Urea-Functionalized Fe4 L6 Cages for Supramolecular Gold Catalyst Encapsulation to Control Substrate Activation Modes

The excellent catalytic performances of enzymes in terms of activity and selectivity are an inspiration for synthetic chemists and this has resulted in the development of synthetic containers for supramolecular catalysis. In such containers the local environment and pre-organization of catalysts and substrates leads to control of the activity and selectivity of the catalyst. Herein we report a supramolecular strategy to encapsulate single catalysts in a urea-functionalized Fe4 L6 cage, which can co-encapsulate a functionalized urea substrate through hydrogen bonding. Distinguished selectivity is obtained, imposed by the cage as site isolation only allows catalysis through π activation of the substrate and as a result the selectivity is independent of catalyst concentration. The encapsulated catalyst is more active than the free analogue, an effect that can be ascribed to transitionstate stabilization rather than substrate pre-organization, as revealed by the MM kinetic data. The simple strategy reported here is expected to be of general use in many reactions, for which the catalyst can be functionalized with a sulfonate group required for encapsulation.

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes: A Mechanistic View

Hydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included.

From Thioureas to Thioquinolines through Isolated Benzothiazines by Gold Catalysis

New benzothiazine heterocycles have been formed from thiourea derivatives by using different gold catalysts. The catalyst and the conditions were optimised towards the selective synthesis of six‐membered benzothiazine heterocycles, characterised by X‐ray diffraction. Interestingly, these organic compounds evolved under gold catalysis in basic medium to achieve the formation of amino thioquinolines through an unprecedented aromatisation process of the heterocycle. The reaction was also carried out stoichiometrically by reaction with gold complexes to afford thioquinolines coordinated to the gold fragment. Benzothiazine, amino thioquinoline heterocycles and gold‐derived species have a great potential for biological applications.

Sequential Oxidative Fragmentation and Skeletal Rearrangement of Peroxides for the Synthesis of Quinazolinone Derivatives

For the first time, the sequential reaction of peroxyoxindole that involves base-promoted oxidative fragmentation to isocyanate formation and primary amine or amino alcohol accelerated skeletal rearrangement to synthesize exo-olefinic-substituted quinazolinone or oxazoloquinazolinone is reported. The advantages of this new reaction include a broad substrate scope and transition-metal-free and room-temperature conditions. The formation of the isocyanate as a key intermediate that accelerates oxidative skeletal rearrangement has been confirmed by trapping experiments and spectroscopic evidence.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Gold-Catalyzed Intermolecular Alkyne Hydrofunctionalizations—Mechanistic Insights

An overview of the current state of mechanistic understanding of gold-catalyzed intermolecular alkyne hydrofunctionalization reactions is presented. Moving from the analysis of the main features of the by-now-generally accepted reaction mechanism, studies and evidences pointing out the mechanistic peculiarities of these reactions using different nucleophiles HNu that add to the alkyne triple bond are presented and discussed. The effects of the nature of the employed alkyne substrate and of the gold catalyst (employed ligands, counteranions, gold oxidation state), of additional additives and of the reaction conditions are also considered. Aim of this work is to provide the reader with a detailed mechanistic knowledge of this important reaction class, which will be invaluable for rapidly developing and optimizing synthetic protocols involving a gold-catalyzed alkyne hydrofunctionalization as a reaction step.

Tuning Activity and Selectivity during Alkyne Activation by Gold(I)/Platinum(0) Frustrated Lewis Pairs

Introducing transition metals into frustrated Lewis pair systems has attracted considerable attention in recent years. Here we report a selection of three metal-only frustrated systems based on Au(I)/Pt(0) combinations and their reactivity toward alkynes. We have inspected the activation of acetylene and phenylacetylene. The gold(I) fragments are stabilized by three bulky phosphines bearing terphenyl groups. We have observed that subtle modifications on the substituents of these ligands proved critical in controlling the regioselectivity of acetylene activation and the product distribution resulting from C(sp)-H cleavage of phenylacetylene. A mechanistic picture based on experimental observations and computational analysis is provided. As a result of the cooperative action of the two metals of the frustrated pairs, several uncommon heterobimetallic structures have been characterized.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

