Gold-Catalyzed Reactions between Alkenyldiazo Carbonyl Species and Acetals

Gold-Catalyzed Synthesis of Diaza-hexatrienes Via Diazo Attack at Vinylgold Carbenes: An Easy Access to 1H-Pyrazolo[4,3-b]pyridine-5-ones

This work reports a gold-catalyzed stereoselective synthesis of highly substituted E-configured 2,3-diaza-1,3,5-hexatrienes using α-diazo nitriles and cyclopropene derivatives; such products arise from an atypical diazo attack of α-aryldiazo nitriles at vinylgold carbenes. For these 2,3-diaza-1,3,5-hexatrienes, we develop a novel anionic cyclization of derivatives of one family to form 1H-pyrazolo[4,3-b]pyridine-5-ones.

Gold-Catalyzed [3+2] Carbocycloaddition Reaction of Pinacol Alkenylboronates: Stereospecific Synthesis of Boryl-Functionalized Cyclopentene Derivatives

Gold-catalysis has enabled new synthetic opportunities in the chemistry of vinyldiazo compounds. Herein, we report the gold-catalyzed reaction of stabilized vinyldiazo compounds with pinacol alkenylboronates to provide boryl-functionalized cyclopentene derivatives through a formal [3+2] carbocycloaddition reaction, a very unusual pathway in alkenylboronate chemistry. This reaction proceeds with high regio- and stereoselectivity. The synthetic usefulness of the resulting borylated cyclopentene derivatives toward the synthesis of densely functionalized cyclopentanoids is also demonstrated.

Ionization of gold (γ-methoxy)vinyl complexes generates reactive gold vinyl carbene complexes

Cationic gold vinyl carbene/allylic cation complexes of the form (E)-[(L)AuC(H)C(H)CAr₂]⁺ OTf^- {L = IPr, Ar = Ph [(E)-5a], L = IPr, Ar = 4-C₆H₄OMe [(E)-5b], L = P(t-Bu)₂ o-biphenyl, Ar = 4-C₆H₄OMe [(E)-5c]} were generated in solution via Lewis acid-mediated ionization of the corresponding gold (γ-methoxy)vinyl complexes (E)-(L)AuC(H)C(H)C(OMe)Ar₂ at or below -95 °C. Complexes (E)-5b and (E)-5c were fully characterized in solution employing multinuclear NMR spectroscopy, which established the predominant contribution of the aurated allylic cation resonance structure and the significant distribution of positive charge into the γ-anisyl rings. Complex (E)-5b reacted rapidly at -95 °C with neutral two-electron, hydride, and oxygen atom donors exclusively at the C1 position of the vinyl carbene moiety and with p-methoxystyrene to form the corresponding vinylcyclopropane. In the absence of nucleophile (E)-5a decomposed predominantly via intermolecular carbene dimerization whereas formation of 1-aryl-5-methoxy indene upon ionization of (Z)-(IPr)AuC(H)C(H)C(OMe)(4-C₆H₄OMe)₂ [(Z)-6b] implicated an intramolecular Friedel-Crafts or electrocyclic Nazarov pathway for the decomposition of the unobserved vinyl carbene complex (Z)-[(IPr)AuC(H)C(H)C(4-C₆H₄OMe)₂]⁺ OTf^- [(Z)-5b].

Recent advances in catalytic asymmetric intermolecular oxidation of alkynes

In recent years, gold-metal-catalyzed intermolecular alkyne oxidation by an N-oxide oxidant, which presumably involves a gold carbenoid intermediate, has attracted increasing attention because it circumvents the employment of hazardous and potentially explosive diazocarbonyl compounds as starting materials for carbene generation.

Cyclopentene Annulations of Alkene Radical Cations with Vinyl Diazo Species Using Photocatalysis

A direct (3+2) cycloaddition between alkenes and vinyl diazo reagents using either Cr or Ru photocatalysis is described. The intermediacy of a radical cation species enables a nucleophilic interception by vinyl diazo compounds, a departure from their traditional electrophilic behavior. A variety of cyclopentenes are synthesized using this method, and experimental insights implicate a direct cycloaddition instead of a cyclopropanation/rearrangement process.

Zinc-Catalyzed Synthesis of Conjugated Dienoates through Unusual Cross-Couplings of Zinc Carbenes with Diazo Compounds

Zinc-catalyzed selective cross-coupling of two carbene sources, such as vinyl diazo compounds and enynones, enabled the synthesis of conjugated dienoate derivatives. This reaction involved the unprecedented coupling of a zinc furyl carbene with vinyl diazo compounds through the γ-carbon. Alternatively, dienoates were also prepared by a commutative cross-coupling of zinc vinyl carbenes generated from cyclopropenes and simple diazo compounds.

