1,3,5-Trienes benzene ring

Although one can envisage dehydrogenation and rearrangement of the coordinated trienes in 149 and 150 to yield 114, no such process has actually been observed to take place. Because both triene complexes are derived from the trans ligand, and not the cis isomer, it is possible that it is the ds-hexa-l,3,5-triene ligand that produces the benzene ring in cluster... 

Ortho photocycloaddition was first reported in a U.S. patent [1] dated September 3, 1957. Irradiation of benzonitrile in the presence of various alkenes resulted in the formation of derivatives of l-cyanobicyclo[4.2.0]octa-2,4-diene. The first ortho photocycloaddition to benzene was reported in 1959 by Angus and Bryce-Smith [2], who discovered that benzene and maleic anhydride react to form a stable adduct at 60°C under the influence of ultraviolet radiation. This 1 2 adduct was formed from one molecule of benzene and two molecules of maleic anhydride. Two years later, Bryce-Smith and Lodge [3] found that acetylenes could also be photoadded to benzene. The isolated products were cyclooctatetraenes, formed by ring opening of the primarily formed bicyclo[4.2.0]octa-2,4,7-trienes. Since those early years, hundreds of examples of ortho photocycloadditions of alkenes to the benzene ring and many mechanistic investigations have been reported and they will be discussed in this chapter. 

In addition to benzene rings, cycloheptatriene is activated or protected by forming the stable if complex 300. An example of the strong stabilization effected by coordination is shown by isolation of the optically active l,3,5-cycloheptatrien-3-ols 301, 304 and 305 as their enol forms. l,3,5-Cycloheptatrien-3-ol was isolated as complex 301 by hydrolysis of silyl enol ether 300. The triene system is stabilized by coordination,... 

Figure 1 shows the geometry of the triene 619 Fig. 2, for comparison, that of the diene 721 In the case of the [2.2.2]phane the three bridges force a chairshape deformation upon the benzene rings, which show a minimum distance of 2.76 A (C3—C15, C5—Cl 1, C7—Cl 3). In contrast, the average van der Waals distance be-... 

Rehybridization would partially displace the intraanular tr-electron density to the outside of the benzene rings, thus reducing destabilizing interaction in the interior of the molecule. In addition, rehybridization rationalizes a striking feature of the triene 6, also found in [2.2]paracyclophane 112 its diene 72V> and in [3.3]paracyclo-phane20 the aromatic C—H-bonds of all phane hydrocarbons mentioned are inclined towards the inside of the molecule, e.g. in triene 6 by an angle of 13°. 

Wagner and Nahm have observed the photoaddition of a remote double bond to the benzene ring of acetophenone derivatives affording (179) and (180) from the irradiation of (181) and (182) respectively. In another publication they have reported further on the process and have demonstrated that the final product from the reaction is a secondary photoproduct. Thus the irradiation of (181) has been shown by n.m.r. spectroscopy to afford an initial photoproduct (183). This is thermally labile and.is converted into the triene (184) which undergoes a photochemical cyclization to yield the stable product (179). ... 

Other irmovative approaches have been used to promote aromatization, once the RCM reaction involving the two alkenes has been accomplished. These two approaches are shown in Scheme 17.4. In the first one, triene 24 underwent an RCM reaction to afford the exocycUc double bond-containing compound 25 [18]. Use of the rhodium catalyst [RhCl(cod)]j then facihtated formation of the desired phenol 26. Another neat way to accomplish formation of the aromatic benzene ring was to allow the intermediate cyclic Michael acceptor 28 (formed by RCM of 27) to undergo a Heck-Mizoroki reaction with p-methoxybenzenediazonium tetrafluoroborate 30 to afford the second phenol 29 [18]. 

Transmetalation of Zirconated Compounds. The reaction of allylzirconation derivatives with allyl chloride can be catalyzed by CuCl to give stereodefined 1,4,7-trienes. Similarly, zirconacyclopentadienes react with 2 equiv of allyl chloride to give 1,4,6,9-decatetraenes in good yields. Reaction of zirconacyclopentadienes with alkynes such as dimethyl acetylenedicar-hoxylate can produce a hexa-substituted benzene ring system in the presence of CuCl/LiCl (eqs 34 and 35). ... 

Although benzene is not a triene and its electrochemical reduction is not always practically facile, the benzenoid ring has been found to be easily reduced by the electrochemical method when magnesium is used as cathode26 (equation 29). As some of the typical... 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

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