a better understanding of cyclometalation reactions This Account summarizes recent research that implicates the occurrence of electrophilic attack at the metal center in reactions leading to metal-electrophile bonding

Abstract: Structural studies of mercury compounds are evaluated to providing definite evidence that the van der Waals radius of mercury is in the range 1.7-2.0 A. A general value for the radius should lie at the conservative end of the range, hence 1.73 A, conveniently corresponding to Grdenic’s largely neglected upper limit for any form of bonding, is proposed.

Abstract: This perspective focuses on the higher oxidation state (III, IV) organometallic chemistry of palladium involving a range of strong oxidants, with consideration of platinum chemistry where it is informative for the evaluation of structure and mechanism. Particular emphasis is placed on hypervalent iodine reagents, halogens and related oxidants of intense current interest in organic synthesis, together with linkages in concepts between this chemistry and recent advances in studies of diaryl disulfides, diaryl diselenides, and diaryl peroxides as oxidants.

Abstract: Convenient and widely applicable synthetic routes to methylhalogenopalladium(II), PdXMe(L2), and dimethylpalladium(II) complexes, PdMe2(L2), have been developed, including complexes of triphenylphosphine and a wide range of bidentate nitrogen donor ligands. These routes involve either the generation of Pd"Me, species at low temperature from methyllithium reagents and trans-PdCl2(SMe2)2 followed by addition of ligand, PdIMe(2,2'-bipyridyl) being synthesized through the oxidative-addition reactivity of Pd2(dba)3(CHCl3), or the facile synthesis of complexes with the reagents [PdIMe(SMe2)]2 and [PdMe2(pyridazine)], in organic solvents at ambient temperature. These reagents are particularly suitable for ligands sensitive to MeLi reagents, and [PdIMe(SMe2)]2 is also a suitable substrate for the synthesis of chloro and bromo complexes, PdXMe(L2), including PPh3 complexes. Several new nitrogen donor bidentate ligands are described, containing 1-methylimidazol-2-y1(mim) and pyridin-2-y1 (py) groups in (mim)2C=CH2 and (py)(mim)C=CH2 as relatives of planar ligands such as 2,2'-bipyridyl and mim, py, and pyrazol-1-yl (pz) groups in (py)(mim)CH2, (py)(mim)CHMe, (mim)2CH2, (mim)2CHMe, and (pz)(mim)CH2 as relatives of ligands such as (py)2CH2 and (pz)2CH2. Methylpalladium(II) complexes of unsymmetrical bidentate ligands exhibit isomerism; e.g., isomers of PdIMe[(py)(mim)C=O] occur in the ratio 9:1, where the dominant isomer has the pyridine ring trans to methyl. The ligands with methane bridges, e.g. (pz)(py)CH2, ethane bridges, e.g. (pz)2CHMe, and propane bridges, e.g. (py)CMe2, form complexes PdMe2(L2) and PdIMe(L2) that exhibit variable-temperature NMR spectra indicating boat-to-boat inversion of the chelate ring, but complexes of (mim)2CHMe and (py)(mim)CHMe appear to adopt only the conformation with the methyl group axial and adjacent to palladium.

Abstract: 1.1 Introduction to Pd-catalyzed directed C–H functionalization The development of methods for the direct conversion of carbon–hydrogen bonds into carbon-oxygen, carbon-halogen, carbon-nitrogen, carbon-sulfur, and carbon-carbon bonds remains a critical challenge in organic chemistry. Mild and selective transformations of this type will undoubtedly find widespread application across the chemical field, including in the synthesis of pharmaceuticals, natural products, agrochemicals, polymers, and feedstock commodity chemicals. Traditional approaches for the formation of such functional groups rely on pre-functionalized starting materials for both reactivity and selectivity. However, the requirement for installing a functional group prior to the desired C–O, C–X, C–N, C–S, or C–C bond adds costly chemical steps to the overall construction of a molecule. As such, circumventing this issue will not only improve atom economy but also increase the overall efficiency of multi-step synthetic sequences. Direct C–H bond functionalization reactions are limited by two fundamental challenges: (i) the inert nature of most carbon-hydrogen bonds and (ii) the requirement to control site selectivity in molecules that contain diverse C–H groups. A multitude of studies have addressed the first challenge by demonstrating that transition metals can react with C–H bonds to produce C–M bonds in a process known as “C–H activation”.1 The resulting C–M bonds are far more reactive than their C–H counterparts, and in many cases they can be converted to new functional groups under mild conditions. The second major challenge is achieving selective functionalization of a single C–H bond within a complex molecule. While several different strategies have been employed to address this issue, the most common (and the subject of the current review) involves the use of substrates that contain coordinating ligands. These ligands (often termed “directing groups”) bind to the metal center and selectively deliver the catalyst to a proximal C–H bond. Many different transition metals, including Ru, Rh, Pt, and Pd, undergo stoichiometric ligand-directed C–H activation reactions (also known as cyclometalation).2,3 Furthermore, over the past 15 years, a variety of catalytic carbon-carbon bond-forming processes have been developed that involve cyclometalation as a key step.1b–d,4 The current review will focus specifically on ligand-directed C–H functionalization reactions catalyzed by palladium. Palladium complexes are particularly attractive catalysts for such transformations for several reasons. First, ligand-directed C–H functionalization at Pd centers can be used to install many different types of bonds, including carbon-oxygen, carbon-halogen, carbon-nitrogen, carbon-sulfur, and carbon-carbon linkages. Few other catalysts allow such diverse bond constructions,5,6,7 and this versatility is predominantly the result of two key features: (i) the compatibility of many PdII catalysts with oxidants and (ii) the ability to selectively functionalize cyclopalladated intermediates. Second, palladium participates in cyclometalation with a wide variety of directing groups, and, unlike many other transition metals, promotes C–H activation at both sp2 and sp3 C–H sites. Finally, the vast majority of Pd-catalyzed directed C–H functionalization reactions can be performed in the presence of ambient air and moisture, making them exceptionally practical for applications in organic synthesis. While several accounts have described recent advances, this is the first comprehensive review encompassing the large body of work in this field over the past 5 years (2004–2009). Both synthetic applications and mechanistic aspects of these transformations are discussed where appropriate, and the review is organized on the basis of the type of bond being formed.

Abstract: Pick your Pd partners: A number of catalytic systems have been developed for palladium-catalyzed CH activation/CC bond formation. Recent studies concerning the palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed. In the past decade, palladium-catalyzed CH activation/CC bond-forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming CC bonds from CH bonds: PdII/Pd0, PdII/PdIV, Pd0/PdII/PdIV, and Pd0/PdII catalysis. A more detailed discussion is then directed towards the recent development of palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

Abstract: s, or keywords if they used Heck-type chemistry in their syntheses, because it became one of basic tools of organic preparations, a natural way to

Abstract: The area of transition-metal-catalyzed direct arylation through cleavage of CH bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross-coupling reactions with organometallic reagents In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross-dehydrogenative arylations Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations

Abstract: Functionalizing traditionally inert carbon–hydrogen bonds represents a powerful transformation in organic synthesis, providing new entries to valuable structural motifs and improving the overall synthetic efficiency. C–H bond activation, however, often necessitates harsh reaction conditions that result in functional group incompatibilities and limited substrate scope. An understanding of the reaction mechanism and rational design of experimental conditions have led to significant improvement in both selectivity and applicability. This critical review summarizes and discusses endeavours towards the development of mild C–H activation methods and wishes to trigger more research towards this goal. In addition, we examine select examples in complex natural product synthesis to demonstrate the synthetic utility of mild C–H functionalization (84 references).