Recent Developments in Coinage Metal Catalyzed Transformations of Stabilized Vinyldiazo Compounds: Beyond Carbenic Pathways

Transition metal-catalyzed transformations of vinyldiazo compounds have become a versatile tool in organic synthesis. Although several transition metals have been investigated for this purpose, this field has been mainly dominated by dirhodium catalysts. Remarkable levels of chemo-, regio-, diastereo- and enantioselectivity have been reached in some of these rhodium-catalyzed transformations. In the last few years coinage metals have also emerged as useful catalysts in transformations involving vinyldiazo compounds. In some cases, highly efficient catalyst-dependent protocols arising from divergent mechanistic pathways have been reported. In this Personal Account, we aim to showcase recent advances in metal coinage catalyzed transformations of vinyldiazoacetates, an exciting field of research to which our group has actively contributed in the last few years.

Gold-catalyzed transformations of α-diazocarbonyl compounds: selectivity and diversity

Metal carbenes, generated from the decomposition of diazo compounds by transition-metals, have a broad range of applications in organic synthesis. The progress of organic transformation via transition metal (such as Rh, Cu, Fe, Ag) carbenes has been well summarized in a few excellent reviews. This review will summarize gold-catalyzed transformations of α-diazocarbonyl compounds by highlighting the specificity and applicability of these diverse transformations such as X-H insertion, C-H functionalization, cyclopropanation, cycloaddition, and coupling reactions. In order to understand these reactions, the mechanistic rationale for selected examples is also provided.

Gold and diazo reagents: a fruitful tool for developing molecular complexity

After a decade since its discovery and being yet at its sunrise, the use of gold catalysis for the transfer of carbene groups from diazo compounds is becoming a powerful synthetic tool. The main advances in the area and future directions are provided in this Feature Article.

A Desulfonylative Approach in Oxidative Gold Catalysis: Regiospecific Access to Donor-Substituted Acyl Gold Carbenes

Donor-substituted acyl gold carbenes are challenging to access selectively by gold-promoted intermolecular oxidation of internal alkynes as the opposite regioisomers frequently predominate. By using alkynyl sulfones or sulfonates as substrates, the oxidative gold catalysis in the presence of substituted pyridine N-oxides offers regiospecific access to acyl/aryl, acyl/alkenyl, and acyl/alkoxy gold carbenes by in situ expulsion of sulfur dioxide. The intermediacies of these reactive species are established by their reactivities, including undergoing further oxidation by the same oxidant, cyclopropanation of styrenes, engaging in a [3+2] cycloaddition with α-methylstyrene, and conversion into dienones.

Gold-catalyzed carboalkoxylations of 2-ethynylbenzyl ethers to form 1- and 3-substituted 2-methoxy-1-H-indenes: Brønsted acids versus gold catalysis

Selective synthesis of 1- and 3-substituted 2-methoxyindenes from the carboalkoxylations of 2-ethynylbenzyl ethers is described; the former is obtained efficiently with P(t-Bu)2(o-biphenyl)AuCl/NaBARF in DCM/MS 4 Å whereas the latter is produced preferably with P(t-Bu)2(o-biphenyl)AuCl/AgNTf2 in pre-dried DCM. Both 1- and 3-substituted 2-indenyl ethers are subjected to ozone oxidations to afford two distinct carbonyl products. Our new data indicate that 1-substituted 2-indenyl ethers are generated from gold catalysts whereas their 3-substituted analogues arise from Brønsted acids.

Enantioselective oxidative gold catalysis enabled by a designed chiral P,N-bidentate ligand

A newly developed P,N-bidentate ligand enables enantioselective intramolecular cyclopropanation by a reactive α-oxo gold carbene intermediate generated in situ. The ligand design is based on our previously proposed structure (with a well-organized triscoordinated gold center) of the carbene intermediate in the presence of a P,N-bidentate ligand. A C2-symmetric piperidine ring was incorporated in the ligand as the nitrogen-containing moiety. A range of racemic transformations of α-oxo gold carbene intermediates have been developed recently, and this new class of chiral ligands could enable their modification for asymmetric synthesis, as demonstrated in this study.

