Formals, aromatic

This is by far the most used type of primary synthesis for quinoxalines. It usually involves the cyclocondensation of an o-phenylenediamine (or closely related substrate) with a synthon containing an oxalyl [—C(=0)—C(=0)—] or equivalent [e.g., HC(=0)—C=N] grouping. For convenience, discussion of this synthesis is subdivided according to the type of synthon used to produce formally aromatic quinoxalines the formation of similar ring-reduced quinoxalines (mostly from related synthons at a lower oxidation state) is included in each such category. 

By contrast, in the formally aromatic 277-electron system of 9886 [Eq. (20)] the average Ge-Ge bond distance is 2.326(4) A (98a) and 2.335(2) A (98b), thus 0.1 A shorter than in the anion 97.81 The three germanium atoms of this free germyl cation form an equilateral triangle. 

Known 1,2,3-dithiazoles are represented by the structures in row (a) of Figure 1. Strictly, these are 1,2,3-dithiazolines, the first one being 1,2,3-dithiazoline-5-one. 1,2,5-Oxathiazoles are represented by l,2,5-oxathiazolidine-4-ones (b) and 3-methylene 1,2,5-oxathiazolines (c). 1,2,3-Oxathiazoles have been obtained only in the form of their S-oxides (d), which should rather be termed -azolidines and -azolidinones. Experimentally prepared S-oxides of 1,2,3-dithiazoles and 1,2,5-oxathiazoles are depicted in row (e) and should be termed dithiazolines and oxathiazolines. 1,2,3-Dithiazolyl radicals (f) are known, as well as 1,2,3-dithiazolium cations (g) which are the only formally aromatic examples of the ring system and which may be represented by a number of resonance structures (h). 

This contribution first surveys some of the attractive properties of boron, briefly describing applications that have been developed mostly with non-aromatic boron-containing compounds. It then examines many of the stable, formally aromatic boron heterocycles that have been reported to dale, covering much of the pertinent literature through the end of 1999. With the sum of these two parts, I hope the reader will gain an appreciation of the untapped potential held by boron heterocycles, especially for constructing new bioactive agents. 

Despite its unfavorable NMR properties, the nO nucleus has attracted considerable interest, since its chemical shifts represent a discriminating probe for structural and molecular properties. In a study of some 5-membered heterocycles (furan and isoxazole methyl derivatives) (840MR(22)55) it was found that the nO chemical shifts are mainly determined by the p-electron density on the oxygen atom. A nO downfield shift of 222 ppm is observed on the formal aromatization of tetrahydrofuran to furan (61HCA865). 

Benzo-fusion to a pyrylium ring can give rise to two isomers, namely the 1- and 2-benzopyrylium ions (6) and (219) both of which are formally aromatic 107r- electron systems. 

Jenks et al. studied the effects of conjugation and aromaticity on the sulfoxide bond by means of ab initio computation <1996JOC1275>. They calculated S-O bond dissociation energies (BDEs) and found that, in a formally aromatic system such as thiophene sulfoxide, the SO BDE is decreased by as much as 25kcalmoP relative to the BDE of DMSO. Although the BDE of the formally antiaromatic thiirene sulfoxide increased by about 15 kcal moP the authors concluded, on the basis of calculated geometries and isodesmic reactions with pure hydrocarbons, that cyclic unsaturated sulfoxides are neither significantly aromatic nor antiaromatic. 

In spite of the fact that it is not formally aromatic, the bisvinylogous porphyrin 4.187 has shown promise as a ligand for the coordination of two metal... 

Both photolysis and thermolysis of 5-diazouracils 132 result in the formation of formal aromatic C—H insertion products, the corresponding in-dolo[2,3-d]pyridines 133, in good yields (77H1911). 

Fusion of cyclobutadiene to another 4n w-system leads to a rr-cycle containing a total number of (4n + 2) rr-electrons, a formally aromatic arrangement. Both 4S60 and 4961 (the latter in the charge-separated form 49 b) could be viewed as planar examples of this class of compounds. Although rather reactive, both 48 and 49 exhibit NMR spectra with unusually low chemical shifts for the cyclobutenyl and... 

