Photooxidative Reaction of ß-Oxoamides with Amines for the Synthesis of Pyrrolin-4-ones under External Photocatalyst-Free Conditions.

The incorporation of oxygen atoms from air under aerobic conditions plays an important role in organic synthesis. Herein, Brønsted acids are found to be a two-in-one strategic catalyst to transform enamines from ß-oxoamides and amines to pyrrolin-4-ones without an external photocatalyst under visible-light conditions. The Brønsted acid can inhibit the C-C bond fragmentation of the [2 + 2] adduct from enamine and 1O2, but most importantly, it can form photosensitizers with enamine and pyrrolin-4-one product by acidochromism to promote the 1O2 generation.

Highly E-Selective Olefin Synthesis Catalysed by Novel Quinoxalinone Photocatalyst under Visible Light Conditions.

In photo-induced olefin synthesis, the photocatalysts with high triplet energy could cause the isomerization of olefins. This study demonstrates a new quinoxalinone photocatalytic system for highly stereoselective alkenes preparation from alkenyl sulfones and alkyl boronic acids. Our photocatalyst could not convert the thermodynamically favored E-olefin to Z-olefin, guaranteeing the high E-configuration selectivity of the reaction. There is weak interaction between boronic acids and quinoxalinone according to NMR experiments, probably decreasing the oxidation potential of boronic acids. This system can be further extended to the allyl and alkynyl sulfones to give corresponding alkenes and alkynes.

Oxidative cross-coupling of quinoxalinones with indoles enabled by acidochromism.

An oxidative cross-coupling of quinoxalinones with indole derivatives via B(C6F5)3·H2O induced acidochromism of quinoxalinone derivatives was developed under mild and external photocatalyst-free conditions. The reaction shows excellent substrate scope, accommodating a wide range of functional groups. The usefulness of this strategy was demonstrated by the synthesis of the natural products Azacephalandole A and Cephalandole A in high yields. Moreover, the products are fluorophores showing prevalent fluorescence properties with a wide emission range and good relative quantum yields.

The B(C6F5)3·H2O promoted synthesis of fluoroalkylated 3,3',3''-trisindolylmethanes from fluorocarboxylic acids and indoles.

Trisindolylmethanes (TIMs) exist in many bioactive natural products and are frequently applied in medicinal chemistry and materials science. Herein, a simple and efficient protocol promoted by B(C6F5)3·H2O for the synthesis of their fluoroalkylated analogues, fluoroalkylated 3,3',3''-TIMs, is reported for the first time. Easily accessible fluorocarboxylic acids are utilized as the fluoroalkyl sources, exhibiting an obvious fluorine effect. This convenient and green process features mild and metal-free conditions, easy scale-up, and an environmentally friendly byproduct.

Synthesis of (2-(Quinolin-2-yl)phenyl)carbamates by a One-Pot Friedel-Crafts Reaction/Oxidative Umpolung Aza-Grob Fragmentation Sequence.

Utilizing the easily available isatin-based propargyl amines prepared from isatins, terminal alkynes, and anilines, (2-(quinolin-2-yl)phenyl)carbamates were prepared by a one-pot reaction in sequence, combining the gold-catalyzed Friedel-Crafts cyclization, oxidative umpolung aza-Grob fragmentation, and nucleophilic addition. In this process, gold-catalyzed cyclization of isatin-based propargyl amines gave 1'H-spiro[indoline-3,2'-quinolin]-2-ones, which were oxidized in situ by hypervalent iodine via the aza-Grob fragmentation to afford isocyano intermediates 2-(2-isocyanatophenyl)quinolines. Followed by the nucleophilic addition with alcohol solvents, (2-(quinolin-2-yl)phenyl)carbamates were synthesized. This procedure features easy operation, a wide substrate scope, and mild conditions.

Divergent Synthesis of 2-Cyanoaryl Carbamate and 2-Cyanoaryl Urea Derivatives via Hypervalent Iodine-Induced C-C Bond Cleavage.

Divergent synthetic methods for transforming isatins to 2-cyanoaryl carbamate and 2-cyanoaryl urea derivatives were developed using ammonium carbamate as the nitrogen source and iodobenzene diacetate as the oxidant. This reaction features mild conditions, broad substrate scope, and moreover, the use of toxic cyano-containing compounds is avoided.

Alkylation of Allyl/Alkenyl Sulfones by Deoxygenation of Alkoxyl Radicals.

A challenging deoxygenation of alkoxyl radicals from readily accessible alcohol derivatives was developed, affording facile synthesis of functionalized alkenes with good functional group tolerance under mild reaction conditions. Because alkoxyl radicals can easily undergo ß-fragmentations or hydrogen abstractions, this new strategy for deoxygenation of alkoxyl radicals is highly valuable. Moreover, mechanistic studies revealed that the electron-neutral phosphine acts as the deoxygenation reagent.

Lewis Acid Catalyzed [4 + 2] Cycloaddition of N-Tosylhydrazones with ortho-Quinone Methides.

