Since Schrock and Grubbs discovered the molybdenum and ruthenium-carbene complexes in 1990 and 1992, metathesis reactions have been a useful tool in synthetic organic chemistry. As the metathesis reaction, olefin metathesis, enyne metathesis and alkyne metathesis are known. The carbon-carbon double or triple bonds are cleaved and the double or triple bonds are formed simultaneously in the metathesis reaction. Novel synthetic methods for the carbo- and hetero-cyclic compounds from diene, enyne and diyne have been developed using these reactions. The tandem metathesis reactions were also developed and many bond cleavages and bond formations occurred in one step reaction to form various functionalized compounds. For development of asymmetric metathesis reaction, novel molybdenum- and ruthenium-catalysts were also synthesized and asymmetric ring-opening and ring-closing metatheses were carried out to form optically active compounds with the high ee. The novel syntheses of the natural products and the biologically active substances have been achieved using these reactions.
Direct borylation and silylation of unreactive C-H bonds catalyzed by transition metal complexes have been extensively studied by several research groups and have become economical, efficient, elegant, and environmentally benign protocols for the synthesis of a variety of organoboron and organosilicon compounds. A number of transition metal complexes catalyzes C-H borylation and silylation of alkanes, arenes, heteroarenes, alkenes, or benzylic positions of alkylarenes by diborons or hydoroboranes and disilanes or hydrosilanes to produce the corresponding alkyl-, aryl-, heteroaryl-, vinyl-, or benzylboron and -silicon compounds, respectively. In this review, seminal early works and exciting recent developments in the area of direct borylation and silylation of hydrocarbons based on C-H activation are summarized.
Organic bases are important class of reagents for synthetic chemistry. Recent developments in optimizing the basicity and in reducing nucleophilicity provide various new synthetic methodologies. This review describes synthetic applications of two excellent organic superbases. One is phosphazene base developed by Schwesinger and the other is proazaphosphatrane base, which was first synthesized by Verkade. The bacisity of the Schwesinger's P 4 base is in the range of organolithium bases and 18 orders of magnitude more basic than conventional DBU. Verkade bases are also strong bases due to the protonation on the bridgehead phosphorus atom with a resultant transannulation to form the corresponding azaphosphatrane structure. Properties, applications, and synthetic benefits of these bases are discussed.
In the early 1970s, intramolecular aldol reaction catalyzed by proline was reported by Hajos et al., and its intermolecular version was discovered by List, Barbas, and Lerner in 2000. In the same year, MacMillan reported the asymmetric Diels-Alder reaction catalyzed by organocatalyst by lowering the LUMO energy of enones by the formation of iminium salt. After these two seminal papers, chiral small organic molecules have been widely employed in asymmetric synthesis because they have several advantages over the conventional transition metal based catalysts. Organic catalysts are inexpensive, readily available, and non-toxic. They are not sensitive to moisture and oxygen. The products are free from the contamination of metals. Because of these advantages, this field has been expanding so rapidly. Though there are so many asymmetric reactions catalyzed by organocatalysts, this article briefly summarizes the recent progress in the following reactions because of the limitation of space : Aldol reaction, Mannich reaction, Michael reaction, functionalization of α-position of carbonyl compounds, cycloaddition reactions such as Diels-Alder reaction, [3+2] cycloaddition reaction, and [4+3] cycloaddition reaction, allylation, Morita-Baylis-Hillman reaction, epoxidation, and phase-transfer reaction.
