Gold-catalyzed cycloisomerizations of enynes: a mechanistic perspective

Cationic gold-catalyzed intramolecular cyclization of substituted 1,5-diynes to access indenone derivatives

An efficient intramolecular cyclization reaction was developed to achive indenone derivatives. The substituted 1,5-diyenes were converted to the cooresponding indenones via gold-catalyzed organic transformation and moderate to excellent yields of the title molecules were obatined via formation of two C=O and one C-C bonds under mild reaction condtions in one-pot.

Synthesis of chiral polycyclic N-heterocycles via gold(I)-catalyzed 1,6-enyne cyclization/intramolecular nucleophilic addition

Cycloisomerization of N-tethered indole- and dihydropyrrole-arylpropargyl substrates into complex polycycles was investigated by using gold(I) catalysis. Enantioselectivities up to 93% ee were obtained in the asymmetric version of this process. DFT calculations rationalized the reactivity observed for the formation of such complex molecular frameworks.

Insights into the gold(I)-catalyzed intermolecular annulations of alkynes with N-allenamides: a mechanistic DFT study

The mechanism of Au(I)-catalyzed intermolecular annulation of 2-(1-alkynyl)-2-alken-1-one with N-allenamide was carefully elucidated using density functional theory (DFT). The reaction is initiated by the binding of the Au(I) catalyst with 2-(1-alkynyl)-2-alken-1-one rather than with N-allenamide. The key intermediate, a gold all-carbon 1,3-dipole species, is revealed to be more reactive than the gold allylic carbocation. The influence of ligands and substituents was rationally analyzed. We believe that our study will provide deeper mechanistic insights into the chemoselective reactions of alkynes with N-allenamide.

Diametric calix[6]arene-based phosphine gold(I) cavitands

We report the synthesis and characterization, in low polarity solvents, of a novel class of diametric phosphine gold(I) cavitands characterized by a 1,2,3-alternate geometry. Preliminary catalytic studies were performed on a model cycloisomerization of 1,6-enynes as a function of the relative orientation of the bonded gold(I) nuclei with respect to the macrocyclic cavity.

Mechanism and Origins of Enantioselectivity of Cobalt-Catalyzed Intermolecular Hydroacylation/Cyclization of 1,6-Enynes with Aldehydes

Density functional theory calculations were performed to investigate the cobalt-catalyzed intermolecular hydroacylation/cyclization of 1,6-enynes. The computations show that the initial oxidative cyclization constitutes the rate-determining step of the overall reaction....

Gold(i) complexes bearing a PNP-type pincer ligand: photophysical properties and catalytic investigations

The synthesis and characterization of two dinuclear and five tetranuclear gold(i) complexes bearing the 2,6-bis(diphenylphosphinomethyl)pyridine diphosphane ligand (DPPMPY) are herein reported.

Efficient synthesis of tetracyclic γ-lactams via gold-catalyzed oxidative cyclization of alkenyl diynes

An efficient gold-catalyzed cascade cyclization of alkenyl diynes involving alkyne oxidation, carbene-alkyne metathesis and cyclopropanation has been developed, furnishing a series of tetracyclic γ-lactams bearing one quaternary carbon center and...

Cobalt-catalyzed cascade hydroalkenylation of 1,6-enynes with chalcones

An efficient stereoselective cobalt-catalyzed hydrovinylative cyclization of 1,6-enynes with chalcones to obtain functionalized pyrrolidines has been developed.

Gold-Catalysed Rearrangement of Unconventional Cyclopropane-Tethered 1,5-Enynes

The synthesis of particular cyclopropane-tethered 1,5-enynes, namely 6-alkynyl-4-alkylidenebicyclo[3.1.0]hex-2-enes, enabled the discovery of an unprecedented gold-catalyzed rearrangment to indenes. A computational study of the mechanism of this profund skeleton rearrangement is...

Rh2(II)-Catalyzed Enantioselective Intramolecular Büchner Reaction and Aromatic Substitution of Donor-Donor Carbenes

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction of donor/donor-carbenes was reported and a series of valuable chiral polycyclic products were synthesized. Both aryloxy enynones and diazo compounds were efficient carbene precursors for this reaction. Excellent yields (up to 99%) and outstanding enantioselectivities (up to >99% ee) were achieved under standard conditions. For furyl substituted chiral cyclohepta[b]benzofurans bearing a substituent at the C4 position on cycloheptatrienes, control reactions showed that the chiral Büchner products could slowly racemize either under dark or natural light conditions. A diradical-involved mechanism rather than a zwitterionic intermediate was proposed to explain the racemization. Furthermore, furyl substituted chiral fluorene derivatives were obtained via asymmetric aromatic substitution when biaryl enynones were employed as carbene precursors.

