Recent advances in photochemical asymmetric syntheses through photosensitization with optically active compounds have been reviewed. In sharp contrast to the flourishing studies on thermal asymmetric synthesis, asymmetric photochmistry does not appear to have attracted much attention of synthetic organic chemists until very recently, except for the diastereodifferentiating photochemical reactions of optically active substrates' carrying chiral handles. This is simply because the other modes of asymmetric photochemistry do not provide satisfactory optical yields applicable to synthetic reactions However, recent efforts in the study of photosensitized asymmetric reactions via intimately interacting exciplexes not only materialized photochemical chirality multiplication with optical yields exceeding 90%, but also revealed that the product's chirality is totally inverted by changing the reaction temperature and consequently the optical yield increases with raising temperature beyond the equipodal temperature (T0). This unusual chirality switching by temperature has been attributed to the vital contribution of the entropy factor in the enantiodifferentiation process, which originates most probably from dynamic, synchronized conformational changes in the exciplex intermediate involving both sensitizer and substrate molecules. The present status and future perspective of photosensitized enantiodifferentiating geometrical isomerization, deracemization, [2+2] and [4+2] cycloadditions, and polar addition are discussed. Also presented are some tentative guidelines for getting higher optical yields in uni- and bimolecular asymmetric photosensitizations. This paper is dedicated to Prof. Dr. Hans-Dieter Scharf on the occasion of his 65th birthday.
Most homogeneous catalysts have higher selectivity but lower rates than heterogeneous catalysts in gas-phase reactions. An ideal catalyst would have the best of both worlds, high selectivity and high rates which may be possible with homogeneous catalysts in supercritical fluids rather than liquid solvents. These fluids are compressed gases heated past their critical point. Preliminary results have confirmed that increased rates and selectivities can be obtained because of several unusual properties of supercritical fluids : high solubility of reactant gases, rapid diffusion between phases, and possibly weaker solvation of the catalyst. This review will describe research on organic reactions and homogeneous catalysis in supercritical fluids, with emphasis on supercritical carbon dioxide and water as reaction media.
Rare earth metal trifluoromethanesulfonates (lanthanide and scandium triflates) were found to be stable Lewis acids in water and can act as Lewis acid catalysts in several carbon-carbon forming reactions In all cases, the reactions proceeded smoothly in the presence of a catalytic amount of the triflate under mild conditions in aqueous solution. For example, in the presence of a catalytic amount of rare earth metal triflate, formaldehyde in water solution (commercial formaldehyde solution) was activated and the hydroxymethylation reaction of silyl enol ethers proceeded smoothly Rare earth metal triflates were also quite effective in the aldol reaction of silyl enol ethers with aldehydes in aqueous media, and water-soluble aldehydes such as acetaldehyde, acrolein, and chloroacetaldehyde could be employed for direct use. Moreover, the allylation reaction of a wide variety of carbonyl compounds with tetraallyltin was successfully carried out in aqueous media by using scandium triflate as a catalyst. Unprotected sugars reacted directly to give the adducts in high yields. In all these reactions, lanthanide triflates were quantitatively recovered after the reactions were completed and could be reused.
In the past decade, in has been established that enzymes exhibit high catalytic activity as a solid state in organic solvents containing little or no water. Because organic solvents are better reaction media than water for most synthetic transformations, such has been applied to a number of organic reactions. This article is intended to provide mechanistic understanding of this phenomenon, which counters conventional wisdom. The key concept here is the high rigidity of protein's conformation under limited water content conditions : the balance of its thermodynamic stability and kinetic rigidity is essential for the enzyme's high catalytic activity and selectivities in organic media. Some applications for organic synthesis are also presented.
From a viewpoint of synthetic organic chemistry is discussed acid catalysis of ion-exchanged clay montmorillonites in some carbon-carbon bond-forming reactions, compared with acid catalysis of conventional homogeneous acids such as CF3SO3H and BF3·OEt2. Acidity of clay depends on the sort of cations exchanged in the clay. Tin (IV), iron (III), and aluminum (III) ion-exchanged montmorillonites show very strong acidity in organic media, promoting the carbon-carbon bond formation involving the reactions of carbonyl compounds with silicon nucleophiles such as cyanotrimethylsilane and silicon enolates. Sulfuric acid-treated montmorillonite, K 10 has been found to have mesopores in nanometers, acting as a suitable template to produce meso-tetraalkylporphyrins from aliphatic aldehydes and pyrroles via oligomerization and intramolecular cyclization in the nanospaces. The advantage of using solid acids over using homogeneous acids in organic synthesis is mentioned as well.