Salen [N, N'-ethylenebis (salicylideneimine)] ligands are readily prepared by condensation of o-hydroxybenzaldehydes and diamines, and are known to form a variety of metal complexes. In the last decade, optically active metallosalen complexes have emerged as very useful catalysts for various asymmetric transformations, and high enantioselectivities and unique catalytic properties have been acquired by dictating the aldehyde, diamine, and central metal units. Of particular note is their tremendous contribution to recent rapid progress in atom-efficient asymmetric reactions, which emit no waste or only nontoxic byproducts of small molecular weight such as water and nitrogen. Described in this review are (1) asymmetric and chemoselective aerobic oxidations under photoactivation, (2) asymmetric oxidations with hydrogen peroxide regulated by cis-β salen complexes, (3) enantio- and diastereo-selective cyclopropanations of alkenes with diazo compounds, (4) asymmetric aminations with azides, and (5) asymmetric ring opening reactions of epoxides and addition reactions to carbonyl substrates by dual activation methods.
A new type of solid acid and base-catalyzed reactions has been selected and addressed from the viewpoint of practical reactions applicable to fine chemicals synthesis. As solid acid catalysts, zeolites (NaY, Sn-β, Ti-β, molecular sieves 4A (MS4A)), mesoporous aluminosilicates (Al-MCM-41, Al-HMS), montmorillonite (Ti-Mont), hydroxyapatite (Ru-HAP), Ru (OH)x/Al2O3, and polystyrene-supported sulfonic acid were reviewed in carbonyl-ene reaction, the Diels-Alder reaction, alkylation, aldol-type condensation, oxidation, reduction, hydration, acetalization, and esterification. Solid base catalysis by alkali-earth metal oxide (MgO, CaO, SrO, Mg-Al mixed oxide, KOH/Fe2O3), hydrotalcite (HT-OH, HT-tBuO), and MS4A was mainly discussed in the carbon-carbon and carbon-oxygen bond forming reactions.
Transition metal catalysis has found a wide range of applications in organic synthesis. However, the separation of the product from the catalyst as well as its reuse is a major drawback of homogeneous catalysis. The recent emergence of ionic liquids as alternatives to volatile organic solvents also provides an opportunity to affect immobilization of homogeneous catalysts in this unique phase. Segregation of the catalyst in the ionic liquid would allow facile separation of the products and the catalyst and recycling of the catalyst and the reaction medium. This review article focuses on catalytic reactions conducted in imidazole-based ionic liquids from the viewpoint of catalyst immobilization. A low viscosity ionic liquid, [bmim] NTf2, and its effective use in a circulatory microflow system are also highlighted.
Organic reactions using microreactors have attracted significant research interest because of their inherent features such as fast mixing, precise temperature control, and precise residence time control. These features are especially advantages for fast and exothermic reactions, which are difficult to perform using conventional macro-scale batch reactors in a controlled manner.Some examples of such reactions including Friedel-Crafts alkylation, cationic polymerization, and Swern oxidation are described in this article. Other examples are also demonstrated with the special emphasis on multiphase reactions. These examples speak well for the potentiality of microreactors in the enhancement of the ability of organic synthesis to meet demanding expectations in future.
Among non-conventional stimulations to accelerate organic reactions, high-pressure, ultrasound irradiation, and microwave irradiation are representative methods and are now becoming more and more popular in laboratories. This review focuses on organic reactions under microwave irradiation since this technique has attracted broad range of interest from the scientific and practical viewpoints. After the brief description on the historical and theoretical backgrounds of the dielectric heating, microwave effects, practical aspects of microwave heating (choice of solvent and microwave reactors), scope, and limitations of microwave-assisted synthetic reactions are discussed.
New palladium-catalyzed reactions recently discovered which involve β-carbon elimination and formation of palladacycles as key reactions are surveyed. Arylation of aromatic ring takes place by the reaction of α, α-disubstituted arylmethanols with aryl halides via β-carbon elimination. Novel methods of functionalization of aromatics were discovered by Catellani by the unique palladium and norbornene-catalyzed coupling reactions of aryl halides. Polyarylations of phenols and other aromatic rings with aryl halides proceed smoothly via facile formation of palladacycles due to the participation of OH groups. These reactions proceed via the following interconversion between Pd(0)-Pd(II)-Pd(IV) species.