Molybdenum-Quinone-Catalyzed Deoxygenative Coupling of Aromatic Carbonyl Compounds.

In the presence of triphenylphosphine as a mild reductant, the use of catalytic amounts of Mo(CO)6 and an ortho-quinone ligand enables the intermolecular reductive coupling of aromatic aldehydes and the intramolecular coupling of aromatic ketones to produce functionalized alkenes. Diaryl- and diheteroaryl alkenes are synthesized with high (E)-selectivity and a tolerance toward bromide, iodide, and steric hindrance. Intramolecular coupling of dicarbonyl compounds under similar conditions affords mono- and disubstituted phenanthrenes.

Interplay of Anion-π+ and π+ -π+ Interactions in Novel Pyrido[2,1-a]isoquinolinium-Based AIEgens - Substituent- and Counterion-Dependent Fluorescence Modulation and Applications in Live Cell Mitochondrial Imaging.

Recently, the concept of anion-π+ interactions has witnessed unique applications in the field of AIEgen development. In this contribution, we disclose a consolidated study of a library of N-doped ionic AIEgens accessed through silver-mediated cyclization of pyridino-alkynes. A thorough photophysical, computational and crystallographic study has been conducted to rationalize the observed substituent- and counterion-dependent fluorescence properties of these luminogens. We further elucidate the prominent role of anion-π+ interactions, π+ -π+ interactions and other non-covalent interactions, in inhibiting the undesired ACQ effect. Finally, we have also demonstrated the application of selected AIEgens for imaging of mitochondria in live cells.

Gold-Catalyzed Alkynylative Meyer-Schuster Rearrangement.

By applying the "interplay" mode, which consolidates two key reactivity modes of gold catalysis, namely π-activation mode and cross-coupling mode, the first alkynylative Meyer-Schuster rearrangement is designed and successfully implemented. The current protocol gives straightforward access to enynones, a highly valuable building block, from easily available propargyl alcohol feedstocks. Control experiments suggest an Au(III) catalyst triggers the Meyer-Schuster rearrangement, whereas monitoring the reaction with ESI-HRMS provided strong evidence in favor of a key alkynylgold(III) intermediate which supports the proposed "interplay" scenario.

Gold and hypervalent iodine(iii): liaisons over a decade for electrophilic functional group transfer reactions.

Over the last two decades, hypervalent iodine(iii) reagents have evolved from being 'bonding curiosities' to mainstream reagents in organic synthesis, in particular, electrophilic functional group transfer reactions. In this context, gold catalysts have not only emerged as a unique toolbox to facilitate such reactions (especially alkynylations) but also opened new possibilities with their different modes of reactivities for other functional group transfer reactions (acetoxylations and arylations). This feature article critically summarizes hitherto all such Au-catalyzed electrophilic functional group transfer reactions with hypervalent iodine(iii) reagents, emphasizing their mechanistic aspects.

External Oxidant-Dependent Reactivity Switch in Copper-Mediated Intramolecular Carboamination of Alkynes: Access to a Different Class of Fluorescent Ionic Nitrogen-Doped Polycyclic Aromatic Hydrocarbons.

An interesting case of external oxidant-controlled reactivity switch leading to a divergent set of ionic nitrogen-doped polycyclic aromatic hydrocarbons (N-doped PAHs), is presented here, which is quite unrecognized in copper-mediated reactions. In the current scenario, from the same pyridino-alkyne substrates, the use of the external oxidant PhI(OAc)2, in combination with Cu(OTf)2, gave N-doped spiro-PAHs via a dearomative 1,2-carboamination process; whereas, without the use of oxidant, an alkyne/azadiene [4 + 2]-cycloaddition cascade occurred to exclusively afford ionic N-doped PAHs. These newly synthesized N-doped PAHs further exhibit tunable emissions, as well as excellent quantum efficiencies.

Ionic Pyridinium-Oxazole Dyads: Design, Synthesis, and Application in Mitochondrial Imaging.

We recently developed an oxidative intramolecular 1,2-amino-oxygenation reaction, combining gold(I)/gold(III) catalysis, for accessing structurally unique ionic pyridinium-oxazole dyads (PODs) with tunable emission wavelengths. On further investigation, these fluorophores turned out to be potential biomarkers; in particular, the one containing -NMe2 functionality (NMe2-POD) was highly selective for mitochondrial imaging. Of note, because of mitochondria's involvement in early-stage apoptosis and degenerative conditions, tracking the dynamics of mitochondrial morphology with such imaging technology has attracted much interest. Along this line, we wanted to build a library of such PODs which are potential mitochondria trackers. However, Au/Selecfluor, our first-generation catalyst system, suffers from undesired fluorination of electronically rich PODs resulting in an inseparable mixture (1:1) of the PODs and their fluorinated derivatives. In our attempt to search for a better alternative to circumvent this issue, we developed a second-generation approach for the synthesis of PODs by employing Cu(II)/PhI(OAC)2-mediated oxidative 1,2-amino-oxygenation of alkynes. Thes newly synthesized PODs exhibit tunable emissions as well as excellent quantum efficiency up to 0.96. Further, this powerful process gives rapid access to a library of NMe2-PODs which are potential mitochondrial imaging agents. Out of the library, the randomly chosen POD-3g was studied for cell-imaging experiments which showed high mitochondrial specificity, superior photostability, and appreciable tolerance to microenvironment changes with respect to commercially available MitoTracker green.

Gold-Catalyzed 1,2-Oxyalkynylation of N-Allenamides with Ethylnylbenziodoxolones.

A gold-catalyzed 1,2-oxyalkynylation of N-allenamides with ethylnylbenziodoxolones (EBXs) has been achieved for the first time. The reaction, which follows a redox-neutral Au(I)/Au(III) catalytic pathway, was enabled in an attempt to exhaust the EBX reagents atom-economically by putting the nucleophilic carboxylate part of EBXs to appropriate use. This constitutes the first example for gold-catalyzed ß-alkynylation of N-allenamides to construct highly valuable 1,3-enynes. The potential of the sequence is further documented by some follow-up transformations.

Oxidant-free oxidative gold catalysis: the new paradigm in cross-coupling reactions.

The construction of C-C and C-X (X = hetero atom) bonds is the core aspect for the assembly of molecules. In this regard, late transition-metal-catalyzed cross-coupling reactions have retained their dominance for decades. Lately, gold catalysts have been emerging as a reliable contender for such transformations owing to the recent findings that reveal gold's potential beyond carbophilic π-Lewis acid. The newfound ability of gold to undergo redox transformations under certain circumstances has allowed us to explore gold catalysts as a viable alternative to other late transition-metals for efficient construction of cross-coupled products. This feature article critically presents an overview of all the redox neutral cross-coupling reactions enabled by gold catalysis, which we believe would stimulate further research activities in this promising area.

Exploiting the dual role of ethynylbenziodoxolones in gold-catalyzed C(sp)-C(sp) cross-coupling reactions.

Reported herein is the gold-catalyzed alkynylation of terminal alkynes using ethynylbenziodoxolones (EBXs), where EBXs serve a dual role as oxidants as well as alkyne transfer agents to access unsymmetrical 1,3-diynes. Hence, the catalytic system requires no external oxidants and is compatible with a broad range of substrates, including those with polar functional groups such as NH, OH and B(OH)2.