Synthetic applications of gold-catalyzed ring expansions

Gold(III) Auracycles Featuring C(sp3)-Au-C(sp2) Bonds: Synthesis and Mechanistic Insights into the Cycloauration Step

The direct auration of arenes is a key step in numerous gold-catalyzed reactions. Although reported more than 100 years ago, understanding of its underlying mechanism has been hampered by the difficulties in the isolation of relevant intermediates given the propensity of gold(III) species to undergo reductive elimination. Here, we report the synthesis and isolation of a new family of intriguing zwitterionic [C(sp3)^C(sp2)]-auracyclopentanes, as well as of their alkyl-gold(III) precursors and demonstrate their value as mechanistic probes to study the C(sp2)-Au bond-forming event. Experimental investigations employing Kinetic Isotope Effects (KIE), Hammett plot, and Eyring analysis provided important insights into the formation of the auracycle. The data suggest a SEAr mechanism wherein the slowest step might be the π-coordination between the arene and the gold(III) center, en route to the Wheland intermediate. We also show that these auracyclopentanes can work as catalysts in several gold-promoted transformations.

A ring expansion strategy towards diverse azaheterocycles

The development of innovative strategies for the synthesis of N-heterocyclic compounds is an important topic in organic synthesis. Ring expansion methods to form large N-heterocycles often involve the cycloaddition of strained aza rings with π bonds. However, in some cases such strategies suffer from some limitations owing to the difficulties in controlling the regioselectivity and the accessibility of specific π-bond synthons. Here, we report the development of a general ring expansion strategy that involves a formal cross-dimerization between three-membered aza heterocycles and three- and four-membered-ring ketones through synergistic bimetallic catalysis. These formal cross-dimerizations of two different strained rings are efficient and scalable, and provide a straightforward and broadly applicable means of assembling diverse N-heterocycles, such as 3-benzazepinones, dihydropyridinones and uracils, which are versatile units in numerous drugs and biologically active compounds. Preliminary mechanistic studies revealed that the C-C bond of strained ring ketones is first cleaved by the Pd⁰ species during the reaction.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Unconventional Gold-Catalyzed One-Pot/Multicomponent Synthesis of Propargylamines Starting from Benzyl Alcohols

A formal homogeneous gold-catalyzed A3-coupling, starting from benzyl alcohols, is reported for the straightforward synthesis of propargylamines. This is the first process where these highly valuable compounds have been synthesized, starting from the corresponding alcohols in a one-pot oxidation procedure using MnO2, followed by a HAuCl4·3H2O catalyzed multicomponent reaction. The final products are obtained with very good yields in short reaction times, which is of fundamental interest for the synthesis of pharmaceuticals. The usefulness and efficiency of our methodology is successfully compared against the same reaction starting from aldehydes.

Adventures and Detours in the Synthesis of Hydropentalenes

Functionalized hydropentalenes (i.e., bicyclo[3.3.0]octanones) constitute important building blocks for natural products and for ligands for asymmetric catalysis. The assembly and tailored functionalization of this convex roof-shaped scaffold is challenging and has motivated a variety of synthetic approaches including our own contributions, which will be presented in this account.

1 Introduction

2 Biosynthesis of Hydropentalenes

3 Hydropentalenes through the Pauson–Khand Reaction

4 Hydropentalenes through Transannular Oxidative Cyclization of Cycloocta-1,4-diene

5 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Dodecahydrocyclopenta[a]indenes

6 Functionalization of Bicyclo[3.3.0]octan-1,4-diones to Crown Ether Hybrids

7 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Cylindramide

8 Tandem Ring-Opening Metathesis/Ring-Closing Metathesis/Cross-Metathesis of Bicyclo[2.2.1]heptanes

9 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Geodin A

10 Hydropentalenes through Enantioselective Desymmetrization of Weiss Diketones

11 Functionalization of Weiss Diketones by Carbonyl Ene Reactions

12 Functionalization of the Weiss Diketone to Cylindramide and Geodin A Core Units

13 Biological Properties of Bicyclo[3.3.0]octanes

14 Hydropentalenes through Vinylcyclopropane Cyclopentene Rearrangement

15 Functionalization of Bicyclo[3.3.0]octanes toward Chiral Dienes

16 Miscellaneous Syntheses of Hydropentalenes

17 Conclusion and Outlook

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

[C^N]-Alkenyl Gold(III) Complexes by Proximal Ring-Opening of (2-Pyridyl)alkylidenecyclopropanes: Mechanistic Insights

