Published papers

The first observation of cleavage of unactivated aromatic C-N bonds on a late-transition-metal center was achieved using o-acylanilines and a ruthenium complex. Use of olefins as additives dramatically improved the rate of C-N bond cleavage. The carbon fragment remaining on the ruthenium after C-N bond cleavage was coupled with a pherylboronate to form a biphenyl framework. The present results suggest that ruthenium-catalyzed C-C bond formation from o-acylaniline and organoboronate proceeds via direct C-N bond cleavage on the ruthenium center.

A unique effect of styrene additive on product selectivity was observed for RuH2(CO)(PPh3)(3)-catalyzed C-H arylation of acetophenone derivatives bearing two ortho C-H bonds. Without styrene, the C-H arylation with arylboronates gives diarylation products as the major products throughout the reaction, but the use of styrene as an additive switches the product selectivity and leads to selective formation of monoarylation products.

Fully pi-conjugated polycyclic hydrocarbons with antiaromatic character have attracted research attention because of their unique properties such as narrow energy gaps, and thus should find application as optical and electronic materials. Although antiaromatic 16 pi-electron frameworks can be constructed by the incorporation of multiple seven-membered rings in a fused fashion to install methylenecycloheptatriene (heptafulvene) segments, the development of corresponding benzo[1,2:4,5]di[7]annulene (BDA)-containing pi-conjugated systems remains challenging due to the difficulty of their molecular design and synthesis. In this study, we develop an unprecedented chemical structure of cycloheptatriene-bis-annelated indenofluorene, which possesses both BDA and indenofluorene frameworks in a fused fashion. Physical measurements and X-ray analyses, along with theoretical calculations, indicated that antiaromaticity appeared in the BDA framework. By using the conjugated polycyclic hydrocarbon possessing both seven-membered and five-membered rings, this study provides fundamental insight into the strong antiaromatic nature of heptafulvene-based BDA framework.

We report a hydroaminative cyclization of enynes using phosphine-quinolinolato rhodium catalysts. The hydroaminative cyclization of 2-vinylphenylacetylene derivatives with secondary amines gives 2-aminoindenes in good yields. The reaction is considered to proceed through carbon-carbon bond formation on a catalytically generated aminocarbene ligand.

Both 2:1 and 1:1 couplings of alkylacetylenes with secondary amines were achieved using 8-quinolinolato rhodium catalysts and CsF. The 2:1/1:1 selectivity was switched by choosing the reaction solvent. In DMA, an unprecedented 2:1 coupling reaction of alkylacetylenes with amines proceeded to give 2-aminodiene products. One-pot 2:1 coupling/reduction provided rapid access to various allylamines, while one-pot coupling/hydrolysis gave enones as products. In toluene, anti-Markovnikov hydroamination occurred under relatively mild conditions to give 1:1 coupling products.

Anti-Markovnikov addition of anilines to aliphatic terminal alkynes proceeded using an 8-quinolinolato rhodium/phosphine catalyst system. The use of a strong organic base, 1,1,3,3,–tetramethylguanidine, in the catalyst system enabled the formation of the aldimine products. Substrates with various functional groups including polar groups such as a phenolic hydroxy group are applicable to the hydroamination.

Ruthenium complexes, e.g., Ru(H)2(CO)(PPh3)3, have been found to catalyze the addition of ortho C–H bonds of aromatic ketones to olefins with a high degree of efficiency and selectivity. 2′-Methylacetophenone reacts with various types of terminal olefins to give 1 : 1 coupling products in good to excellent yields. The C–C bond formation takes place exclusively at the terminal carbon atom of olefins except for styrene which affords a mixture of two regioisomers. Acetylnaphthalenes, cyclic aromatic ketones, and heteroaromatic ketones also react with triethoxyvinylsilane to give 1 : 1 addition products in virtually quantitative yields. From 2′-acetonaphthone or 3-acetylthiophene, in which two different reaction sites are available, only one out of four possible regioisomers is obtained. The importance of the coordination of the oxygen atom of the ketone to ruthenium and the intervention of a cyclometallation intermediate are suggested. A deuterium labeling experiment using acetophenone-d5 and triethoxyvinylsilane shows that an H/D exchange between the aromatic and olefinic positions takes place to some extent, even prior to the formation of the product. This implies that the rate-determining step is not the C–H bond cleavage step, but the product forming step.

During the last decade, catalytic functionalization of C-H bonds has become one of the most attractive research areas in modern organic synthesis. To date, many efficient catalytic reactions have been reported. In almost all of these reactions, heteroatom-directed C-H bond cleavage and/or C-C bond formation are quite important for attaining high regioselectivity and efficiency. In this chapter, several representative results concerning transition metal-catalyzed alkylation of C-H bonds with olefins, alkenylation with acetylenes, and carbonylation with olefins and carbon monoxides will be briefly reviewed. © 2007 Springer-Verlag Berlin Heidelberg.

