DOCSLIB.ORG

Magnetic Criteria of Aromaticity

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Magnetic Criteria of Aromaticity Chemical Society Reviews Magnetic Criteria of Aromaticity Journal: Chemical Society Reviews Manuscript ID: CS-TRV-02-2015-000114.R2 Article Type: Review Article Date Submitted by the Author: 10-May-2015 Complete List of Authors: Stanger, Amnon; Technion, Chemistry Gershno-Poranne, Renana; Technion, Chemistry Page 1 of 34 Chemical Society Reviews Magnetic Criteria of Aromaticity Renana Gershoni-Poranne [a] and Amnon Stanger* [a] Addresses [a] R. Gershoni-Poranne, Prof. A. Stanger Schulich Faculty of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion – Israel Institute of Technology, Haifa 32000, Israel. E-mail: [email protected] Dedication Dedicated to Prof. Yitzhak Apeloig on the occasion of his 70 th birthday. Introduction Preface: Benzene was discovered in 1825 by Faraday. The determination and understanding of its structure presented a great challenge to scientists: the compound was highly unsaturated, yet did not show any of the typical reactions of unsaturated compounds, e.g., addition of bromine. The first suggestion that the molecule is cyclic is traditionally attributed to Kekulé in his 1865 1 and 1866 2 papers, although some argue that this idea was presented earlier by Josef Loschmidt. 3 In any event, accumulating experimental evidences, mainly experiments that were conducted by Ladenburg and Wroblewsky, indicated that the proposed structure was unsatisfactory, as it did not account for there being only three disubstituted isomers of benzene instead of the expected four. It was only in 1872 that Kekulé suggested oscillating structures, in which synchronous 1,2-shifts of the three double bonds occur very rapidly. 4 It took about six decades until Erich Hückel 5-9 and Linus Pauling 10 introduced their quantum mechanical based MO and VB explanations, respectively, which introduced the concept of delocalization (of bonds and electrons). This fundamental property is vital to the topic of the present paper. When a metallic loop (containing delocalized electrons) is inserted into a magnetic field, an electric current is induced. The current in the loop induces a magnetic field that opposes the external magnetic field at the inside of the loop. Aromatic molecules contain cyclically delocalized electrons and therefore, when subjected to a magnetic field, show a ring current and a resulting induced magnetic field. However, there are some fundamental differences between the classic (loop) and quantum (molecular) phenomena. The main one, which is relevant to this paper, is the direction (sense) of the induced current and its resulting induced magnetic field. While the direction of an induced current in a loop is determined by the relative orientation of the magnetic field and the loop’s movement, the direction of the current in a molecule depends also on the type of the system. Generally, an external magnetic field 1 Chemical Society Reviews Page 2 of 34 induces a paratropic ring current (the opposite direction relative to a classic loop) in a system that contains 4n π electron, while a system containing (4n+2) π electrons shows a diatropic ring current (the same direction as a classic loop). A frontier orbitals based explanation has been offered by Fowler and collaborators. 11-13 In a nutshell, this explanation rests on the different symmetry changes that take place when, under an external magnetic field, a HOMO-LUMO transition occurs in the double-degenerated HOMO of a (4n+2) π electrons system to the doubly degenerated LUMO vs. a single HOMO to a single LUMO level in 4n π electrons systems. This, in turn, causes a different balance between the linear momenta of transition, which produce diatropic ring currents, and angular momenta of transition, which produce paratropic ring currents. A later work by Corminboeuf et al 14 studies the angular momentum of transition on NICS using CMO-NICS and suggests similar conclusions. We shall return to the consequences and support for this explanation later. Since the HOMO-LUMO gaps in 4n π electrons systems are smaller than the HOMO-LUMO gaps in (4n+2) π electrons systems it is important to note here that (a) the paratropic currents in the former are generally stronger than the diatropic currents in the latter, since transitions between energy levels are proportional to ∆E-1 between the respective levels, and (b) it is important to include configuration interactions (CI) in the calculations of the induced ring currents (and the resulting induced magnetic fields), due to the energy proximity of the empty orbitals in 4n π electrons systems. Conversely, the calculations of the induced ring currents and induced magnetic field of (4n+2) π electrons systems are much less sensitive to the inclusion of CI. Phenomenologically, it is well established that aromatic compounds exhibit diatropic ring currents under external magnetic field and antiaromatic compounds exhibit paratropic ring currents under the same conditions. The question that is raised here (and will be answered later) is whether the appearance of diatropic and paratropic ring currents under external magnetic fields is limited only to aromatic and antiaromatic compounds. In other words, is the presence of an induced ring current under an external magnetic field a sufficient or only a necessary condition to define a system as aromatic (or antiaromatic)? There are mainly four criteria for aromaticity: energy, structure, electron density based indices 15 and magnetic properties. The first two have been discussed recently 16 and in this issue of Chemical Society Reviews . The third, electron density based criteria, was reported by Solà and coworkers to be “the most accurate among those examined” in the comprehensive comparison they conducted in 2008. 