Silver versus Gold Catalysis in Tandem Reactions of Carbonyl Functions onto Alkynes: A Versatile Access to Furoquinoline and Pyranoquinoline Cores

Regioselective Access to 2-Iminoimidazolidines via AgF-Mediated Cascade Reactions

A convergent access to substituted 2-iminoimidazolidines from aromatic amines and N-propargyl S-methylthiourea is developed via Ag(I)-mediated cascade guanylation-cyclization reactions. This method features high regioselectivity, excellent efficiency, and mild reaction conditions. Subsequent deprotection of the Boc (tert-butyloxycarbonyl) group under acidic conditions provides expedient access to aryl 2-aminoimidazole derivatives in a convenient manner.

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%.

Design, synthesis and biological evaluation of new thiazole scaffolds as potential TRPM8 antagonists

The preliminary results on the development of a viable methodology for the further functionalization of 4-hydroxythiazole derivatives to afford target TRPM8 antagonists are reported. The combined Sonogashira coupling/annulation reactions of the ethyl 2-(3-fluorophenyl)-4-tifluoromethylsulfonyloxy-1,3-thiazole-5-carboxylate have been applied to the synthesis of analogues of the selective blocker of TRPM8 DFL23448. Among all the synthetised derivatives, the most promising compound resulted to be active as TRPM8 blocker (IC50 = 4.06 µM), showing an excellent metabolic stability and no cytotoxic effects. Finally, in silico characterisation of the derivatives showed no violation of the drug-likeness rules.

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.

Enantioselective Friedel-Crafts reaction of hydroxyarenes with nitroenynes to access chiral heterocycles via sequential catalysis

Naphthols, hydroxyindoles and an activated phenol are reacted with differently substituted (E)-nitrobut-1-en-3-ynes using the commercially available Rawal's chiral squaramide. The corresponding β-nitroalkynes were obtained with good yields and excellent enantioselectivities. Moreover, dihydronaphthofurans can be accessed via silver catalysed cyclization in a tandem one-pot procedure, with high preservation of the optical purity.

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.

Silver-catalyzed tandem cycloisomerization/hydroarylation reactions and mechanistic investigations for an efficient access to 1,2-dihydroisoquinolines

A combination of AcOH and Ag catalyst induced an efficient tandem reaction to form 1,2-dihydroisoquinoline derivatives. Highly functionalized multiheterocyclic scaffolds are accessible straightforwardly using readily available starting materials.

Chiral N-heterocyclic Carbene Gold Complexes: Synthesis and Applications in Catalysis

N-Heterocyclic carbenes have found many applications in modern metal catalysis, due to the formation of stable metal complexes, and organocatalysis. Among a myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, inaccessible by other metal complexes. The present review summarizes synthetic efforts towards the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.

Self-assembled coordination thioether silver(I) macrocyclic complexes for homogeneous catalysis

The bis-ortho-thioether 9,10-bis[(o-methylthio)phenyl]anthracene was synthesized as a syn-atropisomer, as revealed by X-ray diffraction. This alkylaryl thioether ligand (L) formed different macrocyclic complexes by coordination with silver(I) salts depending on the nature of the anion: M₂L₂ for AgOTf and AgOTFA, M₆L₄ for AgNO₃. A discrete M₂L complex was obtained in the presence of bulky PPh₃AgOTf. These silver(I) complexes adopted similar structures in solution and in the solid state. As each sulfur atom in the ligand is prochiral, macrocycles L₂M₂ were obtained as mixtures of diastereoisomers, depending on the configurations of the sulfur atoms coordinated to silver cations. The X-ray structures of the two L₂·(AgOTf)₂ stereoisomers highlighted their different geometry. The catalytic activity of all silver(I) complexes was effective under homogeneous conditions in two tandem addition/cycloisomerization of alkynes using 0.5–1 mol % of catalytic loading.

Silver(i)-catalyzed addition of pyridine-N-oxides to alkynes: a practical approach for N-alkenoxypyridinium salts

A smooth catalytic approach to N-alkenoxypyridinium salts by using pyridine-N-oxides as the nucleophilic partner with alkynes under acidic conditions has been developed. This method uses different Ag(i) salts, with 5% AgOAc being the most efficient, to provide an efficient, practical and alternative way to obtain valuable N-alkenoxypyridinium salts with good to excellent yields (up to 93%).

Synthesis of 4-(1H-isochromen-1-yl)isoquinolines through the silver-catalysed homodimerization of ortho-alkynylarylaldehydes and subsequent condensation of the 1,5-dicarbonyl motif with NH3

4-(1H-Isochromen-1-yl)isoquinoline derivatives were synthesized in high yields via the AgBF₄-catalyzed self-reaction of ortho-alkynylarylaldehydes to give isochromene intermediates, followed by the dehydration of the 1,5-dicarbonyl motif with NH₃. Compared with electron-rich aromatic substituents, this strategy can provide the desired isochromene products with an electron-deficient isoquinoline unit. The reactions feature simple experimental operations, mild reaction conditions and high product yields.

