Simple gold-catalyzed synthesis of benzofulvenes--gem-diaurated species as "instant dual-activation" precatalysts

Expanded-ring N-heterocyclic carbenes for the stabilization of highly electrophilic gold(I) cations

Strategies for the synthesis of highly electrophilic Au(I) complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr2 C6 H3 ). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C6 F5 )3 , by contrast, generates a species (at low temperatures) featuring a [HB(C6 F5 )3 ](-) fragment with spectroscopic signatures similar to the "free" borate anion. Subsequent rearrangement involves BC bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr(f) 4 ] (Ar(f) =C6 H3 (CF3 )2 -3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}2 (μ-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr(f) 4 ](-) counterion.

Gold-catalyzed cycloisomerization of 1,6-diyne esters to 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes

A synthetic method to chemoselectively prepare 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes efficiently by gold-catalyzed cycloisomerization of 1,6-diyne esters is described. These three product classes were accessed divergently by taking advantage of the electronic and steric differences between a phosphine and NHC (NHC = N-heterocyclic carbene) ligand in the respective gold(I) complexes and that of gold(III) complex combined with substrate substitution patterns and optimized reaction conditions. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, substrates with a pendant aryl group at the acetate alkynyl position were found to undergo preferential 1,3-acyloxy migration/5-exo-dig cyclization/Friedel-Crafts reaction to give 1H-cyclopenta[b]naphthalenes. In contrast, the analogous reactions with PicAuCl2 (Pic =2-picolinate) were found to proceed by selective 1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to afford cis-cyclopenten-2-yl δ-diketones. Changing the catalyst to [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) and the acetate alkynyl position from an aryl to vinyl substituent in the starting ester led to 1,3-acyloxy migration/5-exo-dig cyclization/Prins-type [2 + 2]-cycloaddition to provide bicyclo[3.2.0]hepta-1,5-dienes.

Dual gold catalysis: synthesis of polycyclic compounds via C-H insertion of gold vinylidenes

New and interesting polycyclic compounds have been synthesized from non-conjugated diyne systems by dual gold catalysis. A quaternary carbon center in the backbone and the accompanying Thorpe-Ingold effect enabled the unprecedented insertion of sp(3) and sp(2) CH bonds that for the first time were incorporated within the backbone of the diyne system and allowed the construction of complex polycyclic carbon scaffolds inaccessible by previous approaches in which the CH bonds for the insertion were situated at the other end of the alkyne.

Intriguing mechanistic labyrinths in gold(I) catalysis

Gold(i) controls complex transformations proceeding through carbocationic species by stabilising the key reactive intermediates.

Many mechanistically intriguing reactions have been developed in the last decade using gold(i) as catalyst. Here we review the main mechanistic proposals in gold-catalysed activation of alkynes and allenes, in which this metal plays a central role by stabilising a variety of complex cationic intermediates.

Mechanism of AuCl₃-catalyzed cyclization of 1-(indol-2-yl)-3-alkyn-1-ols: a DFT study

A computational study with the B3LYP functional was carried out to elucidate the mechanisms of AuCl₃- and AgOTf-catalyzed cyclization of 1-(indol-2-yl)-3-alkyn-1-ols. The theoretical studies suggested that the two main processes, cycloaddition and hydrogen-transfer, are included in all possible reaction pathways. Calculations revealed that AuCl₃ is more effective in catalytic ability than AgOTf to catalyze the cyclization of 1-(indol-2-yl)-3-alkyn-1-ols into carbazole derivatives. More importantly, we found that the ligands of catalysts, Cl⁻ and OTf⁻, are critical in a stepwise proton-transport process involved in intramolecular nucleophilic addition because they act as a proton shuttle to lower the activation free energy barrier of the rate-determining step. The theoretical discovery of the role of ligands of catalysts in hydrogen shift process suggests that AuCl₃- and AgOTf-catalyzed cyclization of 1-(indol-2-yl)-3-alkyn-1-ols can be accelerated when ligands with the property of nucleophile are used. Our theoretical calculations reproduced the experimental results very well. The present study is expected to help understand other transition metal-catalyzed reactions and to give guidance for future design of new catalysts.

