Cations heterocyclic, review

Cations s. a. Radical cations -, heterocyclic, review 22, 261 suppl. 27... 

Thus, most ionic liquids are formed from cations that do not contain acidic protons. A summary of the applications and properties of ionic liquids may be found in a number of recent review articles [3]. The most common classes of cations are illustrated in Figure 2.1-1, although low melting point salts based on other cations, such as complex poly cationic amines [4] and heterocycle-containing drugs [5], have also been prepared. 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

The reactions of heterocyclic cationic substrates such as quinolinium ions have been treated in an authoritative article by Bunting.21 In the present chapter we shall be concerned with the formation of ff-adducts as obtained from neutral heteroaromatic substrates and nucleophilic reagents of all types. This matter has never been reviewed before but is included in part in a comprehensive review article by Terrier.22... 

Among the numerous classes of heterocyclic cations that are now actively studied, pyrylium salts are distinguished by the extraordinary variety of their transformations, making them good synthons and widely applicable. An exhaustive review on this subject has been published by an international group of chemists dealing with pyrylium chemistry [82AHC(Suppl.)]. 

The related family of phosphonio-phospholides also belong to the five-membered aromatic heterocycles, as reviewed comprehensively in the recent work of Gudat [209] The aromaticity of the rings were concluded from stabilization in isodesmic reactions [210] and also from the structure of those rings characterized by X-ray diffraction, exhibiting equalized bond length distribution in the case of the cationic 59 [211] and also for the neutral zwitterionic 60 [212], and its r - and r 5-complexed forms [214] (Scheme 34)... 

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