Published papers

Transition-metal-catalyzed electrochemical C−H functionalizations have been extensively studied as atom- and step-economical clean methods in organic synthesis. In this account, we described our efforts on the palladium-catalyzed electrochemical C−H functionalizations, including C−H halogenations of arylpyridines and benzamide derivatives using HCl/HBr and I2 as a halogen source, a one-pot process giving teraryls via the palladium-catalyzed electrochemical C−H iodination and subsequent Suzuki-Miyaura coupling, and an iodine-mediated oxidative homo-coupling reaction of arylpyridines.

Direct functionalization of unreactive bonds catalyzed by transition metal complexes has become a powerful tool in organic synthesis and has been studied extensively by many researchers. Our research group has been exploring regioselective functionalization via chelation-assisted cleavage of unreactive bonds by transition metal catalysts. Here we describe our recent efforts toward development of efficient methods for construction of π-conjugated systems by catalytic substitution via chelation-assisted cleavage of unreactive aromatic carbon-hydrogen and -heteroatom bonds. Ruthenium catalysts were employed to cleave aromatic carbon-hydrogen, carbon-oxygen, carbon-nitrogen, and carbon-fluorine bonds (aromatic carbon bonds) at ortho positions of directing groups and to introduce aryl, alkenyl, and carbonyl groups onto the aromatic rings. Application of these methods for short syntheses of substituted fused aromatic compounds such as twisted anthracenes, pentacenes, dibenzo[a,h] anthracenes, and picenes is also described.

A series of electron-accepting π-conjugated molecules having fluorine-containing dicyanovinylidene as terminal groups has been synthesized for the application to electron-transporting semiconductors. This terminal group can be easily incorporated into π-conjugated frameworks. Electrochemical measurements indicated that these compounds showed low-lying lowest unoccupied molecular orbital energy levels, which could be fine-tuned by the combination of central unit. The thin films fabricated by solution process showed typical electron-transporting characteristics in field-effect transistors.

A novel method for direct transformation of allyl groups in allylbenzene derivatives to alkenyl groups via rhodium-catalyzed C-C bond cleavage is reported. The alkenylation with styrenes of allylbenzenes containing pyridyl and pyrazolyl groups as a directing group proceeded efficiently to give alkenylation products. We also developed a new protocol for transformation of an ortho-prenylated phenol to an aniline derivative.

An efficient synthesis of tetrabenzo[a,d,j,m]coronene derivatives having alkyl and alkoxy substituents at the 3, 6, 13, and 16-positions was achieved based on the ruthenium-catalyzed coupling reactions of anthraquinone derivatives with arylboronates via C-H and C-O bond cleavage. The reaction sequence involving the arylation, carbonyl methylenation, and oxidative cyclization effectively provided various tetrabenzo[a, d,j, m]coronenes in short steps from readily available starting materials. Tetrabenzo[a,d,j,m]coronenes possessing two different types of substituents were obtained selectively by sequential chemoselective C-O arylation and C-H arylation. The H-1 NMR spectra of the tetrabenzo[a,d,j,m]coronene product indicated its self-assembling behavior in CDCl3.

An ortho-selective C-H alkylation of aromatic ketones with N-alkenylindoles proceeded in the presence of just a Fe(PMe3)(4) catalyst. In addition, C-H indolylation products, which are considered to be formed via 1,4-iron migration, were also observed as minor products.

A new method for the synthesis of multiarylanthracenes was developed by means of the RuH2(CO)(PPh3)(3)-Catalyzed arylation of anthraquinone with arylboronates. This method consists of short and straightforward sequences starting with an easily accessible anthraquinone and is applicable to the syntheses of various multiarylanthracenes including those bearing twisted backbones.

