Aromaticity aromatic carbocycles

Annelation increases the complexity of the spectra just as it does in the carbocyclic series, and the spectra are not unlike those of the aromatic carbocycle obtained by formally replacing the heteroatom by two aromatic carbon atoms (—CH=CH—). Although quantitatively less marked, the same trend for the longest wavelength band to undergo a bathochromic shift in the heteroatom sequence O < NH < S < Se < Te is discernible in the spectra of the benzo[Z>] heterocycles (Table 17). As might perhaps have been anticipated, the effect of the fusion of a second benzenoid ring on to these heterocycles is to reduce further the differences in their spectroscopic properties (cf. Table 18). The absorption of the benzo[c]... 

SCHEME 6. Diels-Alder reactions with benzoid and nonbenzoid aromatic carbocycles as diene... 

Aromatic carbocycles, 9 267-283 Aromatic carboxylic acids, microbial reduction of, 16 402... 

Formation of Six-Membered, Non-Aromatic Carbocycles and Six-Membered Heterocycles... 

All known examples of the Bart and Bechamp reactions in which the product contains an arsonic or arsinic acid group attached directly to an aromatic carbocyclic ring structure have been included. For example, quinoline arsonic acids are included if the arsono group is linked to the 5-, 6-, 7-, or 8-position but are not included if the attachment occurs in the 2-, 3-, or 4-position. 

Ultraviolet absorption spectra have been used for many years in a qualitative way to indicate similarities in the bonding patterns of different compounds. The recorded UV spectra of azoloazines are characterized by a band at 250-280 nm which is ascribed to a n-n transition and which finds similarity in indole as well as other aromatic carbocyclic and heterocyclic compounds (84CHEC-1(4)497). 

Several saturated parent systems shown in Scheme 1, especially (2), (4) and (7), have been incorporated into spiro compounds or have been fused with a variety of non-aromatic carbocyclic or heterocyclic rings. Unsaturated compounds with an endocyclic carbon-carbon double bond, formally possible in all systems with three adjacent heteroatoms, are only known in very few cases (e.g. cyclic sulfites and sulfates of enediols) or in the form of benzo or other condensed systems (mainly cyclic sulfites and sulfates of aromatic 1,2-dihydroxy compounds). For details concerning the various types of derivatives, see Section 4.33.4.2. 

Cyclic 7r-electron conjugation in aromatic carbocycles equalizes the lengths of the ring bonds. On the substitution of a ring carbon atom by a heteroatom, the even distribution of 7r-electron density becomes distorted, conjugation is partially... 

Systems with a fused aromatic carbocyclic ring which have been studied, either theoretically or synthetically, in the period under review include l//-l,3-benzazaphospholes 14 and 15, [l,3]azaphospholo[l,5- ]pyridine 16, [l,3]azapho-spholo[l,5- ]isoquinoline 17, and [l,3]azaphospholo[l,5- 7]quinoline 18. 

In order to obtain analogs of 1,2-dihydroisoquinolines in which the aromatic carbocycle is replaced by various heteroaromatic systems, the furopyridines (19), the thieno[3,2-c]pyridines (20), and the thieno[2,3-c]-pyridines (21) were synthesized (70BSB301 71JHC57). By Grignard coupling of 4-chloro-19 with allylmagnesium chloride in the presence of catalytic... 

Organochlorine insecticides are chlorinated hydrocarbon compounds that fall into three basic structure classifications aryl (aromatic), carbocyclic, and heterocyclic. 

VI. REARRANGEMENTS OF NON-AROMATIC CARBOCYCLES A. DIenone-Phenol Rearrangements... 

Palladium chemistry involving heterocycles has many unique characteristics stemming from the inherently different structural and electronic properties of heterocyclic molecules in comparison to the corresponding aromatic carbocycles. One salient feature of heterocycles is the marked activation at positions a and y to the heteroatom. For N-containing heterocycles, the presence of the N-atom polarizes the aromatic ring, thereby activating the ex and y positions, making them more prone to nucleophilic attack. For example, the order of S Ar displacement of heteroaryl hahdes with EtO is [3] ... 

One class of transformations that illustrate the striking difference in reactivity between heteroarenes and carbocyclic arenes is the heteroaryl Heck reaction, in which an aryl or heteroaryl halide is coupled directly with a heteroaromatic compound to afford a biaryl product (formally a C—H bond functionalization process). Intermolecular Heck reactions involving the functionalization of aromatic carbocycles with aryVheteroaryl halides are rare [70], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles. 

Scheme 22 Aromatic carbocycles and oxepines synthesized via ortho-alkylation/Mizoroki-Heck coupling...

There are an immense number of examples of aldol condensations of aromatic carbocyclic ketones with aromatic aldehydes. These systems usually give excellent yields of aldol condensation products, because the starting ketone can usually enolize in only one direction and because the ketones do not afford... 

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