Counter Ion Effect in Au/Ag-Catalyzed Chemoselective 6-endo-dig N- and O-Cyclizations of Enyne–Urea System: Diversity-Oriented Synthesis of Annulated Indoles

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.

Influence of counterions on the supramolecular frameworks of isoquinoline-based silver(i) complexes

Crystal structures of seven new Ag(i) complexes with bis(1-isoquinolinecarboxamide)alkane derivatives were analyzed in terms of the coordinative ability and participation in non-covalent interactions of several counterions.

Synthesis of indoline-piperidinonesviaa novel Ugi, ring expansion,pseudo-Dieckmann condensation and rearrangement cascade reaction

Synthesis of functionalized indoline-piperidinones through an Ugi, ring expansion,pseudo-Dieckmann and rearrangement cascade reaction in one-pot.

Lactone Synthesis by Enantioselective Orthogonal Tandem Catalysis

In this work, we report enantioselective orthogonal tandem catalysis for the one pot conversion of Meldrum's acid derivatives and alkynes into δ-lactones. This new transformation, which resembles a formal [4+2] cycloaddition with concomitant decarboxylation and loss of acetone, proceeds in high yields and excellent enantioselectivity (up to 99 % ee) over a broad substrate scope. The products are densely functionalized and ripe for further transformations, as demonstrated here by both ring-opening reactions and reduction to saturated lactones. It was discovered that a new and serendipitously formed Ag^I -Me-StackPhos complex efficiently catalyzes the highly selective 6-endo-dig cyclization, completely reversing the regiochemistry that has been previously reported in related systems. More generally, in this study we identify a pair of compatible catalysts for alkyne difunctionalization that operate concurrently, which enable the alkyne to act as both a nucleophile and an electrophile in sequential one-pot transformations.

(CH3)2CuLi/Cu(OTf)2 Mediated N- or O-Cyclization of Urea-Tethered Cyclobuta[ b]indolines

A (CH₃)₂CuLi/Cu(OTf)₂ mediated N- or O-cyclization of urea-tethered cyclobuta[ b]indolines has been reported in this paper, giving a new synthetic protocol for the construction of pyrimido[1,6- a]indolone and cyclic imidate derivatives in moderate to good yields with a broad substrate scope under mild conditions. A plausible reaction mechanism has been also proposed on the basis of previous reports and the control experiments.

Reagent-controlled regiodivergence in the [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides

A regiodivergent [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides based on the N–O bond cleavage is achieved by changing reaction conditions.

Gold-catalyzed post-MCR transformations towards complex (poly)heterocycles

Post multicomponent reaction (MCR) transformations are one of the most successful methods leading to high structural diversity and molecular complexity. A well-known MCR, the Ugi reaction typically affords a linear peptide backbone, enabling post-Ugi transformations as an elegant solution to rigidify the Ugi adduct into more drug-like species. Not surprisingly, the development of such transformations leading to new structural frameworks has expanded rapidly over the last few years. These reactions have reached an impressive level of performance and versatility, particularly in amalgamation with gold catalysis. This review outlines the developments achieved in the past decade, highlighting the modifications that are performed in a sequential or domino fashion with emphasis on major concepts, synthetic applications of the derived products as well as mechanistic aspects.

Reactions of secondary propargylamines with heteroallenes for the synthesis of diverse heterocycles

This focused review aims to summarize recent developments in the processes involving additions of secondary propargylamines to various heteroallenes and subsequent transition metal-catalyzed or electrophile-mediated cyclizations.

Substrate-driven selective mono- and bis-couplings of ortho-(OTf/I/Br) substituted gem-dibromovinylarenes

A one-pot approach with the substrate-driven selective and competitive reactivity of ortho-trifloxy, iodo or bromo functionalized gem-dibromovinylarenes was studied under Pd-catalyzed conditions.