Deuterium-labeling experiments were performed for the Fe(PMe 3) 4-catalyzed C-H/olefin coupling using a deuterium-labeled aromatic ketone with various alkenes. While the reactions with a variety of alkenes provided the linear alkylation products formed via 1,2-insertion of alkene into an Fe-H bond, the reversible 2,1-insertion proceeded during the reaction highly depends on the choice of the alkene. No H/D scrambling resulting from 2,1-insertion/β-elimination was detected for the reactions with a vinylsilane and N -vinylcarbazole, but the reactions with styrenes are considered to involve rapid 2,1-insertion/ β-elimination processes to cause significant levels of H/D scrambling.

We report here a C-H homoallylation reaction of aromatic ketones with methylenecyclopropanes (MCPs) only using a catalytic amount of Fe(PMe3)4. A variety of aromatic ketones and MCPs are applicable to the reaction to form ortho-homoallylated aromatic ketones selectively via regioselective scission of the three-membered rings. The homoallylated products are amenable to further elaborations, providing functionalized 1,2-dihydronaphthalenes.

Transition-metal-catalyzed functionalization of unreactive carbon bonds such as C-H, C-O, C-N, and C-F bonds has been extensively studied over the last two decades. In this account we describe our studies on chelation-assisted catalytic transformations of unreactive carbon bonds in aromatic and olefinic compounds. Various C-C bond forming reactions are achieved by catalytic addition of C-H bonds to alkenes and alkynes, and via coupling reactions such as those of C-H, C-O, C-N, and C-F bonds with boronic esters. Methods for the conversion of C-H bonds into C-Si and C-X (X = halogen) bonds are also developed. The C-C bond forming reactions are applied to short syntheses of polycyclic aromatic hydrocarbons. 1 Introduction 2 Conversion of C-H Bonds into C-C Bonds 2.1 C-H Alkylation 2.1.1 Scope and Limitations 2.1.2 Mechanistic Studies 2.2 C-H Arylation 2.3 C-H Alkenylation 2.4 Introduction of Carbonyl Functionality via C-H Bond Cleavage 3 Conversion of Aromatic C-O, C-N and C-F Bonds into C-C Bonds by Coupling with Organoboronates 3.1 Coupling with Organoboronates via Aromatic C-O Bond Cleavage 3.2 Coupling with Organoboronates via Aromatic C-N Bond Cleavage 3.3 Coupling with Organoboronates via Aromatic C-F Bond Cleavage 4 Introduction of Heteroatoms at C-H Bonds 4.1 Silylation of C-H Bonds 4.1.1 Silylation of Aromatic C-H Bonds with Hydrosilanes 4.1.2 Silylation of Benzylic C(sp3)-H Bonds with Hydrosilanes 4.1.3 C-H Silylation with Vinylsilanes 4.2 Aromatic C-H Halogenation by Palladium-Catalyzed C-H Bond Cleavage and Electrochemical Oxidation 4.2.1 C-H Chlorination and Bromination 4.2.2 C-H Iodination and One-Pot C-H Iodination/Suzuki-Miyaura Coupling by ON/OFF Switching of Electric Current 5 Applications of C-H Functionalization in Short Syntheses of Polycyclic Aromatic Hydrocarbons (PAHs) 5.1 Convenient Synthesis of Multiarylanthracenes 5.2 Short Synthesis of Alkyl-Substituted Anthracenes and Pentacenes 5.3 Short Synthesis of Dibenzo[a,h]anthracenes and Picenes 6 Conclusions

Palladium-catalyzed electrochemical iodination and one-pot arylation of arylpyridines are described. Ortho-selective C-H iodination proceeded via dual activation of each substrate by a palladium catalyst and an electrode. Various aryl groups were introduced at the ortho positions of arylpyridines by ON/OFF switching of two different catalytic cycles using the same palladium catalyst in a one-pot fashion. © 2012 American Chemical Society.

A new convenient method for the synthesis of dibenzo-[a, h]anthracenes and picenes using ruthenium-catalyzed regioselective C-H arylation of aromatic ketones has been developed. Acetophenone derivatives and 1, 4-benzenediboronates were coupled in 2:1 ratios to form p-terphenyl derivatives. Conversion of the acetyl group to an ethynyl group, followed by cycloaromatization provided the desired fused aromatic compounds. An organic field-effect transistor fabricated from one of these products gave moderate hole mobility. © 2011 The Chemical Society of Japan.

