Cationic NHC–gold(I) complexes: Synthesis, isolation, and catalytic activity

Gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers; synthesis of substituted 1,3-dienes

Synthesis of substituted 1,3-dienes was achieved via gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers. The N-heterocyclic carbene gold chloride catalyst (IPrAuCl) was superior in terms of activity and selectivity and afforded the 3,3-product in excellent yields. A proposed cation-π inter-action played a significant role in affecting the reaction rate.

Gold(I)-NHC complexes of antitumoral diarylimidazoles: structures, cellular uptake routes and anticancer activities

Five new heterocyclic gold carbene complexes were prepared, four chlorido-[1,3-dimethyl-4,5-diarylimidazol-2-ylidene]gold complexes 6a-d and a chlorido-[1,3-dibenzylimidazol-2-ylidene]gold complex 11, and three of them were characterised by X-ray single crystal analyses. They were tested for cytotoxicity against a panel of four human cancer cell lines and non-malignant fibroblasts, for tubulin interaction, and for the pathways of their uptake into 518A2 melanoma cells. All complexes showed cytotoxic activity in the micromolar IC(50) range with distinct selectivities for certain cell lines. In stark contrast to related metal-free 1-methyl-4,5-diarylimidazoles, the complexes 6 and 11 did not noticeably inhibit the polymerisation of tubulin to give microtubules. The cellular uptake of complexes 6 occurred mainly via the copper transporter (Ctr1) and the organic cation transporters (OCT-1/2). Complex 11 was accumulated preferentially via the organic cation transporters and by Na(+)/K(+)-dependent endocytosis. The new gold carbene complexes seem to operate by a mechanism different from that of the parent 1-methylimidazolium ligands.

Gold(I)-Catalyzed Intramolecular Hydroamination of N-Allylic,N'-Aryl Ureas to form Imidazolidin-2-ones

Treatment of N-allylic,N'-aryl ureas with a catalytic 1:1 mixture of di-tert-butyl-o-biphenylphoshphine gold(I) chloride and silver hexafluorophosphate (1 mol %) in chloroform at room temperature led to 5-exo hydroamination to form the corresponding imidazolidin-2-ones in excellent yield. In the case of N-allylic ureas that possessed an allylic alkyl, benzyloxymethyl, or acetoxymethyl substituent, gold(I)-catalyzed 5-exo hydroamination leads to formation of the corresponding trans-3,4-disubstituted imidazolidin-2-ones in excellent yield with ≥50:1 diastereoselectivity.

Development of versatile and silver-free protocols for gold(I) catalysis

The use of a versatile N-heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) permits the in situ generation of the [Au(IPr)](+) ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. (1)H NMR studies in several solvent media of the in situ generation of this [Au(IPr)](+) ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}(2)(μ-OH)], which is fully characterized and was tested in gold(I) catalysis.

Gold(I)-catalyzed intramolecular amination of allylic alcohols with alkylamines

A 1:1 mixture of (1)AuCl [1 = P(t-Bu)(2)o-biphenyl] and AgSbF(6) catalyzes the intramolecular amination of allylic alcohols with alkylamines to form substituted pyrrolidine and piperidine derivatives. Gold(I)-catalyzed cyclization of (R,Z)-8-(N-benzylamino)-3-octen-2-ol (96% ee, 95% de) led to isolation of (R,E)-1-benzyl-2-(1-propenyl)piperidine in 99% yield with 96% ee, consistent with the net syn addition of the amine relative to the departing hydroxyl group.

Synthesis and equilibrium binding studies of cationic, two-coordinate gold(I) π-alkyne complexes

Reaction of a 1:1 mixture of (L)AuCl [L = P(t-Bu)₂o-biphenyl or IPr] and AgSbF₆ with internal alkynes led to isolation of the corresponding cationic, two-coordinate gold π-alkyne complexes in ≥ 90% yield. Equilibrium binding studies show that the binding affinities of alkynes to gold(I) are strongly affected by the electron density of the alkyne and to a lesser extent on the steric bulk of the alkyne. These substituent effects on alkyne binding affinity are greater than are the differences between the inherent binding affinities of alkynes and alkenes to gold(I).

