Azulene Moiety as Electron Reservoir: History
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Subjects: Physics, Atomic, Molecular & Chemical
Contributor:
Alexandru C. Razus

The nonalternant aromatic azulene, an isomer of alternant naphthalene, differs from the latter in peculiar properties. The large polarization of the π-electron system over the seven and five rings gives to azulene electrophile property a pronounced tendency to donate electrons to an acceptor, substituted at azulene 1 position. This paper presents cases in which azulene transfers electrons to a suitable acceptor as methylium ions, positive charged heteroaromatics and examples of neutral molecules that can accept electrons. 

  • methylium ion
  • heteroaromatics
  • π-electron transfer
  • azulene

1. Introduction

The stability of organic compounds can be increased or lowered by the presence of π electrons and the conjugation of this type of electron can help the stabilization of an electronic system. Very briefly and as a general rule, the stable aromatic π electron systems need to be coplanar with an alternating conjugation and must follow Huckel’s rule, namely to contain 4n+2 π electrons. For the polycyclic aromatic hydrocarbons, the existence of benzenoid sextets is accepted as a key element in the theory of aromaticity (Clar’s rule) [1]. Thus heptalene (Scheme 1), the polycyclic hydrocarbon composed of two fused cycloheptatriene rings, despite the alternant conjugation, is an unstable, with a non-planar helicity skeleton which also does not satisfy Huckel’s or Clar’s rule. From the last point of view, pentalene can also be excluded because it has only 4n π electrons. Both compounds are rapidly destroyed even at very low temperatures (−78 °C to −100 °C) [2][3]. At the other extreme is found the well-known aromatic planar naphthalene, with alternant conjugation and following Clar’s rule.
Symmetry 13 00526 sch001 550
Scheme 1. Several bicyclic systems with conjugated π electrons.
Azulene is naphthalene’s isomer, built from a fusion of cyclopentadiene and cycloheptatriene rings. Although this planar compound obeys Huckel’s rule, it possesses a nonalternant aromatic structure and at first glance it lacks aromatic sextet [4][5]. However when one electron is transferred from the 7-atom ring to the 5-atom ring, the azulene can be regarded as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron tropylium cation (Scheme 2). The compound polarity is reflected in the presence of the dipole moment (1.08 D).
Symmetry 13 00526 sch002 550
Scheme 2. Azulene: resonance structures and orbital molecular.
Reactivity studies confirm also that the seven-membered ring is electrophilic and the five-membered ring is nucleophilic. The electron transfer can be made to a positively charged substituent (Scheme 3). In this case, the tropylium ion generation provides good stability to the formed cation and, implicitly, an activation of the electrophilic reactions (Scheme 3). While the azulene molecule is symmetrical only with respect to the x-axis, naphthalene is supplementary symmetrical with respect to the y-axis (Scheme 1). For this reason, azulene has three pairs of identical positions (1,3; 4,8; 3,7) and two distinct positions (2 and 6) while naphthalene twice has four identical positions (α and β). These positions have different charges, therefore different reactivity. In comparison with other benzenoid hydrocarbons, the difference between the small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) ensures its remarkable chemical, electronic or optical behavior [6].
Symmetry 13 00526 sch003 550
Scheme 3. Tropylium system involvement as stabilizer.
As a result of these considerations, this short survey is devoted to the behavior of azulene-1-yl moiety as electron reservoir which influences several systems that include it. Special attention was given to the synthesis of such interesting systems. The examination of these topics is not exhaustive and is addressed especially to those who want to obtain information on an aspect of the complex chemistry of azulene.

