Heterocycles definition

Classification Aromatic heterocyclic Definition Ester of benzyl alcohol and nicotinic acid Empirical C13H11NO2... 

Ciassification 6-membered aromatic heterocyclic Definition Avail, commercially as the hydrochloride, phosphate, or tripalmitate derivs. Empihcai CsHnNOe... 

Constable EC. Nucleophihc attack upon coordinated heterocycles definitive evidence for the enhanced electrophilicity of coordinated pyridines. Inorg Chim Acta. 1984 82 53-57. 

Constable, E.C. (1984) Nucleophilic attack upon coordinated heterocycles definitive evidence for the enhanced electrophilicity of coordinated pyridines, Inorg, Chim, Acta, 82, 53-57 Constable, E.C. and Leese, T.A. (1988) Ligand reactivity in polypyridine complexes [Ru(bipy)3] " analogs incorporating pendant polyamine substituents, Inorg, Chim, Acta, 146, 55-58... 

Condensa.tlon, This term covers all processes, not previously iacluded ia other process definitions, where water or hydrogen chloride is eliminated ia a reaction involving the combination of two or more molecules. The important condensation reactions are nitrogen and sulfur heterocycle formation, amide formation from acid chlorides, formation of substituted diphenyl amines, and misceUaneous cyclizations. 

The term mesoionic was suggested originally to describe compounds of the sydnone (129) or sydnone imine (130) type (49JCS307, 50JCS1542). Since then this term has been applied somewhat indiscriminately to a variety of heterocyclic molecules which can be represented formally as zwitterions. The original, rather restrictive definition put forward by Baker and Ollis (55CI(L)910) was later made even more so by Ollis and Ramsden (76AHC(19)1), and it is this most recent definition which now appears to be the most useful. 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

Since Huisgen s definition of the general concepts of 1,3-dipolar cycloaddition, this class of reaction has been used extensively in organic synthesis. Nitro compounds can participate in 1,3-dipolar cycloaddition as sources of 1,3-dipoles such as nitronates or nitroxides. Because the reaction of nitrones can be compared with that of nitronates, recent development of nitrones in organic synthesis is briefly summarized. 1,3-Dipolar cycloadditions to a double bond or a triple bond lead to five-membered heterocyclic compounds (Scheme 8.12). There are many excellent reviews on 1,3-dipolar cycloaddition, in particular, the monograph by Torssell covers this topic comprehensively. This chapter describes only recent progress in this field. Many papers have appeared after the comprehensive monograph by Torssell. Here, the natural product synthesis and asymmetric 1,3-dipolar cycloaddition are emphasized.630 Synthesis of pyrrolidine and -izidine alkaloids based on cycloaddition reactions are also discussed in this chapter. 

Rings incorporating [4 +2] rt-electrons are aromatic according to the Hiickel definition and on this basis 1,2,3-thiadiazoles can be considered as aromatic. This is supported by 13C and 111 NMR chemical shifts. In 1990, the aromaticities of some five- and six-membered ring heterocycles including 1,2,3-thiadiazole were studied by computational methods and found to correlate well with their chemical natures <1990JPR885>. 

In the last ten years arylation has been tbe most studied homolytic aromatic substitution, also in the heteroaromatic series. Numerous data concerning a large variety of heterocycles have permitted the definition of many details for the individual substrates, without adding, however, anything particularly new as regards the general characteristics of the reaction, already outlined in the previous review of Norman and Radda. These characteristics are substantially the same as those observed in the homocyclic aromatic series, for which comprehensive reviews are available. There is therefore a sharp difference in behavior between arylation and other homolytic substitutions described in the previous sections. These latter have quite different characteristics, and sometimes they are not known, in the homocyclic series. 

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

Although this was not appreciated at the time, these compounds were first handled by Busch and Becker in 1896. Their study continued until a more extensive report by Busch and Schmidt was published in 1929,195 However, Busch and his co-workers were apparently unaware of some closely related work reported by von Pechmann in 1896. These earlier studies have been interpreted by Farrar, extended by Christopherson and Treppendahl, and brought to a definitive conclusion by the Sheffield group. This history is an interesting facet of the development of the chemistry of meso-ionic heterocycles. 

In the definition of the term meso-ionic as originally proposed by Baker and Ollis, it was stated that a compound may be appropriately called meso-ionic if it is a five- or possibly a six-membered heterocyclic compound which cannot be represent satisfactorily by any one covrdent or polar structure and possesses a sextet of electrons in association with the atoms comprising the ring. This definition has formed the basis of this review, but we now propose a modification. 

One hundred and forty-four meso-ionic heterocycles of type A (13, 19) and 84 meso-ionic heterocycles of type B (14, 20) are possible. The numbers of these two types which are now known (Table I type A, 44 representatives) and (Table II type B, 8 representatives) encourage us to put forward the proposal that the term meso-ionic should in future be restricted to five-membered heterocycles belonging to type A (13, 19) and type B (14,20). This clear restriction upon the use of the term meso-ionic has obvious advantages. It still embraces 228 different classes of heterocycles with a common structural characteristic, and the many types of meso-ionic compounds included in the authoritative review by Ohta and Kato " are included. Needless to say, the restriction upon the definition of the term meso-ionic to five-mem red heterocycles of type A and type B still includes, for example, benz derivatives such as the compounds 67, 71, 110, 123, 133, 151, 206, 209, 226, 255, and 258. 

We must note that we are dealing here not with static molecules, as no molecule is stationary even at the absolute zero of temperature, but rather with non-reacting molecules. This will be extended, however, to include mass spectrometry and the reactions which proceed within the mass spectrometry tube, as these are used to define the structure of the parent molecule. Obviously, though, such reactions have an importance of their own which is not neglected. Details of species involved as reactive intermediates, which may exist long enough for definition by physical techniques, will also be considered. For example, the section on ESR (Section 2.04.3.7) necessarily looks at unpaired electron species such as neutral or charged radicals, while that on UV spectroscopy (Section 2.04.3.3) considers the structure of electronically excited heterocyclic molecules. 

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