Diversity Oriented Synthesis of Indoloazepinobenzimidazole and Benzimidazotriazolobenzodiazepine from N(1)-Alkyne-1,2-diamines

A one-pot protocol for the diversity oriented synthesis of two N-polyheterocycles indoloazepinobenzimidazole and benzimidazotriazolobenzodiazepine from a common N(1)-alkyne-1,2-diamine building block is described. The approach involves sequential formation of benzimidazole through cyclocondensation and oxidation, which is followed by the formation of either an azepine ring (through alkyne activation and 6-endo-dig cyclization, 1,2-migration with ring expansion, and re-aromatization), or diazepine and triazole rings through 1,3-dipolar cycloaddition.

MediaChrom: Discovering a Class of Pyrimidoindolone-Based Polarity-Sensitive Dyes

A small library of six polarity-sensitive fluorescent dyes, nicknamed MediaChrom, was prepared. This class of dyes is characterized by a pyrimidoindolone core fitted out with a conjugated push-pull system and a carboxy linker for a conceivable coupling with biomolecules. The optimized eight-step synthetic strategy involves a highly chemo- and regioselective gold-catalyzed cycloisomerization reaction. The photophysical properties of MediaChrom dyes have been evaluated in-depth. In particular, the MediaChrom bearing a diethylamino as an electron-donating group and a trifluoromethyl as an electron-withdrawing group displays the most interesting and advantageous spectroscopic features (e.g., absorption and emission in the visible range and a good quantum yield). Promising results in terms of sensitivity have been obtained in vitro on this dye as a membrane/lipophilic probe and as a peptide fluorescent label.

Silver-catalysed reactions of alkynes: recent advances

Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon-carbon triple bond makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010-2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e., C-C, C-N, C-O, C-Halo, C-P and C-B). Compared with other coinage metals such as Au and Cu, silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions.

Counterion influence on the N-I-N halogen bond

A detailed investigation of the influence of counterions on the [N-I-N]+ halogen bond in solution, in the solid state and in silico is presented. Translational diffusion coefficients indicate close attachment of counterions to the cationic, three-center halogen bond in dichloromethane solution. Isotopic perturbation of equilibrium NMR studies performed on isotopologue mixtures of regioselectively deuterated and nondeuterated analogues of the model system showed that the counterion is incapable of altering the symmetry of the [N-I-N]+ halogen bond. This symmetry remains even in the presence of an unfavorable geometric restraint. A high preference for the symmetric geometry was found also in the solid state by single crystal X-ray crystallography. Molecular systems encompassing weakly coordinating counterions behave similarly to the corresponding silver(i) centered coordination complexes. In contrast, systems possessing moderately or strongly coordinating anions show a distinctly different behavior. Such silver(i) complexes are converted into multi-coordinate geometries with strong Ag-O bonds, whereas the iodine centered systems remain linear and lack direct charge transfer interaction with the counterion, as verified by 15N NMR and DFT computation. This suggests that the [N-I-N]+ halogen bond may not be satisfactorily described in terms of a pure coordination bond typical of transition metal complexes, but as a secondary bond with a substantial charge-transfer character.

Switchable stereocontrolled divergent synthesis induced by aza-Michael addition of deactivated primary amines under acid catalysis

Switchable tandem intramolecular aza-Michael/Michael and double aza-Michael reactions allow the oriented synthesis of highly functionalised cyclic skeletons. Conjugate addition of deactivated anilines triggers chemo- and stereo-divergent ring-closure reaction pathways with a striking selectivity depending on reaction conditions.

Diversity-oriented synthesis of ketoindoloquinoxalines and indolotriazoloquinoxalines from 1-(2-nitroaryl)-2-alkynylindoles

A one-pot protocol for the diversity-oriented synthesis of two indole-based annulated polyheterocycles, ketoindoloquinoxalines and indolotriazoloquinoxalines, has been described. The salient features of the methodology involves either a metal/O2-catalyzed aminooxygenation or a [3 + 2] cycloaddition pathway.