Heteroaromatic compounds reduction

The validity of the Hammett relationship log K/Ko = pa- has been extensively investigated for five-membered heteroaromatic compounds and their benzo analogues. The ratio Pheterocycie/Pbenzene is closest to Unity for thiophene. Judged from work on the polarographic reduction of nitro compounds, the ability to transmit electronic effects is HC=CH = S < O < NH. 

Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons 0), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for NO reduction (3) For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. 

The enantioselective hydrogenation of olefins, ketones and imines still represents an important topic and various highly enantioselective processes based on chiral Rh, Ru or Ir complexes have been reported. However, most of these catalysts failed to give satisfactory results in the asymmetric hydrogenation of aromatic and heteroaromatic compounds and examples of efficient catalysts are rare. This is especially the case for the partial reduction of quinoline derivatives which provide 1,2,3,4-tetrahydroquinolines, important synthetic intermediates in the preparation of pharmaceutical and agrochemical products. Additionally, many alkaloid natural products consist of this stmctural key element. 

The Brpnsted acid catalyzed hydrogenation of quinolines with Hantzsch dihydropyridine as reducing agent provides a direct access to a variety of substituted tetrahydroquinolines (Table 4.2). The mild reaction conditions of this metal-free reduction of heteroaromatic compounds, high yields, operational simplicity and practicability, broad scope, functional group tolerance and remarkably low catalyst loading render this environment-friendly process an attractive approach to optically active tetrahydroquinolines and their derivatives (Table 4.3) (see page 176). ... 

Polarography of Heterocyclic Compounds I, Polarographic Behavior of Heterocycles and Electrochemical Reduction of Heteroaromatic Compounds J. Stradins, V. Kadis and S. Hillers, Khim. Geterotsikl. Soedin., 1973, 1587-1603. 

The asymmetric reduction of aromatic and heteroaromatic compounds still represents a great challenge (Glorius 2005 Noyori 2002 Knowles 2002 Ohkuma et al. 2000 Ohkuma and Noyori 2004 Nishiyama and... 

Sometimes the polarographic behavior of heteroaromatic compounds is complicated by adsorption of depolarizer and/or products the adsorbability generally increases with the number of aromatic nuclei in the molecule. In the reduction of benzoyl pyiidines270 it was found that the adsorption of both depolarizer and products was important in acid solution, whereas only the former was adsorbed at high pH. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

In the laboratory of G.A. Posner, semisynthetic antimalarial trioxanes in the artemisinin family were prepared via an efficient Friedel-Crafts alkylation using a pyranosyi fluoride derived from the natural trioxane lactone artemisinin. The alkylating agent, pyranosyi fluoride, was prepared from the lactone in two steps reduction to the lactol followed by treatment with diethylaminosulfur trifluoride. The highly chemoselective alkylation was promoted by BF3-OEt2 and several electron-rich aromatic and heteroaromatic compounds were alkylated in moderate to high yield using this method. 

Cosmao, J. M., Collignon, N. and Queguiner, G. (1979) Alkylating reduction of some polynitrogen heteroaromatic compounds. J. Fleterocycl. Chem., 16, 973-976. 

Enantioselective Reduction of Nitrogen-Based Heteroaromatic Compounds... 

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