Carbocycles, planar

The structural elucidation of many azacyclooctatetraenes reveals that the tub conformation is the preferred conformation of the eight-membered-ring. As in the case of the carbocyclic analog, the compounds avoid a planar situation with an antiaromatic, 8 n-electron system and exhibit rather normal polyolefinic character. 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

From the results we have obtained for the systems C,H4, Q,H6, and CKH8 we can infer a rule, first discovered by Huckel and now rather well known, concerning the aromaticity of planar, carbocyclic systems of the type (CH) . 

Anti peri planarity of double-bonds seems to be an important factor also in carbocyclic cases. Indeed, Harding and collaborators (13) have reported that the intramolecular cyclization of cyclohexenols 36 gave spiroisomer 36 in preference to 37 as the major product (ratio 4 1 when R=H and 9 1 when R CHg or nyC Hg). 

The structure of a potassium salt of a cyclooctatetraene dianion has been determined by Raymond and co-workers (269). In this bright yellow complex (XlXb in Fig. 3) a diglyme molecule is coordinated to each of the potassium atoms through all three oxygen atoms. Two potassium-diglyme units lie on either side of the planar carbocyclic ring equidistant from the ring center. 

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