Yoshiaki Nishibayashi

A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia

The synthesis of transition metal-dinitrogen complexes and the stoichiometric transformation of their coordinated dinitrogen into ammonia and hydrazine have been the subject of considerable research, with a view to achieving nitrogen fixation under ambient conditions. Since a single example in 2003, no examples have been reported of the catalytic conversion of dinitrogen into ammonia under ambient conditions. The dimolybdenum-dinitrogen complex bearing PNP pincer ligands was found to work as an effective catalyst for the formation of ammonia from dinitrogen, with 23 equiv. of ammonia being produced with the catalyst (12 equiv. of ammonia are produced based on the molybdenum atom of the catalyst). This is another successful example of the catalytic and direct conversion of dinitrogen into ammonia under ambient reaction conditions. We believe that the results described in this Article provide valuable information with which to develop a more effective nitrogen-fixation system under mild reaction conditions.

Visible-Light-Mediated Utilization of α-Aminoalkyl Radicals: Addition to Electron-Deficient Alkenes Using Photoredox Catalysts

Synthetic use of α-aminoalkyl radicals formed by single electron oxidation of amines is quite limited. Here we demonstrate addition of α-aminoalkyl radicals to electron-deficient alkenes by visible-light-mediated electron transfer using transition metal polypyridyl complexes as photocatalysts, via a sequential redox pathway.

Catalytic Propargylic Substitution Reactions

In sharp contrast to the well‐established transition metal‐ catalyzed allylic substitution reactions, the study of the corresponding propargylic substitution reactions has been quite limited for constructing carbon–carbon and carbon–heteroatom bonds at the propargylic position. However, during the last decade, general and useful catalytic reactions have been developed extensively. In this Review, we summarize recent advances in catalytic propargylic substitution reactions of propargylic alcohols and their derivatives with heteroatom‐ and carbon‐centered nucleophiles. In addition to a variety of transition metals, such as Ru, Re, Rh, Ir, Ni, Pd, Pt, and Cu, which form organometallic species as intermediates, several Lewis acids and Brønsted acids are also applicable as catalysts in these reactions. The reaction mechanism and the scope of nucleophiles that can be used depend on the nature of the catalysts. Some examples of enantioselective substitution reactions at the propargylic position are also described.

Copper-Catalyzed Enantioselective Propargylic Amination of Propargylic Esters with Amines: Copper−Allenylidene Complexes as Key Intermediates

The scope and limitations of the copper-catalyzed propargylic amination of various propargylic esters with amines are presented, where optically active diphosphines such as Cl-MeO-BIPHEP and BINAP work as good chiral ligands. A variety of secondary amines are available as nucleophiles for this catalytic reaction to give the corresponding propargylic amines with a high enantioselectivity. The results of some stoichiometric and catalytic reactions indicate that the catalytic amination proceeds via copper-allenylidene complexes formed in situ, where the attack of amines to the electrophilic gamma-carbon atom in the allenylidene complex is an important step for the stereoselection. Investigation of the relative rate constants for the reaction of several para-substituted propargylic acetates with N-methylanilines reveals that the formation of the copper-allenylidene complexes is involved in the rate-determining step. The result of the density functional theory calculation on a model reaction also supports the proposed reaction pathway involving copper-allenylidene complexes as key intermediates. The catalytic procedure presented here provides a versatile and direct method for the preparation of a variety of chiral propargylic amines.

Cooperative Catalytic Reactions Using Organocatalysts and Transition‐Metal Catalysts: Enantioselective Propargylic Alkylation of Propargylic Alcohols with Aldehydes

The enantioselective propargylic alkylation of propargylic esters with aldehydes in the presence of a copper complex and an optically active secondary amine as cocatalysts has been found to give the corresponding propargylic alkylated products in good yields as a mixture of two diastereoisomers with a high enantioselectivity.

Ruthenium-Catalyzed Enantioselective Carbon−Carbon Bond Forming Reaction via Allenylidene-Ene Process: Synthetic Approach to Chiral Heterocycles Such As Chromane, Thiochromane, and 1,2,3,4-Tetrahydroquinoline Derivatives

Our previously disclosed ruthenium-catalyzed carbon-carbon bond forming reactions between propargylic alcohols and alkenes via an allenylidene-ene type pathway have been successfully applied to an enantioselective intramolecular cyclization for a variety of chiral heterocycles such as chromane, thiochromane, and 1,2,3,4-tetrahydroquinoline derivatives (up to 99% ee) by use of a suitable optically active diruthenium complex as a catalyst. The methodology described in this paper becomes a novel synthetic approach to chiral heterocycles, the structures of which are widely found in many natural and biologically active compounds.

Catalytic Reduction of Dinitrogen to Ammonia by Use of Molybdenum–Nitride Complexes Bearing a Tridentate Triphosphine as Catalysts

Newly designed and prepared molybdenum-nitride complexes bearing a mer-tridentate triphosphine as a ligand have been found to work as the most effective catalysts toward the catalytic reduction of dinitrogen to ammonia under ambient conditions, where up to 63 equiv of ammonia based on the Mo atom of the catalyst were produced.

Recent Progress in Transition-Metal-Catalyzed Reduction of Molecular Dinitrogen under Ambient Reaction Conditions

This paper describes our recent progress in catalytic nitrogen fixation by using transition-metal-dinitrogen complexes as catalysts. Two reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and its equivalent such as silylamine under ambient reaction conditions have been achieved by the molybdenum-, iron-, and cobalt-dinitrogen complexes as catalysts. Many new findings presented here may provide new access to the development of economical nitrogen fixation in place of the Haber-Bosch process.

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Catalytic formation of ammonia from molecular dinitrogen by use of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing PNP-pincer ligands: remarkable effect of substituent at PNP-pincer ligand.

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In accordance with a theoretical study, the catalytic activity is enhanced by the introduction of an electron-donating group to the pyridine ring of PNP-pincer ligand, and the complex bearing 4-methoxy-substituted PNP-pincer ligands is found to work as the most effective catalyst, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-product. Time profiles for the catalytic reactions indicate that the rates of the formation of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is complementary in the reaction system.