[Pd(IPr*)(cinnamyl)Cl]: an efficient pre-catalyst for the preparation of tetra-ortho-substituted biaryls by Suzuki-Miyaura cross-coupling

Quantifying and understanding the steric properties of N-heterocyclic carbenes

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include (a) the percent buried volume (%V_bur), which provides a convenient single number to measure steric impact and (b) steric maps, which provide a graphical representation of the steric profile of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs, from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare their steric properties.

Fluorinated liquid crystals: evaluation of selectively fluorinated facially polarised cyclohexyl motifs for liquid crystal applications

This paper explores the synthesis of a series of prototype negative dielectric liquid crystalline (LC) compounds which contain fluorinated cyclohexane motifs. The series are analogues and differ only in that they contain between one to four fluorine atoms. The stereochemistry is such that all of the fluorines/C-F bonds are on one face of the cyclohexane ring. This follows from the recent recognition that cyclohexanes with fluorines orientated in one direction, perpendicular to the ring have a strong molecular dipole, a characteristic that might be an advantage in the design of negative dielectric materials. However it is found that the increased polarity, particularly with two or more oriented C-F bonds, leads to higher melting points and poorer solubility in test matrix formulations, relative to hydrocarbon liquid crystals. This arises due to electrostatic intermolecular interactions between the polarised cyclohexyl and aryl rings. The study highlights that in order to take advantage of these polar cyclohexane motifs for liquid crystal design, appropriate scaffolds are required which are free of aromatic rings and contain peripheral solubilising motifs.

Designing Pd-N-Heterocyclic Carbene Complexes for High Reactivity and Selectivity for Cross-Coupling Applications

Over the past decade, the use of Pd-NHC complexes in cross-coupling applications has blossomed, and reactions that were either not previously possible or possible only under very forcing conditions (e.g., > 100 °C, strong base) are now feasible under mild conditions (e.g., room temperature, weak base). Access to tools such as computational chemistry has facilitated a much greater mechanistic understanding of catalytic cycles, which has enabled the design of new NHC ligands and accelerated advances in cross-coupling. With these elements of rational design, highly reactive Pd-NHC complexes have been invented to catalyze the selective formation of single products in a variety of transformations that have the potential to afford multiple compounds (e.g., isomers). Pd-NHC catalysts may be prepared as stable Pd(II) precatalysts that are readily reduced to the active Pd(0) species in the presence of an organometallic cross-coupling partner or nucleophile possessing β-hydrogens. It has been found from computational and experimental results that Pd-NHC complexes bearing a single bulky NHC ligand are well-suited to tackle challenging cross-coupling reactions. N-Aryl-substituted imidazole-2-ylidenes with branched alkyl chains at the ortho positions of the aryl group are effective for the challenging couplings of hindered biaryls, secondary alkyl organozincs, electron-deficient anilines, α-amino esters, primary alkylamines, and ammonia. The bulk of the NHC has been tuned by increasing the size of the alkyl groups at the ortho positions and substituting the NHC core with chlorine substituents. All of the cross-coupling transformations studied benefit from the increased bulk when the ortho groups are changed from methyl to 2-propyl to 3-pentyl. However, there is a limit to the positive effect of steric bulk, as some reactions do not benefit from the increased size of the 4-heptyl group compared with 3-pentyl. Thus, there is an optimum size for the NHC ligand that depends upon whether reactivity (turnover frequency and turnover number), selectivity, or both are needed to obtain the desired reaction outcome. In the cases that we have studied, reactivity and selectivity increase together (i.e., the fastest catalyst is also the most selective), allowing cross-couplings to be carried out under mild conditions to obtain one product with high selectivity. This Account focuses on seminal literature reports that have disclosed new Pd-NHC complexes that have led to significant breakthroughs in efficacy for challenging couplings while demonstrating high selectivity for the desired target. These catalysts have been used widely in materials science, pharmaceutical, and agrochemical applications.