2. Triaryl-methylium Ion with at Least One Azulen-1-yl Moiety

One of the best known and most commonly studied effects of azulen-1-yl moiety as electron donor is the high stability that it gives to the attached cation generating the highly stable pseudo-base, (azulen-1-yl)methylium ions (Scheme 4), whereas from the pKR+ = −6.4 found for triphenylmethylium ion, the value of this parameter increases to 11.5 for the tri(azulen-1-yl)methylium ion [7]. As can be seen in Scheme 4 the pKR+ value increases dramatically with the number of azulen-1-yl at the cation center. Thus, the presence of two such moieties ensures a high cation stability reflected by the pKR+ = 7.3. Introduction of aryl, heteroaryl, but especially of azulen-1-yl as the third substituent raises this parameter over 10. Therefore, obtaining these last chemical species and the study of their behavior have been and continue to be a concern for several research groups.
Symmetry 13 00526 sch004 550
Scheme 4. pKR+ values for some methylium ions.
Most synthesis of (azulen-1-yl)methylium ions are based on the reaction of nucleophilic azulene with electrophiles obtained in different ways, some of which will be presented in the following.
The carbocation resulting from the protonation of carbonyl double bond (compound 1 in Scheme 5) or derived from carbinlos (compound 2) reacts with azulene and, depending on the conditions used, a series of reactions stop at the carbenium salts (step a in Scheme 5) while others directly give neutral product 3 (step a+b or b’). From the compounds 3, the corresponding salts 4 are synthetized by hydride extraction with DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) in the presence of HPF6 or, less often, using Ph3C+ PF6− (step c or c’). When azulenes unsubstituted in positions 1 and 3 were condensed with carbonylic compounds, both these positions can be occupied and oligomers may be also present in the reaction mixture, or are the only products.
Symmetry 13 00526 sch005 550
Scheme 5. General route for the synthesis of azulen-1-yl(s) methylium ions.
Azulenes were reacted with mono or diphenyl carbinols with the aim to obtain (azulen-1-yl)methane with one or two unsubstituted or substituted phenyl moieties at the central carbon atom (Scheme 5) [8][9][10]. Neither the generation of primary carbocation nor the intermediate azulenium ion were highlighted. The reactions occurred with good yields in acetic acid with the addition of a few drops of H2SO4 and, together with the product substituted in position 1, the reaction mixture contains a reduced amount of 1,3-disubstituted product.
The reaction of carbonylic derivatives with azulenes represents and continues to be a constant focus of interest for researchers. Both the scientific importance and the practical applications of the obtained compounds contribute to this. Several examples will illustrate the synthesis pathways, the nature of the resulting products and some of their properties.
The condensation of azulenes with formaldehyde and other aldehydes started in the 1950s–1960s [11]. The early reactions occurred either in ether in the presence of HCl [11] or with HClO4 in acetic acid or in tetrahydrofuran [12]. The latter reaction conditions were applied to reactions of guaiazulene with aryl or heteroaryl aldehydes with good yields (between 20% and 97%), giving perchlorates of (guaiazulen-3-yl)methylium ion with other substituent(s) in positively charged carbon. In attempts with azulene instead of guaiazulene the yield decreased due to a larger or smaller number of formed oligomers or polymers. The presence of di(azulen-1-yl)methane from the azulene-1-carbaldehyde and azulene via perchlorate intermediate was also reported, however without the product characterization. Here it must be noted that for a long time the absence of elaborate procedures for product characterization (e.g., NMR or MS spectroscopy) has been a serious obstacle to the unequivocal assignment of the structure of some obtained products. This assignment was made mainly on the basis of UV-Vis/IR spectra and elemental analysis. Several reactions of azulene also used benzaldehydes [10] or aliphatic aldehydes and ketones [13]. In 1961 Hafner and al. reacted azulen-1-carbaldehyde and 4,6,8-trimethylazulen-1-carbaldehyde with azulene in the presence of POCl3 followed by treatment with NaI to obtain di(azulen-1-yl)methylium iodides [14]. The same authors synthesized tri(azulen-1-yl)methylium chloride (87% yield) after the reaction of azulene with tetra-ethoxy-methane in the presence of HCl in nitromethane.
