Currently, flow chemistry attracts much attention in academia and industry. We are interested in continuous flow synthesis with heterogeneous catalysts. To construct efficient flow reactors, preparation of active heterogeneous catalysts is one of the most important tasks. In general, heterogeneous catalysts have lower reactivity and selectivity compared with homogeneous catalysts. We designed active heterogeneous catalysts for continuous flow synthesis. Here, we describe development of polysilane-supported palladium/alumina hybrid catalysts and hydrogenation reactions under continuous flow conditions. These systems produced that the desired hydrogenation products in high yields with high selectivities without palladium leaching. We also developed PS-Pybox-calcium chloride catalyst system and applied it to asymmetric 1,4-addition reactions under continuous flow conditions. The substrate scope of these reactions is broad, and higher TON than batch systems has been attained. This is the first example using chiral calcium chloride as a catalyst in continuous flow synthesis.
Having a variety of species including radicals, cations, anions, and transition metal species as part of the repertoire, carbonylation chemistry is becoming more and more rich in synthetic methodology. Recent innovations with regard to the reaction devices available for carbonylation have been significant, including autoclaves that have the ability to permit light irradiation under CO pressure, compact CO boosters to create very high CO pressure conditions, flow reactor systems equipped with a mass flow controllers to ensure excellent gas-liquid mixing, and twin-tube reactors available for carbonylation with ex-situ generated CO. In this account, we survey the recent evolution of modern carbonylation techniques beginning with novel carbonylation reactions developed in our laboratory, like borohydride-mediated carbonylation, light induced carbonylation, and carbonylation based on radical/ion hybrid reactions. We believe that with innovations in synthetic methods and equipment, each carbonylation reaction will be easily run in a device specifically suitable for optimal performance in that reaction.
Electron transfer is one of the most typical driving factors for organic reaction, and organic electrosynthesis serves as a straightforward and powerful method for organic electron transfer processes. On the other hand, microreactor technology for organic synthesis has been significantly investigated for the past two decades, however, the integrated use of microreactor technology with organic electrosynthesis has been quite limited so far. We report here that the integrated use of microreactor technology with organic electrosynthesis which offers one of the most sophisticated processes in organic chemistry. In particular, (a) Organic electrosynthetic processes without using intentionally added supporting electrolyte, (b) Organic electrosynthetic processes utilizing liquid-liquid parallel laminar flow, (c) Novel paired electrolytic processes, and (d) Multi-step reaction systems via electrogenerated species, have been described.
Ionic liquids have been investigated extensively in recent years. They are a class of solvents with peculiar properties and have many advantages in comparison to classical organic solvent from the standpoint of green chemistry. For the aim to discuss potential of ionic liquids for organic syntheses, this mini review has focused on three topics: the first is how to use ionic liquids as solvents for transition metal catalysis mediated reactions, and the second is recent progress using supported ionic liquid phase catalysis. The last topic is activation of several types of reactions using ionic liquids.
Recent progress in catalytic nitrogen fixation by using transition metal-dinitrogen complexes is reviewed. Two effective reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and ammonia equivalent such as silylamine under ambient reaction conditions have been achieved by the use of molybdenum- and iron-dinitrogen complexes as catalysts. Both reaction systems provide a new aspect in the development of novel nitrogen fixation under mild reaction conditions, instead of the Haber-Bosch process.
Organometallic photocatalysis developed in the authors’ laboratory, which is mediated by [Ru(bipy)3]2+ (TB)-type sensitizers, is reviewed. After characteristics of photophysical properties of TB ((1) effective MLCT transition caused by visible light, (2) long lifetime of the excited triplet states, and (3) redox activities of the excited states) are briefly summarized, discussion is devoted to two types of catalytic systems. One is “photoredox catalysis”, where the photoexcited TB species serves as a 1e-oxidant as well as a 1e-reductant (SET (single electron transfer) processes) in a single catalytic cycle. The reactions are further divided into two types: reductive and oxidative quenching cycles. The former is effective for generation of radical species from electron-rich substrates such as amine and organoborate, whereas the latter providing carbon-centered radicals is effective for C-C bond formation, in particular, C-CF3 bond formation by the action of electrophilic trifluoromethylating reagents. The present photoredox-catalytic system turns out to be very green in terms of (1) generation of organic radicals under mild conditions without use of harmful reagents, (2) no need of special equipment, (3) no need of sacrificial redox reagent (redox-neutral and atom-economic), and (4) sunlight-promoted reactions (no need of energy from oil and coal). The other catalytic system is “difunctional dinuclear catalyst system” consisting of a TB-type photo-harvesting unit and a reactive Pd center connected by a bipyrimidine bridge. The dinuclear catalyst turns out to be effective for dimerization/oligomerization/polymerization of olefins, and the photoexcitation promotes insertion of olefin into the Pd-C bond. Studies on the substituent effects and DFT analysis reveal that photoexcitation at the TB-like moiety induces MLCT transition toward the bridging bmp part, which causes perturbation of the electronic structure of the reactive Pd center to promote the insertion.