Metal Confinement through N-(9-Alkyl)fluorenyl-Substituted N-Heterocyclic Carbenes and Its Consequences in Gold-Catalysed Reactions Involving Enynes

A series of gold(I) and gold(III) complexes containing bulky bis-N,N'-(9-alkylfluorenyl) heterocyclic carbene (^R F-NHC) ligands have been prepared in high yields from appropriate imidazolinium, imidazolium and benzimidazolium salts. In all complexes, the carbene ligand provides high steric protection of the Au-X bond trans to the carbenic C atom. Irrespective of the metal oxidation state, the complexes showed high efficiency in a tandem 3,3-rearrangement/Nazarov reaction of an enynyl acetate. One of the Au^III complexes, [AuCl₃ (^R F-NHC)], was further found to be suitable for the efficient cyclisation of a propargylcarboxamide. Furthermore, unlike related NHC-gold(I) complexes based on conventional bulky N-heterocyclic carbenes (notably, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-(bis-tert-butyl)imidazol-2-ylidene (ItBu)), the studied [Au^I Cl(^R F-NHC)] complexes catalysed the conversion of a 1,6-enyne in the presence of indole into a single product; this arises from the embracing character of the ligand, which prevents indole addition on one of the catalytic intermediates. A structure/selectivity relationship was established for all carbenes tested that took into account percent buried volumes and topographic steric maps. The results illustrate the high potential of confining NHCs in organic synthesis.

Well-defined nickel and palladium precatalysts for cross-coupling

Transition metal-catalysed cross-coupling is one of the most powerful synthetic methods and has led to vast improvements in the synthesis of pharmaceuticals, agrochemicals and precursors for materials chemistry. A major advance in cross-coupling over the past 20 years is the utilization of well-defined, bench-stable Pd and Ni precatalysts that do not require the addition of free ancillary ligand, which can hinder catalysis by occupying open coordination sites on the metal. The development of precatalysts has resulted in new reactions and expanded substrate scopes, enabling transformations under milder conditions and with lower catalyst loadings. This Review highlights recent advances in the development of Pd and Ni precatalysts for cross-coupling, and provides a critical comparison between the state of the art in Pd- and Ni-based systems.

A simple synthetic entryway into palladium cross-coupling catalysis

The simple synthesis of a family of palladates containing an imidazolium counterion is presented. These “ate” complexes can be easily converted into well-defined palladium–N-heterocyclic carbene (NHC) complexes.

Pd-PEPPSI-IPentCl: a new highly efficient ligand-free and recyclable catalyst system for the synthesis of 2-substituted indoles via domino copper-free Sonogashira coupling/cyclization

One pot synthesis of 2-substituted indolesviaSonogashira coupling (without a copper catalyst) followed by intermolecular cyclization using Pd-PEPPSI-IPent^Cl.

Effect on orthometallation of NHC palladium complexes toward the catalytic activity studies in Suzuki coupling reaction

Herein, we report an approach to improve catalytic activity for cross-coupling reactions.

New, potentially chelating NHC ligands; synthesis, complexation studies, and preliminary catalytic evaluation

Two new, potentially chelating NHC ligands are synthesised, characterised and their preliminary catalytic activity in NHC–Pd and NHC–Ru complexes is evaluated.

Deuterodechlorination of Aryl/Heteroaryl Chlorides Catalyzed by a Palladium/Unsymmetrical NHC System

The catalytic deuterodechlorination of aryl/heteroaryl chlorides was developed with a palladium/unsymmetrical NHC system, and the precisely controlled introduction of deuterium into a variety of aryl/heteroaryl compounds was achieved with a high level of efficiency, selectivity, and deuteration degree. This method was also successfully applied to the transformation of bioactive agents even in a gram-scale synthesis. The crystal structure analysis of Pd-NHC complexes led to the observation of Pd-arene interaction.

In Situ Generation of ArCu from CuF2 Makes Coupling of Bulky Aryl Silanes Feasible and Highly Efficient

A bimetallic system of Pd/CuF2, catalytic in Pd and stoichiometric in Cu, is very efficient and selective for the coupling of fairly hindered aryl silanes with aryl, anisyl, phenylaldehyde, p-cyanophenyl, p-nitrophenyl, or pyridyl iodides of conventional size. The reaction involves the activation of the silane by Cu(II), followed by disproportionation and transmetalation from the Cu(I)(aryl) to Pd(II), upon which coupling takes place. Cu(III) formed during disproportionation is reduced to Cu(I)(aryl) by excess aryl silane, so that the CuF2 system is fully converted into Cu(I)(aryl) and used in the coupling. Moreover, no extra source of fluoride is needed. Interesting size selectivity towards coupling is found in competitive reactions of hindered aryl silanes. Easily accessible [PdCl2 (IDM)(AsPh3)] (IDM = 1,3-dimethylimidazol-2-ylidene) is by far the best catalyst, and the isolated products are essentially free from As or Pd (<1 ppm). The mechanistic aspects of the process have been experimentally examined and discussed.