A Simple Metal-Free Cyclization for the Synthesis of 4-Methylene-3-Substituted Quinazolinone and Quinazolinthione Derivatives: Experiment and Theory

A new series of 3-substituted 4-methylene-quinazolinthiones and 4-methylene-quinazolinones were synthesized in moderate to excellent yield through a simple reaction of 2-aminoacetophenones with isocyanates or isothiocyanates. The reaction shows good tolerance of many important functional groups in the presence of air and water under metal-free conditions. Only water is produced as a coproduct, rendering this “green” methodology a highly versatile and eco-friendly alternative to the existing methods for the construction of the quinazolinone/quinazolinthione framework. We have interpreted the reaction mechanism by use of quantum chemical calculations on the basis of state-of-the-art computational methods SMD-B3LYP-D3(BJ)/BS1//B3LYP/BS1.

Dinuclear Gold Complexes Supported by Wide Bite Angle Diphosphines for Preorganization-Induced Selective Dual-Gold Catalysis

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual-gold catalysis is explored. Using the preorganizing abilities of well-known wide bite angle diphosphine ligands, DBFPhos and DPEPhos, dinuclear Au(I)–Au(I) complexes 1 and 2 are used as precursors to form well-defined monocationic species with either a chlorido- or acetylido-ligand bridging the two gold centers. These compounds are active catalysts for the dual-gold heterocycloaddition of a urea-functionalized alkyne, and the preorganization of both Au-centers affords efficient σ,π-activation of the substrate, even at high dilution, significantly outperforming benchmark mononuclear catalysts.

Ring strain strategy for the control of regioselectivity. Gold-catalyzed anti-Markovnikov cycloisomerization initiated tandem reactions of alkynes

Gold-catalyzed nucleophilic addition to terminal alkynes has received considerable interest in the past decade, as this chemistry offers a highly efficient and regioselective way for CC, CH and CX bond formation. However, such a nucleophilic addition mainly involves a Markovnikov addition. In this short review, the recent progress of the gold-catalyzed 5-endo-dig cycloisomerization-initiated tandem reactions by utilizing the steric strain in ring formation to achieve an anti-Markovnikov regioselectivity was reviewed, including the scope of reactions, mechanism and synthetic applications.

Catalytic domino amination and oxidative coupling of gold acetylides and isolation of key vinylene digold intermediates as a new class of ditopic N-heterocyclic carbene complexes

A catalytic domino amination and oxidative coupling of in situ prepared gold acetylides has been developed for the synthesis of abnormal NHC (aNHC) gold complexes, and key vinylene digold intermediates are isolated.

A palladium and gold catalytic system enables direct access to O- and S-linked non-natural glyco-conjugates

Here we report a straightforward cross-coupling method for the synthesis of non-natural glycoamino acids from alkyne-bearing monosaccharides and p-iodophenylalanine. Pd/Au-catalyzed Sonogashira coupling is tolerant to both O- and S-glycosides without any epimerization. In addition, no racemization of the amino acid was observed allowing direct access to the homogeneous glyco-conjugate in a single step. Notably, this Pd/Au catalytic system presents enhanced catalytic activity than conventional Pd/Cu and Pd-only platforms, and it further enables the convergent synthesis of glycodipeptides.

Synthesis of tetra- and octa-aurated heteroaryl complexes towards probing aromatic indoliums

Polymetalated aromatic compounds are particularly challenging synthetic goals because of the limited thermodynamic stability of polyanionic species arising from strong electrostatic repulsion between adjacent carbanionic sites. Here we describe a facile synthesis of two polyaurated complexes including a tetra-aurated indole and an octa-aurated benzodipyrrole. The imido trinuclear gold(I) moiety exhibits nucleophilicity and undergoes an intramolecular attack on a gold(I)-activated ethynyl to generate polyanionic heteroaryl species. Their computed magnetic properties reveal the aromatic character in the five-membered ring. The incorporation of the aurated substituents at the nitrogen atom can convert non-aromaticity in the parent indolium into aromaticity in the aurated one because of hyperconjugation. Thus, the concept of hyperconjugative aromaticity is extended to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. This work highlights the difference in aromaticity between polymetalated aryls and their organic prototypes.

