Carbon-hydrogen bond activation which hopefully enables to directly utilize natural hydrocarbon resources is a subject of considerable importance in the future chemical industry. In particular, organometallic chemistry involving carbon-hydrogen bond activation is a field which is experiencing vigorous growth today. The article reviews the current status of the following aspects in this challenging field; 1) methodology to activate carbonhydrogen bonds, 2) selectivity and thermodynamic considerations, and 3) applications to functionalize hydrocarbons in the presence of transition metal complexes as reagents or catalysts.
A general and convenient method for the stereo-and regioselective synthesis of conjugated alkadienes, alkenynes, arylated alkenes, and heterobiaryls is described. The reaction of (E) -or (Z) -1-alkenylboronates obtainable by hydroboration, with either (E) -or (Z) -1-alkenyl halides in the presence of a catalytic amount of tetrakis (triphenylphosphine) palladium and bases such as sodium alkoxides gives the corresponding (E, E) -, (E, Z) - (Z, E) -and (Z, Z) -alkadienes stereo-and regioselectively, while retaining the configuration of both the starting alkenylboronates and haloalkenes The similar reactions of (E) -and (Z) -1-alkenylboronates with 1-alkynyl, aryl, allylic, and benzylic halides also provide the corresponding couplig products stereosoelectively. The versatility of this method has been demonstrated by the synthesis of some natural products bearing conjugated alkadiene structures. The reaction of arylboronic acids with aryl halides to give unsymmetrical biaryls is also presented. A mechanism of this cross-coupling reaction, which involves the transmetallation between a 1-alkenylborane and an alkoxopalladium (II) complex generated through the metathetical displacement of a halogen atom of RPd (II) X with sodium alkoxide, is proposed.
Origin and application of oxidative cleavages of the silicon-carbon bonds have been reviewed.While siliconcarbon bonds have been well known to be extremely stable to oxidizing agents or aerobic oxidation, we have recently found facile oxidative cleavages of the silicon-carbon bonds in certain silafunctional compounds by hydrogen peroxide, peracid, and dioxygen.The presence of at least one functional group (hetero atom) on silicon is essential for the oxidation.Fluoride ions promote the oxidation. The article includes the following topics. (1) An important role of penta- and hexa-coordinate species for the activation of the silicon-carbon bond toward oxidizing agents. (2) Characteristic features, stereochemistry, and general scope- and-limitations of the hydrogen peroxide oxidation. (3) Applications of the hydrogen peroxide oxidation to regio- and stereo-selective synthesis of polyhydroxylated molecules by the use of multi-functional silicon reagents.Hydrosilanes, silyl anions, silylmethyl Grignard reagents, silylmethyl bromide, 1-bromovinylsilanes, and allylsilanes have been used to be synthons of HO (+) , HO (-) , HOCH2 (-) , HOCH2 (·) , MeCO (·) , α-hydroxyallyl anion etc. (4) Oxidation by dioxygen under a biomimetic flavin-based redox condition.
The role of reversible oxygen-binding in hemoglobin is appeared since heme is fixed in the higher ordered structure of globin protein. In this review, the newly synthesized heme derivatives, so-called “lipid-heme” embedded in polymerized assembly of phospholipid molecules are explained, which is firstly able to realize oxygen transport in aqueous system comparable to the transporting efficiency of hemoglobin. Also the preliminary results of in vivo tests for aqueous (lipid-heme/polylipid-liposome) systems are added briefly
Recent advances in the chemistry of 3- sulfolene, 2, 5- dihydrothiophene 1, 1-dioxide, are reviewed from a view point of diene synthesis. Reaction of 3- sulfolene α- carbanion generated under a variety of basic conditions with various electrophiles followed by a stereoselective desulfonylation provides a new method for synthesizing (E) -, (E, Z) -, and (E, E) -conjugated dienes with extremely high stereoselectivity. The method is applied to the syntheses of various natural products having a conjugated diene structure. 3-Sulfolenes are used as masked dienes in the syntheses of cyclic compounds including natural products via inter- and intramolecular Diels-Alder reactions. Some other developments of the chemistry of 3-sulfolenes and its benzo analogs, benzo [c] -1, 3-dihydrothiophene 2, 2-dioxides, are briefly reviewed.
Stereoselective routes to conjugated unsaturated compounds from sec-propargylic alcohols (1) are described. Thermal treatments of β-allenic esters, prepared by the reaction of 1 with trialkyl orthoacetates, with alumina gave (2 E, 4 Z) -alkadienoic esters with high stereoselectivity. β-Allenic amides also rearranged with alumina to give (2 E, 4 Z) -alkadienoic amides. Direct oxidation of 2, 4-alkadienoates with SeO2 afforded furans and selenophenes. Iodolactonization of, β-allenic acids gave γ-alkylidene butenolide in one step. Stereoselective rearrangements of 2, 4-alkadienoates to 3, 5-alkadienoates with lithium diisopropylamide are described. Application of (2 E 4Z) -dienoates to the synthesis of natural products such as insect pheromones, flavors, octadecapolyenoic acids and leukotriene A4 methyl ester are described. Base-catalyzed condensation of sec-pro-pargylic alcohols with 1-cyanopyrrolidine gave imidates, which undergo thermal rearrangement to give pyridones or dienyl-ureas. Reaction of 1-alkyn-3-ol with benzonitrile gave 4-methylene-4, 5-dihydrooxazoles, which serve as useful intermediates for the synthesis of highly functionalized oxazoles.