17 However, all three of these criteria are beyond the scope of this article. Magnetic criteria are currently the most popular methods and include NMR chemical shifts of 1H, 3He and Li +, magnetic susceptibility anisotropy, magnetic susceptibility exaltation, Nucleus Independent Chemical Shifts (NICS), Aromatic Ring Current Shielding (ARCS) and Current Density Analysis (CDA) plots. We will focus our attention on 1H-NMR techniques and the two computational methods NICS and CDA, which are most in use today. NMR chemical shifts . The ring current model (RCM) 18 suggests that, for a (4n+2) π electrons system, the diatropic ring current will form an induced magnetic field which is opposite to the external field at the center of the current. The lines of this magnetic field are curved (see Figure 1) and outside of the ring they are in same direction as the external field. Thus, protons that are outside of the ring experience a 2 Page 3 of 34 Chemical Society Reviews downfield shift, while protons which are inside the ring experience an upfield shift. In 4n π electrons systems a paratropic ring current is induced, thus the effect on the external and internal protons is exactly the opposite. This model was challenged, 19 but a response to this challenge advocated that the model is valid. 20, 21 Thus, chemical shifts of protons can and have been used to identify aromatic and antiaromatic compounds. An extension of the use of NMR to assess aromaticity was developed mainly by R. Mitchell. 22 This and the conclusions regarding NMR as an aromaticity-determining tool are discussed below. Figure 1. Ring current and induced magnetic field in benzene. Magnetic susceptibility exaltation :23 This method is based on the empirical observation that for non- aromatic systems the experimental magnetic susceptibility can be predicted from the magnetic susceptibilities of the atoms with some correction factors. Aromatic systems show a magnetic susceptibility that is larger than the one calculated from the atoms and this difference is termed exaltation. This method was popular during the 60s-80s of the last century. Nowadays, mainly due to the difficultly of quantifying aromaticity with this method and the need for an empirical reference, together with the development of computational methods that allow a more detailed and direct study of the magnetic properties, in a qualitative and quantitative fashion, this method is rarely used. Thus, a CAS search of the topic “Magnetic susceptibility exaltation” in the last ten years yielded 51 results. None of them is an experimental measurement of the property and most of them are used as computational evidence for aromaticity of the studied compounds, mainly in comparison to other criteria of aromaticity, such as the recent demonstration by Andjelkovic et al. 24 Since the experimental measurement of magnetic susceptibility exaltation is almost non-existent and computationally there are better methods for assessing aromaticity (see below), this topic will not be further discussed here. It should be emphasized that neither aromaticity nor ring currents are measureable quantities. However, both NMR chemical shifts and diamagnetic susceptibility exaltation are experimentally measurable quantities that result from ring currents. The following two methods, current density analysis (CDA) and nucleus independent chemical shifts (NICS) are computational only. Current Density Analysis (CDA). This method visualizes the ring current via arrows. The direction of the arrows (clockwise or anticlockwise) describes the type of the current (paratropic or diatropic) while the size of the arrows indicates the strength of the current. The most popular methods for current density analysis are the Continuous Transformation of Origin of Current Density – Diamagnetic Zero (CTOCD- DZ) 25-27 also known as the ipsocentric method. 11, 12, 28 Two other CDA methods are the anisotropy of 3 Chemical Society Reviews Page 4 of 34 induced current density (ACID) 29 and gauge inlcuding magnetically induced current (GIMIC).30 The main advantage of the CDA methods is the pictorial representation of the ring current/s in the system. For example, it is easy to see that, pentalene has a global paratropic ring current and two small local paratropic currents, 31, 32 something that would be difficult (although not impossible, see below) to see with other methods.