We demonstrate a strategy that can provide isochromenes with an electron-deficient isoquinoline unit. The reactions feature simple operations, mild conditions and high yields.

Catalytic formal [4 + 1] isocyanide-based cycloaddition: an efficient strategy for the synthesis of 1H-cyclopenta[b]quinolin-1-one derivatives

An unprecedented catalytic isocyanide-based cycloaddition of alkyl and aryl isocyanides with (Z)-2-(2-hydroxy-2-alkylvinyl)quinoline-3-carbaldehydes was developed for the direct and efficient synthesis of 1H-cyclopenta[b]quinolin-1-one derivatives. On the basis of DFT calculations, a mechanism is proposed involving formal [4 + 1] cycloaddition of isocyanides, followed by imine-enamine tautomerization and air oxidation.

Ag2O nanoparticle-catalyzed substrate-controlled regioselectivities: direct access to 3-ylidenephthalides and isocoumarins

Herein, we disclose the first example of an efficient, silver oxide nanoparticle-catalyzed, direct regioselective synthesis of 3-ylidenephthalides 11–16 and isocoumarins 17–20via sonogashira type coupling followed by substrate-controlled 5-exo-dig or 6-endo-dig cyclization reaction, respectively. This one pot coupling involves reaction of substituted 2-halobenzoic acid with meta/para-substituted and ortho-substituted terminal alkynes, which proceeded in a regioselective manner resulting in the formation of 3-ylidenephthalides or isocoumarins, respectively, in excellent yields (up to 95%) with complete Z-selectivity. This protocol features relatively broad substrate scope, mild conditions, operational simplicity, and is favourable with aromatic/alicyclic terminal alkynes. The competition experiments and gram-scale synthesis further highlight the importance and versatility of the methodology. The proposed mechanistic pathways illustrate that the regioselectivity is substantially being controlled by the substituent(s) present on the acetylenic phenyl ring.

We report the first example of an efficient, Ag₂O nanoparticle-catalyzed, direct regioselective synthesis of 3-ylidenephthalides and isocoumarins via Sonogashira type coupling followed by substrate-controlled 5-exo-dig or 6-endo-dig cyclization reaction, respectively.

Indium-Catalyzed Intramolecular Hydroamidation of Alkynes: An Exo-Dig Cyclization for the Synthesis of Pyranoquinolines through Post-Transformational Reaction

An efficient approach for the synthesis of pyranoquinolines through the indium-catalyzed activation of alkynes is reported. Intramolecular hydroamidation of alkynes can proceed through alkyne activation by indium(III) and then 6-exo-dig cyclization, leading to a fused pyran ring with high selectivity, high atom economy, and good to excellent yields. The cyclization was accomplished through the oxygen, not the nitrogen, of the amide functional group.

C,O-Chelated BINOL/Gold(III) Complexes: Synthesis and Catalysis with Tunable Product Profiles

Unprecedented stable BINOL/gold(III) complexes, adopting a novel C,O-chelation mode, were synthesized by a modular approach through combination of 1,1'-binaphthalene-2,2'-diols (BINOLs) and cyclometalated gold(III) dichloride complexes [(C^N)AuCl₂ ]. X-ray crystallographic analysis revealed that the bidentate BINOL ligands tautomerized and bonded to the Au^III atom through C,O-chelation to form a five-membered ring instead of the conventional O,O'-chelation giving a seven-membered ring. These gold(III) complexes catalyzed acetalization/cycloisomerization and carboalkoxylation of ortho-alkynylbenzaldehydes with trialkyl orthoformates.

Synthesis of fused tricyclic indolizines by intramolecular silver-mediated double cyclization of 2-(pyridin-2-yl)acetic acid propargyl esters

A silver-mediated intramolecular double cyclization of 2-(pyridin-2-yl)acetic acid propargyl esters for the synthesis of fused tricyclic indolizines is reported.

Water-assisted metal-free catalyzed cyclization of 2-alkynylarylketones: a facile approach to indenones

A facile approach starting from 2-alkynylarylketones was developed for the construction of functionalized indenones under metal-free and water-assisted conditions.

TBAB-catalyzed cascade reactions: facile synthesis of 1-trifluoromethyl-3-alkylidene-1,3-dihydrofuro[3,4-b]quinolines via 5-exo-dig cyclization of o-arylalkynylquinoline aldehydes

TBAB-catalyzed cascade reaction condition is developed for the synthesis of 1-trifluoromethyl-3-alkylidene-1,3-dihydrofuro[3,4-b]quinolines from o-arylalkynylquinoline aldehydes via 5-exo-dig cyclization.