Kazunari Nakajima

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 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.

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.

Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia

It is vital to design effective nitrogen fixation systems that operate under mild conditions, and to this end we recently reported an example of the catalytic formation of ammonia using a dinitrogen-bridged dimolybdenum complex bearing a pincer ligand, where up to twenty three equivalents of ammonia were produced based on the catalyst. Here we study the origin of the catalytic behaviour of the dinitrogen-bridged dimolybdenum complex bearing the pincer ligand with density functional theory calculations, based on stoichiometric and catalytic formation of ammonia from molecular dinitrogen under ambient conditions. Comparison of di- and mono-molybdenum systems shows that the dinitrogen-bridged dimolybdenum core structure plays a critical role in the protonation of the coordinated molecular dinitrogen in the catalytic cycle.

Direct sp3 CH Amination of Nitrogen‐Containing Benzoheterocycles Mediated by Visible‐Light‐Photoredox Catalysts

Visible-light-mediated direct sp(3) C-H amination of benzocyclic amines via α-aminoalkyl radicals by using photoredox catalysts is described here. The obtained N,N-acetals were also successfully applied for carbon-carbon bond forming reactions with carbon nucleophiles. The procedure is suitable for a late-stage modification of C-H bonds to C-C bonds.

Synthetic Utilization of α-Aminoalkyl Radicals and Related Species in Visible Light Photoredox Catalysis

Single electron oxidation of amines provides an efficient way to access synthetically useful α-aminoalkyl radicals as reactive intermediates. After the single electron oxidation of amines, fragmentation of the resulting radical cations proceeds to give the α-aminoalkyl radicals along with generation of a proton. In the synthetic utilization of the α-aminoalkyl radicals, precise control of single electron transfer is essential, because further oxidation of the α-aminoalkyl radicals occurs more easily than the starting amines and the α-aminoalkyl radicals are converted into the corresponding iminium ions. As a result, photoinduced single electron transfer is quite attractive in the synthetic utilization of the α-aminoalkyl radicals. Recently, visible light-photoredox catalysis using transition metal-polypyridyl complexes and other dyes as catalysts has attracted considerable attention, where useful molecular transformations can be achieved through the single electron transfer process between the excited catalysts and substrates. In this context, MacMillan et al. ( Science 2011, 334 , 1114 , DOI: 10.1126/science.1213920 ) reported an aromatic substitution reaction of cyanoarenes with amines, where α-aminoalkyl radicals work as key reactive intermediates. Pandey and Reiser et al. ( Org. Lett. 2012 , 14 , 672 , DOI: 10.1021/ol202857t ) and our group ( Nishibayashi et al. J. Am. Chem. Soc. 2012 , 134 , 3338 , DOI: 10.1021/ja211770y ) independently reported reactions of amines with α,β-unsaturated carbonyl compounds, where addition of α-aminoalkyl radicals to alkenes is a key step. After these earliest examples, nowadays, a variety of transformations using the α-aminoalkyl radicals as reactive intermediates have been reported by many groups. The α-aminoalkyl radicals are usually produced from amines by single electron oxidation and the subsequent deprotonation of the C-H bond adjacent to the nitrogen atom. In addition, the α-aminoalkyl radicals are also produced from α-silylamines and α-amino acids in high efficiency through desilylation or decarboxylation after the single electron oxidation. The generated α-aminoalkyl radicals are utilized in a variety of reaction systems. In fact, reactions based on the addition of α-aminoalkyl radicals to alkenes and other unsaturated bonds have been extensively studied. Aromatic and other types of substitution reactions have also been investigated. Some of these transformations are achieved by combination of photoredox catalysts and other catalysts such as Brønsted and Lewis acids, organocatalysts, and transition metal catalysts. It is also noteworthy that the enantioselective reactions have been accomplished by combination of photoredox catalysts and chiral catalysts. The strategy for the generation of α-aminoalkyl radicals can be applied to utilize other types of alkyl radicals. In the generation of α-aminoalkyl radicals, the bond dissociation of the radical cations occurs at the α-position of amines. In relation to this process, synthetic utilization of other types of alkyl radicals generated by the bond dissociation of the radical cations at a remote position has been also investigated. These alkyl radicals have been applied to molecular transformations in a manner similar to the α-aminoalkyl radicals. Recently, organic synthesis using the α-aminoalkyl radicals and related alkyl radicals has been studied extensively. In this Account, we describe recent advances in photoredox-catalyzed synthetic utilization of these alkyl radicals.

Copper-Catalyzed Enantioselective Propargylic Etherification of Propargylic Esters with Alcohols

Enantioselective propargylic etherification of propargylic esters with not only aliphatic alcohols but also phenols in the presence of a catalytic amount of copper-Pybox complex gives the corresponding propargylic ethers in good to high yields with a high to excellent enantioselectivity (up to 99% ee). The result described here provides the first successful example of enantioselective propargylic etherification.

Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP-type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).

Cleavage and Formation of Molecular Dinitrogen in a Single System Assisted by Molybdenum Complexes Bearing Ferrocenyldiphosphine

The N≡N bond of molecular dinitrogen bridging two molybdenum atoms in the pentamethylcyclopentadienyl molybdenum complexes that bear ferrocenyldiphosphine as an auxiliary ligand is homolytically cleaved under visible light irradiation at room temperature to afford two molar molybdenum nitride complexes. Conversely, the bridging molecular dinitrogen is reformed by the oxidation of the molybdenum nitride complex at room temperature. This result provides a successful example of the cleavage and formation of molecular dinitrogen induced by a pair of two different external stimuli using a single system assisted by molybdenum complexes bearing ferrocenyldiphosphine under ambient conditions.