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction and aromatic substitution of donor/donor-carbenes were reported and a series of valuable chiral polycyclic products were synthesized.

Highly Diastereoselective Synthesis of Octahydro-1H-cyclpenta[c]pyridine Skeleton via Pd/Au-Relay Catalyzed Cascade Reaction of (Z)-1-Iodo-1,6-diene and Alkyne

The Octahydro-1H-cyclopenta[c]pyridine Skeletons exist in a broad spectrum of bioactive natural products, and the development of efficient and convenient protocols to construct this skeleton remains a challenging task. Herein, we...

Mechanistic insights into nickel- and gold-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds: a DFT study

Density functional theory (DFT) calculations were performed to gain an in-depth mechanistic understanding of Ni(0)- and Au(i)-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds.

Iron-Catalyzed Ring Opening of Cyclopropanols and Their 1,6-Conjugate Addition to p-Quinone Methides

A novel iron-catalyzed ring opening of cyclopropanols and their 1,6-conjugate addition to p-quinone methides for accessing substituted phenols is disclosed. In this protocol, various cyclopropanols are converted to alkyl radicals and undergo 1,6-conjugate addition to p-quinone methides toward C-C bond formation. The salient features of this methodology include operationally simple and mild reaction conditions, environmentally benign protocol, high efficiency, inexpensive catalyst, good to excellent yield, and a wide range of substrate scope.

Gold-Catalyzed Domino Cycloisomerization/Alkoxylation: An Entry to 3,4-Dihydro-1H-[1,4]oxazino[4,3-a]indole

A novel and mild synthetic route for the preparation of functionalized polycyclic indole skeletons via a gold-mediated cycloisomerization/alkoxylation of 1,6-aldehyde-yne has been developed. This atom-economical catalytic process that associates IPrAu(MeCN)BF₄ and an alcohol demonstrated remarkable selectivity in accessing functionalized 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole derivatives of high synthetic utility (21 examples, yields of ≤96%) and could be optimized under asymmetric conditions with an enantiomeric excess of ≤86%.

Gold (I)-Promoted Intermolecular Cascade Annulation to Access 2-Hydroxybenzocarbazoles via a Meyer-Schuster Rearrangement

An efficient cascade annulation protocol was established to access substituted 2-hydroxybenzocarbazoles from alkynylcyclohexadienones and substituted 2-aminophenols under gold catalysis. In this transformation a new C-C, two C-N bonds were formed sequentially and moderate to excellent yields of 2-hydroxybenzocarbazole derivatives were obtained selectively via Meyer-Schuster rearrangement in one-pot.

TfOH catalysed domino-double annulation of arenes with propargylic alcohols: a unified approach to indene polycyclic systems

The design and development of a TfOH catalysed domino strategy for the double annulation of arenes with propargylic alcohols for the rapid generation of indene based polycyclic systems is reported. The dehydration, intramolecular 6-endo-dig hydroarylation, and cationic cyclization were consecutively promoted by TfOH. The key features of this strategy are the formation of two C-C bonds, unified access to indene polycyclic systems, excellent yields (up to 95%), high atom economy (>90%), an operationally simple procedure, and water being the only byproduct. By extending this strategy, a two-step synthesis of the pentacyclic systems of hypoxylonol A (43% overall yield from α-tetralone), daldinone A (63% overall yield from β-tetralone) and spiro-tetracyclic framework of incarviatone A has also been achieved.

Precious metal complexes of bis(pyridyl)allenes: synthesis and catalytic and medicinal applications

The incorporation of donor-type substituents on the allene core opens up the possibility of coordination complexes in which the metal is bonded to the donor groups, with or without interaction with the double bond system. Despite the challenges in the synthesis of such allene-containing metal complexes, their unique 3D environments and dual functionality (allene and metal) could facilitate catalysis and interaction with chemical and biological systems. Bis(pyridyl)allenes are presented here as robust ligands for novel Pd(II), Pt(IV) and Au(III) complexes. Their synthesis, characterisation and first application as catalysts of benchmark reactions for Pd, Pt and Au are presented with interesting reactivity and selectivities. The complexes have also been probed as antimicrobial and anticancer agents with promising activities, and the first studies on their unusual interaction with several DNA structures will open new avenues for research in the area of metallodrugs with new mechanisms of action.