Load more

Recommended publications
  • Ring Currents Modulate Optoelectronic Properties of Aromatic Chromophores at 25 T
    Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T Bryan Kudischa, Margherita Maiuria,b, Luca Morettia,b, Maria B. Oviedoa,c,d,e, Leon Wangf, Daniel G. Oblinskya, Robert K. Prud’hommef, Bryan M. Wongc, Stephen A. McGillg, and Gregory D. Scholesa,1 aDepartment of Chemistry, Princeton University, Princeton, NJ 08540; bDipartimento di Fisica, Politecnico di Milano, 20133 Milano, Italy; cDepartment of Chemical & Environmental Engineering, University of California, Riverside, CA 92521; dDepartment of Materials Science & Engineering, University of California, Riverside, CA 92521; eInstituto de Investigaciones Fisicoquímicas de Cordoba, Consejo Nacional de Investigaciones Científicas y Técnicas, Departamento de Química Teorica y Computacional, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, X5000HUA Córdoba, Argentina; fDepartment of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08540; and gNational High Magnetic Field Laboratory, Tallahassee, FL 32310 Edited by Catherine J. Murphy, University of Illinois at Urbana–Champaign, Urbana, IL, and approved April 1, 2020 (received for review October 16, 2019) The properties of organic molecules can be influenced by magnetic This perspective in many ways is inspired by some of the sa- fields, and these magnetic field effects are diverse. They range lient features that govern the magneto-optical responses in from inducing nuclear Zeeman splitting for structural determina- magnetic circular dichroism (MCD) spectroscopy. While best tion in NMR spectroscopy to polaron Zeeman splitting organic known for its ability to interrogate the degeneracy of electronic spintronics and organic magnetoresistance. A pervasive magnetic states, many electronic transitions will display a weak MCD due field effect on an aromatic molecule is the aromatic ring current, to the magnetic field-induced mixing of electronic states that which can be thought of as an induction of a circular current of gives rise to the B-terms, even in diamagnetic systems (19).
    [Show full text]
  • Electrooxidation Enables Highly Regioselective Dearomative Annulation of Indole and Benzofuran Derivatives
    ARTICLE https://doi.org/10.1038/s41467-019-13829-4 OPEN Electrooxidation enables highly regioselective dearomative annulation of indole and benzofuran derivatives Kun Liu1, Wenxu Song1, Yuqi Deng1, Huiyue Yang1, Chunlan Song1, Takfaoui Abdelilah1, Shengchun Wang 1, Hengjiang Cong 1, Shan Tang1 & Aiwen Lei1* 1234567890():,; The dearomatization of arenes represents a powerful synthetic methodology to provide three-dimensional chemicals of high added value. Here we report a general and practical protocol for regioselective dearomative annulation of indole and benzofuran derivatives in an electrochemical way. Under undivided electrolytic conditions, a series of highly functio- nalized five to eight-membered heterocycle-2,3-fused indolines and dihydrobenzofurans, which are typically unattainable under thermal conditions, can be successfully accessed in high yield with excellent regio- and stereo-selectivity. This transformation can also tolerate a wide range of functional groups and achieve good efficiency in large-scale synthesis under oxidant-free conditions. In addition, cyclic voltammetry, electron paramagnetic resonance (EPR) and kinetic studies indicate that the dehydrogenative dearomatization annulations arise from the anodic oxidation of indole into indole radical cation, and this process is the rate- determining step. 1 College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, P. R. China. *email: aiwenlei@whu. edu.cn NATURE COMMUNICATIONS | (2020) 11:3 | https://doi.org/10.1038/s41467-019-13829-4 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-13829-4 reaking the aromatic systems of electron-rich arenes or fused indolines (Fig. 1a)39–44. Therefore, it is highly appealing to heteroarenes provides three-dimensional chemicals of high develop efficient approaches to allow for their preparation.
    [Show full text]
  • Robert Burns Woodward
    The Life and Achievements of Robert Burns Woodward Long Literature Seminar July 13, 2009 Erika A. Crane “The structure known, but not yet accessible by synthesis, is to the chemist what the unclimbed mountain, the uncharted sea, the untilled field, the unreached planet, are to other men. The achievement of the objective in itself cannot but thrill all chemists, who even before they know the details of the journey can apprehend from their own experience the joys and elations, the disappointments and false hopes, the obstacles overcome, the frustrations subdued, which they experienced who traversed a road to the goal. The unique challenge which chemical synthesis provides for the creative imagination and the skilled hand ensures that it will endure as long as men write books, paint pictures, and fashion things which are beautiful, or practical, or both.” “Art and Science in the Synthesis of Organic Compounds: Retrospect and Prospect,” in Pointers and Pathways in Research (Bombay:CIBA of India, 1963). Robert Burns Woodward • Graduated from MIT with his Ph.D. in chemistry at the age of 20 Woodward taught by example and captivated • A tenured professor at Harvard by the age of 29 the young... “Woodward largely taught principles and values. He showed us by • Published 196 papers before his death at age example and precept that if anything is worth 62 doing, it should be done intelligently, intensely • Received 24 honorary degrees and passionately.” • Received 26 medals & awards including the -Daniel Kemp National Medal of Science in 1964, the Nobel Prize in 1965, and he was one of the first recipients of the Arthur C.
