A versatile reducing agent, diimide, can be generated efficiently by the aerobic oxidation of hydrazine with neutral and cationic synthetic flavin catalysts 1 and 2. This technique provides a convenient and safe method for the aerobic reduction of olefins, which proceeds with 1 equiv of hydrazine under an atmosphere of O2 or air. The synthetic advantage over the conventional gas-based method has been illustrated through high hydrazine efficiency, easy and safe handling, and characteristic chemoselectivity. Vitamin B2 derivative 6 acts as a highly practical, robust catalyst for this purpose because of its high availability and recyclability. Association complexes of 1 b with dendritic 2,5-bis(acylamino)pyridine 15 exhibit unprecedented catalytic activities, with the reduction of aromatic and hydroxy olefins proceeding significantly faster when a higher-generation dendrimer is used as a host pair for the association catalysts. Contrasting retardation is observed upon similar treatment of non-aromatic or non-hydroxy olefins with the dendrimer catalysts. Control experiments and kinetic studies revealed that these catalytic reactions include two independent, anaerobic and aerobic, processes for the generation of diimide from hydrazine. Positive and negative dendrimer effects on the catalytic reactions have been ascribed to the specific inclusion of hydrazine and olefinic substrates into the enzyme-like reaction cavities of the association complex catalysts.
Recent progress in the development of flavin-catalyzed oxidations and related reactions is described with respect to scope, limitation, and reaction mechanism. The 4a-hydroperoxyflavins, which are the most simplified model compounds of flavoenzymes, act as catalytically active species for the oxidation of organic substrates with the help of H2O2 or O2 as a mild oxidant. This principle behind the simulation of flavoenzymes led to the discovery of a variety of environmentally benign, oxidative transformations of secondary amines to nitrones, tertiary amines to N-oxides, sulfides to sulfoxides, and Baeyer–Villiger oxidations of ketones. Asymmetric oxidation of sulfides can also be performed with several chiral flavin catalysts. One of the fortunate outcomes of this study is the development of an environmentally friendly (“green”) method for the “aerobic hydrogenation” of olefins, which is achieved by in situ generation of diimide with the aid of the flavin-catalyzed oxidation of hydrazine under an O2 atmosphere. © 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 354–361; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20135
Flavin-catalyzed green oxidation of heteroatom compounds such as sulfides and amines with molecular oxygen and even air in the presence of hydrazine monohydrate in a fluorous solvent such as 2,2,2-trifluoroethanol at room temperature gives the corresponding oxidation products highly efficiently and selectively along with water and molecular nitrogen, which are environmentally benign by-products. The proposed reaction mechanism is based on the kinetics, solvent effect, and redox properties of flavin catalysts.
Highly chemoselective Baeyer–Villiger oxidations can be performed in the presence of other reactive functionalities such as alcohols, olefins, and sulfides, which would undergo electrophilic oxidation under conventional conditions (see scheme). [DMRFlEt]+[ClO4]− (depicted blue) is a new class of flavin compound that catalyzes aerobic Baeyer–Villiger oxidations in the presence of Zn dust as the electron source.
Empfindliche Substrate wie Alkohole, Olefine und Sulfide, die unter gewöhnlichen Baeyer-Villiger-Bedingungen elektrophile Oxidationen eingehen würden, bleiben in einer hoch chemoselektiven katalytischen Baeyer-Villiger-Oxidation unversehrt (siehe Schema). Die Flavin-Verbindung [DMRFlEt]+[ClO4]− (blau) katalysiert die aerobe Baeyer-Villiger-Oxidation in Gegenwart von Zinkstaub.
Der chirale Organokatalysator Bisflavin 1 katalysiert asymmetrische Baeyer-Villiger-Reaktionen von Cyclobutanonen mit H2O2 (siehe Schema). Die entsprechenden Lactone werden mit bis zu 74 % ee erhalten.
The chiral organocatalyst bisflavin 1 catalyzes the asymmetric Baeyer–Villiger reaction of cyclobutanones with H2O2 (see scheme). The corresponding lactones are obtained with up to 74 % ee.
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