Reversal of Regioselectivity in Asymmetric C-H Bond Annulation with Bromoalkynes under Cobalt Catalysis.

Metal-catalyzed asymmetric C-H bond annulation strategy offers a versatile platform, allowing the construction of complex P-chiral molecules through atom- and step-economical fashion. However, regioselective insertion of π-coupling partner between M-C bond with high enantio-induction remain elusive. Using commercially available Co(II) salt and chiral-Salox ligands, we demonstrate an unusual protocol for the regio-reversal, enantioselective C-H bond annulation of phosphinamide with bromoalkyne through desymmetrization. The reaction proceeds through ligand-assisted enantiodetermining cyclocobaltation followed by regioselective insertion of bromoalkyne between Co-C, subsequent reductive elimination, and halogen exchange with carboxylate resulted in P-stereogenic compounds in excellent ee (up to >99 %). The isolation of cobaltacycle involved in the catalytic cycle and the outcome of control experiments provide support for a plausible mechanism.

Cp*Co(III)-Catalyzed C(8)-Nucleophilic Cascade Cyclization of Quinoline N-Oxide with 1,6-Enyne.

The C(8)-selective nucleophilic cascade cyclization of quinoline N-oxide with easily derived 1,6-enyne from phenol derivatives is demonstrated. A variety of quinoline N-oxide and alkynes are discovered to be suitable for producing a library of quinoline N-oxide tethered cis-hydrobenzofurans with high yields and excellent functional group tolerance. The utility of the protocol has been accomplished by post-synthetic modification of the cyclized product. The mechanistic studies indicate a base-assisted internal electrophilic-type substitution (BIES)-type pathway for C-H bond activation, and electrospray ionization mass spectrometry (ESI-MS) analysis of the stoichiometric reaction confirmed the formation of a key five-membered cobaltacycle.

C-H bond functionalization by dual catalysis: merging of high-valent cobalt and photoredox catalysis.

The merger of transition metal catalysis and photocatalysis has emerged as a versatile platform that opened the gateway to diverse low-energy pathways for several synthetic transformations. However, amidst the first-row transition metals, directed C-H bond functionalization mediated by high-valent cobalt catalysis has advanced with rising momentum owing to its unique reactivity and the ability to participate in both one- and two-electron transfer reactions. However, the use of expensive, privileged Cp* ligands or use of stoichiometric silver(I) or manganese(III) is unavoidable. Despite significant advances in their respective fields, the combination of these two "green" approaches to further the vested interest of the scientific research community towards the development of ecofriendly and sustainable protocols is noticeably limited. Thus, the methodology based on high-cobalt-photoredox dual-catalytic strategy has high dormant potential and is worthy to explore. Herein, we highlight the recent advances in the high-valent cobalt-catalyzed sustainable catalytic approach by harnessing light energy for oxidative C-H bond functionalization. With this, we hope to inspire the development of unexplored cobalt-photoredox-catalyzed reactions with improved efficiency and selectivity.

Room-temperature C-H bond alkynylation by merging cobalt and photocatalysts.

A new protocol is developed for the mono- and bis-ortho-C-H alkynylation of easily accessible benzamide derivatives using alkynyl bromides at room temperature by merging cobalt and photocatalysts. The diverse reactivity of various alkynyl bromides towards the C-H alkynylation and competing C-H/N-H bond annulation reactions has been demonstrated to give the corresponding products in good yields with excellent functional group tolerance.

Cp*CoIII-Catalyzed C(7)-H Bond Annulation of Indolines with Alkynes.

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*CoIII catalysis, which involves a cascade reaction of C(7)-H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.

Si Nanocrystals/ZnO Nanowires Hybrid Structures as Immobilized Photocatalysts for Photodegradation.

Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV and white light in comparison to the pristine ZnO NWs. Based on the optical measurements, we hypothesize the band structures of Si-NCs/ZnO-NWs and the potential mechanism for the improved photodegradability.

Cp*Co(III)-Catalyzed C-H Alkylation with Maleimides Using Weakly Coordinating Carbonyl Directing Groups.

A novel protocol for ortho-C-H alkylation of aromatic and heteroaromatic ketones and esters under Cp*Co(III) catalysis has been developed for the first time. The reaction proceeds through initial cyclometalation via weak chelation-assisted C-H bond activation, followed by coordination of activated alkene, insertion between Co-C, and protodemetalation.

Cp*CoIII -Catalyzed Bis-isoquinolone Synthesis by C-H Annulation of Arylamide with 1,3-Diyne.

Cp*CoIII -catalyzed highly regioselective mono- and bis-annulation of arylamides with 1,3-diynes using N-OMe as an internal oxidant is demonstrated. This atom-economical transformation does not require any external oxidant and tolerates many functional groups. Various symmetrical and unsymmetrical heterocycles (homo and hetero) are accessed with predictable regio- and chemoselectivity.

Cp*Co(III)-Catalyzed Annulation of Carboxylic Acids with Alkynes.

A new procedure for oxidative coupling of aromatic and acrylic acids with alkynes has been developed using abundant, nontoxic, and air stable Cp*Co(III) catalyst. The coupling involves initial cyclometalation via weak chelation-assisted C-H bond activation followed by alkyne coordination, insertion, and reductive elimination leading to diverse isocoumarins (α-pyranones) in good yields under mild conditions.

Aging of Silicon Nanocrystals on Elastomer Substrates: Photoluminescence Effects.

Nanocrystalline silicon is widely known as an efficient and tunable optical emitter and is attracting great interest for applications such as light-emitting devices (LEDs), electronic displays, sensors, and solar-photovoltaics. To date, however, luminescent silicon nanocrystals have been used exclusively in traditional rigid devices, leaving a gap in knowledge regarding how they behave on elastomeric substrates. The present study shows how the optical and structural/morphological properties of plasma-synthesized silicon nanocrystals (SiNCs) change when they are deposited on stretchable substrates made from polydimethylsiloxane (PDMS). Our results indicate that SiNCs deposited directly from the gas phase onto PDMS exhibit morphological changes, as well as modified aging characteristics due to enhanced oxidation. These results begin to fill the knowledge gap and point to the potential of using luminescent SiNC layers for flexible and stretchable electronics such as LEDs, displays, and sensors.

Ruthenium-Catalyzed Intramolecular Hydroarylation of Arenes and Mechanistic Study: Synthesis of Dihydrobenzofurans, Indolines, and Chromans.

A ruthenium-catalyzed, amide-directed intramolecular hydroarylation of alkene-tethered benzamide derivatives is discussed. This method proficiently constructs dihydrobenzofuran, indoline, and chroman skeletons of biological significance in good to excellent yields; the overall process is atom-economical and step-efficient. The reaction exhibits broad scope, tolerating common functional groups, labile protecting units, and heteroaryl motifs. The use of a catalytic amount of base suffices the need. Deuterium scrambling and kinetic studies offer valuable facts for understanding the reaction mechanism.