Systematic research into building and studying the salts of triaryl-methylium ions with one to three azulen-1-yl contribution, and with other aryl or heteroaryl moieties at charged carbon, has been undertaken especially by Asao et al. since 1990 [7][15]. In addition to very high stability achieved by the azulen-1-yl moiety mentioned above [16], another target pursued by the authors was the study of the propeller shaped structure which can be adopted by some triaryl-methylium ions, mainly those containing three azulen-1-yl moieties at the cationic center. The results obtained indicate that favorable conjugation contributes to the transition state of the ring flipping as well as to the ground state. Therefore, these cations show a unique dynamic stereochemical behavior [17][18][19][20]. The general route for the synthesis of these salts involves two steps: the condensation of azulene with different aromatic or heteroaromatic aldehydes in acetic acid, followed by hydride removal from the substituted methane. An example of this reaction sequence is given in Scheme 6 for obtaining methylium salts substituted with three azulen-1-yl moieties, 9, or with two azulen-1-yl and phenyl, 10 via neutral intermediates 5 and 7. The hydride removal with DDQ gave the expected result, whereas attempts using Ph3P+ depended on a neutral compound. Hydride elimination with the help of Ph3P+proceedes normally starting from compound 7 but using compound 5 one azulen-1-yl group is substituted by phenyl, leading to product 11 (azulenes 12 and 13 were formed as by-products) [21]. Working with azulene without substituents in position 1 and 3, the azulene can act as a spacer and compounds with the structures 6 and 8 were highlighted in low amounts. In this case, as well as when azulene has a substituent that can be replaced electrophilically (e.g., tBu group) in position 1 and/or 3, the yield decreases due to the formation of oligomers. Oligomers such as 6A and 8A were converted into bis methylium derivatives, 6B and 8Bb, after hydride(s) removal.
Symmetry 13 00526 sch006 550
Scheme 6. Synthesis of methylium salts incorporated in azulen-1-yl moieties.
Another objective pursued was the obtaining of as varied information as possible when both azulene and phenyl moieties included in the methylium ion were substituted. Nevertheless, because of the large volume of the reported data, only some of the most illustrative compounds, 9, 10 and 14, will be briefly discussed below (Scheme 7).
Symmetry 13 00526 sch007 550
Scheme 7. Methylium ions with substituted azulene and/or phenyl moieties.
As mentioned above and as a result of the values for pKR+ collected in Table 1,the increased number of azulen-1-yl moieties at the methylium center increases the stability of the system and the azulene substituents can decisively influence this parameter. Thus, the pKR+ for 9 (Raz = 3-Me) (Scheme 7) is similar to the compound without substituent 9 (Raz = H). However, the large tBu groups in position 3, 9 (Raz = 3-tBu), increases substantially the pKR+ and, for the presence of two such groups in positions 3 and 6, 9 (Raz = 3,6-di tBu), the value exceeds that of the unsubstituted compound by three units. It has been suggested that this increase can be attributed to the steric effect and less to hyperconjugation [16]. The contribution of the donor substituents such as OMe [22][23] or NMe2 [24][25] to phenyl and/or azulene is more important. As shown in Scheme 7, for cation 10 (Rph = Raz = Me2N) the charge is distributed between the resonance structures A, A’ and A”, namely methylium, tropylium, and ammonium ions. Although the contribution of the ammonium group is remarkable, it is smaller than that exerted by the tropylium. Thus, the difference between the pKR+ values of the tri(4-dimethylaminophenyl)methylium ion [26] and that for the corresponding tri(azulen-1-yl)methylium ion, 9 (Raz = 6-Me2N), namely 9.36 and 24.3, can be a suggestive example. Therefore, it can be concluded that the extreme stability of the last methylium ion is due to the dipolar structure of the azulene rings, in addition to the contribution of the mesomeric effect of the di-methyl-amino groups.
Table 1. pKR+ for the compounds 9, 10 and 14.
Raz Rph
for 10 and 14		 pKR+
9 10 14
H H 11.3 10.5 3.0
3-Me H 11.4 10.8 3.7
3-tBu - 13.2 - -
3,6-di tBu H 14.3 12.4 4.6
6-OMe - >14.0 - -
H 4-OMe - 11.7 -
6-OMe 4-OMe - >14.0 -
3,6-di tBu 4-OMe - 13.4 -
H 4-Me2N - 13.2 -
3,6-di tBu 4-Me2N - 13.8 -
H 4-Me2N - - 12.6
3,6-di tBu 4-Me2N - - 13.3
6- Me2N - 24.3 - -
6- Me2N 4-Me2N - 21.5 -
6- Me2N 4-Me2N - - 14.0
3-Me (18a) 2-thienyl - 11.2 -
3-tBu (18a) 2-thienyl - 11.8 -
3-Me (18b) 3-thienyl - 11.4 -
3- tBu (18b) 3-thienyl - 12.4 -
The influence of azulen-1-yl moieties on the positive charge at the cationic carbon is also well reflected by the 13C-NMR spectra in series of compounds 9, 10 and 14. The up-field chemical shifts for C+ belonging to these compounds observed at 151.82–161.38, 161.11–169.83 and 168.58–177.68 ppm, respectively, indicates a decrease in charge at C+ in the series and an enhanced thermodynamic stability. The chemical shift of C+ in 9 (Raz = 3,6-di tBu) at 151.82 ppm (in DMSO-d6) is slightly up-field compared with those in 9 (Raz = H and Me) at 157.40 and 154.17 ppm. Considerable up-fields occur for 10 (Raz = H and Me) at 165.54 and 161.58 ppm in CDC13) [7][15][16]. It should be noted that, despite the impressive stability of compounds 9, 10 and 14 with Rph = Raz = NMe2 the chemical shifts for C+ for these cations (156.44, 162.08, and 169.98 ppm, respectively) are almost comparable with those with Rph = Raz = H [25].
Several compounds have also been reported in which phenyl has been replaced by another aryl or heteroaryl group. In addition to the partially characterized compounds reported by Kirby and Reid in 1960 [12], more recently phenyl was replaced with 2- and 3-thienyl as in compounds 15a,b depicted in Scheme 8 [27]. These substituents should have a stabilizing effect and, indeed, the chemical shifts of C+ for compounds 15a (Raz = 3-Me) and 15b (Raz = 3-Me) at 151.8 and 153.7 ppm, respectively, showed significant up-field shift compared with those for the corresponding benzyl cations 10 (Raz = 3-Me, Rph = H) at 161.1 ppm. However, despite their stabilizing effect, as seen in Table 1, the values of pKR+ belonging to the thienyl compounds are comparable with those of the analogous phenyl moiety.
Symmetry 13 00526 sch008 550
Scheme 8. Methylium ions substituted with heteroaromatic moiety.
Another type of (azulen-1-yl)methylium ion was obtained by the acid catalyzed condensation of methyl 3-formylazulene-1-carboxylate with indoles (Scheme 8) [28]. The resulting triarylmethanes were oxidized with DDQ to the corresponding azulene-1-yl-di(indol-3-yl)methylium hexafluorophosphates, 16, with high pKR+ values (12–14 units), comparable to those of the triazulen-1-ylmethylium ion. Unprotected indoles do not give clean condensation due to their capacity to deformylate the azulenic system. Therefore, indole must be substituted at the nitrogen atom.
In addition to the bis methylium ions with azulene as spacer, 6B and 8B (Scheme 6), other spacers as 1,4- or 1,3-phenylene, 17 or 18 [29], as well as 2,5-thiophenediyl, 19 and 2,5-thienothiophenediyl, 20 [27] were synthesized and studied (Scheme 9). A special case is represented by the spectacular tri-methylium ion 21 where the positive charged carbons are placed in 1,3,5 positions of phenyl as spacer [30].
Symmetry 13 00526 sch009 550
Scheme 9. Methylium ions substituted at spacers.
The special properties such as conductivity and ferro-magnetism of several multistage redox systems [31] contributed to the desire to build such stable cationic multistage redox systems by connecting redox systems with an extremely stable carbocation and examining how the redox properties would be changed. In this direction the research must be mentioned which took into account redox properties of the ferrocene and extreme stabilities of the di(1-azulenyl)methylium units [32]. In this aim (Scheme 10) di(azulen-1-yl)ferrocenylmethylium hexafluorophosphate, 22 (R1 = R2 = H or tBu) and 22 (R1 = Me; R2 = H) as well as the di-cation 23 were synthesized. As expected, due to the stabilizing effect of the electron donor ferrocene moiety, the pKR+ of 22 (R1 = R2 = H) slightly increased, compared to those of the phenyl analogues, although the chemical shift for 13C of the central cationic carbons of salts 22 are almost comparable with those in the phenyl analogues.
Symmetry 13 00526 sch010 550
Scheme 10. Di(azulen-1-yl)ferro-cenyl-methylium hexafluorophosphate.

3. Charged Hetero-aryls Stabilized by Azulen-1-yl Moieties

Two reviews on the synthesis of azulenes substituted with heterocyclic moieties were recently published [33][34]. Consequently, in this chapter only the syntheses of compounds with charged heterocycles will be briefly examines and attention will be paid to the consequences of their substitution with azulen-1-yl moieties.
Pyranylium salts, which present interesting properties that make them useful for various technical purposes, are stabilized by substitution with electron donating (or releasing) groups. One of the substituents tested for this purpose was azulen-1-yl moiety situated in 2- or 4-position towards the oxonium center of pyranylium. The reported azulenylpyranylium system reported earlier described syntheses for 4-(azulen-1-yl) 2,6-diphenyl-pyranylium salts [35][36]. Chloropyranylium salt or even pyranylium salt were starting compounds. The obtained products were characterized by melting points, elemental analysis and sometimes infrared spectra. In the desire to obtain a large and varied number of pyranylium salts with azulene moieties as substitute and to start from these salts in the synthesis of pyridines, pyridinium salts or bis azulene, Razus et al. varied both the heterocycle and azulene substituents [37]. The reaction route and the obtained 4-(azulen-1-yl)pyranylium salts are described in Scheme 11. Thus, 4H-pyran-4-ones 1 was reacted with PCl5 or POCl3 and perchloric acid. When Raz = Ph the intermediate 2 was separated and characterized whereas for Raz = Me the reaction was continued without separation of unstable 2. The redoutable nucleophilicity of azulene chlorine leads to substitution yields ranging from satisfactory to very good. From the Scheme 11 results that the obtained pyranylium salts have various substituents at azulene and the 2 and 6 positions of the heterocycle are substituted with phenyl, methyl, thiophen-2-yl or furan-2-yl. Position 3 in the heterocycle was also occasionally substituted.
Symmetry 13 00526 sch011 550
Scheme 11. Synthesis of 4-(azulen-1-yl)pyranylium salts.
Table 2. 1H-NMR spectra of substituted azulenic compounds (chemical shifts in ppm).
1H Azulene 1-Phenyl-
azulene		 4-(Azulen-1-yl)-
2,6-diphenylpyridine		 Salt 3 a Salt 5 b Salt 10 c
3′/5′H - - 7.95 8.83 8.06 8.32
2 7.81 8.02 8.18 8.92 8.45 8.18
3 7.30 7.43 7.53 7.77 7.51 7.35
4 8.23 8.34 8.44 8.89 8.65 8.44
5 7.05 7.14 7.28 8.03 7.77 7.28
6 7.45 7.58 7.45 8.31 8.07 7.72
7 7.05 7.14 7.29 8.14 7.80 7.29
8 8.23 8.55 8.71 9.57 9.12 8.71
The beneficial effect on the pyranylium ring salt stability can also be exerted by the azulen-1-yl moiety placed in position 2 of the ring as in products 8, 9. The reaction started from 2H-pyran-2-one and occurred as shown in Scheme 12 with good yields when R = Ph (36–45%) and only with 17% for R = Me [37][38].
Symmetry 13 00526 sch012 550
Scheme 12. Synthesis of 2-(azulen-1-yl)pyranylium salts.
The pyranylium salts were then used as valuable synthons for obtaining a series of pyridines and pyridinium salts [34]. The general pathway for the synthesis of pyridinium salts in two steps [39] was applied to the generation of salts substituted with azulen-1-yl moieties (Scheme 13) [38][40][41]. In the first step the pyranylium ring is opened by amine in the presence of triethylamine followed by the ring closure of the intermediate enamine. Good or even quantitative yields were reported for products 10, 10′ and 11, 12 and the generation of pyridinium salts is not influenced by the amine except when tBuNH2 was used, which lowers the yield. When started from 2,6-dimethylpyranilium salts alongside the pyrylium salts 13, traces of anilines 14 were present in almost all reactions with amines; however, using iPrNH2 the amount of aniline reaches a 5–25% yield. The route for generation of compound 14 is described in Scheme 13.
Symmetry 13 00526 sch013 550
Scheme 13. Synthesis of 4-(azulen-1-yl)pyridinium salts.
Research has also been pursued on oxygen replacement from 2,4-diphenylpyranylium salts, 8, with N+Rpy, just as for 2.6-pyranilium salts (Scheme 14). The yields of products 15 were very good except when Rpy = Ph (yields: 26–34%) [38].
Symmetry 13 00526 sch014 550
Scheme 14. Synthesis of 2-(azulen-1-yl)pyridinium salts.
In addition to the research on pyranylium and pyridinium salts with phenyl or methyl at positions 2 and 6, these positions were also replaced by 5-membered heterocycles, thiophen-2-yl and furan-2-yl as shown for compounds 12 and 13 in Scheme 11 [42]. The ability to donate electrons of azulen-1-yl moiety suggested the idea of stabilizing positively charged heteroaromatic compounds, not only with one of this group but with several azulen-1-yl groups. Because the attempts to obtain 2,4,6-tri(azulen-1-yl)pyranylium salts were unsuccessful, attention was directed to the synthesis of some of their vinylogs [43]. Thus, the synthesis started from 2,6-dimethyl-4-(azulen-1-yl)pyranylium salts or from 2,4,6-trimethylpyranylium salts which are condensed with azulene-1-carbaldehyde (Scheme 15).
Symmetry 13 00526 sch015 550
Scheme 15. Viny-logs of tri(azulen-1-yl)pyranylium salts.
The reaction of salts 5 with azulene-1-carbaldehyde (AzCHO) occurred in acetic anhydride and a mixture of products 16 and 17 resulted, depending on the reaction temperature. At 100 °C mono condensation generates product 17 at about 50% and 16 resulted in trace, whereas at reflux after bis condensation, the amount of compound 16 increased to about 45% and 17 decreased to 5–10%. A similar behavior was reported when 2,4,6- trimethylpyranylium perchlorate reacted with AzCHO. At 100 °C only compound 20 resulted, at 160 °C compound 20 disappears being replaced by compounds 18 and 19, and under microwave irradiation salt 18 remained the only product (50–55%).
Recently, this class of compounds has been expanded by the generation of 4-(azulen-1-yl)pyranylium and pyridinium salts substituted in 2- and 6-positions with 2-(furan-2-yl)vinyl, 2-(thiophene-2-yl)vinyl or 2-(3-thienyl)vinyl moieties [44].
The distribution of π electrons resulting from resonance structures of charged heterocycles substituted in position 4 with azulen-1-yl moiety, shown in Scheme 16, significantly influence the chemical shifts of 1H and 13C in the NMR spectra of both azulene and heterocycle moieties of the salts. Because proton spectra of the studied salts (Table 2) are more suggestive in this regard and provide more valuable information, they will be briefly discussed.
Symmetry 13 00526 sch016 550
Scheme 16. The most important resonance structures of charged heterocycles.
From Table 2 it can be seen that the good electron-acceptor pyranylium/pyridinium moiety strongly deshielded all azulenic protons compared with neutral 1-phenylazulene [45] or with 4-(azulen-1-yl)-2,6-diphenylpyridine [41]. On the other hand, the donor-electron contribution of azulene can be highlighted by comparing the chemical shift of protons in the pyranylium ring of compounds 3 and 5 (δ = 8.83 and 8.06 ppm, respectively) and for the same protons in 2,4,6-triphenyl-pyranylium perchlorate (δ = 9.17 ppm) [46][47].
The dihedral angle determines the intensity of the push-pull effect between the heterocycle and the aromatic groups as well as the direction of the magnetic fields generated by the heteroaromatic ring current, well reflected in the 1H-NMR spectra. For this reason, the dihedral angle was calculated [48] for several (azulen-1-yl)heteroaromatic systems and results are included in the Table 3 [37]. The data in the table highlight some features of these systems. The angle for neutral 4-azulen-1-yl-2,6-diphenylpyridine exceeds by 16o that of corresponding pyridinium salt 10 (R2 = Me; Rpy = nBu; Raz = R3 = H) attesting the contribution of higher conjugation between the two moieties for salt.
Table 3. Dihedral angle between the azulen-1-yl and the charged heterocycle moiety.
Compd. * Raz R3 Dihedral Angle
(in °) [48]
3 (R2 = Ph) H H 24
H Me 38
2-tBu-6-Me H 50
5 (R2 = Me) H H 22
2-tBu-6-Me H 45
10 (R2 = Me;
Rpy = nBu)		 H H 29
H Me 42
2-tBu-6-Me H 54
10 (R2 = Ph; Rpy = Me) H H 39
The involvement of the charged heteroaromatic moiety decreases from pyranylium to pyridinium salts due to the releasing electron effect, +I, of alkyl substituent at quaternary nitrogen. As a result, the resonance structure with tropylium system Scheme 16 decreases in contribution and so the dihedral angle increases followed by the deshielding of pyridinium protons and the shielding of azulene protons for pyridinium salts 10 in order of inductive effect of substituent Rpy at N+, namely iPr < nBu < Bn < Ph.
Another disturbing factor of the dihedral angle is the occupation of the azulenic position 2 or of position 3(5) of the heterocycle. As an example, the presence of tBu at C2 in salts 3 or 5 with RAz = 2-tBu,6-Me shielded azulenic protons and deshielded pyranylium protons, and the Me in position 3 of pyranylium moiety shields all azulenic protons.
There are no significant differences between the chemical shifts of azulene protons for the pyranylium and pyridinium salts with thiofen-2-yl or furan-2-yl in 2 and 6-positions, 6, 7, 12 and 13 and those of the corresponding compounds with phenyl in these positions [42].
Without going into details, a remark must be made regarding the influence of the heteroaromatic magnetic field, which exerts influence mainly on the neighboring azulene protons 2, 7 and 8, which are more deshielded [40][41].
As expected, the chemical shifts of carbon atoms belonging to azulene for all salts included in the pyranylium and pyridinium series 3–13 undergo an important deshielding as compared with the reduced effect when azulene is coupled with milder electron withdrawing heterocycles, such as pyridines, thiophene or with phenyl an (effect present especially at C5, C7 and C8a where an increase to 12 ppm was observed for chemical shifts) [37].
With the decrease of push-pull efficiency exerted by the π-electronic system at the increase in dihedral angle, a bathochromic effect results, observed in UV-Vis spectra in the order: pyridines, pyridinium salts, pyranylium salts [37][38][40][41]. The values of λmax (in methanol) for these compounds with Raz = R3 = H; R2 = Ph (Rpy = Me) are 370, 434 and 530 nm, respectively. The stabilization of tropylium structure by alkylation produces a bathochromic displacement; for example, for pyranylium salts 3 (Raz = H), 3 (4,6,8-Me3) or 3 (5-iPr,3,8-Me2) λmax increases in the order: 530, 543 and 576 nm. As an interesting observation, for the 2,4,6-tri(azulen-1-ylvinyl) pyranylium salt 18, due to the very large conjugation of the π-electronic system, λmax reaches a high value, 690 nm [43].
The research on charged heteroaryls stabilized by azulen-1-yl moieties was recently enlarged with a new class, namely chalcogeno-pyranylium salts with 4-position occupied by azulen-1-yl group [49]. 4-(azulen-1-yl)-2,6-diphenyl-thiopyranylium and 4-(azulen-1-yl)-2,6-diphenyl-selenopyrylium perchlorates, 21 and 22 (Scheme 17) were synthesized with good yields on the same route as that used for the corresponding pyranylium salts.
Symmetry 13 00526 sch017 550
Scheme 17. 4-(Azulen-1-yl)chalco-geno-pyranylium salts.
The lower chalcogeno-pyrylium salts’ polarization compared with that for the pyrylium salts is reflected both in UV-Vis spectra as well as in the proton chemical shifts. Thus, the bathochromic effect noticed in order O+ < S+ < Se+ (λmax for 3 (Raz = R3 = H; R2 = Ph), 21 (Raz = H and 22 (Raz = H) are 530, 572 and 600) could be associated with the decrease in the electronegativity of heteroatom (O, 3.5; S, 2.5; Se, 2.4) [50]. The 1H-NMR spectra of chalco-geno-pyrylium salts show the same characteristics and trends as in the case presented above for pyranylium salts.
  1. Other systems stabilized by azulen-1-yl moieties

                   In this last chapter reference will be made to some special cases of azulen-1-yl moiety participation to the stability of some carbocations and even of some neutral molecules.

                 In connection with the stabilization of triarylmethylium ions by at least one azulen-1-yl moiety discussed before, the delocalized monocarbenium ions, salt 23, stabilized by largely conjugate π electrons including azulen-1-yl moieties next is presented. The salt 23 was obtained by condensation of all-trans-retinal with azulenes (Scheme 18) [50].

Scheme 18.  Condensation of all-trans-retinal with azulenes.

                   Considering the good electron donor property of both ferrocene and of azulen-1-yls, Farrell et al. [51] used ferrocene-azulenium carbocations and their vinylogs as tetrafluoroborates to investigate a novel series of monometallic monocations, 24n. Starting from formyl ferrocene and azulenes as precursors, the yields of products 24n were between 58 % and 68 % (Scheme 19).

Scheme 19. Ferrocene-azulenium carbocations.

                   Contrary to the very low stability of most carbocations with hydrocarbon skeleton, some cyclic cations as cyclopropenyl [52] have remarkable stability. In fact, this system is the simplest aromatic Hückel system. The substitution of this cation with three cyclopropyl [53] or three guaiazulene moieties [54] dramatically increases the value of pKR+ compared with the compound with three phenyl at ring as it can be seen in Scheme 20.  From the data in Scheme 20 it results the high difference between the pKR+ for triphenylmethylium and the tri(azulen-1-yl)methylium cation as compared with the difference between cyclopropenylium cations 25(R = Ph) and 25(R = guaiazulene-3-yl). The compound 25(R = guaiazulene-3-y) was obtained in the Friedel-Crafts reaction of C3Cl3+AlCl4 (prepared from tetrachlorocyclopropene and AlCl3) with 3 molar equivalents of guaiazulene in dichloromethane at -70 oC [54].

Scheme 20. pKR+ values for some carbocations with hydrocarbon skeleton.

                   Regarding the azulen-1-yl system contribution to the charge distribution in neutral molecules, without detailing this extensive subject that goes beyond the subject of this review, only some informative example will be presented below.

                   The azulenyl squarines, that will be discussed briefly, already synthesized in 1966 [55], have interested a number of researchers due to the optical properties of the compounds as well as for some biological applications. It should be mentioned that in the over 50 years since the birth of the first azulenyl squarines a huge number of compounds have been reported, some of them being covered by patents and in 2017 a consistent review was published on this topic [56]. The general procedure for obtaining these compounds, 26, is shown in Scheme 21 [57]. The presence of electron-donating groups in an aromatic system employed in a squaraine synthesis tends to favor the reaction and indeed the yield of reactions between azulene and squaric acid proceed satisfactorily.

Scheme 21. Azulen-1-yl squarines.

                   Without going into details, some information on the structure of the azulenyl squarines is required. The X-ray crystal structure of compound 26(Raz = H or 3,8-Me2, 5-iPr) [56,58] shows the shorter C1-C2 bond as compared with the other five ring bonds. At the same time the bond length for six bonds in big ring are reduced as compared with the distance between the atoms in five membered ring and the longer azulene bond is between C3a and C8a. (Scheme 21). These observations seem to suggest a more efficient π–electron conjugation over C3a-C4-C5-C6-C7-C8-C8a than a tropylium structure.

                   The presence of [18]annulene substructures with 26π electrons within the macrocycle is responsible for the aromatic characteristics of porphyrins. According to this model, a large number of other compounds, including some containing azulene systems in the molecule, have been made over time. Lash et al. had the idea of replacing one or more pyrrole rings in the porphyrin macrocycle with the five ring of azulene so edifying a new class of compounds, azuliporphyrins and related carbaporphyrinoid compounds [58].

                   One of the synthesis of azuliporphyrins is based on the application of the “3+1”variant of the MacDonald reaction as it can be seen in Scheme 22 [58]. Azulene-1,3-dialdehyde was condensed with tripyrrane in the presence of HBF4, in dichloromethane followed by oxidation with DDQ. The compounds 27, porphyrin analogues, due to the participation of the five azulene ring at the stabilization of product, were generated in good yields. A large number of azuliporphyrins were also obtained by Lash's research team from which only two more examples, 28 and 29 were selected. In the last compound two azulenes moieties were inserted into the molecular structure.

Scheme 22. Synthesis of azuliporphyrins.

                   Finally and as a curiosity, the attempts to realize a system analog to that of porphyrin, in which the four five-membered rings belong to azulenes [59,60] will be presented. Starting from theoretically exciting however presumed quatyrin system (Scheme 23), analog to that of porphyrin, the beginning was the preparation of calix[4]azulenes, 30, by reacting azulenes with paraformaldehyde in the presence of florisil. However the realized molecules did not have the desired structure namely [18]annulene substructures with 26π electrons.  Using the tetraaryl compound 31, after oxidation with DDQ and an addition of HBF4·Et2O, the tetracation 32 was generated. Surprisingly, the chemical shifts in 1H-NMR spectrum of 32 are consistent with a non-aromatic structure. This finding as well as the lack of coplanarity of the macrocycle, determined by X-ray spectra, seems to indicate the predominant participation of the structure 32B with the charges located at the four Csp2 compared to the structure 32A with the tropylium ions. Thus, the macrocyclic aromatic ring current is essentially absent.

Scheme 23. Macrocycles including azulene moieties.

 4. Conclusions

              Azulenes, these exceptional compounds, related to natural derivatives, have a special structure that gives them properties totally different from those of alternating aromatic hydrocarbons and even from the nonalternating ones with which they are related. The paper proposes a brief look at one of the properties of azulenes. The configuration of π-electron system over the seven and five azulene rings develops a dipole moment that favors a displacement of the electrons towards an acceptor substituted in position 1 of the azulene. In this way the seven-ring adopts the stable structure of tropylium ion and consequently stabilizes the entire system in which it is involved. This explains the ease of azulenes to react in electrophilic substitutions as well as to donate electrons to acceptor systems. This paper is focused on the donor property of azulene and is limited to the most relevant acceptors, namely methylium ions and positive charged heteroaryls. Several cases of neutral molecules benefiting from azulen-1-yl assistance are also quickly described. It was considered useful to briefly summarize the synthesis of the compounds considered. The change in the charge distribution on the studied systems results in variations in the chemical shifts of protons and carbon atoms in the NMR spectra as well as some changes in the UV-Vis spectra. The pKR+ value of some pseudobases, such as cations, is a valuable indicator of their stability. The analysis of the mentioned parameters highlights the role of azulene as an electron donor on the stability of the systems in which it is included especially in those with positive charge. Certainly this property of azulene will further stimulate the building of chemical systems belonging to a series of areas of theoretical interest but also in those of technical interest such as dyes, active materials for electronic, optoelectronic and electrochromic devices.

This entry is adapted from the peer-reviewed paper 10.3390/sym13040526

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