Dimerization azulenes

C-TMS protection of the alkyne provided acceptable yields of 3-substituted indole as long as the hydroxy group was protected with a stable group. Purple colored impurities, one of which has been identified as azulene 45, were seen in both coupling reactions using C-TMS-alkynes such as 36 and 40d (Scheme 4.9). The azulene was presumably formed through the dimerization of acetylenes... 

Pham W, Weissleder R, Tung CH (2002) An azulene dimer as a near-infrared quencher. Angew Chem Int Ed Engl 41 3659-3662... 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

The reaction of arenes with aryllead tricarboxylates performed in trifluoroacetic acid, affording biarenes, takes place via a cationic 7t-complex.162 Since azulenes form 7r-donor/acceptor complexes with various 7r-acids, the arylation of 4,6,8-trimethylazulene 108 was attempted with />-methoxyphenyllead triacetate 1 (Equation (135)).163 Only one isomer of 1-arylazulene 109 was formed although in a modest 27% isolated yield. Based on recovered unreacted azulene, the effective yield was 43%. A dimer 110, 3,3 -dianisyl-l,l -biazulene (4% yield), was suggested to result from the one-electron oxidation of the intermediate 4-methoxyphenyl cation in the 7t-complex. 

Thiophene 1,1-dioxide did not undergo cycloaddition with electron-deficient dienophiles. In most of the cases the dihydrobenzothiophene derivative 109 was obtained as the major product. This shows that self-dimerization is faster than cycloaddition with a different molecule. In the case of dimethyl acetylenedicarboxylate (DMAD) and 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione (PTAD), the Diels-Alder adducts 111 and 112 of 109 were obtained <1997JA9077>. However, cyclopentadiene gave the Diels-Alder adduct 113 with thiophene 1,1-dioxide. The DMAD adduct 111 on thermolysis undergoes a retro-Diels-Alder reaction to give dimethyl phthalate and thiophene 1,1-dioxide. Azulene was isolated in the thermolysis of 108 in the presence of 6-(dimethylamino)-fulvene this was the result of a [4-1-6] cycloaddition of the thiophene 1,1-dioxide formed in the reaction followed by elimination of SO2 and dimethylamine (Scheme 28) <1999BCJ1919>. 

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

Similar azulene dimers arise on irradiation of 1,2-diethynylbenzenes (equation 65) , and an intramolecular napthalene-type adduct from a 1,8-diethynylnaphthalene (equation 66) . Interestingly, the latter reaction can also be brought about thermally. 

Ultraviolet irradiation of dialkynylbenzenes produces polymers along with small amounts of dimers. The dimer fraction consists of azulenes and, in some cases, naphthalenes. The product distribution in the dimer fraction depends on the relative orientation of the alkynyl groups as well as the substituents on the alkynyl groups and on the ring. o-Diethynylbenzene (365) produces azulenes 366 and 367 while p-diethynylbenzene yields 368 and 369 These products can be accounted for in... 

Allene 68, formed in situ from l-(2,4,6-cydoheptatrien-l-yl)-2-phenyl-acetylene (67) in the presence of DBU in diethyl ether, underwent dimerization to give compounds 69,70, and 71 in 27, 43, and 4% yields, respectively (79CL171). Heating of compound 69 with DBU in benzene afforded a 2 1 isomeric mixture of azulenes 73 and 74 in 90% yield. The formation of the azulene derivatives (73 and 74) probably proceeds via the norcaradiene tautomer (72) of 69 (Scheme 2). 

Mori et report that the azulene quinone (276) in methylene chloride solution undergoes photochemical dimerization on irradiation using a 400 W mercury arc lamp. Three products were isolated from this process and were identified as (277), (278) and (279) in 6, 10 and 24%, respectively. The outcome of the reaction shows some dependence upon the solvent used for the irradiations. A time-resolved single photon study of the photophysics of hypericin and its methylated analogues has been reported. ... 

The azulene anion 752- was reinvestigated by Edlund 92) by the lithium reduction of the neutral azulene (75) in THF-d8. This research followed an earlier study in which a dimeric dianion 192 was mistaken for the dianion 152 93). This anion is a 4nic-... 

Fig. 5. 250.13 MHz H NMR spectrum of the lithium dianion of azulene (152 ) in [2H8]tetrahydro-furan. Residual signals are due to the doubly charged dimer I92...

Azulene can dimerise and stable dimers have been obtained in matrices at low temperatures[79]. 

Lithium reacts with azulene to give an azulenyl radical which is in equilibrium both with an azulenide dianion and a dimeric dianion [143], Aqueous work-up provides a 6,6 -bis(dihydroazuIenyl). 

The unusual keten (270) adds a variety of olefins to give the expected cyclobuta-nones, With vinyl ethers, the keten rea.cts regiospecifically to give 3-alkoxycyclo-butanones (271). The cyclobutane-1,3-dione dimer of (270) is not a stable compound and attempts to isolate a dimer resulted only in the isolation of small amounts of an azulene derivative. (272)... 

Azulene oligomers and polymers are intriguing molecules in terms of tiie construction of functional substances. Some synthetic studies on azulene dimers have been reported. The synthetic methods, however, were considered to restrict the development of further studies. Thus, Ullman coupling reactions at high temperature or some sophisticated reactions based on Hafner s azulene synthesis, starting from bipyridyls, were used. To obtain azulene oligomers, the Suzuki coupling has been applied recently (Equations 10 and 11) [llj. 

Diphenylacetylene undergoes unusual dimerization forming 1,2,3-triphenyl-azulene upon treatment with aluminum trichloride and sulfenyl chloride (Scheme 7.39) [54]. Although the reaction mechanism is unclear, one benzene ring is expanded into a seven-membered ring. An analogous ring expansion process is observed in a platinum-catalyzed cyclization reaction of arylalkynes [55]. 

Coa(CO)8 decomposes in toluene to produce Co4(CO)ia and carbon monoxide. The reaction is reversible the rate-determining step in the Co2(CO)s to Co4(CO)ia direction is said to be dimerization of Coa(CO)6 fragments. Rus(CO)ia reacts with azulene to form Ru4(azulene)(CO)9, in which the azulene is bonded to one face of the tetrahedral Rui cluster. The mechanism is unknown, except that Ru8(azulene)(CO)7 is though to be an intermediate. ... 

Dinitrobenzenesulfenyl chloride aluminum chloride Azulenes hy dimerization... 

Apart from the usual spectroscopic structural evidence, other support for a-polymer-linkages for most CPs is fairly strong 2,5-disubstituted pyrrole monomers fail to polymerize, and the polymerization of 2-monosubstituted monomers stops at the dimer stage 1- and 1,3-substituted azulenes (Scheme 5-Y) fail to polymerize [58] theoretical electron density studies on azulene indicate that electrophilic attack, as with a radical cation on a neutral monomer, would occur at the 4-position, yielding a 1,4-linkage rather than the 1,3-linkage actually observed [59]. 

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