Steven P. Nolan

N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous cross-coupling catalysis: a perfect union

Cross-coupling reactions using Pd-NHC (NHC = N-heterocyclic carbene) catalysts are discussed in this critical review and examined in terms of catalytic activity and how these have permitted advances in the area as they developed (95 references).

N-Heterocyclic carbenes in gold catalysis

The appealing properties of N-heterocyclic carbenes (NHC) as ancillary ligands and the high potential of gold as an organometallic catalyst have made their encounter inevitable. Still in its infancy, NHC-gold catalysis is nevertheless growing rapidly. In this tutorial review, catalytic transformations involving NHC-containing gold(I) and gold(III) complexes are presented. Particular attention is drawn to the versatility and selectivity of these catalysts.

Percent buried volume for phosphine and N-heterocyclic carbene ligands: steric properties in organometallic chemistry.

Electronic and steric ligand effects both play major roles in organometallic chemistry and consequently in metal-mediated catalysis. Quantifying such parameters is of interest to better understand not only the parameters governing catalyst performance but also reaction mechanisms. Nowadays, ligand molecular architectures are becoming significantly more elaborate and existing models describing ligand sterics prove lacking. This review presents the development of a more general method to determine the steric parameter of organometallic ligands. Two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.

Quantifying and understanding the electronic properties of N-heterocyclic carbenes

The use of N-heterocyclic carbenes (NHCs) in chemistry has developed rapidly over the past twenty years. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centres, and as organocatalysts able to mediate an exciting range of reactions. However, the sheer number of NHCs known in the literature can make the appropriate choice of NHC for a given application difficult. A number of metrics have been explored that allow the electronic properties of NHCs to be quantified and compared. In this review, we discuss these various metrics and what they can teach about the electronic properties of NHCs. Data for approximately three hundred NHCs are presented, obtained from a detailed survey of the literature.

Catalytic cross-coupling reactions mediated by palladium/nucleophilic carbene systems

Abstract In this mini-review, we present a summary of our recent work in the field of palladium-catalyzed cross-coupling reactions, with emphasis on the use of nucleophilic N-heterocyclic carbenes (NHC) as ancillary ligand. The palladium-mediated coupling reactions investigated include the Suzuki–Miyaura, Kumada–Tamao–Corriu, Heck, Sonogashira, Stille, Hiyama and aryl amination reactions.

Carboxylation of C−H Bonds Using N-Heterocyclic Carbene Gold(I) Complexes

A highly efficient [(NHC)Au(I)]-based (NHC = N-heterocyclic carbene) catalytic system for the carboxylation of aromatic and heteroaromatic C-H bonds was developed. The significant base strength of the Au-OH species is at the origin of the activation process and permits the facile functionalization of C-H bonds without the use of other organometallic reagents.

[(NHC)Au-I]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings

A highly efficient [(NHC)Au(I)]-based (NHC = N-heterocyclic carbene) catalytic system for the hydration of an array of alkynes that operates under acid-free conditions and at very low catalyst loadings (typically 50-100 ppm and as low as 10 ppm) was developed. Terminal and internal alkynes possessing any combination of alkyl and aryl substituents (alkyl/H, aryl/H, alkyl/alkyl, alkyl/aryl, and aryl/aryl) were found suitable substrates in the present catalytic system.

A sterically demanding nucleophilic carbene : 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) : thermochemistry and catalytic application in olefin metathesis

Abstract The sterically demanding nucleophilic carbene ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, 4) has been synthesized. The reaction of [Cp*RuCl]4 (5; Cp*=η5-C5Me5) with this ligand affords a coordinatively unsaturated Cp*Ru(IPr)Cl (6) complex. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand. Steric parameters associated with this ligand are determined from the X-ray crystal structure study. The carbene ligand reacts with RuCl2(C(H)Ph)(PCy3)2 (1) to yield a mixed carbene–phosphine ruthenium complex RuCl2(C(H)Ph)(IPr)(PCy3) (9). A single-crystal X-ray diffraction study has been performed on 9. The thermal stability of 9 has been studied at 60°C and its catalytic activity has been evaluated for the ring closing metathesis of diethyldiallylmalonate.

