S S symmetry Review Review AzuleneAzulene MoietyMoiety asas ElectronElectron ReservoirReservoir inin PositivelyPositively ChargedCharged Systems; A Short SurveySurvey Alexandru C.C. RazusRazus Romanian“C.D. Nenitzescu” Academy, Institute “C.D. Nenitzescu” of Organic Chemistry, Institute of Romanian Organic Chemistry, Academy, 202202 BB Spl.Spl. Independentei,Independentei, P.O. Box 35-108, 060023 Bucharest, Romania;Romania; [email protected]@yahoo.com Abstract: The non-alternant aromatic azulene, an isomer of alternant naphthalene, differs from the latter in peculiar properties. The The large large polarization polarization of the π-electron system over the seven and fivefive rings gives to azuleneazulene electrophileelectrophile property property a a pronounced pronounced tendency tendency to to donate donate electrons electrons to to an an acceptor, accep- tor,substituted substituted at azulene at azulene 1 position. 1 position. This This paper paper presents presents cases cases in whichin which azulene azulene transfers transfers electrons electrons to toa suitablea suitable acceptor acceptor as as methylium methylium ions, ions, positive positive charged charged heteroaromatics heteroaromaticsand and examplesexamples ofof neutral molecules that can accept electrons. The proposed product synthesis was outlined and the expected R+ electron transfer was highlightedhighlighted by analyzing the NMR, UV-Vis spectraspectra andand thethe pKpKR values.values. Keywords: azulene; methylium ion; heteroaromatics; π-electron-electron transfertransfer 1. Introduction The stability stability of of organic organic compounds compounds can can be beincreased increased or lowered or lowered by the by presence the presence of π electronsof π electrons and the and conjugation the conjugation of this of type this of type electron of electron can help can the help stabilization the stabilization of an elec- of antronic electronic system. system. Very briefly Very and briefly as a and general as a generalrule, the rule, stable the aromatic stable aromatic π electronπ electronsystems Citation: Razus,Razus, A.C. A.C. Azulene Azulene systemsneed to needbe coplanar to be coplanar with an with alternating an alternating conjugation conjugation and must and mustfollow follow Huckel’s Huckel’s rule, Moiety as Electron Reservoir in rule,namely namely to contain to contain 4n+2 4n+2π electrons.π electrons. For the For polycyclic the polycyclic aromatic aromatic hydrocarbons, hydrocarbons, the exist- the Positively Charged Systems; A Short Positive Charged Systems; Short existenceence of benzenoid of benzenoid sextets sextets is accepted is accepted as a key as element a key element in the theory in the theoryof aromaticity of aromaticity (Clar’s Survey. SymmetrySymmetry 20212021,, 1313,, x. 526. (Clar’srule) [1]. rule) Thus [1 heptalene]. Thus heptalene (Scheme (Scheme1), the polycyclic1), the polycyclic hydrocarbon hydrocarbon composed composed of two fused of https://doi.org/10.3390/ https://doi.org/10.3390/xxxxx two fused cycloheptatriene rings, despite the alternant conjugation, is an unstable, with a sym13040526 cycloheptatriene rings, despite the alternant conjugation, is an unstable, with a non-planar non-planarhelicity skeleton helicity which skeleton also does which not also satisfy does Huck not satisfyel’s or Huckel’sClar’s rule. or From Clar’s the rule. last From point the of Academic Editor: Anthony Harriman last point of view, pentalene can also be excluded because it has only 4n π electrons. Both Academic Editor: Anthony Harriman view, pentalene can also be excluded because it has only 4n π electrons. Both compounds compounds are rapidly destroyed even at very low temperatures (−78 ◦C to −100 ◦C) [2,3]. Received: 12 March 2021 are rapidly destroyed even at very low temperatures (−78 °C to −100 °C) [2,3]. At the other At the other extreme is found the well-known aromatic planar naphthalene, with alternant Received:Accepted: 1220 MarchMarch 20212021 extreme is found the well-known aromatic planar naphthalene, with alternant conjuga- Accepted:Published: 2024 MarchMarch 20212021 conjugationtion and following and following Clar’s rule. Clar’s rule. Published: 24 March 2021 Publisher’s Note: MDPI stays neu- Publisher’stral with regard Note: MDPIto jurisdictional stays neutral y withclaims regard in published to jurisdictional maps and claims institu- in Pentalene x x publishedtional affiliations. maps and institutional affil- iations. Naphthalene Copyright: © 2021 by the authors. Azulene Heptalene Submitted for possible open access Copyright: © 2021 by the author. publication under the terms and Licensee MDPI, Basel, Switzerland. Scheme 1. Several bicyclic systems with conjugated π electrons. conditions of the