Annelation, effects

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

Also, in agreement with our concepts on annelation effects, the pK s of 2-X-benzimidazoles are linearly related to the pK s of 2-X-pyridines through Eq. (40). 

The n.m.r. spectra for (112 X = O, NH, S, or SO) have been recorded and compared with that for (112 X = CH2). The annelation effects of the three-membered rings were determined. Using 350 MHz n.m.r. and dipole moments, (113 R = H) and (113 R = OMe) were shown to adopt a boat conformation, with the 0 of the oxiran ring in a pseudo-axial position. The absolute configuration of alliacolide (116) has been established by c.d. 

Cyclohexenone ring from a,/ -ethyleneketones Stereospecific Robinson annelation Effect of solvent... 

Clar E, Guye-Vuilleme JF, McCallum S, Macpherson lA (1963) Annellation effects in the pyrene series and the classification of absorption spectra. Tetrahedron 19 2185-2197... 

Hammett equation, 258 free energy of adsorption of, 230 halogeno-, annelation effect on, 346 halogeno-, aza-effects in, 317 halogeno-, basicity of, 288, 289 halogeno-, ethanolysis of, 298 halogeno-, hydrolysis of, 294 hedogeno-, nucleophilic substitution of, 290, 305, 306... 

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