Iridium(III)-Catalyzed Regioselective B(4)-H Allenylation of o-Carboranes by Ball Milling.

We disclose an iridium(III)-catalyzed regioselective B(4)-H allenylation of o-carboranes from o-carborane acids and propargyl carbonates under ball-milling conditions. This study sheds light on a novel method to accomplish regioselective B(4)-allenylation of o-carborane that cannot be realized through conventional solution-based reactions. Control experiments, including solution, neat, microwave, and high-temperature reactions, reveal that the B(4)-H allenylation of o-carborane is unique for ball-milling conditions.

Iridium(III)-Catalyzed B(4)-Acylmethylation and B(3,5)-Diacylmethylation from o-Carboranes and Sulfoxonium Ylides.

An iridium(III)-catalyzed regioselective acylmethylation of the cage B(4)-H bond in o-carborane acids with sulfoxonium ylides is demonstrated through B(4)-H activation in ethanol under very mild conditions, affording a number of B(4)-acylmethylated o-carboranes. Additionally, the selective sequential B(4)- and B(6)-acylmethylation reactions finally gave B(3,5)-diacylmethylated o-carboranes in one pot.

Expansion of Azulenes as Nonbenzenoid Aromatic Compounds for C-H Activation: Rhodium- and Iridium-Catalyzed Oxidative Cyclization of Azulene Carboxylic Acids with Alkynes for the Synthesis of Azulenolactones and Benzoazulenes.

Rhodium-catalyzed oxidative [4 + 2] cyclization reactions through the C-H activation of azulene carboxylic acids as nonbenzenoid aromatic compounds with symmetrical and unsymmetrical alkynes were developed under aerobic conditions, which produced azulenolactone derivatives with a wide substrate scope and excellent functional group tolerance. Interestingly, azulenic acids in reaction with alkynes underwent iridium-catalyzed [2 + 2 + 2] cyclization accompanied by decarboxylation to afford tetra(aryl)-substituted benzoazulene derivatives. The reactivity order for C-H activation reaction is greater toward azulene-6-carboxylic acid, azulene-1-carboxylic acid, and azulene-2-carboxylic acid. For the first time, the expansion of azulenes having directing group as nonbenzenoid aromatic compounds for C-H activation was successful, indicating that nonbenzenoid aromatic compounds can be used as good substrates for the C-H activation reaction. Therefore, the research area of C-H activation will certainly expand to nonbenzenoid aromatic compounds in future.

Selective C-C bond formation from rhodium-catalyzed C-H activation reaction of 2-arylpyridines with 3-aryl-2H-azirines.

A novel method for the synthesis of acylmethyl-substituted 2-arylpyridine derivatives using 3-aryl-2H-azirines was developed by exploring a prototype reaction using DFT-calculations and carrying out targeted experiments guided by the calculated mechanism. 2H-Azirine was initially hypothesized to ring-open at the metal center to furnish familiar metal nitrene complexes that may undergo C-N coupling. Computational studies quickly revealed and prototype experimental work confirmed that neither the formation of the expected metal nitrene complexes nor the C-N coupling were viable. Instead, azirine ring-opening followed by C-C coupling was found to be much more favorable to give imines that readily underwent hydrolysis in aqueous conditions to form acylmethyl-substituted products. This new method was highly versatile and selective toward a wide range of substrates with high functional group tolerance. The utility of the new method is demonstrated by a convenient one-pot synthesis of biologically relevant heterocycles such as pyridoisoindole and pyridoisoqunolinone.

Synthesis of Diaryl Ketones through Oxidative Cleavage of the C-C Double Bonds in N-Sulfonyl Enamides.

An oxidative cleavage of a C-C double bond is developed from the photochemical [2+2]-cycloaddition of diaryl N-tosyl enamides, aryl heteroaryl N-tosyl enamides, and N-tosyl cyclic enamides with singlet molecular oxygen, followed by a ring-opening reaction mediated by Cs2CO3 under air and sunlight without the use of photosesitizer, producing symmetrical and unsymmetrical diaryl, heterodiaryl, and cyclic ketones in good to excellent yields. Moreover, the oxidative cleavage of C-C triple bonds from 1-alkynes is demonstrated for the synthesis of symmetrical and unsymmetrical ketones from the Cu-catalyzed [3+2]-cycloaddition, Rh-catalyzed alkoxyarylation, photooxygenation, and ring-opening reaction in one-pot. Because the synthesis of the symmetrical and unsymmetrical diaryl and/or heterodiaryl ketones bearing an electron-donating group is not easy, the present method is notable.

One-Pot Synthesis of Indolizines via Sequential Rhodium-Catalyzed [2 + 1]-Cyclopropanation, Palladium-Catalyzed Ring Expansion, and Oxidation Reactions from Pyridotriazoles and 1,3-Dienes.

An efficient, one-pot synthetic method for producing functionalized indolizine derivatives was developed via a Rh-catalyzed [2 + 1]-cyclopropanation, Pd-catalyzed ring expansion, and subsequent oxidation using manganese dioxide from pyridotriazoles and 1,3-dienes.

Synthesis of Imidazopyridines via Copper-Catalyzed, Formal Aza-[3 + 2] Cycloaddition Reaction of Pyridine Derivatives with α-Diazo Oxime Ethers.

The Cu-catalyzed, formal aza-[3 + 2] cycloaddition reaction of pyridine derivatives with α-diazo oxime ethers in trifluoroethanol was used to synthesize imidazopyridines via the release of molecular nitrogen and elimination of alcohol. These methods enabled modular synthesis of a wide range of N-heterobicyclic compounds such as imidazopyridazines, imidazopyrimidines, and imidazopyrazines with an α-imino Cu-carbenoid generated from the α-diazo oxime ethers and copper.

Synthesis of Bicyclic Isothiazoles through an Intramolecular Rhodium-Catalyzed Transannulation of Cyanothiadiazoles.

An intramolecular rhodium-catalyzed transannulation of readily available cyanothiadiazoles containing an ester, amide, or ether as a linker is described. It provides a wide range of bicyclic isothiazoles in good to excellent yields together with the release of molecular nitrogen. These results indicate that the carbon atom in the α-thiavinyl carbene is nucleophilic and that the sulfur atom is electrophilic.

Synthesis of Azepinoindoles via Rhodium-Catalyzed Formal Aza-[4 + 3] Cycloaddition Reaction of 3-Diazoindolin-2-imines with 1,3-Dienes in One-Pot.

Rhodium-catalyzed formal aza-[4 + 3] cycloaddition reaction of 3-diazoindolin-2-imines with 1,3-dienes was demonstrated for the synthesis of azepinoindoles in good to excellent yields in one-pot. First, rhodium-catalyzed [2 + 1] cycloaddition reaction smoothly took place to produce iminyl vinyl cyclopropane intermediate at room temperature in chlorobenzene for 1 h, which was thermally converted to azepinoindoles via aza-Cope rearrangement.