A formal [4 + 2] cycloaddition of N-tosylhydrazones with ortho-quinone methides was developed, affording the facile synthesis of diverse 1,3-oxazine derivatives under mild conditions. In this transformation, N-tosylhydrazones are used as a 1,2-dipole synthon under base-free conditions. Moreover, the substrate scope is broad, and the products are formed with high diastereoselectivities in most of the cases.

Boron-Catalyzed Azide Insertion of α-Aryl α-Diazoesters.

A challenging metal-free azide insertion of α-aryl α-diazoesters in the presence of B(C6F5)3 (5 mol %) was developed for the first time. The reaction features an easy operation, wide substrate scope, and mild conditions and affords the corresponding products in moderate to high yields. More importantly, alkene and alkyne functional groups were well tolerated because no cyclopropanation or cyclopropenation was observed. Furthermore, the corresponding azide products could be converted to primary amines or 1,2,3-triazole derivatives after simple transformations.

Rhodium/Silver Synergistic Catalysis in Highly Enantioselective Cycloisomerization/Cross Coupling of Keto-Vinylidenecyclopropanes with Terminal Alkynes.

A rhodium/silver synergistic catalysis has been established, enabling cycloisomerization/cross coupling of keto-vinylidenecyclopropanes (VDCPs) with terminal alkynes toward the regio- and enantioselective formation of diversified tetrahydropyridin-3-ol tethered 1,4-enynes in good yields and high ee values. In this synergistic catalysis, Rh(I) and Ag(I) catalysts selectively activate keto-VDCP substrates and terminal alkynes to generate the π-allyl Rh(III) complex of oxa-rhodacyclic intermediate and Ag alkynyl intermediate, respectively. The rapid transmetalation of alkynyl groups from Ag to Rh is proposed to play a key role in realizing the regioselective cleavage of the distal bond of the three-membered ring in this transformation.

Gold(I)-Catalyzed Cycloisomerization of ortho-(Propargyloxy)arenemethylenecyclopropanes Controlled by Adjacent Substituents at Aromatic Rings.

Gold(I)-catalyzed cycloisomerization of ortho-(propargyloxy)arenemethylenecyclopropanes afforded two different types of products, that is, products of methylenecyclopropane migration and cycloisomerization products of the methylenecyclopropane moiety, controlled jointly by electronic and steric effects of the adjacent substituents. Furthermore, the corresponding cycloisomerization products could be also produced in an enantiomerically enriched manner.

Recent Advances in the Synthesis of Heterocycles and Related Substances Based on α-Imino Rhodium Carbene Complexes Derived from N-Sulfonyl-1,2,3-triazoles.

In recent years, α-imino rhodium carbene complexes derived by ring-opening of N-sulfonyl-1,2,3-triazoles have attracted much attention from organic chemists. Many transformations of these species have been reported that involve, in most cases, nucleophilic attack at the carbene center of the α-imino rhodium carbene, facilitating the synthesis of a wide range of novel and useful compounds, particularly heterocycles. This Minireview mainly focuses on advances in the transformation of N-sulfonyl-1,2,3-triazoles during the past two years.

Rhodium(II)-Catalyzed and Thermally Induced Intramolecular Migration of N-Sulfonyl-1,2,3-triazoles: New Approaches to 1,2-Dihydroisoquinolines and 1-Indanones.

New rhodium(II)-catalyzed or thermally induced intramolecular alkoxy group migration of N-sulfonyl-1,2,3-triazoles has been developed, affording divergent synthesis of 1,2-dihydroisoquinoline and 1-indanone derivatives according to different conditions. N-Sulfonyl keteneimine is the key intermediate for the synthesis of dihydroisoquinoline, whereas the aza-vinyl carbene intermediate results in the formation of 1-indanone.

A Selective Rh(I) -Catalyzed Substrate-Controlled C-C Bond Activation of Benzyl Sulfonamide/Alcohol-Tethered Alkylidenecyclopropanes.

Benzyl sulfonamide/alcohol-tethered alkylidenecyclopropanes undergo a rhodium-catalyzed and substrate-controlled selective C-C bond activation, producing three types of common organic structural units: benzo[c]azepine/oxepines, dihydronaphthalen-1-amines, and conjugated dienes. Epoxidation and aromatization of these products to construct two useful compounds have also been achieved.

Unprecedented Oxycyanation of Methylenecyclopropanes for the Facile Synthesis of Benzoxazine Compounds Containing a Cyano Group.

A novel intramolecular oxycyanation of methylenecyclopropanes is reported that proceeds through oxidative cleavage of the N-CN bond and subsequent palladium transfer from N to O of the amide group. A range of substituted benzo[d][1,3]oxazines with a cyano group are readily furnished by this newly developed oxycyanation reaction. Tris(4-trifluoromethylphenyl)phosphine as a ligand has been found to be crucial to effectively promote the transformation with high chemo- and regioselectivity. Moreover, the reaction outcome can be significantly affected by the electronic effect of the acyl group attached to the nitrogen atom of methylenecyclopropanes. When R(3) is a chloromethyl group, the pyrrolo[2,3-b]quinoline derivative is obtained by thermal-induced [3+2] cycloaddition of methylenecyclopropane to the methanediimine intermediate.