Thiadiazines have been the subject of numerous review articles.1 In this section, only the formally aromatic or antiaromatic thiadiazines of the types A, B, D, and E are discussed. On the basis of formula C2, the 1,2,5-thiadiazines may be considered as antiaromatic compounds, but their chemistry involves mainly the hydrogenated derivatives and is therefore not included here. 

In this review an aromatic compound is any five- or six-membered cyclic structure that is formally aromatic or that one of its tautomers or resonance forms is aromatic (76M11), that is, not only p nidine but also pyridones, and not only thiazole but also thiazolinethiones. We will include systems, like phthalimide, that although nonaromatic have all its ring atoms in a plane or close to it, that is, the critical condition is the absence of sp atoms in the ring, thus 3,5-dihydrox5 yrazoles will be considered but their tautomers pyrazolidine-3,5-diones will not. 

A variety of biochemical pathways are known which may lead to reactive quinoid derivatives. They include dihydroxylation of aromatic or heterocyclic compounds and epoxide formation and hydrolysis to -diphenolic compounds (Booth and Boyland 1957) o- and p-hydroxylations of phenols or arylamines (In-SCOE et al. 1965 Miller et al. 1960 Booth and Boyland 1957) and rearrangement of -hydroxyarylamines to o-aminophenols (Miller and Miller 1960). It now appears that aromatic hydroxylations proceed via highly reactive arene oxides, i.e., compounds in which a formal aromatic double bond has undergone epoxidation. Depending on the compound, arene oxides may give rise to other electrophilic reactive species, including quinoid structures, but react as such readily with nucleophiles and thus provide a basis for understanding covalent attachment of aromatic hydrocarbon derivatives to protein and nucleic acids (Jerina and Daly 1974). 

Kurbatov et al. [122] studied properties of 4-nitrobenzodifuroxan 139 (NBDF) and found that, in spite of the formal aromatic structure, the C=C-N02 fragment has pronounced nitroalkene character. NBDF was found to undergo cycloaddition with dienes (in accord with NED), and, as a heterodiene, it is capable of reacting with ethyl vinyl ether (in accord with lED) to give polycondensed heterocycles 140 and 141, respectively (Scheme 70). 

The a-(phenylsulfonyl)- and a-(ethoxyphosphoryl)-diazoacetamides 84d/e are exclusively converted to formal aromatic C-H insertion products 86d/e upon rhodium(II) perfluorobutyramide (Rh2(pfb)2> catalysis. The unsubstituted diazoacetamide 84a affords exclusively the Buchner ring expansion product 85a, and the Buchner reaction remains the favorable pathway with diazo substrates 84b/c, which bear relatively small a-substituents. The predominant formation of the Buchner products in these cases can be rationalized on the basis of steric effects. Various isoquinolinones are synthesized intramolecularly via six-membered ring formation with high regioelectivity and diastereoselectivity, while averting the common Buchner reaction. 

Ferrocene is also called "dicyclopentadienyl iron" which accurately describes its sandwich structure. What may not be apparent is that the molecule is quite nonpolar it is soluble in hexane. The two cyclopentadienyl rings are formally aromatic, six-electron systems and as such are extremely electron rich. The ferrocene system can be functionalized by electrophilic aromatic substitution chemistry. The property of ferrocene most relevant to our efforts, however, is that an electron may be lost reversibly from iron [Fe(II) = e + Fe(ni)] so that the entire molecule becomes positively charged. The half wave potential for ferrocene is observed at about 400 mV (vs. calomel) in aqueous solution and the ferricinium species is stable in water for hours. This contrasts, for example with nitrobenzene or most quinones, which are generally less to much less stable than this upon reduction. These properties make ferrocene particularly attractive for applications in redox-altered chemisuy. 

Pentaphenylborole, a formally anti-aromatic compound that can be prepared by reaction of 1,1-dibutyl-2,3,4,5-tetraphenylstannole with dichloro(phenyl)-borane, is reduced by potassium to the formally aromatic dianion, and acts as a four-electron ligand in forming transition-metal complexes. ... 

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