Gold-Catalyzed Reactions Towards Diversity: From Simple Substrates to Functionalized Carbo- and Heterocycles

The field of gold catalysis has been in constant expansion during the last twenty years. Based on the precept of π-activation of unsaturated simple substrates, several new rearrangements have been discovered, implying aryl, alkyne, alkene or keto derivatives as key partners. In this personal account, the main contributions in the field of gold catalysis from our group will be highlighted, emphasizing the recent reports, starting from 1,6- and 1,5-enynes and then moving to keto-ynes derivatives. The gold-catalyzed reactions will be presented starting from classical skeletal rearrangements (cycloisomerization) and then domino processes. In each part, the presentation of asymmetric versions will be highlighted.

Inter/Intramolecular Cascade of 1,6-Enynes Catalyzed by All-Metal Aromatic Tripalladium Complexes and Carboxylic Acids

Trinuclear all-metal aromatic clusters are an original class of molecules with a cyclic and planar metal core. Characterized by peculiar metal-metal delocalized bonds, they represent a new frontier in transition-metal catalysis. We report a study on C-C-forming reactions of polyunsaturated substrates catalyzed by trinuclear all-metal aromatic palladium clusters. The synthesis of two new families of tricyclic compounds was obtained with a broad functional group tolerance under mild reaction conditions. A peculiar regio- and diastereoselectivity characterized the method, demonstrating that trinuclear palladium complexes are complementary to their popular mononuclear peers. Furthermore, preliminary studies on the mechanism of these polycyclization reactions revealed unique features of the homogeneous catalytic system.

Triaminocyclopentadienyl Ruthenium Complexes - New Catalysts for Cascade Conversions of Propargyl Alcohols

Various triaminocyclopentadienyl ruthenium complexes have been synthesized from Ru₃ (CO)₁₂ . The new complexes were tested for their ability to catalyze cascade conversions of propargyl alcohols. Their associated catalytic activities complement the activities of known diaminocyclopentadienone ruthenium complexes. In particular, the substrate scope of catalytic cycloadditions with 3-ketolactones or phloroglucinol derivatives is extended to terpenoid-derived propargyl alcohols containing an internal alkyne moiety. A wide range of cyclic terpenoid and phloroglucinol adducts are obtained by complementary application of both types of catalysts.

Alumina-Mediated π-Activation of Alkynes

The ability to induce powerful atom-economic transformation of alkynes is the key feature of carbophilic π-Lewis acids such as gold- and platinum-based catalysts. The unique catalytic activity of these compounds in electrophilic activations of alkynes is explained through relativistic effects, enabling efficient orbital overlapping with π-systems. For this reason, it is believed that noble metals are indispensable components in the catalysis of such reactions. In this study, we report that thermally activated γ-Al₂O₃ activates enynes, diynes, and arene-ynes in a manner enabling reactions that were typically assigned to the softest π-Lewis acids, while some were known to be triggered exclusively by gold catalysts. We demonstrate the scope of these transformations and suggest a qualitative explanation of this phenomenon based on the Dewar-Chatt-Duncanson model confirmed by density functional theory calculations.

Enantioselective Oxidative Multi-Functionalization of Terminal Alkynes with Nitrones and Alcohols for Expeditious Assembly of Chiral α-Alkoxy-β-amino-ketones

Catalytic oxidative functionalization of alkynes has emerged as an effective method in synthetic chemistry in recent decades. However, enantioselective transformations via metal carbene intermediates are quite rare due to the lack of robust chiral catalysts, especially in the intermolecular versions. Herein, we report the first asymmetric three-component reaction of commercially available alkynes with nitrones and alcohols, which affords α-alkoxy-β-amino-ketones in good yields with high to excellent enantioselectivity using combined catalysis by an achiral gold complex and a chiral spiro phosphoric acid (CPA). Mechanistically, this atom-economic reaction involves a catalytic alkyne oxidation/ylide formation/Mannich-type addition sequence that uses nitrone as the oxidant and the leaving fragment imine as the electrophile, providing a novel method for multi-functionalization of commercially available terminal alkynes.

Enantioselective Rh(II)-Catalyzed Desymmetric Cycloisomerization of Diynes: Constructing Furan-Fused Dihydropiperidines with an Alkyne-Substituted Aza-Quaternary Stereocenter