Gold-Catalyzed Fluorination-Hydration: Synthesis of α-Fluorobenzofuranones from 2-Alkynylphenol Derivatives.

The Au(I) -catalyzed fluorination-hydration of 2-alkynylphenol derivatives in the presence of Selectfluor [1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-[2.2.2]octane bis(tetrafluoroborate)] has been developed. This method provides straightforward access to α-fluorobenzofuranones with the construction of C-O, C=O, and C-F bonds in a single step on the basis of an Au(I) /Au(III) redox catalytic cycle. Several control experiments, including the asymmetric variant of this reaction, were also conducted to gain insight into the reaction mechanism.

Metal-Free Cross-Coupling of Arylboronic Acids and Derivatives with DAST-Type Reagents for Direct Access to Diverse Aromatic Sulfinamides and Sulfonamides.

We have developed a simple and convenient method for the cross-coupling of arylboronic acids and their derivatives with DAST-type reagents under mild and metal-free conditions to directly afford sulfinamides in moderate to good yields. Moreover, sulfonamides were obtained after a simple oxidation reaction. The reaction mechanism was investigated by (18) O-labeling experiments, and the synthetic utility was demonstrated by the sulfoxidation of natural products.

Catalyst-Dependent Stereodivergent and Regioselective Synthesis of Indole-Fused Heterocycles through Formal Cycloadditions of Indolyl-Allenes.

Stereo- and regioselective construction of poly-heterocycles, especially those with several contiguous stereocenters, is still a challenge. In this paper, catalyst-dependent stereodivergent and regioselective synthesis of indole-fused heterocycles through formal cycloadditions of indolyl-allenes has been developed. The reaction features total reversion of an all-carbon quaternary stereocenter when a gold or platinum complex was employed as the catalyst through [3 + 2] cycloaddition of allene with indole, affording different diazabenzo[a]cyclopenta[cd]azulenes as epimers, respectively. In addition, in the presence of IPrAuCl and AgNTf2, highly regioselective exo-type [2 + 2] cycloaddition was observed, in which allene served as a 2C synthon. This methodology provides a simple and straightforward approach for the construction of indole-fused tricyclic systems under mild conditions in an atom-economical way.

Gold-catalyzed tandem reactions of methylenecyclopropanes and vinylidenecyclopropanes.

Gold catalysis is often the key step in the synthesis of natural products, and is a powerful tool for tandem or domino reaction processes. Both gold salts and complexes are among the most powerful soft Lewis acids for electrophilic activation of carbon-carbon multiple bonds toward a variety of nucleophiles. The core of these reactions relies on the interaction between gold catalysts and π-bonds of alkenes, alkynes, and allenes. Activation of functional groups by gold complexes provides a useful and important method for facilitating many different organic transformations with high atom efficiency. Although they are highly strained, methylenecyclopropanes (MCPs) and vinylidenecyclopropanes (VDCPs) are readily accessible molecules that have served as useful building blocks in organic synthesis. Because of their unique structural and electronic properties, significant developments have been made in the presence of transition metal catalysts such as nickel, rhodium, palladium, and ruthenium during the past decades. However, less attention has been paid to the gold-catalyzed chemistry of MCPs and VDCPs. In this Account, we describe gold-catalyzed chemical transformations of MCPs and VDCPs developed both in our laboratory and by other researchers. Chemists have demonstrated that MCPs and VDCPs have amphiphilic properties. When MCPs or VDCPs are activated by a gold catalyst, subsequent nucleophilic attack by other reagents or ring-opening (ring-expansion) of the cyclopropane moiety will occur. However, the C-C double bonds of MCPs and VDCPs can also serve as nucleophilic reagents while more electrophilic reagents are present and activated by gold catalyst, and then further cascade reactions take place as triggered by the release of ring strain of cyclopropane. Based on this strategy, both our group and others have found some interesting gold-catalyzed transformations in recent years. These transformations of MCPs and VDCPs can produce a variety of polycyclic and heterocyclic structures, containing different sized skeletons. Moreover, we have carried out some isotopic labeling experiments and computational studies for mechanistic investigation. These reactions always give the desired products with high level control of chemo-, regio-, and diastereoselectivities, making them highly valuable for the synthesis of natural products and to the pharmaceutical industry and medicine in general.

Gold(I)-catalyzed selective heterocyclization of propargylic thioureas: mechanistic study of competitive gold-activation mode.

A new selective gold(I)-catalyzed intramolecular heterocyclization of propargylic thioureas has been developed, efficiently affording two kinds of cycloadducts in moderate to excellent yields with a broad substrate scope. Further mechanistic investigations indicate that competitive different gold activation modes feature in these cyclization processes. Kinetic experiments reveal that the gold activation mode is influenced by the ligand of the gold catalyst and the reaction conditions.