The reaction of [Ru(CO)(2)(PPh3)(3)] (1) with o-styryldiphenylphophine (SP) (2) gave [Ru(CO)(2)(PPh3)(SP)] (3) in 83% yield. This styrylphosphine ruthenium complex 3 can also be synthesized by the reaction of [Ru(p-MeOC6H4NN)(CO)(2)(PPh3)(2)]BF4 (4) with NaBH4 and 2 in 50% yield. When "Ru(CO)(PPh3)(3)" generated by the reaction of [RuH2(CO)(PPh3)(3)] (8) with trimethylvinylsilane reacted with 2, [Ru(CO)(PPh3)2(SP)] (10) was produced in moderate yield as an air sensitive solid. The spectral and X-ray data of these complexes revealed that the coordination geometries around the ruthenium center of both complexes corresponded to a distorted trigonal bipyramid with the olefin occupying the equatorial position and the C-C bonding in the olefin moiety in 3 and 10 contained a significant contribution from a ruthenacyclopropane limiting structure. Complexes 3 and 10 showed catalytic activity for the hydroamination of phenylacetylene 11 with aniline 12. Ruthenium complex 3 in the co-presence of NH4PF6 or H3PW12O40 proves to be a superior catalyst system for this hydroamination reaction. In the case of the reaction using H3PW12O40 as an additive, ketimines (13) was obtained in 99% yield at a ruthenium-catalyst loading of 0.1 mol%. Some aniline derivatives such as 4-methoxy, 4-triftuoromethyl-, and 4-bromoanilines can also be used in this hydroamination reaction. (c) 2006 Elsevier B.V. All rights reserved.

The regioselective Ru(cod)(cot)-catalyzed alkenylation of arylpyridines and related compounds with alkenyl acetates to give good-to-excellent yields of ortho-alkenylation products was developed. Several arylpyridines having electron-donating (CH3 and OCH3) and -withdrawing (CF3, CN, and Ac) groups and azoles, such as oxazoline, tetrazole, and thiazole, can be used in this coupling reaction. A variety of alkenyl acetates, such as 1-propenyl, 1-butenyl, 1-hexenyl, styryl, 2-methyl-1-propenyl, and 1-cyclohexenyl acetates, reacted with arylpyridines to give the corresponding ortho-alkenylation products in high yields. This direct alkenylation of aromatic C-H bonds with alkenyl acetates provides pi-conjugated aromatic and heteroaromatic compounds under halogen-free reaction conditions.

Ruthenium- and rhodium-catalyzed intramolecular C–H/olefin coupling reactions of 1-(2-pyridyl)-, 1-(2-imidazolyl)-, and 1-(2-oxazolyl)-1,5-dienes proceeded in a regiospecific manner to give 5-membered ring products. Their 1,6-diene analogues are also applicable to the cyclization reaction to give the corresponding 5- and 6-membered carbocycles. For the cyclization of the pyridine derivatives, [RhCl(PPh3)3] showed the highest catalytic activity with respect to efficiency and selectivity. When the imidazole derivatives were employed to the cyclization reaction, the combination of the rhodium(I) complex and the relatively sterically bulky phosphine (e.g., PCy3 and PPFOMe) was a quite effective catalyst system. In the case of the oxazoles, the ruthenium complex, [Ru(H)2(CO)(PPh3)3], showed the highest activity among the catalysts examined. These catalytic reactions usually proceed at 120 °C. The [RhCl(PPh3)3]-catalyzed cyclization reaction of 2-[(1E)-4,4-dimethyl-1,5-hexadienyl]pyridine smoothly proceeded even at room temperature to give the corresponding cyclized product in 93% yield after 24 h. A hybrid catalyst system containing the rhodium(I) and both PPh3 and P(o-tolyl)3 had a slightly better catalytic activity compared with that of [RhCl(PPh3)3]. The intramolecular cyclization can be also applied to the C–H/CO/olefin coupling reactions. These carbonylation reactions gave the 5-membered ring ketones exclusively. The intramolecular C–H/olefin coupling reactions provide a new entry for constructing carbocyclic compounds from 1,n-dienes.

Ruthenium-catalyzed alkenylation of 2'-alkoxyacetophenones with alkenylboronates provides ortho C-H alkenylation products without sacrificing an ether functional group at the other ortho position. Both excellent chemoselectivity and high product yields are achieved with an aryloxo ruthenium complex. The effective suppression of the C-O bond cleavage was attained by coordination of the alkenyl moiety in the C-H alkenylation product to the ruthenium center.

© 2019 American Chemical Society. A method for the synthesis of N-arylpyrazoles by palladium-catalyzed coupling of aryl triflates with pyrazole derivatives is described. Using tBuBrettPhos as a ligand, the palladium-catalyzed C-N coupling of a variety of aryl triflates including ortho-substituted ones with pyrazole derivatives proceeded efficiently to give N-arylpyrazole products in high yields. 3-Trimethylsilylpyrazole was found to be an excellent pyrazole substrate for the coupling, and the corresponding product, 1-aryl-3-trimethylsilylpyrazole, also served as a great template for the syntheses of N-arylpyrazole derivatives, as demonstrated by regioselective halogenation at the 3-, 4-, and 5-positions of the pyrazole ring.

A novel (PNO)Rh/CuBr (PNO: phosphine-quinolinolate) tandem catalyst system was developed for facile synthesis of α-monosubstituted propargylamines from aliphatic terminal al-kynes and secondary amines. Various terminal alkynes and amines were applicable to this reaction and the corresponding propargylamines were obtained in good yields.