Described herein is an enantioselective dirhodium(II)-catalyzed cycloisomerization of diynes achieved by the strategy of desymmetrization, which not only represents a new cycloisomerization reaction of diynes but also constitutes the first Rh(II)-catalyzed asymmetric intramolecular cycloisomerization of 1,6-diynes. This protocol provides a range of valuable furan-fused dihydropiperidine derivatives with an enantiomerically enriched alkynyl-substituted aza-quaternary stereocenter in high efficiency, complete atom economy, and excellent enantioselectivity (up to 98% ee). Besides, the highly functionalized products could be easily transformed into various synthetically useful building blocks and conjugated with a series of pharmaceutical molecules. The mechanism involving a concerted [3+2] cycloaddition/[1,2]-H shift of the Rh(II) carbenoid intermediate was elucidated by DFT calculations and mechanistic studies. More importantly, the first single crystal of alkyne-dirhodium(II) was obtained to show that a η²-coordinating activation of alkynal by dirhodium(II) was involved. Weak hydrogen bondings between the carboxylate ligands and alkynal were found, which probably made the well-defined paddlewheel-like dirhodium(II) distinctive from other metal complexes in catalyzing this transformation. Furthermore, the origin of the enantioselectivity was elucidated by a Rh₂(R-PTAD)₄-alkyne complex and additional calculational studies.

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

Gold(I)-Catalyzed Intermolecular Formal [4+2] Cycloaddition of O-Aryl Ynol Ethers and Enol Ethers: Synthesis of Chromene Derivatives

Gold(I)-catalyzed formal [4+2] cycloaddition of O-aryl ynol ethers 1 and enol ethers 2 is described. This intermolecular reaction between two electron-rich unsaturated systems takes place, under mild conditions, in the presence of 5 mol% [IPrAu(CH3 CN)]SbF6 as catalyst giving chromene derivatives with good yields. The cycloaddition is completely regio- and stereoselective, as well as versatile for both reactives. Silyl enol ethers can also react in the same way and under the same reaction conditions with quantitative yields. A plausible mechanism through a selective addition of the enol ether to the alkyne gold activated complex followed by an intramolecular aromatic electrophilic substitution is proposed. Several experimental results support the presence of a cationic oxonium intermediate prior to the aromatic substitution. The reaction represents a new entry to the chromene core.

Azido-Alkynes in Gold(I)-Catalyzed Indole Syntheses

The exploitation of nitrogen-functionalized reactive intermediates plays an important role in the synthesis of biologically relevant scaffolds in the field of pharmaceutical sciences. Those based on gold carbenes carry a strong potential for the design of highly efficient cascade processes toward the synthesis of compounds containing a fused indole core structure. This personal account gives a detailed explanation of our contribution to this sector, and embraces the reaction development of efficient gold-catalyzed cascade processes based on diversely functionalized azido-alkynes. Challenging cyclizations and their subsequent application in the synthesis of pharmaceutically relevant scaffolds and natural products conducted in an intra- or intermolecular fashion are key features of our research.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

Gold-Catalyzed Transformation of Ynamides

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N-bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom-transfer reagents, like nitrones, sulfoxides, and pyridine N-oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed.

The interplay of carbophilic activation and Au(I)/Au(III) catalysis: an emerging technique for 1,2-difunctionalization of C-C multiple bonds

Gold complexes have emerged as the catalysts of choice for various functionalization reactions of C-C multiple bonds due to their inherent carbophilic nature. In a parallel space, efforts to realize less accessible cross-coupling reactivity have led to the development of various strategies that facilitate the arduous Au(i)/Au(iii) redox cycle. The interplay of the two important reactivity modes encountered in gold catalysis, namely carbophilic activation and Au(i)/Au(iii) catalysis, has allowed the development of a novel mechanistic paradigm that sponsors 1,2-difunctionalization reactions of various C-C multiple bonds. Interestingly, the reactivity as well as selectivity obtained through this interplay could be complementary to that obtained by the use of various other transition metals that mainly involved the classical oxidative addition/migratory insertion pathways. The present review shall comprehensively cover all the 1,2-difunctionalization reactions of C-C multiple bonds that have been realized by the interplay of the two important reactivity modes and categorized on the basis of the method that has been employed to foster the Au(i)/Au(iii) redox cycle.

Merging Molecular Recognition and Gold(I) Catalysis with Triphoscalix[6]arene Ligands

We report the synthesis and characterization of novel triphosphine calix[6]arene ligands. These supramolecular wheels, with recognition features governed by the hydrogen-bonding domain, were employed to synthesize multitasking trinuclear gold(I) complexes as a new platform for the synthesis of interwoven (pseudo)rotaxane species. In parallel, the multivalent, metal-bonded upper rim displayed catalytic features promoting highly selective gold-catalyzed cycloisomerization reactions of 1,6-enynes.

