Published papers

Regioselective hydroaminative cyclizations of 1,5- and 1,6-diynes via double functionalization of an alkyne carbon were achieved using a phosphine-quinolinolato (PNO) rhodium catalyst. While the reaction of 1,6-diynes with secondary amines provided cyclic 2-aminodienes, phenylene-tethered 1,5-diynes were transformed into benzofulvene derivatives. The reaction is considered to proceed via in situ construction of an aminocarbene ligand, [2 + 2] addition with an internal alkyne moiety, and isomerization to an aminodiene structure. This hydroaminative cyclization proceeds just by heating the substrate with the rhodium catalyst without adding any additive and provides a convenient route to access cyclic 2-aminodiene and aminobenzofulvene derivatives.

Density functional theory (DFT) calculations were conducted by using designed alkane templates, 2-methylpentane and 2-methylhexanes, which allowed for effective comparison of relative stabilities of alkylpalladium complexes with various numbers of branches at the α-carbon and the agostic β-carbon. The calculation results suggested that the alkylpalladium complexes bearing a 1,10-phenanthroline ligand are more stabilized when they have more branches at the α-carbon. In addition, the complexes with an agostic β-C(2°)-H or β-C(3°)-H bond generally have Gibbs free energies lower than those with an agostic β-C(1°)-H bond.

This article presents our findings on catalytic ortho-selective C-C bond formations through the cleavage of C-H bonds in aromatic ketones using low-valent iron-phosphine complexes. We describe four types of catalytic transformations that convert C-H bonds into C-C bonds. Reactions involving terminal alkenes, such as vinyl and allylsilanes, aliphatic alkenes, styrenes, vinyl ethers, and enamines, yield the corresponding linear alkylation products. The coupling reaction using methylenecyclopropanes yields homoallylation products via alkene 1,2-insertion, followed by β-carbon elimination. The C-H alkenylation can be achieved by the reaction with internal alkynes. The use of 1, n -enynes (where n = 6, 7) as coupling partners for the C-H bond led to hydroarylative enyne cyclization products. The results of deuterium-labeling experiments of these reactions are also discussed.

We report here an iron-catalyzed C[sbnd]H functionalization of aromatic ketones via cyclization of enynes. The reaction is applicable to a variety of aromatic ketones and enables regioselective introduction of a hydrogen atom and an aryl group at the alkyne moiety and the olefin moiety, respectively.

Palladium-catalyzed electrochemical C(sp3)-H acetoxylation to prepare 1,2-diol derivatives was achieved using oxime ethers as exo-directing groups. Various substrates containing alkoxy groups with a methyl branch at the α-position as well as norbornan-2-ol derivative were acetoxylated only using a catalytic amount of Pd(OAc)2 along with NaOAc/Ac2O/AcOH under anodic oxidation conditions.

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

The palladium-catalyzed C-H iodination of 1-arylpyridine N-oxides proceeded under electrochemical oxidation conditions using I2 as an iodine source. The reaction of isoquinoline N-oxides possessing various para- or meta-substituted aryl groups at the 1-position proceeded to give the corresponding iodination products. Electron-donating groups on the aryl group facilitated the reaction to give relatively high yields of the product. The reaction was also found to be applicable to 2-aryl-3-picoline N-oxides.

We report here a method for in situ generation of various ruthenium carbonyl phosphine catalysts for arylation via cleavage of inert aromatic carbon-oxygen bonds. The use of catalyst systems consisting of [RuCl2(CO)(p-cymene)], CsF, styrene, and phosphines enabled facile screening of phosphine ligands. Asymmetric C-O arylation was also achieved for atropo-enantioselective biaryl synthesis using a chiral monodentate phosphine ligand.

Ortho-selective aromatic CH alkenylation with alkenyl carbonates proceeds in the presence of a catalytic amount of [Ru(cod)(cot)]. Arylpyridines and an aryloxazoline were regioselectively alkenylated without adding external bases. Aromatic CH alkylation also occurs depending on the reaction conditions. © 2014 The Chemical Society of Japan.

A direct, regioselective alkenylation of aromatic C-H bonds of aryl- and heteroarylpyridines and related compounds with alkenyl esters was developed using Ru(cod)(cot) as a catalyst. Aromatic compounds bearing electronically diverse substituents and various heterocyclic directing groups are reacted with alkenyl acetates bearing mono- and disubstituted alkenyl groups with aliphatic and aromatic substituents to give ortho-alkenylation products in high yields. The results of deuterium-labeling experiments and competition reactions using different ratios of the E and Z isomers of β-styryl acetate suggested that the C-H alkenylation proceeded via a ruthenium-alkene intermediate and the C-O bond of the alkenyl acetate was cleaved by β-acetoxy elimination. Two types of catalytically relevant species were identified, and the reactivities of these species, combined with the results of the kinetic studies, suggest that the rate-limiting step is the exchange of the COD ligand with the alkenyl ester. On the basis of the elucidated mechanism, the first catalytic coupling reaction using aromatic C-H bonds with C-O bonds of ethers was also developed. © 2013 American Chemical Society.