Development of Methods for Novel Construction of Ring Compounds by Environmentally Benign Transition Metal Catalysts

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The creation of novel transition metal complexes which show high catalytic performance is essential for development of highly atom-economical and environmentally benign synthetic methods. In this study, we first succeeded in preparation of novel Ru (0) complexes bearing arenes, p-quinones, tridentate pyridyl ligands, and maleimides via selective displacement of either1,3,5-cyclooctatriene (cot) or dimethyl fumarate (dmfm) ligands in Ru (η^6-1,3,5-cyclooctatriene) (η^2-dimethyl fumarate)_2 (1). In sharp contrast, a similar Ru (0) complex, Ru (η^4-1,5-cyclooctadiene) (η^6-1,3,5-cyclooctatriene) (2) smoothly reacted with maleimides to give novel divalent ruthenacyclopentane complexes (3) via oxidative cyclization of a cot ligand and maleimides on the ruthenium, which may realize the development of novel transition-metal catalyzed [6+2] cocyclization reaction.Based on the above described stoichiometric reactions, novel methods for construction of various ring compounds using environmentally benign ruthenium and rhodium catalysts have been developed in this study. Representative examples include 1) first ruthenium-catalyzed [2+2+1] cocyclization of isocyanates, alkynes, and carbon monoxide enables the rapid synthesis of polysubstituted maleimides, 2) rhodium-catalyzed cyclocotrimerization of isocyanates and alkynes leading to the selective formation of 2-pyridones and pyrimicline-2,4-diones by controlling the molar ratio of isocyanates and alkynes, 3) rhodium-catalyzed direct synthesis of substituted phenols from cyclobutenones and electron-deficient alkenes via C-C bond cleavage/dehydrogenation, and 4) selective codimerization and cotrimerization of different alkenes, as well as selective trimerization of ethylene, via ruthenacyclic intermediates. All reactions proceed with high atom-efficiency and give functional monomers without the formation of byproducts.