PEPPSI-SONO-SP2: a new highly efficient ligand-free catalyst system for the synthesis of tri-substituted triazine derivatives via Suzuki–Miyaura and Sonogashira coupling reactions under a green approach

Consecutive Suzuki–Miyaura and Sonogashira cross coupling reactions catalyzed by a new competent palladium catalyst PEPPSI-SONO-SP² under mild and green reaction conditions.

An efficient indenyl-derived phosphine ligand for the Suzuki–Miyaura coupling of sterically hindered aryl halides

An air-stable aryl substituted indenyl phosphine used in combination with Pd(OAc)₂ provides a highly efficient catalyst for the Suzuki–Miyaura cross-coupling reaction of sterically hindered aryl halides with aryl boronic acids.

A benzo[c]carbazolyl-based phosphine ligand for Pd-catalyzed tetra-ortho-substituted biaryl syntheses

A benzo[c]carbazolyl-based phosphine ligand has been developed for the preparation of tetra-ortho-substituted biaryls.

Highly water-dispersible magnetite nanoparticle supported-palladium–β-cyclodextrin as an efficient catalyst for Suzuki–Miyaura and Sonogashira coupling reactions

A novel highly water-dispersible and recoverable magnetite supported palladium–β-cyclodextrin complex as efficient catalyst in Suzuki–Miyaura and Sonogashira carbon–carbon coupling reactions.

Influence of bulky yet flexible N-heterocyclic carbene ligands in gold catalysis

Three new Au(I) complexes of the formula [Au(NHC)(NTf₂)] (NHC = N-heterocyclic carbene) bearing bulky and flexible ligands have been synthesised. The ligands studied are IPent, IHept and INon which belong to the ‘ITent’ (‘Tent’ for ‘tentacular’) family of NHC derivatives. The effect of these ligands in gold-promoted transformations has been investigated.

The use of the sterically demanding IPr* and related ligands in catalysis

This account highlights the synthesis and applications of one of the very bulky NHC ligands, IPr* (1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-ylidene). This ligand and some of its derivatives have been found very effective in several catalytic applications and have enabled the isolation of highly reactive organometallic complexes. More specifically, applications of this ligand in Pd and Ni chemistry have permitted challenging transformations under mild reaction conditions and low catalyst loadings. We report the successes as well as the limitations encountered using transition-metal systems bearing this ligand-type. This report will hopefully serve as a guide to synthetic chemists, providing insights as to when the very sterically demanding IPr* ligand (and its congeners) and in a broader context, very bulky NHC ligands, should be used.

Recent developments in the Suzuki-Miyaura reaction: 2010-2014

The Suzuki-Miyaura reaction (SMR), involving the coupling of an organoboron reagent and an organic halide or pseudo-halide in the presence of a palladium or nickel catalyst and a base, has arguably become one of most utilized tools for the construction of a C-C bond. This review intends to be general account of all types of catalytic systems, new coupling partners and applications, including the literature between September 2010 and December 2014.

Conditions for palladium-catalyzed direct arylations of 4-bromo and 4-iodo N-substituted pyrazoles without C–Br or C–I bond cleavage

The Pd-catalyzed arylation at the C5 position ofN-protected pyrazole derivatives bearing bromo or iodo substituents at the C4 position is described.

N-Alkylfluorenyl-substituted N-heterocyclic carbenes as bimodal pincers

Two N-heterocyclic carbene precursors having their nitrogen atoms substituted by the expanded 9-ethyl-9-fluorenyl group, namely imidazolinium chloride 6 and imidazolium chloride 7, have been synthesized in high yields from fluorenone (1).