Recent progress in genomic DNA sequencing technology has provided rapid access to the biosynthetic genes of the bioactive secondary metabolites found in plants and microorganisms. In addition, a methodology to rationally decipher the substrate structure and biosynthetic pathway from genomic data based on chemical structures and bioinformatics on natural product biosynthesis has been developed. Using these tools and technology, reconstitution of biosynthetic machinery (a set of biosynthetic enzymes) in an appropriate host cell allows us to synthesize various type of natural products. In this review, we briefly explain mechanism of key skeletal construction enzymes for typical natural products and introduce recent results on the enzymatic total synthesis of representative natural products, such as polyketides, terpenes, peptides and alkaloids.
CS-1036 was a candidate compound for antidiabetic drug and synthesized with 3 key intermediates. The 5-membered iminocyclitol intermediate (C unit), one of the key intermediates for CS-1036 synthesis, was prepared by chemical synthesis in 12 steps previously. By using microbial metabolite, Nectrisine, as a starting material, the manufacturing process of C unit was shortened to 5 steps. Thus, the hybrid process between biological synthesis of Nectrisine and chemical synthesis from Nectrisine to C unit improved the productivity of C unit production dramatically. Nectrisine was originally purified with 2 ion-exchange chromatographies which posed disadvantage for large scale production to remove the residual protein impurities. The combination of denaturation by MeOH and crystallization from brine gave the important intermediate of C unit synthesis with high quality and yield without ion-exchange chromatographies.
Theory has become one of important tools in synthetic organic chemistry. The transition state (TS) of chemical reactions can be visualized by geometry optimizations. This helps to design novel reactions. However, there has been a serious problem. That is, how to prepare initial structures for all relevant TSs. An optimization starting from a bad structure fails to converge. Moreover, geometry optimization cannot find unexpected TSs. The latter prohibits reliable analysis and prediction of mechanisms in highly complicated, multistep reactions. In order to solve this problem, we have developed automated reaction path search methods. In this paper, the artificial force induced reaction (AFIR) method and some examples of its applications are described. The AFIR method would expand applicability of theory dramatically.
The chiral BINOL-phosphoric acid is a well-designed asymmetric organocatalyst because it features the bifunctionality of the Brønsted acid (proton)/base (phosphoryl oxygen) and the conformational rigidity of the 3,3-substituents of the BINOL unit controlling the stereoselectivity through the hydrogen bonding network. Though the bifunctional activation has emerged as an important paradigm of chiral phosphoric acid catalyzed asymmetric reactions, it has remained elusive in most cases. Recently, we revealed the reaction mechanism as well as the stereocontrol mechanism by our computational two-step approach using (1) simplified chemical model and (2) realistic chemical model.
Although many non-peptidic drugs target biological membrane and membrane proteins, it is still difficult to determine the membrane-bound conformation of the drugs. To solve those problems, we have utilized bicelles as a membrane model, since the bicelles, which have planar lipid bilayer portions, are thought to be a more appropriate membrane model than micelles. Small-sized bicelles allow for liquid NMR measurements due to isotropic fast tumbling in solution. We have applied small bicelles to erythromycin A, salinomycin, and amphidinol 3, and determined their membrane-bound structures as well as their positions and orientations in the membranes using coupling constants, NOEs, and paramagnetic relaxation methods. Recently, we found that sphingomyelin, a major lipid constituent of lipid rafts, also forms bicelles, and established its conformation in the bicelles. These studies show the general utility of small bicelles for detailed conformation and orientation analysis of membrane-associated drugs and lipid molecules. We are now developing a bicelle-based crystallization method for membrane proteins, which will facilitate the cocrystalization of membrane proteins and hydrophobic drugs.
NMR (Nuclear Magnetic Resonance) is a useful tool to analyze products of chemical synthesis. However, when by-products are synthesized simultaneously, analyses by NMR become a challenge due to spectral overlapping of each component. For analyses of such mixtures by NMR, very useful method DOSY (Diffusion Ordered SpectroscopY) was proposed in 1990’s. It enables to separate spectra of mixtures into each component by using the difference of individual self diffusion coefficients. In this article, I introduce DOSY from its principle to practical experiments, and show an example of application for analysis of reaction pathway.
Single crystal X-ray structure analysis is an indispensable analytical method to obtain three-dimensional structure of materials. Especially to determine the absolute structure of a novel compound, single crystal analysis is virtually the sole method to achieve the goal without any preliminary information. There is no opposition to the potency of single crystal structure analysis, but the use of the method was limited only to crystallographers in the past. However recently, due to the development of equipment and software, it is becoming an analytical technique that everyone can use. In this article, key components of single crystal analysis systems, detectors, X-ray source and software, will be introduced.