Creative Commons Scheme 1. Several bicyclic systems with conjugated π electrons. This article is an open access article Attribution (CC BY) license distributed under the terms and Azulene is naphthalene’s isomer, built from a fusion of cyclopentadiene and cyclo- (http://creativecommons.org/licenses Azulene is naphthalene’s isomer, built from a fusion of cyclopentadiene and cyclo- conditions of the Creative Commons heptatriene rings. Although this planar compound obeys Huckel’s rule, it possesses a /by/4.0/). heptatriene rings. Although this planar compound obeys Huckel’s rule, it possesses a Attribution (CC BY) license (https:// non-alternant aromatic structure and at first glance it lacks aromatic sextet [4,5]. However non-alternant aromatic structure and at first glance it lacks aromatic sextet [4,5]. However creativecommons.org/licenses/by/ when one electron is transferred from the 7-atom ring to the 5-atom ring, the azulene when one electron is transferred from the 7-atom ring to the 5-atom ring, the azulene can 4.0/). can be regarded as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron Symmetry 2021, 13, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/symmetry Symmetry 2021, 13, 526. https://doi.org/10.3390/sym13040526 https://www.mdpi.com/journal/symmetry Symmetry 2021, 13, x FOR PEER REVIEW 2 of 23 Symmetry 2021, 13, x FOR PEER REVIEW 2 of 23 Symmetry 2021 13 , , 526 be regarded as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron2 of 21 tropylium cation (Scheme 2). The compound polarity is reflected in the presence of the dipolebe regarded moment as (1.08 the fusionD). of a 6 π-electron cyclopentadienyl anion and a 6 π-electron tropyliumtropylium cationcation (Scheme(Scheme2 ).2). The The compound compound polarity polarity is is reflected reflected in in the the presence presence of of the the dipoledipole momentmoment (1.08(1.08 D).D). Scheme 2. Azulene: resonance structures and orbital molecular. Scheme 2. Azulene: resonance structures and orbital molecular. SchemeReactivity 2. Azulene: studies resonance confirm structures also that and the orbital seven-membered molecular. ring is electrophilic and the Reactivity studies confirm also that the seven-membered ring is electrophilic and five-membered ring is nucleophilic. The electron transfer can be made to a positively the five-membered ring is nucleophilic. The electron transfer can be made to a positively chargedReactivity substituent studies (Scheme confirm 3). alsoIn this that case, the seven-memberedthe tropylium ion ring generation is electrophilic provides and good the charged substituent (Scheme3). In this case, the tropylium ion generation provides good stabilityfive-membered to the formed ring is cation nucleophilic. and, implicitly, The electr anon activation transfer ofcan the be electrophilic made to a positivelyreactions stabilitycharged tosubstituent the formed (Scheme cation 3). and, In implicitly,this case, the an tropylium activation ion of the generation electrophilic provides reactions good (Scheme 33).). WhileWhile thethe azuleneazulene moleculemolecule isis symmetricalsymmetrical onlyonly withwith respectrespect toto thethe x-axis, naphthalenestability to the is supplementary formed cation and,symmetrical implicitly, with an respect activation to the of ythe-axis electrophilic (Scheme 1). reactions For this naphthalene(Scheme 3). isWhile supplementary the azulene symmetrical molecule iswith symmetrical respect to only the ywith-axis respect (Scheme to1 ).the For x-axis, this reason, azuleneazulene hashas three three pairs pairs of of identical identical positions positions (1,3; (1,3; 4,8; 4,8; 3,7) 3,7) and and two two distinct distinct positions posi- tionsnaphthalene (2 and 6) is while supplementary naphthalene sy mmetricaltwice has four with identical respect topositions the y-axis (α (Schemeand β). These 1). For posi- this (2reason, and 6) azulene while naphthalene has three pairs twice of hasidentical four identical positions positions (1,3; 4,8; (α3,7)and andβ). two These distinct positions posi- tionshave differenthave different charges, charges, therefore therefore different differe reactivity.nt reactivity. In comparison In comparison with other with benzenoidother ben- zenoidtions (2 hydrocarbons, and 6) while naphthalene the difference tw icebetween has four the identical small highest positions occupied (α and βmolecular). These posi- or- hydrocarbons,tions have different the difference charges, between therefore the differe smallnt highest reactivity. occupied In comparison molecular with orbital–lowest other ben- bital–lowestunoccupied molecularunoccupied orbital molecular (HOMO–LUMO) orbital (HOMO–LUMO) ensures its remarkable ensures its chemical, remarkable electronic chem- ical,zenoid