Dual gold catalysis

For more than a decade the innovative field of homogeneous catalysis by gold was dominated by the interaction of the substrate molecule with one gold center, in most cases in mononuclear gold complexes. The initial interaction was typically a π-coordination of a carbon-carbon double bond to the gold, an activation of the unsaturated substrate molecule by a π-acidic metal center. Only recently clear evidence for reactions that involve the activation of organic substrates by two gold centers was obtained. In that new class of gold-catalyzed reactions the two gold centers interact with the substrate in a very different way. One gold complex is σ-bonded to a terminal alkynyl group in the substrate, the other one is π-coordinated. Only in a few cases, a combination π-coordination and σ-coordination to the same alkyne, which is the energetically preferred mode of interaction with two gold centers, initiates the reaction. In most of the cases, the reaction proceeds through an intermediate with one alkyne σ-bonded to one gold complex and a different alkyne π-coordinated to the second gold complex. Experimental and computational results for many new reactions provide a clear picture of the overall sequence of elemental steps of these conversions; some of the steps are unprecedented in organometallic catalysis and chemistry. For example, the reaction of diynes can involve gold vinylidene-like species as very reactive substructures formed by a 5-exo-dig cyclization. NBO analysis indicates no gold-carbon double bond character in these "vinylidenes". In other reactions, a 6-endo-dig cyclization is energetically preferred; after that gold aryne complexes are not local minima but transition states of a 1,2-shift of gold. Computational studies showed a good correlation of the cyclization mode with the aromaticity of the intermediate. For both the 5-exo-dig and the 6-endo-dig cyclization modes, the intermediates are able to react even with unactivated alkyl-C,H bonds, in low yields even in intermolecular reactions. The final step of the catalytic cycles is also remarkable, because the protodeauration has to occur with the next alkyne substrate molecule. Only then the next gold acetylide is formed directly and a loss of selectivity can be avoided. A computational study suggests that two gold complexes are on the substrate throughout the catalytic cycle. The most recent results indicate that analogous intermediates can be accessible by the reaction of other electrophiles with gold acetylides. With regard to organic synthesis, the overall catalytic conversions open up a universe of new possibilities. Selective C,H-activations now allow to one use usually innocent alkyl side chains for additional anellation reactions by an sp(3)-C,H activation. The C,H activation can even be combined with halogen transfer reactions, directly providing vinyl iodides as versatile building blocks. Short and efficient routes to different carbo- and heterocycles including benzocyclobutenes, fulvenes, and pentalenes demonstrate the synthetic potential not only for total synthesis but also for material science.

Molecular complexity from polyunsaturated substrates: the gold catalysis approach

Over the last two decades, electrophilic catalysis relying on platinum(II), gold(I), and gold(III) salts has emerged as a remarkable synthetic methodology. Chemists have discovered a large variety of organic transformations that convert a great assortment of highly functionalized precursors into valuable final products. In many cases, these methodologies offer unique features, allowing access to unprecedented molecular architectures. Due to the mild reaction conditions and high function compatibility, scientists have successfully developed applications in total synthesis of natural products, as well as in asymmetric catalysis. In addition, all these developments have been accompanied by the invention of well-tailored catalysts, so that a palette of different electrophilic agents is now commercially available or readily synthesized at the bench. In some respects, researchers' interests in developing homogeneous gold catalysis can be compared with the Californian gold rush of the 19th century. It has attracted into its fervor thousands of scientists, providing a huge number of versatile and important reports. More notably, it is clear that the contribution to the art of organic synthesis is very valuable, though the quest is not over yet. Because they rely on the intervention of previously unknown types of intermediates, new retrosynthetic disconnections are now possible. In this Account, we discuss our efforts on the use of readily available polyunsaturated precursors, such as enynes, dienynes, allenynes, and allenenes to give access to highly original polycyclic structures in a single operation. These transformations transit via previously undescribed intermediates A, B, D, F, and H that will be encountered later on. All these intermediates have been determined by both ourselves and others by DFT calculations and in some cases have been confirmed on the basis of experimental data. In addition, dual gold activation can be at work in some of these transformations, for instance, from E to F. Strikingly, we have found propargyl acetates to be particularly productive precursors. In a preliminary step upon electrophilic activation (complex I), they can lead to oxonium J or a vinylcarbenoid species K after 1,2-migration or complexed allenylester M from a formal 1,3-migration. All of them can serve as versatile entries for multievent processes. The propargyl cycle, sometimes called the golden carousel, involves species I-N), which lie in a close equilibrium. The control of this merry-go-round and its offshoots depends on the energy barriers associated with the subsequent reactions of these intermediates. We illustrate these themes in this Account, focusing on the intriguing characteristics of gold catalysis.

