6-Membered rings aromatic ring

Arguing that the MNDO method is more suitable than the AMI method for predicting the heats of formation of five-membered nitrogenated aromatic rings, Garcia and Vilarrasa (88H1803) calculated that 4-fluoroimidazole 14a (R = F, = H) is 2.5 kJ mol more stable than its tautomer 14b,... 

The Diels-Alder reaction with triple bond dienophiles gives access to cyclo-hexa-1,4-diene derivatives. Further reaction of a reactive intermediate thus produced or a subsequent oxidation step can then lead to a six-membered ring aromatic target molecule. 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

Membered Non-Aromatic Ring Compounds a) Pyrazoline Compounds... 

Ring transformations are common in pharmaceuticals. The API lorazepam containing a seven-membered non-aromatic ring can lose a molecule of water and rearrange with the driving force being formation of a six-membered aromatic pyrimidine ring (Fig. 115) (165). 

The effect of conjugation is even more pronounced in aromatic compounds, which have three conjugated double bonds in a six-membered ring. Aromatic C=C bonds are more like bonds than true double bonds, and their reduced pi bonding results in less stiff bonds with lower stretching frequencies, around 1600 cm-1. 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

Organogermanium Complexes Containing Cyclopentadienyl and Other Five- and Six-membered Ring Aromatic Heterocyclic Ligands... 

The base unit of polypyrrole is pyrrole, the structure of which is a five member hetero-aromatic ring containing a nitrogen atom, as shown in Fig. 13a. Polypyrrole is synthesized from the pyrrole monomer by mild oxidation, using chemical or electrochemical technique. After the oxidation of the monomer, a black solid polymer is precipitated from the solution. The polypyrrole structure in its oxidized form is shown in Fig. 13b. Film thicknesses on the order of 1-1.5 pm, using in situ deposition, were obtained for our application [19]. 

It has been found that the cationic polymerization of 2,6-diphenyl-l,6-heptadiene, CH2 =C( )—(CH2)3—C( )=CH2, yields a polymer whose repeat unit consists of six-membered non-aromatic ring structure. Propose a structure of the repeat unit and a mechanism of this polymerization. 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

The influence of the heteroatom on the structure of a series of dinaphtho-fused five membered potentially aromatic ring systems (317), including the phosphorus and arsenic systems, has been studied by crystallographic techniques. A theoretical study has shown that the incorporation of two o, X -phosphorus atoms into the phosphole ring system decreases the ring strain and significantly lowers the inversion barrier about the phosphole pyramidal phosphorus. Pyramidalisation at phosphorus in phospholes has been shown to increase on... 

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