Rhodium-catalyzed carbon-cyano bond cleavage reactions using organosilicon reagents are described. When disilane is used as the silicon reagent, the C-CN bonds in aryl and alkenyl cyanides are silylated to form the corresponding silylated products along with silyl cyanide. Reductive decyanation of nitriles is achieved when hydrosilane is used as the silicon reagent. In addition, this catalytic system can be applied to the C(sp3)-CN bonds in simple aliphatic nitriles. In both reactions, the silylrhodium species generated in situ serves as a catalytically active species, cleaving C-CN bonds via an η2-iminoacyl complex. Addition of external electrophiles allowed the silicon-assisted cleavage of C-CN bonds to be applied to C-C bond formation reactions. Intramolecular arylation of benzonitrile bearing a tethered chlorophenyl group and decyanative Mizoroki-Heck-type alkenylation of nitriles with vinylsilanes have been developed.
The chromophores of the chromoprotein antibiotics C-1027, kedarcidin, and maduropeptin feature a unique nine-membered enediyne structure in the ansa-macrolide ring and show extremely potent cytotoxicity. Their complicated fused ring systems were recently synthesized and the total synthesis of (−)-maduropeptin chromophore was achieved, which thereby led to the revision of its stereochemical structure.
In this account, the early stages of our campaign (2006-2009) to develop catalysts able to realize ideal arene assembly through catalytic C-H bond arylation of aromatic compounds are described. New Rh, Ir, and Ni catalysts have been developed for the C-H bond arylation of heteroarenes with haloarenes. It was also found that Cu(OCOCF3)2 can promote the C-H bond arylation of electron-rich arenes with aryl boronic acids. During these studies, we accidentally discovered that KOt-Bu alone can promote the C-H bond arylation of electron-deficient nitrogen heterocycles with haloarenes. Through a sequence of three consecutive bond-selective C-H arylations, a programmed synthesis of tetraarylthiophenes has been established. During this study, we discovered metal-and ligand-controlled regiodivergency in C-H bond arylation.
In this account, work in our group on the development and applications of a new family of bimetallic, bifunctional asymmetric catalysts based on dinucleating Schiff bases is described. Suitable design of the dinucleating Schiff bases led to the successful development of heterobimetallic transition metal/rare earth metal catalysts as well as homobimetallic transition metal/transition metal catalysts. The concepts involved in catalyst design, applications to asymmetric reactions, mechanistic insights into bimetallic catalysis as well as the catalytic asymmetric synthesis of biologically active compounds are introduced.
Recent successful examples for synthesis of new polyolefins containing polar functionalities by adopting the approaches by (a) direct copolymerization of olefin with polar monomer using living radical or coordination insertion methods, (b) direct alkane C-H activation/functionalization (post functionalization of polyolefins), and by (c) controlled incorporation of reactive functionalities (and the subsequent introduction of polar funtionalities under mild conditions) by coordination polymerization in the presence of transition metal complex catalysts have been described. In particular, our recent efforts for precise synthesis of polyolefins containing polar functionalities by (i) efficient incorporation of reactive functionality by copolymerization of ethylene with nonconjugated diene using nonbridged half-titanocene catalysts and (ii) subsequent chemical modifications under mild conditions have been introduced.
Asymmetric transformations catalyzed by small chiral organic molecules, the so-called enantioselective organocatalysis, have been developed as an efficient protocol for obtaining optically active products in modern organic synthesis. In organocatalytic transformations, the activation of reactants through hydrogen bonding interactions has proven to be a brilliant strategy to provide a chiral environment for the transient assembly of reactants and organocatalysts. In this context, we were interested in the characteristic properties of guanidines, namely, their strong basic character and their ability to act as recognition elements through two parallel hydrogen bonds, which can be manipulated to enable their use as enantioselective organocatalysts. Thus, guanidines are expected to capture nucleophilic components through hydrogen bonding interactions without forming loose ion pairs, after deprotonation from pro-nucleophiles. In addition, the N-H proton would function as a Brønsted acidic site and hence could convey the acid/base dual function even to monofunctional guanidine catalysts. From these basic ideas, we aimed to develop novel axially chiral guanidines as enantioselective Brønsted base catalysts. In this account article, we present our recent achievements in the development of enantioselective transformations using these axially chiral guanidine catalysts.
In this review, we describe some of our recent approaches to the development of functional liquid-crystalline (LC) materials and fibrous aggregates. A variety of electro-active and photo-active π-conjugated molecules that exhibit nanostructured LC phases such as micellar cubic, columnar, and smectic structures have been designed and prepared. The formation of controlled phase-segregated structures has led to the efficient 1D and 2D transportation of charges. Redox-driven molecular shuttling for an LC bistable rotaxane has been achieved in the LC phase. Mechanoresponsive photoluminescent liquid crystals have also been developed. Moreover, we have demonstrated the formation of aligned electro-active fibrous aggregates either using liquid crystals as anisotropic templates or applied electric fields. These anisotropic self-assembled materials may have great potential applications as organic semiconductors, polarizers, sensors, and memories. This kind of approach to the use of self-organization processes for the development of functional molecules may open up new avenues in the fields of supramolecular chemistry and materials science.
Fluorine-containing heterocyclic and carbocyclic compounds attract widespread attention as important components of agrochemicals, pharmaceuticals, materials, and catalysts. To provide a general route to these compounds, we have developed a new methodology to construct fluorocarbon-bearing cyclic systems based on ring closure of functionalized 2-trifluoromethyl-1-alkenes, readily obtained from commercially available CH2=C(CF3)Br or CF3CO2Et. Their intramolecular reactions, such as nucleophilic substitution or addition, alkene insertion, and electrocyclization, provide a wide variety of five- and six-membered heterocycles and carbocycles bearing a fluorinated one-carbon unit (a =CF2, CHF2, or CF3 group). To construct five-membered rings, we successfully accomplished a nucleophilic 5-endo-trig cyclization and 5-endo alkene insertion, normally disfavored processes, as well as fluorine-directed Nazarov cyclization.
A series of P-spiro chiral tetraaminophosphonium salts have been designed and synthesized as a new class of organic molecular catalysts, and their inherent abilities to exert four different, synthetically relevant asymmetric catalyses have been brought out through the unique molecular design on a single core structure, N4P+. Further, these catalyses have been applied to the development of various C-C, C-P, and C-N bond-forming reactions, in which the remarkably high catalytic performances and stereocontrolling abilities of the chiral tetraaminophosphonium salts have been successfully demonstrated.
A revolutionary new methodology for introducing a short-lived radionuclide 11C into carbon frameworks of biologically significant organic compounds has been established by developing rapid C-[11C]methylations and C-[18F]fluoromethylations using Pd0-mediated cross-coupling reactions between [11C]methyl iodide or [18F]fluoromethyl iodide (or bromide) and excess amounts of organostannane or organoboron compounds. These rapid reactions provide firm chemical bases for an efficient, general synthesis of short-lived 11C- and 18F-labeled PET molecular probes in order to strongly promote in vivo molecular science.