Cyclization of gold acetylides: synthesis of vinyl sulfonates via gold vinylidene complexes

Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β-carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold-catalyzed reactions, but for the new substrates, only one gold atom is needed for substrate activation. After formation of a gold vinylidene complex, which forms a tight contact ion pair with the sulfonate leaving group, recombination of the two parts delivers vinyl sulfonates, which are valuable targets that can serve as precursors for cross-coupling reactions, for example.

Synthesis of (spiro)cyclopentapyridinones via C(sp3)-H functionalization: a post-Ugi gold-catalyzed regioselective tandem cyclization

A gold-catalyzed regioselective tandem cyclization of N-propynylbutynamide via Csp(3)-H functionalization has been described, providing a distinctive methodology for the architecture of cyclopentapyridinones as well as spirocyclopentapyridinones.

Reaction mechanism for the dual gold-catalyzed synthesis of dibenzopentalene: a DFT study

A wide range of gold-catalyzed reactions based on a dual activation mechanism has recently been reported in the literature. Herein, we present a computational investigation of the mechanism for the formation of dibenzopentalenes from 1-ethynyl-2-(phenylethynyl)benzene. Transition states have been found, which substantiate the dual activation mechanism previously published and furthermore point towards a continuous presence of two gold moieties throughout the mechanistic cycle, an observation of high importance for all reactions in the field of dual activation. The initial activation of the diyne has been shown to proceed via an intermolecular transfer of a cationic gold catalyst from the thermodynamically preferred geminal-σ,π-acetylide complex to the active non-geminal analogue. Furthermore, the regioselectivity of a 5-endo versus a 6-endo cyclization has been addressed, and the 5-endo cyclization was found to be most favorable both thermodynamically and with regard to the activation barrier.

Gold-catalyzed cyclization of diynes: controlling the mode of 5-endo versus 6-endo cyclization--an experimental and theoretical study by utilizing diethynylthiophenes

Herein, a dual-gold catalyzed cyclization of 3,4-diethynylthiophenes generating pentaleno[c]thiophenes through gold-vinylidenes and C-H bond activation is disclosed. Various new heteroaromatic compounds--substrate classes unexplored to date--exhibiting three five-membered annulated ring systems could be synthesized in moderate to high yields. By comparison of the solid-state structures of the corresponding gold-acetylides, it could be demonstrated that the cyclization mode (5-endo versus 6-endo) is controlled by the electronic and not steric nature of the diyne backbone. Depending on different backbones, we calculated thermodynamic stabilities and full potential-energy surfaces giving insight into the crucial dual-activation cyclization step. In the case of the 3,4-thiophene backbone, in which the initial cyclization is rate and selectivity determining, two energetically distinct transition states could be localized explaining the observed 5-endo cyclization mode by classical transition-state theory. In the case of vinyl and 2,3-thiophene backbones, the theoretical analysis of the cyclization mode in the bifurcated cyclization area demonstrated that classical transition-state theory is no longer valid to explain the high experimentally observed selectivity. Herein, for the first time, the influence of the backbone and the aromatic stabilization effect of the 6-endo product in the crucial cyclization step could be visualized and quantified by calculating and comparing the full potential-energy surfaces.