Copper, Silver, and Gold Complexes in Hydrosilylation Reactions

The reduction of diverse functional groups is an essential protocol in organic chemistry. Transition-metal catalysis has been successfully applied to the reduction of olefins, alkynes, and many carbonyl compounds via hydrogenation or hydrosilylation; the latter presenting several advantages over hydrogenation. Notably, hydrosilylation generally occurs under mild reaction conditions, and consequently over-reduced products are rarely detected. Moreover, the great majority of hydrosilanes employed in this reaction are easily handled, inexpensive, or both. A large number of multiple bonds can be involved in this context, and the hydrosilylation reaction can be regarded as a useful method for the synthesis of silicon-containing organic molecules or a convenient way of reducing organic compounds. Furthermore, the silyl group can also be retained as a protecting group, a strategy that can be of great usefulness in organic synthesis. Since the first Wilkinsons catalyst-mediated hydrosilylation of ketones in 1972, metals such as rhodium and iridium have attracted most of the attention in this area. A wide array of catalytic systems for hydrosilylation reactions is nowadays available, which has allowed for a great expansion of the synthetic scope of this transformation. After having been overlooked in the early years, group 11 metals (Cu, Ag, and Au), especially copper, have emerged as appealing alternatives for hydrosilylation. The use of a stabilized form of copper hydride, the hexameric [(Ph3P)CuH]6, by Stryker represented a breakthrough in copper-catalyzed reduction reactions. Nowadays, several copper-based catalytic systems compare well with a variety of reported rhodium-based catalysts, which generally suffer from the high cost of the catalyst. Tertiary phosphine ligands are the most widely used in these transformations. However, other families such as N-heterocyclic carbenes (NHCs) have shown promising activities. Compared with copper, little attention has been paid to silver- or gold-based catalysts. Silver salts have been considered inert towards hydrosilylation, and they are often employed as innocent anion exchange reagents for the in situ generation of cationic transition metal catalysts. Despite the rare reports available, they have already shown interesting reactivity profiles, for example, in the chemoselective reduction of aldehydes in the presence of ketones. Furthermore, 1,2-hydride delivery is favored over 1,4-reductions for alpha,beta-unsaturated carbonyl compounds, in contrast with most copper-based systems.

Carboxylation of N-H/C-H bonds using N-heterocyclic carbene copper(I) complexes.

Transition-metal-mediated carboxylation of N H and C H bonds represents a nascent area in organic chemistry, because these reactions enable the efficient construction of valuable synthons. Palladium-catalyzed N-carbonylation–oxidation sequences are well-documented, but they often require high catalyst loadings and the use of either gaseous carbon monoxide or Group VI metal–carbonyl complexes. An analogous transformation sequence is also promoted by molybdenum and tungsten carbonyl amine species under forcing temperatures. Important advances in C-carboxylation reactions have been made using ruthenium and nickel complexes; however, examples under mild conditions are elusive. The carboxylation of allylstannanes, organozincs, and organoboronic esters have been described as a new method to improve functional group tolerance, but the stoichiometric consumption of an organometallic reagent remains a disadvantage. The reactivity of allylstannanes and organozinc compounds necessitates handling under an inert atmosphere, while organoboronic esters are expensive. A protocol has recently been developed for the C-carboxylation of simple aromatic groups under very mild reaction conditions. In this case the strongly basic [Au(IPr)(OH)] (IPr=1,3-bis(diisopropyl)phenylimidazol-2-ylidene) complex (pKaDMSO= 30.3(2)) was used, [11] which contains an N-heterocyclic carbene (NHC) ligand [Eq. (1)].