Pyridine-substituted alkylidenecyclopropanes (Py-ACPs) react with gold(III) salts under mild reaction conditions through an unprecedented, proximal ring-opening pathway, to generate highly appealing, catalytically active pyridine alkenyl [C^N]-gold(III) species. Mechanistic studies reveal that the activation of the C-C bond of the ACP takes place through an unusual concerted, σ-bond metathesis type-process.

Efficient Gold(I) Acyclic Diaminocarbenes for the Synthesis of Propargylamines and Indolizines

Mononuclear gold(I) acyclic diaminocarbenes (ADCs) were prepared by the reaction of 1,2-cyclohexanediamine with the corresponding isocyanide complexes [AuCl(CNR)] (R = Cy, t Bu). The three-component coupling of aldehydes, amines, and alkynes was investigated by using these gold(I) ADC complexes. The new gold(I) metal complexes are highly efficient catalysts for the synthesis of propargylamines and indolizines in the absence of solvent and in mild conditions. This method affords the corresponding final products with excellent yields in short reaction times. Additionally, chiral gold(I) complexes with ADCs have been prepared and tried in the enantioselective synthesis of propargylamines.

Ligand Effect on Bonding in Gold(III) Carbonyl Complexes

We quantitatively assess the Dewar-Chatt-Duncanson (DCD) components of the Au(III)-CO bond and the charge density polarization at the CO, in a series of neutral, cationic, and dicationic bis- and monocyclometalated gold(III) complexes via charge-displacement (CD) analysis. A striking feature concerns the very small net electron charge flux from CO to the metal fragment which is unexpectedly stable toward both the charge of the complex and the oxidation state of gold (I, III). All systems exhibit a similar trend for the σ charge rearrangement in the region of the carbonyl bond, where, by contrast, the π back-donation trend variation is large, which is strictly correlated to the change in CO bond distance and the shift in CO stretching frequencies, in close analogy with the gold(I) carbonyl complexes. In the whole series of gold(III) compounds, a large Au(III) ← CO σ donation is measured (from 0.19 to 0.31 electrons), as well as a significant Au(III) → CO π back-donation (from -0.09 to -0.22 electrons), which however is not generally able to completely balance the polarization of the CO π electrons in the direction from oxygen to carbon (C ← O) induced by the presence of the metal fragment [LAu(III)]^0/+1/+2. Surprisingly, all the gold(III) complexes in the series are characterized by a very small anisotropy in the Au(III) → CO in-plane and out-of-plane π back-donation components, in sharp contrast with the marked anisotropy found before for the experimentally characterized [(C^N^C)Au(III)CO]⁺ complex. A first attempt to figure out a rationale on the bonding/reactivity relationship for Au(III)-CO is made by performing a comparative study with an isostructural [(N^N^C)Pt(II)CO]⁺ complex in a model water-gas shift (WGS) reaction.

Enantioselective gold(i)-catalyzed rearrangement of cyclopropyl-substituted 1,6-enynes into 2-oxocyclobutyl-cyclopentanes

A gold(i)-catalyzed cycloisomerization/ring expansion sequence allows the highly enantioselective synthesis of 2-oxocyclobutylcyclopentane derivatives from cyclopropyl-substituted enynes (up to 99% ee).

The synthesis of cycloalka[b]furans via an Au(i)-catalyzed tandem reaction of 3-yne-1,2-diols

A rapid and convenient approach toward substituted cycloalka[b]furan compounds has been developed via an Au(i)-catalyzed cyclization/1,2-rearrangement/aromatization cascade.

