Heterocyclic compounds electron-deficient heterocycles

The structural principles and reaction chemistry of B-8 compounds have recently been reviewed. This includes not only electron-precise 4-, 5- and 6-membered heterocycles of the types described above, but also electron-deficient polyhedral clusters based on closo-. 

An interesting intermediate 30 was proposed to result from the sequential addition of pyridine to tetrachlorocyclopropene (31). Compound 30 represents an alkyl nitrogen ylide with two 1-chloroalkyl pyridinium moieties in the same molecule. Pyridines with electron-withdrawing groups and heterocycles with an electron-deficient nitrogen, for example, pyridine-3-carbaldehyde or quinoline, react with 31 to yield the corresponding mono-substituted products 32a and 32b (83JOC2629) (Scheme 8). 

The site of dihydroxylation in heterocycles depends on the nature of the heteroaromatic system (Scheme 9.31) usually, electron-rich heterocycles like thiophene are readily biooxidized but give conformationally labile products, vhich may undergo concomitant sulfoxidation [241]. Electron deficient systems are not accepted only pyridone derivatives give corresponding cis-diols [242]. Such a differentiated behavior is also observed for benzo-fused compounds biotransformation of benzo[b] thiophene gives dihydroxylation at the heterocyclic core as major product, while quinoline and other electron-poor systems are oxidized at the homoaromatic core, predominantly [243,244]. 

Cycloadditions and cyclization reactions are among the most important synthetic applications of donor-substituted allenes, since they result in the formation of a variety of carbocyclic and heterocyclic compounds. Early investigations of Diels-Alder reactions with alkoxyallenes demonstrated that harsh reaction conditions, e.g. high pressure, high temperature or Lewis acid promotion, are often required to afford the corresponding heterocycles in only poor to moderate yield [12b, 92-94]. Although a,/3-unsaturated carbonyl compounds have not been used extensively as heterodienes, considerable success has been achieved with activated enone 146 (Eq. 8.27) or with the electron-deficient tosylimine 148 (Eq. 8.28). Both dienes reacted under... 

The versatility of these [4+2] heterocyclization reactions is a consequence of the wide range of ene and diene components which can be used. In addition to alkenes and alkynes functioning as ene components, a variety of heterodienophiles is available such as electron-deficient imines (e.g. equation 89), nitriles e.g. equation 90), electrophilic carbonyl compounds (e.g. equation 91), thiocarbonyl compounds (e.g. equation 92), singlet oxygen (e.g. equation 93), nitroso compounds (e.g. equation 94), sulfenylsulfonamides (e.g. equation 95) and azo compounds (e.g. equation 96). Many of these reactions proceed with excellent regioselectivity and stereoselectivity, probably because in many instances they involve... 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

The chemistry of azaquinones centers around the electron-deficient imine double bond. For example, water, methanol, ammonia, methyl-amine, nitromethane, m-xylene, and enamines all add readily to the imine double bond in aza-3-phenyl-l, 4-naphthoquinone. On the other hand azaquinones are also potent dienophiles and thus can function as starting materials for a large variety of highly substituted new heterocyclic compounds. 

The 3-phospholene 1-oxide derivative is a potential heterocycle for easily producing chemically modified phosphorus heterocycles because the compound possesses a reactive C=C double bond, allylic methylene, an electron-deficient methylene group a-positioned to phosphorus, etc. 4-Chloro-l,6-dihydrophosphinie derivatives 16A and 16B are prepared from dichloro-carbene adducts 15 with l-(R)-3-ethylphospholene 1-oxide (Scheme 5) [4]. 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

The cr ./ -phospholes 3a [7b, 7d] and 3b [7c, 7d] (Scheme 12.1) bearing electron-rich and electron-deficient substituents, respectively, were characterized by x-ray diffraction studies (Figure 12.1). In spite of the different electronic natures of the two 2,5-substituents, compounds 3a and 3b share some important structural features in the solid state. Their molecular crystal structures reveal that the three heterocycles are almost coplanar while the phosphorus atom s environment is strongly pyramidalized [7b-7d]. 

Electronically excited carbonyl chromophores in ketones, aldehydes, amides, imides, or electron-deficient aromatic compounds may act as electron acceptors (A) versus alkenes, amines, carboxylates, carboxamides, and thioethers (D, donors). In addition, PET processes can also occur from aromatic rings with electron-donating groups to chloroacetamides. These reactions can be versatile procedures for the synthesis of nitrogen-containing heterocyclic compounds with six-membered (or larger) rings [2],... 

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