Catherine S. J. Cazin

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)].

N-Heterocyclic carbenes in transition metal catalysis and organocatalysis

1. N-Heterocyclic Carbenes - An Introductory Overview.- 2. N-Heterocyclic Carbene Complexes in Additions to Multiple Bonds.- 3. N-Heterocyclic Carbene Complexes in Olefin Metathesis.- 4. N-Heterocyclic Carbene Complexes in Polymerisation, Oligomerisation and Telomerisation Reactions.- 5. N-Heterocyclic Carbene Complexes in Cyclisation Reactions.- 6. N-Heterocyclic Carbene Complexes in Cross-Coupling Reactions.- 7. N-Heterocyclic Carbene Complexes in Arylation Reactions Other than Cross-Couplings.- 8. N-Heterocyclic Carbene Complexes in Dehalogenation Reactions.- 9. N-Heterocyclic Carbene Complexes in Reactions Involving Carbon Monoxide.- 11. N-Heterocyclic Carbene Complexes in Other Transition Metal Mediated Reactions.- 12. N-Heterocyclic Carbenes in Organocatalysis.- 13. N-Heterocyclic Carbene Complexes - Decomposition Pathways.- 14. N-Heterocyclic Carbene Complexes in Industrial Processes.

Copper N-heterocyclic carbene complexes in catalysis

N-Heterocyclic carbene ligated copper complexes act as catalysts in a variety of reactions. A brief overview of this rich chemistry is given here. Of particular note is the ability of Cu(NHC) complexes to functionalize carbonyls, alkenes and alkynes. With growth in the number of Cu(NHC) derived complexes, the catalytic possibilities involving these complexes are ever growing. We feel the full potential of these (for the most part) simply accessed complexes has yet to be fully achieved. The litany of reactions which Cu(NHC) catalyst facilitate are outlined here.

[Pd(IPr*)(cinnamyl)Cl]: An Efficient Pre‐catalyst for the Preparation of Tetra‐ortho‐substituted Biaryls by Suzuki–Miyaura Cross‐Coupling

The bigger the better: The new well-defined [Pd(IPr*)(cin)Cl] pre-catalyst is described. This complex proves to be highly active in the Suzuki-Miyaura cross-coupling for the synthesis of tetra-ortho-substituted biaryls under mild conditions. IPr* is reported as the largest N-heterocyclic carbene (NHC) to date for [Pd(NHC)(cin)Cl] complexes, explaining the high reactivity observed for this complex in this challenging transformation.

Room-temperature activation of aryl chlorides in Suzuki–Miyaura coupling using a [Pd(μ-Cl)Cl(NHC)]2 complex (NHC = N-heterocyclic carbene)

A straightforwardly synthesised complex, [Pd(micro-Cl)Cl(NHC)](2) (NHC = bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr), has been employed to mediate Suzuki-Miyaura reactions involving aryl chlorides at very low catalyst loadings and at room temperature.

Silica-supported imine palladacycles : recyclable catalysts for the Suzuki reaction?

Silica-supported, imine-based palladacyclic catalysts have been synthesised and the crystal structure of complex 9, the triphenylphosphine adduct of the pre-supported precursor complex 8, has been determined. The solid-supported catalysts show considerably lower activity in the Suzuki reaction than their homogeneous counterparts. Poor recyclability of the silica-immobilised catalysts and the presence of active catalysts in solution indicate that imine-based palladacyclic catalysts are unstable with respect to liberation of zero-valent palladium species. Whilst the solid-supported complexes are not useful as catalysts, they do function as excellent mechanistic probes. Studies on model complexes give further information on the processes that cause the liberation of zero-valent species not only from the solid-supported catalysts, but also from homogeneous systems. In all cases it appears that a reductive-elimination event occurs to generate the active catalyst.

Copper N-heterocyclic carbene (NHC) complexes as carbene transfer reagents

A method is described that makes use of easily prepared, inexpensive copper synthons as N-heterocyclic carbene (NHC) transfer agents to generate catalytically relevant gold and palladium complexes.

Simple and versatile synthesis of copper and silver N-heterocyclic carbene complexes in water or organic solvents

A novel synthetic route leading to N-heterocyclic carbene copper complexes has been developed by using air-stable and commercially available copper(I) oxide and imidazolium salts starting materials.

Mixed N-heterocyclic carbene/phosphite ruthenium complexes: towards a new generation of olefin metathesis catalysts.

The synthesis, characterisation and catalytic behaviour of ruthenium indenylidene complexes bearing an N-heterocyclic carbene and triisopropylphosphite are described.

Decarboxylation of aromatic carboxylic acids by gold(I)-N-heterocyclic carbene (NHC) complexes

A recently isolated gold(I) complex, [Au(IPr)(OH)], permits the transformation of carboxylic acids to the corresponding decarboxylated gold(I)-aryl complex without the use of silver co-catalyst under mild reaction conditions.