Photoredox meets gold Lewis acid catalysis in the alkylative semipinacol rearrangement: a photocatalyst with a dark side

The alkylative semipinacol rearrangement of a variety of TMS protected α-styrenyl substituted cyclic alcohols with unactivated bromoalkanes that merge photoredox and Au(i)/Au(iii) catalysis has been achieved.

C-C Bond Migration in the Cycloisomerization of 1,6-Enynes

A full account of our investigation of C-C bond migration in the cycloisomerization of oxygen-tethered 1,6-enynes is described. Under Pt(II) and/or Ir(I) catalysis, cyclic and acylic alkyl groups were found to undergo 1,2-shifts into metal carbenoid intermediates. Interestingly, this process does not appear to be driven by the release of ring strain, and thus provides access to large carbocyclic frameworks. The beneficial effect of CO on the Pt(II) and Ir(I) catalytic systems is also evaluated.

The fluxional amine gold(III) complex as an excellent catalyst and precursor of biologically active acyclic carbenes

A new amine gold(III) complex [Au(C6F5)2(DPA)]ClO4 with the di-(2-picolyl)amine (DPA) ligand has been synthesised. In the solid state the complex has a chiral amine nitrogen because the ligand coordinates to the gold centre through one nitrogen atom from a pyridine and through the NH moiety, whereas in solution it shows a fluxional behaviour with a rapid exchange between the pyridine sites. This complex can be used as an excellent synton to prepare new gold(III) carbene complexes by the reaction with isocyanide CNR. The resulting gold(III) derivatives have unprecedented bidentate C^N acyclic carbene ligands. All the complexes have been spectroscopically and structurally characterized. Taking advantage of the fluxional behaviour of the amine complex, its catalytic properties have been tested in several reactions with the formation of C-C and C-N bonds. The complex showed excellent activity with total conversion, without the presence of a co-catalyst, and with a catalyst loading as low as 0.1%. These complexes also present biological properties, and cytotoxicity studies have been performed in vitro against three tumour human cell lines, Jurkat (T-cell leukaemia), MiaPaca2 (pancreatic carcinoma) and A549 (lung carcinoma). Some of them showed excellent cytotoxic activity compared with the reference cisplatin.

Ring expansion of epoxides under Brønsted base catalysis: formal [3+2] cycloaddition of β,γ-epoxy esters with imines providing 2,4,5-trisubstituted 1,3-oxazolidines

A novel ring-expansion reaction of epoxides under Brønsted base catalysis was developed. The formal [3+2] cycloaddition reaction of β,γ-epoxy esters with imines proceeds in the presence of triazabicyclodecene (TBD) as a superior Brønsted base catalyst to afford 2,4,5-trisubstituted 1,3-oxazolidines in a highly diastereoselective manner. This reaction involves the ring opening of the epoxides with the aid of the Brønsted base catalyst to generate α,β-unsaturated esters having an alkoxide at the allylic position, which would formally serve as a synthetic equivalent of the 1,3-dipole, followed by a cycloaddition reaction with imines in a stepwise fashion. This methodology enables the facile synthesis of enantioenriched 1,3-oxazolidines from easily accessible enantioenriched epoxides.

Synthesis of furans and pyrroles via migratory and double migratory cycloisomerization reactions of homopropargylic aldehydes and imines

A novel gold-catalyzed divergent sysnthesis of furans and pyrroles employing readily available homopropargylic aldehydes and imines have been developed. The regiochemical outcome of this reaction is dependent on the substituent on the terminal alkyne of substrate. Thus, substrates possessing alkyl and aryl substituent at the alkyne moiety produce 2,3,5-substituted furans and pyrroles via a migratory cycloisomerizaton reaction. Whereas, their silicon analogues are capable to undergo a double migratory process leading to 2,3,4-substituted heterocycles.

1,2-boryl migration empowers regiodivergent synthesis of borylated furans

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C2- and C3-borylated furans has been developed. It was found that the copper catalyst as well as the gold catalyst with more basic triflate counterion favor boryl migration toward C3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate affords C2-borylated furan via a formal 1,2-hydrogen shift.

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.