The role of weak intermolecular interactions in “crystal engineering” is briefly reviewed. The selective complexation of TNF (1) and mNBA (2) with 2, 6-DMN was studied in detail to test the validity of C-H··O hydrogen bonding in determining the molecular arrangement in multi-component organic solids. In order to find the new interacting sites in crystal 1, 2, 5-chalcogenadiazoles substituted with a dicyanomethylene group were designed. Systematic structural analyses of 4-12 indicated the presence of novel chalcogen-cyano interaction. This interaction is one of the sources of the directionality in crystal packing and caused the formation of “ribbon” -like networks in the anion-radical salts as well as the CT crystals of 7. By the network were formed the inclusion cavities in the latter, which can be used for the isomer recognition or as unique reaction centers for topochemical transformation. The single crystal-to-single crystal reaction occurred in a certain case, and the S··N≡C interaction was suggested to take part in maintaining the crystal matrix during the solid-state reaction.
Effects of high magnetic field (<14 T) on the primary process of organic photochemical reactions and related phenomena are reviewed. With increasing a magnetic field, lifetimes of triplet radical pairs in micellar solution and triplet biradicals in homogeneous solution increase significantly, reach their maximum values at ca. 2 T, and then decrease gradually up to 14 T. These results are interpreted in terms of the radical pair mechanism. The effects at low field are attributable to the reduction of spin transitions due to the isotropic and anisotropic hyperfine interaction. The inversion of the lifetime is attributable to the enhancement of spin relaxation induced by the anisotropic g-value.
Spontaneous polymerization is defined as the thermally initiated polymerization of pure monomers without initiators being added in the dark. The present review concerns with the spontaneous polymerizations of vinyl monomers in three kinds of anisotropic systems : a micellar solutions of some amphiphilic monomers capable of forming micelles, b emulsion of oil-soluble monomers such as styrene and methyl methacrylate with conventional surfactants, and c aqueous suspension of the oil-soluble monomers in the presence of water-soluble polymers (polysoaps). In a, as a feature of the polymerization systems, the monomer aggregation state has a very important role in controlling the generation of initiating radical species. In b and c, the monomers solubilized in the micelles formed by the surfactants or the polysoaps are assumed to be active for the radical formation. The generalized initiation mechanisms are discussed.
This article reviews the recent development of the weak-bond-induced organization of well-defined structures, or supramolecular self-assembly. This phenomenon has been showing remarkable potential to construct architectural structures, such as helices, macrocycles, cages, tubes, grids, interlocked systems, etc., from programmed components through weak bond interactions. Examples disclosed here show that, of many weak bonds, coordinate bonds are particularly useful for the supramolecular self-assembly owing to their versatile “algorisms” (e.g., linear, trigonal, square planer, tetrahedral, etc.) in bond formation.
Studies on hydrogen-atom abstraction from organic molecules in cryogenic solids are reviewed here. Hydrogen atoms generated in cryogenic solids are shown to abstract hydrogen atoms from organic molecules even at 4.2 K due to quantum-mechanical tunneling. The location of a hydrogen-abstracted carbon atom in an alkane molecule is determined not only by the activation energy for hydrogen abstraction but also by the degree of steric hindrance which prevents the deformation of the molecule accompanied by the abstraction. The decrease in the deformation slows down the dissipation of the heat of hydrogen abstraction to the product molecule. The rate of hydrogen abstraction thereby decreases with increasing steric hindrance which increases with increasing number and length of alkyl chains bonded to a carbon atom. The tertiary carbon atom of 3-methylpentane is therefore not hydrogen-abstracted, though it has the lowest activation energy for the abstraction and is the easiest to be hydrogen-abstracted in liquid.
Basic principles for the design of artificial ion channels which produce large ionic flux across lipid bilayer membranes were described. Several supramolecular approaches were successful to observe single ion channel currents across planar lipid bilayers with characteristics in a close analogy to those of natural ion channels. Stable and constant conductance levels were observed with transitions between open and closed states at a ms to s range. The channels were cation selective over anion. A unimolecular channel was also designed by using macrocyclic resorcinol tetramer which gave a single conductance and a sharp ion selectivity. Ionic currents across the membrane through ion channel mechanism were also modulated by the application of membrane potential or photochemical isomerization.