1,3-bis(diarylmethylidene)-2-methylidenecyclohexanes in cycloaddition and cyclodimerization reactions. The role of stereoelectronic factors.
Cycloaddition and cyclodimerization reactions of 1,3-dibenzylidene- and 1,3-bis(p-methoxybenzylidene)-2-methylidenecyclohexanes, i.e., the diaryltrienes with fixed S-cis-configurations of exocyclic double bonds are studied. These compounds undergo in situ cyclodimerization of the [4 + 2]-type upon dehydration of the precursor tertiary alcohols and form endo-adducts in the Diels-Alder reaction with N-phenylmaleimide. In the presence of CF3COOH, acid-catalyzed cyclodimerization takes place leading to methylidene-substituted spirocyclodimers, which undergo intramolecular cyclization upon prolonged contact with CF3COOH to form a fused carbocyclic system containing a central, 'three-petal' fragment of six-membered rings.
3-Methylene-2-ferrocenylmethylenequinuclidine in cycloaddition and dimerization reactions
Abstract Cycloaddition, cyclodimerization, and cationic dimerization reactions of 3-methylene-2-ferrocenylmethylenequinuclidine as a ferrocenyl substituted 1,3-diene possessing a fixed s -cissoidal conformation of the double bond system have been investigated. This prepared diene has been shown to form Diels-Alder adducts with azodicarboxylic and/or maleic acid N-phenylimides. It has been shown also that the diene undergoes thermal [4 + 2]-cyclodimerization and links 3-methyl-2,5-(1-pipyridinium-1,4-diyl)-1-ferrocenylallylic citation. The latter reaction results in formation of an adduct of chain structure. The products of alkylation of N,N-dimethylaniline with the intermediate allylic cation have been isolated and characterized for the first time.
Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules
The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 A with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 A) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity.