Azulene, reduction

Further purification of azulene may be achieved by sublimation at reduced pressure, mp 99 C. The checkers found that mechanical losses, particularly as mentioned in Note 9, lead to reduction in yield with reduction in scale (0.1 mol, 39% yield 0.5 mol, 43% yield 0.8 mol, 79% yield). 

Shemdal considers the formation of a crystalline picrate, melting at 122°, the best method of identifying azulene. On reduction azulene yields a dihydro-sesquiterpene, and in Sherndal s opinion, it is... 

The 14 re-electron [S Ns]"1" cation is formed by the reaction of S4N4 with the [SN]+ cation.42 The planar, 10-membered ring usually has an azulene shape (19), with alternating sulfur and nitrogen atoms. Electrochemical reduction of S5N5+ salts in acetonitrile produces the polymer (SN)X. 

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. 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

Table I summarizes the pKR+ values and redox potentials for the tri(l-azulenyl)methyl cations. The oxidation exhibited a barely separate two-step, two-electron oxidation wave. This wave is ascribed to the oxidation of two azulene rings to generate a tricationic species. The reduction showed a one-electron wave, which is ascribed to the formation of a neutral radical.

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Arylbenzotriazoles (797) are prepared via 2-nitro- and 2-amino-diphenylamines (Scheme 161). The 2-nitrodiphenylamines (796) are prepared from the appropriate aniline by reaction with 2-fluoronitrobenzene in the presence of KF <808215,85JCS(Pl)2725>. Azo-coupling of 2-amino-1-cyano-azulene (798) with p-tolyldiazonium chloride gives (799) (Scheme 162). Catalytic reduction of (799) quantitatively yields the diamino derivative (800), which on diazotization affords 9-cyano-azuleno[l,2-J]triazole (801) in 77% yield <85TL335>. 

The azulenes 68 and 69 displayed a reversible reduction wave at —1.48 V for 68 and —1.42 V for 69, which have been attributed to the delocalization of the radical anion between the azulene and 1,2-thiazine ring systems (Scheme 9) <2003T4651>. 

Photoreduction was quenched by high concentrations of biacetyl, slightly retarded by iodonaphthalene, but not affected by azulene or anthracene.113 These observations led to the unsatisfying conclusion that reduction proceeded via a triplet state which could be only selectively quenched. However, later work114 using flash photolysis showed that the benzophenone ketyl radical was generated upon irradiation of solutions of benzophenone and acridine, and that its predominant mode of disappearance was by reaction with... 

Figure 3. Regression line of the solution reduction potential versus the hmo-lumo energy (eLUM0, / negative) for a series of benzenoid hydrocarbons (Streitwieser Schwager 1962 Streitwieser 1962). The half-wave potentials for azulene (1), acepleiadylene (2), pyracyclene (3), and C60 are also shown (see text).

Each of the two terpenes (+)-sabinene 153 and the azulenic (+)-aromadendrene 154 gave two diasteromeric ozonides in pentane in a combined high yield. They were characterized individually by 111, 13C, and 170 NMR and by reduction to the corresponding ketones (Scheme 53). 

In some cases, like reduction of azulene or for anodic waves, corresponding to mercury salt formations with various ligands, two or even three consecutive adsorption waves can be observed at gradually increased concentration. Two or three adsorbed layers can be formed, which can differ in chemical composition, in number and structure of adsorbed layers, or in orientation of compounds in such layers. 

It was also demonstrated that MM quadruple bonds could facilitate electron transfer between redox-active centres. Single-electron reduction of [W2(TiPB)2(L10)2] results in the population of an azulene 71 orbital, with NIR electronic absorption spectroscopy indicating that the radical is fully delocalised over both azulene ligands by ligand 7i-M28-ligand 7i conjugation.25... 

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-... 

The TCT method of obtaining relative molecular electron affinities and gas phase acidities has a demonstrated precision of 0.05 to 0.10 eV in the midrange of values from 0.5 eV to 3.0 eV. At the extremes the precision is less, 0.2 eV. Most of the TCT Ea are ground-state electron affinities. The exceptions are the HPMS electron affinities determined for azulene, anthracene, QJv, and CS2, and the ICR value for fluoroanil. The TCT method has been applied to more than 200 molecules. About 30 have been determined by the HPMS and ICR methods and many have been confirmed by the ECD method. Many have also been confirmed by the half-wave reduction potential method and/or solution charge transfer complex spectra. These will be discussed in Chapter 10. The colli-sional ionization method of measuring relative electron affinities can produce inverted orders of intensities and give excited-state Ea rather than ground-state Ea. 

Fewer than 300 Ea for organic molecules have been determined in the gas phase. The majority of the Ea have been determined by the ECD and/or TCT methods. The direct capture magnetron, AMB, photon, and collisional ionization methods have produced fewer than 40 values. Only the Ea of p-benzoquinone, nitrobenzene, nitromethane, azulene, tetracene, and perylene have been determined by three or more methods. Excited-state Ea have been obtained by each of these methods. Half-wave reduction potentials have determined the electron affinities of 50 aromatic hydrocarbons. The electron affinities of another 50 organic compounds have been determined from half-wave reduction potentials and the energies of charge transfer complexes. It is a manageable task to evaluate these 300 to 400 Ea. 

By using triisobutyl- and diisobutylalane such reductions may be extended to other aldehydes and ketones 206). Some benzaldehyde derivatives, furfural, and some azulene aldehydes are reduced without the occurrence... 

Deliberate dehydrogenation of natural hydroazulenes and other sesquiterpene derivatives, sometimes after preceding dehydration, hydrogenation, and reduction, has been the first synthetic path to azulenes (55FCF(3)334, p. 355 59MI2, p. 301). Similar reactions yielded... 

Besides reduction/dehydration (Section 4.3) and deamination (Section 4.4.2), hydrolysis/decarboxylation of carboxylic esters or nitriles is the most widely used method to degrade primary synthetic azulene derivatives to form simpler species or parent ring systems. The reason is the easy accessibility and usefulness of mono- and di(alkoxycarbonylated)... 

It is possible to selectively reduce an unsaturated side-chain catalytically without reducing the azulene rings (66MI1). Recently, an effective metal-free method has been published for the selective reduction of the C = C double bond even in azulenic enones, for instance, thiophenes 54 (Scheme 81) or 55a and 55b, using the hydride donor cycloheptatriene and protic acid (06OL3137). 

Selective reduction. NaBH4 added with stirring at 5° under Ng to a soln. of ethyl 3-ethyl-4-acetyl-5-methylpyrrole-2-carboxylate in tetrahydrofuran, then BFg-etherate added dropwise at a rate to keep the temp, near 10°, warmed to 25°, and stirred 1 hr. at this temp. ethyl 3,4-diethyl-5-methylpyrrole-2-carboxylate. Y ca. 100%. H. W. Whitlock and R. Hanauer, J. Org. Chem. 33, 2169 (1968) also with reduction of carbalkoxyl groups, azulenes, s. A. G. Anderson, Jr., and R. D. Breazeale, Am. Soc. 91, 2375 (1969). 

The reactivity of azulene towards electrophiles resembles that of activated benzene derivates in that it couples with dCazo-comoounds [47,83,91,92]. Reduction of 1-arylazoazulenes with sodium bisulphite gives 1-aminoazulenes which can only be isolated as salts or as derivatives such as acetyl derivatives [93]. 

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