Gold-catalyzed selective oxidation of 4-oxahepta-1,6-diynes to 2H-pyran-3(6H)-ones and chromen-3(4H)-ones via β-gold vinyl cation intermediates

α-Oxo gold carbenes generated in situ via the gold-catalyzed selective oxidation of 4-oxahepta-1,6-diynes were effectively trapped by internal alkynes, resulting in the formation of 2H-pyran-3(6H)-ones, 3, and chromen-3(4H)-ones, 4, upon facile trapping the vinyl cation intermediates.

Gold-catalyzed α-furanylations of quinoline N-oxides with alkenyldiazo carbonyl species

Gold-catalyzed α-furanylations of 8-alkylquinoline N-oxides have been achieved using various alkenyldiazo carbonyl species as nucleophiles.

Expanding the horizon of intermolecular trapping of in situ generated α-oxo gold carbenes: efficient oxidative union of allylic sulfides and terminal alkynes via C-C bond formation

With a new P,S-bidentate phosphine as the ligand to gold(I), the α-oxo gold carbenes generated in situ via gold-catalyzed intermolecular oxidation of terminal alkynes were effectively trapped by various allylic sulfides, resulting in the formation of α-aryl(alkyl)thio-γ,δ-unsaturated ketones upon facile [2,3]sigmatropic rearrangements.

NHC-AuCl/selectfluor: a highly efficient catalytic system for carbene-transfer reactions

The combination of NHC-gold complex and Selectfluor has been found to be a highly efficient catalyst system for carbene-transfer reactions, with a turnover number (TON) up to 990000 and a turnover frequency (TOF) up to 82500 h(-1).

A non-diazo approach to α-oxo gold carbenes via gold-catalyzed alkyne oxidation

For the past dozen years, homogeneous gold catalysis has evolved from a little known topic in organic synthesis to a fully blown research field of significant importance to synthetic practitioners, due to its novel reactivities and reaction modes. Cationic gold(I) complexes are powerful soft Lewis acids that can activate alkynes and allenes toward efficient attack by nucleophiles, leading to the generation of alkenyl gold intermediates. Some of the most versatile aspects of gold catalysis involve the generation of gold carbene intermediates, which occurs through the approach of an electrophile to the distal end of the alkenyl gold moiety, and their diverse transformations thereafter. On the other hand, α-oxo metal carbene/carbenoids are highly versatile intermediates in organic synthesis and can undergo various synthetically challenging yet highly valuable transformations such as C-H insertion, ylide formation, and cyclopropanation reactions. Metal-catalyzed dediazotizations of diazo carbonyl compounds are the principle and most reliable strategy to access them. Unfortunately, the substrates contain a highly energetic diazo moiety and are potentially explosive. Moreover, chemists need to use energetic reagents to prepare them, putting further constrains on operational safety. In this Account, we show that the unique access to the gold carbene species in homogeneous gold catalysis offers an opportunity to generate α-oxo gold carbenes if both nucleophile and electrophile are oxygen. Hence, this approach would enable readily available and safer alkynes to replace hazardous α-diazo carbonyl compounds as precursors in the realm of gold carbene chemistry. For the past several years, we have demonstrated that alkynes can indeed effectively serve as precursors to versatile α-oxo gold carbenes. In our initial study, we showed that a tethered sulfoxide can be a suitable oxidant, which in some cases leads to the formation of α-oxo gold carbene intermediates. The intermolecular approach offers excellent synthetic flexibility because no tethering of the oxidant is required, and its reduced form is not tangled with the product. We were the first research group to develop this strategy, through the use of pyridine/quinolone N-oxides as the external oxidants. In this manner, we can effectively make a C-C triple bond a surrogate of an α-diazo carbonyl moiety in various gold catalyses. With terminal alkynes, we demonstrated that we can efficiently trap exclusively formed terminal carbene centers by internal nucleophiles en route to the formation of cyclic products, including strained oxetan-3-ones and azetidin-3-ones, and by external nucleophiles when a P,N-bidentate ligand is coordinated to gold. With internal alkynes, we generated synthetically useful regioselectivities in the generation of the α-oxo gold carbene moiety, which enables expedient formation of versatile enone products. Other research groups have also applied this strategy en route to versatile synthetic methods. The α-oxo gold carbenes appear to be more electrophilic than their Rh counterpart, which many chemists have focused on in a large array of excellent work on metal carbene chemistry. The ease of accessing the reactive gold carbenes opens up a vast area for developing new synthetic methods that would be distinctively different from the known Rh chemistry and promises to generate a new round of "gold rush".