Palladium catalysts possessing pyridineoxazoline ligands were found to catalyze the chain-walking cycloisomerization of 1,n-dienes to form five-membered rings and were particularly effective for the reaction of 1,14-dienes. Asymmetric synthesis of cyclopentane derivatives was also achieved for the reaction of 1,n-dienes using chiral pyridineoxazoline ligands.

Carbon-carbon bond formation at unreactive sp3-carbons in small organic molecules via chain walking was achieved for the palladium-catalyzed cycloisomerization of 1,n-dienes. Various 1,n-dienes (n = 7-14) such as those containing cyclic alkenes, acyclic internal alkenes, and a trisubstituted alkene can be used for the chain-walking cycloisomerization/hydrogenation process, and five-membered ring compounds including simple cyclopentane and pyrrolidine derivatives can easily be prepared. Chain walking over a tertiary carbon was also found to be possible in the cycloisomerization. It is not necessary for the linker portion of the diene to contain a quaternary center, and diene substrates with two alkene moieties linked by a tertiary carbon or a nitrogen atom can also be used as substrates. Column chromatography using silica gel containing silver nitrate was found to be effective for isolating some of the cycloisomerization products without hydrogenation. Deuterium-labeling experiments provided direct evidence to show that the reaction proceeds via a chain-walking mechanism.

We report here a method for direct catalytic introduction of simple α-acylalkyl groups via rhodium-catalyzed C-H functionalization with cyclic alkenyl carbonates, synthetic equivalents to enolates bearing leaving groups. The reaction proceeded smoothly without using bases to give α-aryl ketones in high yields. Various nitrogen-containing aromatic rings and amide groups serve as directing groups. 3-Substituted isocoumarins can also be prepared by one-pot C-H functionalization/cyclization.

8-Quinolinolate is a monoanionic, hard, strongly 6-donating ligand that can form strong chelates with various metals. However, studies of the catalytic activities of soft transition metal complexes such as greater than second row, low-valent, late-transition metals having 8-quinolinolate ligands had not been well explored until recently. In recent years, several research groups including our own have studied and developed various reactions of terminal alkynes using rhodium(I) catalysts containing an 8-quinolinolate ligand. In this Perspective, we surveyed the transformations of terminal alkynes using 8-quinolinolato rhodium(I) catalyst such as additions of alcohols, amines, and thiols as well as polymerization, trimerization, dimerization, and alkyne/alkene [2 + 2] cycloaddition. Mechanistic studies based on theoretical calculations of hydroalkoxylation of alkynes and alkyne/alkene [2 + 2] cycloaddition reactions have also been described.

A method for catalytic conversion of C(aryl)-C(alkenyl) bonds in styrene derivatives to new C-C bonds is developed. In the presence of a rhodium catalyst, the alkenyl groups of styrenes bearing a pyrazolyl directing group were efficiently converted to other carbon substituents upon reacting with various alkenes including styrenes, aliphatic alkenes, and allyl alcohols. It is also indicated that the C-C bond cleavage proceeded via a hydrometalation/beta-carbon elimination pathway.

Catalytic, regioselective C-H functionalizations have been extensively studied over the past two decades. In this account, we report our results on catalytic C-C, C-Si, and C-halogen bond formation reactions by substitution via the cleavage of C-H bonds. Catalytic arylation with arylboronates was developed and applied to the convenient synthesis of polycyclic aromatic hydrocarbons. Alkenylation reactions were also realized by the use of alkenylboronates and alkenyl esters. Various carbonyl-containing substituents were introduced using carbamoyl chlorides, alkyl chloroformates, acyl chlorides, and cyclic alkenyl carbonates. C-Si bonds were formed by reactions with hydrosilanes and vinylsilanes. C-H halogenation reactions were achieved by a combination of C-H bond cleavage and electrochemical oxidation and were applied to one-pot arylation reactions using the Suzuki-Miyaura coupling prodecure.