Fused azulenes

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

The bonding situation and the thermodynamic stability of the cations were examined and calculated. The bond alternation in the fused azulene... 

Azulene can not only be linked directly or via conjugated linkers to form oligo-azulenes but also be fused with benzene rings to form larger polycyclic conjugated molecules. Shown in Figure 4.5 are a few examples of fused azulene (15-19),... 

When the carbotricyclization reaction of 82 is carried out in the absence of hydrosilane using [Rh(COD)Cl]2 as the specific catalyst, a novel [2-i-2-i-2-i-1] reaction (CO-CaT) takes place to give the same cyclopenta[e]azulene 83 accompanied by a small amount of 5-6-5 fused tricyclic product 86 (Scheme 7.26) [53]. It is noteworthy that an attempted CO-... 

Problein 10.32 The deep-blue compound azulene (C,yHg) has five- and seven-membered rings fused through two adjacent C s. It is aromatic and has a significant dipole moment of 1.0 D. Explain. M... 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

Nonbenzenoids These compounds generally have two or more rings fused together, but none of the rings is a benzene structure, and they conform to HiickeTs rule, i.e. they have (4u + 2) tt electrons, and are aromatic compounds, e.g. azulene. 

There are several compounds that possess some measure of aromatic character typical of benzene, but do not possess a benzenoid ring. Appropriately, they have (4n + 2) it electrons and are classified as nonbenzenoid aromatic compounds (see Section 21-9). An example is azulene, which is isomeric with naphthalene and has a five- and a seven-membered ring fused through adjacent carbons ... 

Dibenzo-fused dithiepins undergo photochemically induced ring contractions. Thus, irradiation of 2-phenyldibenzo-[df [l,3]dithiepin 1-oxide 72 <1999T5027> and 9-phenyl-4,8,9-trithiadibenzo[i2/,9]azulene 8-oxide 70 <1996CL655> using a high-pressure mercury lamp (400 W) gave disulfides 99 and 100 (Scheme 22). 

For a number of [c]-fused 1,3,5,7-tetramethyl derivatives having azulene-type electronic spectra, the Amax values of the longwave maxima increase from 540 to 706 nm in the following series (El tsov and co-workers Table... 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Azulene (Figure 7.48) is an intriguing variant wherein the cyclohep-tatrienyl unit is fused with a five-membered ring. The organometallic chemistry of this hydrocarbon and related derivatives is less intensely studied but includes the 18VE derivatives shown, obtained from reactions with simple binary carbonyls. 

Morrill, T. C Opitz, R Reploglc, L. L., Kat-sumoto, K., Schroeder, W., and Hess, B. A., Correspondence between theoretically predicted and experimentally observed sites of electrophilic sub.stitution on a fused tricyclic hetcroaromatic (azulene) system. Tetrahedron Lett., 2077, 1975. 

Kuroda, C., Shimizu, S., Haishima, T., and Satoh, J.Y, Synthesis of a stereoisomer of frullanolide utilizing the intramolecular cyclization of co-formyl-2-alkenylsilane. Bull. Chem. Soc.. Jpn., 66, 2298, 1993. Kuroda, C., Inoue, S., Kato, S., and Satoh, J.Y, Synthesis of a-methylene-y-lactones fused to a perhydro azulene carbon framework through intramolecular cyclization of allylsilanes, 7. Chem. Res. (M), 458, 1993. 

Aromatic polycychc compounds need not contain contain benzene rings. For example, purine, which contains two fused heterocyclic rings, is aromatic. Azulene, named for its deep blue color, is also aromatic, although one ring has five carbon atoms and the other has seven. It is not, however, as aromatic as its isomer naphthalene, which has two fused benzene rings. 

The HOMO-LUMO gap decreases as the number of fused rings increases. The decreasing gap is reflected in the hardness values shown in Scheme 8.4, as assigned by Zhou and Parr. The values for phenanthrene and acenaphthene, which have more localized double bonds, are 0.315 and 0.151, respectively. Similarly, the nonbenzenoid hydrocarbon azulene is calculated to be softer. 

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