Development of methodology directed toward environmentally benign molecular transformations is one of the most important subjects in modern synthetic organic chemistry. As a promising method for environmentally benign transformation of organic molecules, we have focused on the transition metal catalyzed "Hydrogen Transfer Reactions". Here, we report our recent studies on hydrogen transfer reactions catalyzed by group 9 metal complexes bearing Cp*(η5-pentamethyl-cyclopentadienyl) ligand and their application to organic synthesis, which include (1) Cp*Ir complex-catalyzed oxidation of alcohols, (2) Cp Ir complex-catalyzed transfer hydrogenation of nitrogen heteroaromatics using 2-propanol as a hydrogen donor, (3) Cp Ir complex-catalyzed carbon-nitrogen and carbon-carbon bond formation using alcohols as alkylating reagents, (4) Cp Ir and Cp Rh complex-catalyzed synthesis of nitrogen heterocycles by intra- and intermolecular carbon-nitrogen bond formation.
Metal-mediated or -catalyzed retro-allylation of homoallyl alcohol has emerged as a new useful method for preparing allylmetals. The metal alkoxides of well-designed homoallyl alcohols undergo retro-allylation via chairlike six-membered transition states. Thanks to the rigid transition states, the retro-allylation reactions proceed regio- and stereo specifically, yielding regio- and stereochemically well-defined allylmetals. The allylmetals thus generated are used in situ, which can afford allylated products with high selectivity. Retro-allylation reactions mediated by zirconium, gallium, palladium, and rhodium are described. Reversible additions of pentamethylcyclopentadienyl anion to carbonyl compounds are also described.
Marine polycyclic ethers exemplified by brevetoxin B, a red tide toxin, have a unique trans-fused polycyclic ether ring system. Their synthetically challenging complex structures and potent bioactivities have attracted the attention of numerous synthetic organic chemists. We have already developed several efficient methods for synthesis of cyclic ethers based on the SmI2-induced reductive cyclization of β-alkoxyacrylate, β-alkoxyvinyl sulfone, and β-alkoxyvinyl sulfoxide with high stereoselectivity. Iterative and two-directional strategies for the construction of polycyclic ethers were developed based on the above methods. Several convergent strategies for the synthesis of polycyclic ethers were also developed based on acetylide-triflate coupling, acetylide-aldehyde coupling, or intramolecular Barbier reaction, respectively. Total syntheses of brevetoxin B, mucocin, and pyragonicin were accomplished as application of the developed methods.
Poly(substituted methylene) synthesis (PSMS) is a method for preparing carbon-carbon main chain polymers, where the main chain is constructed from one carbon unit. If we can find monomers and initiators suitable for the process, PSMS could be a powerful tool for polymer synthesis, providing a method for preparing C-C main chain polymers which cannot be obtained by vinyl polymerization. This review describes recent development for PSMS, particularly focusing on transition metal initiated polymerization of diazocarbonyl compounds, which can afford polymers bearing polar substituents such as ester and acyl groups on all the main chain carbons. A variety of poly(substituted methylene) can be prepared by Pd-mediated (co) polymerization of the monomers with various substituents on their carbonyl carbons, although the molecular weights of the products are a few thousands. Pd-mediated and thermally-induced copolymerizations of diazocarbonyl compounds with alkynes and styrene, respectively, are also possible. On the other hand, Rh-mediated polymerization of ethyl diazoacetate proceeds in a highly stereospecific manner, giving syndiotactic poly(ethoxycarbonyl methylene)s with high molecular weights (Mn100000).
Introduction of lateral intermolecular interactions (dipole-dipole interactions, hydrogen bonds, and perfluoroarene-arene interactions) are attempted in rodlike and half-disklike molecules. In the cases of dicyanothiophenes, introduction of long cores stabilized their liquid crystal phases to give smectic and columnar phases, though the large lateral dipole in short rodlike molecules suppressed the molecular movements to destabilize the liquid crystal phases. Trialkoxybenzoic anhydrides were also organized into columnar aggregates by intermolecular dipole-dipole interaction. Further, molecules introduced a large lateral dipole at the center were useful for phase transition from smectic C to smectic CA phase. On the other hand, introduction of intermolecular hydrogen bonds in the direction of the molecular short axis generated novel smectic and cubic phases with strong hydrogen bonding. In the case of the liquid crystalline ureas, a switching behavior of the polarities was observed. Perfluoroarene-arene interaction was also useful to organize small molecules into a columnar superstructure.
N-heterocyclic carbenes have been studied for their abilities to catalyze C-C bond formation/cleavage and asymmetric reactions. N-heterocyclic carbenes generated from imidazolium, benzimidazolium, triazolium, pyrido[1, 2-c]imidazolium, pyrido[2, 1-c]triazolium, and dipyrido[1, 2-c : 2'1'-e]imidazolium iodide were found to be effective catalysts in benzoin condensation. The acidity of the azolium salts at the C2-hydrogen of the imidazolium or the triazolium moieties relates to the base effect on the reaction. These carbenes can also be employed in retro-benzoin condensation, which is a new method of synthesizing ketones by C-C bond cleavage of α-substituted benzoins. Chloro and fluoro groups of haloarenes are nucleophilically substituted by aroyl groups originated from aromatic aldehydes by catalytic action of imidazolidenyl and triazolidenyl carbenes to afford ketones. N-heterocyclic carbenes mediate the addition of trimethylsilylcyanide to aldehydes to yield cyanohydrin trimethylsilyl ethers. The use of chiral imidazolidenyl carbene derived from (R, R)-1, 3-bis[(1-naphthyl)ethyl] imidazolium chloride led to enantioseletive cyanosilylation. C2-symmetric imidazolidenyl carbenes catalyze asymmetric acylation of racemic secondary alcohols.
Recent development of synthetic studies on novel marine alkaloids, haouamine A and B, is described. Haouamines have attracted considerable attention of synthetic chemists because of their unique structure and selective activity against human colon carcinoma cell line HT-29, however, only two groups have so far reported successful formation of their highly strained 3-aza--paracyclophane core. This review focuses on synthetic efforts to construct the core skeleton of haouamines reported by Baran and Wipf.