Synthesis and Characterization of New Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Bidentate Schiff-Base Ligands

A novel series of Schiff-base-substituted ruthenium carbene complexes have been prepared by the treatment of RuCl_2(=CHPh)(PCy_3)_2 with a variety of Schiff-base ligands as the thallium salts. Modification of the Schiff-base electronic and/or structural substituents allowed for the tuning of the complexes activities in olefin metathesis reactions. The structures of the complexes were unambiguously characterized by NMR studies and X-ray analysis. These newly prepared complexes are highly stable to air, moisture, and temperature and even exhibit catalytic activity in polar protic solvents

Metal-Free Carbocyclization of Homoallylic Silyl Ethers Leading to Cyclopropanes and Cyclobutanes

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimWe have developed a Hosomi-Sakurai type carbocyclization of homoallylic silyl ethers in reaction with silyl nucleophiles, catalyzed by Lewis acidic silylium salt. It offers cyclopropane and cyclobutane products with high efficiency and selectivity. A range of silyl nucleophiles could be engaged in this transformation to give small-sized carbocycles incorporating allyl, allenyl, carbonyl, indole or thioether groups. Diastereoselectivity in the cyclobutane formation was observed to be dependent on the steric bulkiness of incoming nucleophiles.11sciescopu

Synthesis and Characterization of New Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Bidentate Schiff-Base Ligands

A novel series of Schiff-base-substituted ruthenium carbene complexes have been prepared by the treatment of RuCl_2(=CHPh)(PCy_3)_2 with a variety of Schiff-base ligands as the thallium salts. Modification of the Schiff-base electronic and/or structural substituents allowed for the tuning of the complexes activities in olefin metathesis reactions. The structures of the complexes were unambiguously characterized by NMR studies and X-ray analysis. These newly prepared complexes are highly stable to air, moisture, and temperature and even exhibit catalytic activity in polar protic solvents

Charting a course for chemistry

To mark the occasion of Nature Chemistry turning 10 years old, we asked scientists working in different areas of chemistry to tell us what they thought the most exciting, interesting or challenging aspects related to the development of their main field of research will be — here is what they said

Asymmetric formation of γ-lactams via C–H amidation enabled by chiral hydrogen-bond-donor catalysts

Chiral γ-lactams are effective structural motifs found in numerous pharmaceutical agents. Despite their importance, current approaches mostly necessitate laborious synthetic steps employing pre-functionalized starting materials under demanding conditions. In this regard, asymmetric C−H amidation can provide an ideal platform for rapid construction of this valuable scaffold from unactivated materials, but unsolved issues have hampered the strategy. Here, we report iridium catalysts that overcome these challenges by utilizing chiral hydrogen-bond-donor ligands. The protocol makes use of easily accessible substrates derived from carboxylic acid, which display excellent efficiency and enantioselectivity towards direct amidation of prochiral sp 3 C−H bonds. Desymmetrization of meso-substrates is also achieved, where two consecutive stereogenic centres are selectively introduced in a single transformation. Computational investigations reveal the presence of crucial hydrogen bonding in the stereo-determining transition states and spectroscopic analysis of the structural analogues further corroborate this non-covalent interaction. © 2019, The Author(s), under exclusive licence to Springer Nature Limite

Transition-Metal-Mediated Direct C-H Amination of Hydrocarbons with Amine Reactants: The Most Desirable but Challenging C-N Bond-Formation Approach

Cross-dehydrogenative couplings (CDCs) have become one of the most straightforward protocols in the C-H bond functionalizations, showing step- and atom-efficiency without need of prefunctionalization of substrates and reactants. However, catalytic C-H amination procedures based on the CDC strategy by employing amine reactants are considered to be challenging mainly due to the highly nucleophilic character of parent amines to inhibit the catalytic turnovers and the difficulty in optimizing proper oxidative conditions. In spite of these concerns, recent efforts have led to notable advances in the C-H amination procedures, particularly in the intermolecular reaction system. In this Perspective, we address the recent achievements in the transition-metal-mediated CDC amination reactions with two types of hydrocarbon substrates: (i) direct amination of acidic C-H bonds with parent amines and (ii) chelation-assisted CDC amination/amidation of nonacidic C-H bonds. Mechanistic aspects are also briefly delineated in representative amination reactions to provide insights for the future development of highly practical and more environmentally benign processes. © 2016 American Chemical Society11031001sciescopu

The Use of Ammonia as an Ultimate Amino Source in the Transition Metal-Catalyzed C-H Amination

The development of efficient and elective transition metal catalyst systems enabling a direct C-H amination of hydrocarbons using ammonia is highly desirable considering the fact that ammonia is the most readily available nitrogen source. It is anticipated that a new mechanistic scaffold distinct from the currently applicable ones will guide this research eventually to have broad applicability in synthetic methodology, medicinal chemistry, and materials science. © 2017 American Chemical Society120211sciescopu

Comparison of the Reactivities and Selectivities of Group9 [Cp*M-III] Catalysts in C-H Functionalization Reactions

Pentamethylcyclopentadienyl (Cp*)-based Group9 metal (Co, Rh, or Ir) catalysts have emerged as powerful tools for C-H functionalization reactions. Whilst a diverse range of organic transformations have been developed by using [Cp*M-III] catalysts, they have often exhibited orthogonal reactivities and varied selectivities that depended on the choice of the central metal atom. An understanding of the physicochemical properties of the metals, as well as of their reaction mechanisms, has led to significant expansion of the synthetic scope of C-H functionalization reactions. This Focus Review summarizes and discusses the comparative catalytic reactivities and selectivities of the [Cp*M-III] catalysts, with an emphasis on metal-dependent pathway-switching by considering the mechanistic rationale ⓒ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Iridium-Catalyzed Direct C-H Amination with Alkylamines: Facile Oxidative Insertion of Amino Group into Iridacycle

Described herein is the development of Cp∗Ir(III)-catalyzed direct arene C-H amination using alkylamines as an amino source. This C-N bond formation showcases a notable example of cross-dehydrogenative coupling to install an amino functionality at the ortho-position of benzamide substrates. Mechanistic studies including the isolation of an amine-bound iridacyclic intermediate along with a set of chemical oxidations demonstrated the Ir-catalyzed inner-sphere C-H amination with primary alkylamines for the first time. © 2015 American Chemical Society343