Photochemical Initiation and Reactions of Thiyl Radicals Studied with S2' Radical Traps.

A radical trapping method based on an S2' homolytic substitution reaction was applied to study the mechanism of a photochemical spirocyclisation of indole-ynones in the presence of thiols. Starting material, products and a range of trapped radical intermediates were simultaneously detected in reaction mixtures by mass spectrometry (MS). The trapped intermediates included both initiating and main chain propagating radicals.

Indole-ynones as Privileged Substrates for Radical Dearomatizing Spirocyclization Cascades.

Indole-ynones have been established as general substrates for radical dearomatizing spirocyclization cascade reactions. Five distinct and varied synthetic protocols have been developed─cyanomethylation, sulfonylation, trifluoromethylation, stannylation and borylation─using a variety of radical generation modes, ranging from photoredox catalysis to traditional AIBN methods. The simple and easily prepared indole-ynones can be used to rapidly generate diverse, densely functionalized spirocycles and have the potential to become routinely used to explore radical reactivity.

Pd-Catalyzed Indolization/-C-H Annulation/-Dealkylation Cascade to Cyclopenta-Fused Acenaphtho[1,2-]indole Scaffold.

A novel Pd-catalyzed cascade reaction of ,-dialkyl-substituted -alkynylanilines involving an indolization/-C-H annulation/-dealkylation sequence has been developed to construct a cyclopenta-fused acenaphtho[1,2-]indole (ANI) scaffold. A variety of aromatic hydrocarbons having a -C-H bond at the alkynyl terminus, such as naphthalene, phenanthrene, pyrene, and fluoranthene, were employed, affording the corresponding π-extended ANI derivatives. The ANI molecules showed relatively narrow energy gaps by increasing HOMOs and lowering LUMOs, implying their potential applications as π-segments in low-band-gap materials.

Visible-light-induced intramolecular charge transfer in the radical spirocyclisation of indole-tethered ynones.

Indole-tethered ynones form an intramolecular electron donor-acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols. This initiates a novel radical chain sequence, based on dearomatising spirocyclisation with concomitant C-S bond formation. Sulfur-containing spirocycles are formed in high yields using this simple and mild synthetic protocol, in which neither transition metal catalysts nor photocatalysts are required.

Indole Synthesis Using Silver Catalysis.

Indoles are amongst the most important classes of heteroaromatics in organic chemistry, commonly found in biologically active natural products and therapeutically useful compounds. The synthesis of indoles is therefore important and several methods for their synthesis that make use of silver(I) catalysts and reagents have been developed in recent years. This Minireview contains, to the best of our knowledge, a comprehensive coverage of silver-mediated indole forming reactions since the first reaction of this type was reported in 2004.

Pd-Catalyzed cascade cyclization of o-alkynylanilines via C-H/C-N bond cleavage leading to dibenzo[a,c]carbazoles.

A new and efficient Pd-catalyzed cascade cyclization of biaryl-tethered o-alkynylanilines for the formation of dibenzo[a,c]carbazole derivatives has been reported. The use of the alkyl-substituted tertiary anilines together with the combination of the PdCl2 catalyst with the MnO2 oxidant and PivOH is vital for giving rise to 5-endo cyclization, C-N bond cleavage, and C-H bond activation in a cascade manner to produce the corresponding products with structural diversity.

Ag(I)-Catalyzed Synthesis of Azabicyclic Alkaloid Frameworks from Ketimine-Tethered Ynones: Total Synthesis of Indolizidine 209D.

An efficient Ag(I)-catalyzed π-acid activation method for the cyclization of cyclic ketimine-tethered ynones is reported. Various nitrogen-containing scaffolds commonly found in bioactive alkaloids can be prepared in high yields, and the utility of the method is demonstrated by a formal synthesis of (±)-lasubine II and in a short total synthesis of (±)-indolizidine 209D.

N-Methyl Transfer Induced Copper-Mediated Oxidative Diamination of Alkynes.

A novel intramolecular oxidative diamination of bis(2-aminophenyl)acetylene for the synthesis of the structurally intriguing π-conjugated polyheterocyclic scaffold, 5,10-dihydroindolo[3,2-b]indole (DHII), has been developed under Cu(hfacac)2/O2 oxidation systems. The structure design of bis(2-aminophenyl)acetylene bearing both N,N-dimethylamine and primary amine groups is crucial for constructing the corresponding DHII scaffold. Notably, an intermolecular N-methyl transfer from the nitrogen atom of N,N-dimethylamine to the primary amine takes place, which is a critical step for the successful implementation of the present annulation process.

Highly efficient heterogeneous aerobic cross-dehydrogenative coupling via C-H functionalization of tertiary amines using a nanoporous gold skeleton catalyst.

We report for the first time that zero-valent nanoporous gold (AuNPore) is a robust and green heterogeneous catalyst for α-C-H functionalization of various tertiary amines. AuNPore combines with molecular oxygen at 80 °C or tert-butyl hydrogen peroxide at room temperature and catalyses the heterogeneous cross-dehydrogenative coupling (CDC) reaction efficiently to afford the corresponding C-C and C-heteroatom coupling products in good to excellent yields with excellent reusability.

Carboxylic acid-catalyzed highly efficient and selective hydroboration of alkynes with pinacolborane.

We have demonstrated for the first time that carboxylic acids are able to catalyze the direct hydroboration of various terminal and internal alkynes with pinacolborane without using any metal catalysts. This unprecedented catalytic hydroboration exhibits a broad functional groups compatibility, giving the corresponding alkenyl diboronates and monoboronates in good to high yields with exclusive regio- and stereoselectivities.

Unsupported nanoporous gold catalyst for highly selective hydrogenation of quinolines.

For the first time, the highly efficient and regioselective hydrogenation of quinoline derivatives to 1,2,3,4-tetrahydroquinolines using unsupported nanoporous gold (AuNPore) as a catalyst and organosilane with water as a hydrogen source is reported. The AuNPore catalyst can be readily recovered and reused without any loss of catalytic activity.