The development and catalytic uses of N-heterocyclic carbene gold complexes

Gold has emerged as a powerful synthetic tool in the chemist's arsenal. From the early use of inorganic salts such as AuCl and AuCl(3) as catalysts, the field has evolved to explore ligands that fine-tune reactivity, stability, and, more recently, selectivity in gold-mediated processes. Substrates generally contain alkenes or alkynes, and they usually involve straightforward protocols in air with solvents that can often times be of technical grade. The actual catalytic species is the putative cationic gold(I) complex [Au(L)](+) (where L is a phosphorus-based species or N-heterocyclic carbene, NHC). The early gold systems bearing phosphine and phosphite ligands provided important transformations and served as useful mechanistic probes. More recently, the use of NHCs as ligands for gold has rapidly gained in popularity. These two-electron donor ligands combine strong σ-donating properties with a steric profile that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the gold-NHC complexes have been used as well-defined precatalysts and have permitted the isolation of reactive single-component systems that are now used instead of the initial [Au(L)Cl]/silver salt method. Because some are now commercially available, NHC-containing gold(I) complexes are gathering increasing interest. In this Account, we describe the chronological development of this chemistry in our laboratories, highlighting the advantages of this family of gold complexes and reviewing their synthesis and applications in catalysis. We first outline the syntheses, which are straightforward. The complexes generally exhibit high stability, allowing for indefinite storage and easy handling. We next consider catalysis, particularly examining efficacy in cycloisomerization, other skeletal rearrangements, addition of water to alkynes and nitriles, and C-H bond activation. These processes are quite atom-economical, and in the most recent C-H reactions the only byproduct is water. State-of-the-art methodology now involves single-component catalysts, precluding the need for costly silver co-catalysts. Remarkably, the use of an NHC as a supporting ligand has permitted the isolation of [Au(L)(S)](+) species (where S is a solvent molecule such as a nitrile), which can act as single-component catalysts. Some improvements are still needed, as the single components are most often synthesized with a silver reagent. Owing to the stabilizing effect of NHC coordination, some NHC-containing systems can catalyze extremely challenging reactions (at temperatures as high as 140 °C) and react at very low loadings of gold (ppm levels). Our latest developments deal with C-H bond functionalization and hold great promise. We close with a selection of important developments by the community with gold-NHC complexes. As demonstrated by the turns and twists encountered during our own journey in the gold-NHC venture, the chemistry described here, combining fundamental organometallic, catalytic, and organic methodology, remains rich in opportunities, especially considering that only a handful of gold(I) architectures has been studied. We hope this Account will encourage young researchers to explore this emerging area, as the adage "the more you do, the more you have to do" surely holds true in gold-mediated catalysis.

[{Au(IPr)}2(μ-OH)]X complexes: synthetic, structural and catalytic studies

The synthesis of a series of dinuclear gold hydroxide complexes has been achieved. These complexes of type [{Au(IPr)}(2)(μ-OH)]X (X=BF(4), NTf(2), OTf, FABA, SbF(6); IPr=2,6-bis(disopropylphenyl)imidazol-2-ylidene; NTf(2)=bis(trifluoromethanesulfonyl)imidate; OTf=trifluoromethanesulfonate; FABA=tetrakis(pentafluorophenyl)borate) are easily formed in the presence of water and prove highly efficient in the catalytic hydration of nitriles. Their facile formation in aqueous media suggests they are of relevance in gold-catalyzed reactions involving water. Additionally, a series of [Au(IPr)(NCR)][BF(4)] (R=alkyl, aryl) complexes was synthesized as they possibly occur as intermediates in the catalytic reaction mechanism. (1)H and (13)C NMR data as well as key bond lengths obtained by X-ray diffraction studies are compared and reveal an interesting structure-activity relationship. The collected data indicate a negligible effect of the nature of the nitrile on the reactivity of [Au(L)(NCR)][X] complexes in catalysis.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Alkynes as equivalents of alpha-diazo ketones in generating alpha-oxo metal carbenes: a gold-catalyzed expedient synthesis of dihydrofuran-3-ones

An expedient and reliable method for accessing reactive alpha-oxo gold carbenes via gold-catalyzed intermolecular oxidation of terminal alkynes has been developed. Significantly, this method offers a safe and economical alternative to the strategies based on diazo substrates. Its synthetic potential is demonstrated by expedient preparation of dihydrofuran-3-ones containing a broad range of functional groups.

Gold(I)-catalyzed amination of allylic alcohols with cyclic ureas and related nucleophiles

A 1:1 mixture of [P(t-Bu)(2)-o-biphenyl]AuCl and AgSbF(6) catalyzes the intermolecular amination of allylic alcohols with 1-methylimidazolidin-2-one and related nucleophiles that, in the case of gamma-unsubstituted or gamma-methyl-substituted allylic alcohols, occurs with high gamma-regioselectivity and syn-stereoselectivity.

Syntheses, X-ray crystal structures, and solution behavior of monomeric, cationic, two-coordinate gold(I) pi-alkene complexes

Treatment of a suspension of (IPr)AuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine] and AgSbF(6) (1:1) with isobutylene at room temperature for 12 h led to isolation of [(NHC)Au(eta(2)-H(2)C=CMe(2))](+) SbF(6)(-) (1a) in 98% yield, which was characterized by spectroscopy and X-ray crystallography. A number of cationic gold pi-alkene complexes were isolated employing a procedure similar to that used to isolate 1a, two of which were analyzed by X-ray crystallography. Spectroscopy, X-ray crystallography, and alkene binding studies were in accord with a gold-(pi-alkene) interaction dominated by sigma-donation from the alkene to gold.