Dissecting Anion Effects in Gold(I)-Catalyzed Intermolecular Cycloadditions

From a series of gold complexes of the type [t-BuXPhosAu(MeCN)]X (X=anion), the best results in intermolecular gold(I)-catalyzed reactions are obtained with the complex with the bulky and soft anion BAr4F- [BAr4F-=3,5-bis(trifluoromethyl)phenylborate] improving the original protocols by 10-30% yield. A kinetic study on the [2+2] cycloaddition reaction of alkynes with alkenes is consistent with an scenario in which the rate-determining step is the ligand exchange to generate the (η2-phenylacetylene)gold(I) complex. We have studied in detail the subtle differences that can be attributed to the anion in this formation, which result in a substantial decrease in the formation of unproductive σ,π-(alkyne)digold(I) complexes by destabilizing the conjugated acid formed.

Gold-catalyzed C(sp3)–H bond functionalization

Homogeneous gold-catalyzed sp³ C–H bond functionalization strategy opens a new avenue for economical and sustainable construction of fine chemicals.

Synthesis of spiroaminals by bimetallic Au/Sc relay catalysis: TMS as a traceless controlling group

An efficient synthesis of spiroaminals over fused aminals has been successfully developed by bimetallic Au/Sc catalysis by using TMS as a traceless controlling group.

Formal hydrogenation of arynes with silyl Cβ–H bonds as an active hydride source

In stark contrast to the effective 1°, 2°, and 3° C–H bond insertion of alkyl groups tethered to arynes, the 2° and 3° C–H bonds on the β-carbon of silyl groups show high tendency for hydride transfer rather than C–H insertion, whereas the corresponding 1° C–H bonds exclusively undergo C–H insertion.

Gold-catalyzed [2+2+1] cycloaddition of 1,6-diyne carbonates and esters with aldehydes to 4-(cyclohexa-1,3-dienyl)-1,3-dioxolanes

A synthetic method to stereoselectively prepare 4-(cyclohexa-1,3-dienyl)-1,3-dioxolanes in good to excellent yields by gold(I)-catalyzed [2+2+1] cycloaddition of 1,6-diyne carbonates and esters with aldehydes is described. The cascade process involves 1,2-acyloxy migration followed by cyclopropenation and cycloreversion. This leads to an unprecedented [2+2+1] cycloaddition of the resulting alkenylgold carbenoid species, examples of which are extremely rare, with two aldehyde molecules at catalyst loadings as low as 1 mol %. The usefulness of this cycloisomerization chemistry was further demonstrated by the transformation of one example to the corresponding phenol.

Gold-catalyzed reaction of oxabicyclic alkenes with electron-deficient terminal alkynes to produce acrylate derivatives

Oxabicyclic alkenes can react with electron-deficient terminal alkynes in the presence of a gold catalyst under mild conditions, affording the corresponding addition products in moderate yields. When using alkynyl esters as substrates, the (Z)-acrylate derivatives are obtained. Using but-3-yn-2-one (ethynyl ketone) as a substrate, the corresponding addition product is obtained with (E)-configuration. The proposed mechanism of these reactions is also discussed.

Synthesis of axially chiral gold complexes and their applications in asymmetric catalyses

Several novel chiral N-heterocyclic carbene and phosphine ligands were prepared from (S)-BINOL. Moreover, their ligated Au complexes were also successfully synthesized and characterized by X-ray crystal diffraction. A weak gold-π interaction between the Au atom and the aromatic ring in these gold complexes was identified. Furthermore, we confirmed the formation of a pair of diastereomeric isomers in NHC gold complexes bearing an axially chiral binaphthyl moiety derived from the hindered rotation around C–C and C–N bonds. In the asymmetric intramolecular hydroamination reaction most of these chiral Au(I) complexes showed good catalytic activities towards olefins tethered with a NHTs functional group to give the corresponding product in moderate yields and up to 29% ee.