Heteroaromatic compounds directed metalation

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). ... 

Seregin IV, Gevorgyan V (2007) Direct transition metal-catalyzed functionalization of heteroaromatic compounds. Chem Soc Rev 36 1173-1193... 

Chemical shifts for C6F5M and C6F X5 BM derivatives (where M is a transition metal group directly attached to the fluoroaryl ring) are summarized in Table XVIII and with structures [342] to [351]. (193— 200) Compounds with heteroaromatic rings directly attached to the metal are included here. 

Direct transition metal[Pg.21]

Direct arylation of aromatic compounds by C-H activation with aryl halides by transition metal catalysis has become an important synthetic process. However until recently the reported procedures were limited to only electron-rich aromatic rings and heteroaromatic compounds. Biaryl compounds containing pyrazine were isolated in moderate to high yields when potassium... 

These and many other examples show a growing interest in both metal-catalyzed and metal-free C-H functionahzations of aromatic and heteroaromatic compounds. However, as mentioned in Sect. 3 of this chapter, this metal-catalyzed reactions are beyond our consideration in this book which focuses on direct nucleophilic attack on unsubstituted carbon of an activated aromatic ring. 

The chapter The SN -Amination of Aromatic Compounds , authored by Anna Gulevskaya and Alexander Pozharsky (Rostov-on-Don University, Russia), presents a comprehensive review on the direct Sn amination of electron-deficient heteroaromatic compounds. Recent advances in this area and many new aspects of the Sn amination are discussed, including new types of reagents, metal-free catalysts, solvents and the hydride ion acceptors. The review shows that the amination is rather promising synthetic alternative to both classic and transition metal-catalyzed amino-dehalogenation reactions. 

A variety of methods for the catalytic direct arylation of aromatic and heteroaromatic compounds via the cleavage of C—H bonds has been developed during recent years. As complementary synthetic tools for conventional cross-couplings in the preparation of biaryls and arylated heteroarenes that require neither stoichiometric metallation nor halogenation, these reactions may be both useful and economical in a variety of situations. Consequently, it is highly likely that a major effort will be made in the near future to enhance the catalytic efficiency and regioselectivity of these reactions. 

Once it was realized that heteroaromatic sulfonamides could be prepared and converted to sulfonylureas, and that these compounds possessed many of the desirable hebicidal qualities of the benzene sulfonylureas, a large synthetic effort was started to prepare more examples. Using thiophene and pyrazole as representative examples, we have been able to synthesize all of the positional isomers of the mono- and di-substituted sulfonamides. This paper will outline the major synthetic pathways that we have discovered, emphasizing directed metallation processes and nucleophilic substitution reactions, leading to the preparation of these heterocyclic sulfonamides. Many of tiie methods developed for thiophene and pyrazole sulfonamide synthesis have been extended to the synthesis of pyridine and other heteroaromatic sulfonamides. 

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