A Rhodamine-based Fluorescent Chemodosimeter for Au3+ in Aqueous Solution and Living Cells

A highly selective rhodamine hydrazide-based fluorescent chemosensor for Au3+ detection was developed. The aqueous solution of rhodamine N-hydroxysemicarbazide (RHS), in the presence of Au3+, exhibited a significant 55-fold turn-on fluorescence response at 591 nm and a colorimetric change from colorless to pink. Other interested ions including Li+, Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Pb2+, Mn2+, Co2+, Ni2+, Ag+, Cd2+, Cu2+, Hg2+, Zn2+, Sn2+, Fe2+, Fe3+, Cr3+, Ce3+ did not induce any distinct color/spectral changes. The irreversible detection mechanism occurred via Au3+-promoted 5-exo-trig ring closure to yield 1,3,4-oxadiazole-2-one product. The RHS probe is non-responsive to other biologically relevant metal ions and the limit of detection for Au3+ was calculated to be 0.5 µM with a linear range of 0 to 90 µM. Fluorescence bioimaging of Au3+ in HepG2 cells was also successfully demonstrated.

Chiral Gold(III) Complexes: Synthesis, Structure, and Potential Applications

Since the beginning of the 2000's, homogeneous gold catalysis has emerged as a powerful tool to promote the cyclization of unsaturated substrates with excellent regioselectivity allowing the synthesis of elaborated organic scaffolds. An important goal to achieve in gold catalysis is the possibility to induce enantioselective transformations by the assistance of chiral complexes. Unfortunately, the linear geometry of coordination for gold usually encountered in complexes at the +1 oxidation states renders this goal very challenging. In consequence, the interest toward the synthesis of chiral gold(III) complexes is steadily growing. Indeed, the square planar geometry of the gold(III) cation appears more suitable to promote chiral induction. Beside catalysis, gold(III) complexes have also shown promising potential in the field of pharmacology. Herein, syntheses and applications of well-defined gold(III) complexes reported over the last fifteen years are summarized.

Indium(III)-Catalyzed Stereoselective Synthesis of Tricyclic Frameworks by Cascade Cycloisomerization Reactions of Aryl 1,5-Enynes

The indium(III)-catalyzed cascade cycloisomerization reaction of 1,5-enynes with pendant aryl nucleophiles is reported. The reaction proceeds in cascade under mild reaction conditions, using InI₃ (5 mol %) as a catalyst with a range of 1,5-enynes furnished with aryl groups (phenyl and phenol) at alkene (E and Z isomers) and with terminal and internal alkynes. Using 1-bromo-1,5-enynes, a one-pot sequential indium-catalyzed cycloisomerization and palladium-catalyzed cross-coupling with triorganoindium reagents were developed. The double cyclization is stereospecific and operates via a biomimetic cascade cation-olefin through 1,5-enyne cyclization (6-endo-dig) and subsequent C-C hydroarylation or C-O phenoxycyclization. Density functional theory (DFT) computational studies on 1,5-enynyl aryl ethers support a two-step mechanism where the first stereoselective 1,5-enyne cyclization produces a nonclassical carbocation intermediate that evolves to the tricyclic reaction product through a S_EAr mechanism. Using this approach, a variety of tricyclic heterocycles such as benzo[b]chromenes, phenanthridines, xanthenes, and spiroheterocyclic compounds are efficiently synthesized with high atom economy.

Kinetics and Mechanism of the Gold-Catalyzed Hydroamination of 1,1-Dimethylallene with N-Methylaniline

The mechanism of the intermolecular hydroamination of 3-methylbuta-1,2-diene (1) with N-methylaniline (2) catalyzed by (IPr)AuOTf has been studied by employing a combination of kinetic analysis, deuterium labelling studies, and in situ spectral analysis of catalytically active mixtures. The results of these and additional experiments are consistent with a mechanism for hydroamination involving reversible, endergonic displacement of N-methylaniline from [(IPr)Au(NHMePh)]⁺ (4) by allene to form the cationic gold π-C1,C2-allene complex [(IPr)Au(η² -H₂ C=C=CMe₂ )]⁺ (I), which is in rapid, endergonic equilibrium with the regioisomeric π-C2,C3-allene complex [(IPr)Au(η² -Me₂ C=C=CH₂ )]⁺ (I'). Rapid and reversible outer-sphere addition of 2 to the terminal allene carbon atom of I' to form gold vinyl complex (IPr)Au[C(=CH₂ )CMe₂ NMePh] (II) is superimposed on the slower addition of 2 to the terminal allene carbon atom of I to form gold vinyl complex (IPr)Au[C(=CMe₂ )CH₂ NMePh] (III). Selective protodeauration of III releases N-methyl-N-(3-methylbut-2-en-1-yl)aniline (3 a) with regeneration of 4. At high conversion, gold vinyl complex II is competitively trapped by an (IPr)Au⁺ fragment to form the cationic bis(gold) vinyl complex {[(IPr)Au]₂ [C(=CH₂ )CMe₂ NMePh]}⁺ (6).