Polymetalated aromatic compounds are challenging synthetic goals. Here, the authors describe the synthesis of tetra-aurated indole and octa-aurated benzodipyrrole complexes, in which incorporation of the aurated substituents at the nitrogen atoms leads to hyperconjugative aromaticity in the five-membered rings.

Oxyboration with and without a Catalyst: Borylated Isoxazoles via B-O σ-Bond Addition

Herein we report an oxyboration reaction with activated substrates that employs B–O σ bond additions to C–C π bonds to form borylated isoxazoles, which are potential building blocks for drug discovery. Although this reaction can be effectively catalyzed by gold, it is the first example of uncatalyzed oxyboration of C–C π bonds by B–O σ bonds—and only the second example that is catalyzed. This oxyboration reaction is tolerant of groups incompatible with alternative lithiation/borylation and palladium-catalyzed C–H activation/borylation technologies for the synthesis of borylated isoxazoles.

Catalytic σ-activation of carbon–carbon triple bonds: reactions of propargylic alcohols and alkynes

A variety of organic transformations have been developed based on conceptually novel catalytic σ-activation of CC bonds of alkynes.

Methyl-, Ethenyl-, and Ethynyl-Bridged Cationic Digold Complexes Stabilized by Coordination to a Bulky Terphenylphosphine Ligand

Reactions of the gold(I) triflimide complex [Au(NTf2 )(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)] (1) with the gold(I) hydrocarbyl species [AuR(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)] (2 a-2 c) enable the isolation of hydrocarbyl-bridged cationic digold complexes with the general composition [Au2 (μ-R)(PMe2 Ar${{^{{\rm Dipp}{_{2}}}}}$)2 ][NTf2 ], where Ar${{^{{\rm Dipp}{_{2}}}}}$=C6 H3 -2,6-(C6 H3 -2,6-iPr2 )2 and R=Me (3), CHCH2 (4), or CCH (5). Compound 3 is the first alkyl-bridged digold complex to be reported and features a symmetric [Au(μ-CH3 )Au](+) core. Complexes 4 and 5 are the first species of their kind that contain simple, unsubstituted vinyl and acetylide units, respectively. In the series of complexes 3-5, the bridging carbon atom systematically changes its hybridization from sp(3) to sp(2) and sp. Concomitant with this change, and owing to variations in the nature of the bonding within the [Au(μ-R)Au](+) unit, there is a gradual decrease in aurophilicity, that is, the strength of the Au⋅⋅⋅Au bonding interaction decreases. This change is illustrated by a monotonic increase in the Au-Au distance by approximately 0.3 Å from R=CH3 (2.71 Å) to CHCH2 (3.07 Å) and CCH (3.31 Å).

Dispersion, solvent and metal effects in the binding of gold cations to alkynyl ligands: implications for Au(I) catalysis

The coordination modes of the [Au(PPh3)](+) cation to metal alkynyl complexes have been investigated. On addition to ruthenium, a vinylidene complex, [Ru(η(5)-C5H5)(PPh3)2([double bond, length as m-dash]C[double bond, length as m-dash]CPh{AuPPh3})](+), is obtained while addition to a gold(iii) compound gives di- and trinuclear gold complexes depending on the conditions employed. In the trinuclear species, a gold(i) cation is sandwiched between two gold(iii) alkynyl complexes, suggesting that coordination of multiple C-C triple bonds to gold is facile.

Catalyst-controlled divergence in cycloisomerisation reactions of N-propargyl-N-vinyl sulfonamides: gold-catalysed synthesis of 2-sulfonylmethyl pyrroles and dihydropyridines

Gold-catalysed, divergent synthesis of 2-sulfonylmethyl pyrroles and dihydropyridines from readily available N-propargyl-N-vinyl sulfonamides is reported.

Alkynyl-functionalized gold NHC complexes and their coinage metal clusters

Phenylpropynyl-functionalized N-heterocyclic carbenes as ligands for the synthesis of heterometallic hexanuclear coinagemetal clusters which exhibit mixed metallophillic interactions and intense white photoluminescence at low temperature.