    [Show full text]
  • Annulation of Propargylic Dithioacetals Leading to Furan-Containing Oligoaryls*
    Pure Appl. Chem., Vol. 77, No. 7, pp. 1213–1219, 2005. DOI: 10.1351/pac200577071213 © 2005 IUPAC Annulation of propargylic dithioacetals leading to furan-containing oligoaryls* Tien-Yau Luh Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Abstract: Reaction of propargylic dithioacetals with BuLi followed by treatment with di- aldehydes yields the corresponding allenyl carbinols which can be cyclized to give the 2,3,5-trisubstituted furans. The use of this strategy for the synthesis of a range of alternating benzene-furan oligoaryls is described. Keywords: annulation; propargylic dithioacetals; oligoaryls; furans; optoelectronic applica- tions; cyclophanes; convergent synthesis. INTRODUCTION Direct conversion of carbon–sulfur bonds in dithioacetal functionality into the corresponding car- bon–carbon bonds has been extensively studied in recent years [1]. Representative examples are shown in Scheme 1. The substituents R1 and/or R2 can be aryl, alkenyl alkynyl, or, more recently, alkyl groups. Since carbon and sulfur atoms have similar electronegativities, the polarity of the carbon–sulfur bond can vary depending on the nature of the substrates and reaction conditions. In general, the carbon end of a carbon–sulfur bond can serve as a carbocationic equivalent to reaction with a nucleophile. Alternatively, the nucleophile can attack at the sulfur end of a carbon–sulfur bond to generate the car- banion, which can then react with an electrophile leading to the corresponding substitution product. In this case, the carbanionic moiety must be stabilized by a neighboring stabilizing substituent(s). Ikehira and Krief have shown that benzylic dithiolanes react with butyllithium followed by protonolysis to give the corresponding mono-desulfurized products (eq.
    [Show full text]
  • Synthesis of an Unsymmetrically Pentafunctionalized Corannulene Derivative (Part I) Synthesis of Platinum and Ethynyl-Platinum Corannulenes (Part II)
    Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2012 Synthesis of an Unsymmetrically Pentafunctionalized Corannulene Derivative (Part I) Synthesis of Platinum and Ethynyl-Platinum Corannulenes (Part II) Maag, Roman M Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-164179 Dissertation Published Version Originally published at: Maag, Roman M. Synthesis of an Unsymmetrically Pentafunctionalized Corannulene Derivative (Part I) Synthesis of Platinum and Ethynyl-Platinum Corannulenes (Part II). 2012, University of Zurich, Faculty of Science. Part I: Synthesis of an Unsymmetrically Pentafunctionalized Corannulene Derivative and Part II: Synthesis of Platinum and Ethynyl-Platinum Corannulenes Dissertation zur Erlangung der naturwissenschaftlichen Doktorwurde¨ Dr. sc. nat. vorgelegt der Mathematisch-naturwissenschaftlichen Fakult¨at der Universit¨at Zurich¨ von Roman M. Maag von Winkel ZH Promotionskommitee: Prof. Dr. Jay S. Siegel (Vorsitz) Prof. Dr. Kim K. Baldridge Prof. Dr. Cristina Nevado Prof. Dr. Roger Alberto Zurich,¨ 2012 Abstract of the Dissertation Part I: Synthesis of an Unsymmetrically Pentafunctionalized Corannulene Derivative and Part II: Synthesis of Platinum and Ethynyl-Platinum Corannulenes by Roman M. Maag University of Zurich, 2012 Prof. Dr. Jay S. Siegel, Chair Corannulene (C20H10) is a polyaromatic hydrocarbon that can be considered as the smallest fragment of Buckminsterfullerene exhibiting a curved surface. Among the in- teresting properties of corannulene are rapid bowl inversion and esthetically appealing fivefold symmetry (C5v), which is rare in chemistry. Whereas the first synthesis in 1968 only afforded milligram quantities, several improvements in the synthetic strategy finally culminated in the development of an efficient process which today furnishes corannulene in kilogram quantities.
    [Show full text]
  • Molecular Assembly of Polycyanoarenes with Silver Salts
    MOLECULAR ASSEMBLY OF POLYCYANOARENES WITH SILVER SALTS AND SYNTHESIS OF POLYCYCLIC AROMATIC HYDROCARBONS A DISSERTATION IN Chemistry and Pharmaceutical Sciences Presented to the Faculty of the University of Missouri-Kansas City in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY by GERARDO B. MÁRQUEZ B.Sc. Central University of Venezuela, 1993 Kansas City, Missouri 2012 © 2012 GERARDO B. MÁRQUEZ ALL RIGHTS RESERVEDED MOLECULAR ASSEMBLY OF POLYCYANORENES WITH SILVER SALTS AND SYNTHESIS OF POLYCYCLIC AROMATIC HYDROCARBONS Gerardo Batalla Márquez, Candidate for the Doctor of Philosophy Degree University of Missouri-Kansas City, 2012 ABSTRACT This dissertation encompasses the investigation of two distinct subjects. In the first part, which is in the area of molecular self-assembly, the complexation of organonitrile aryl compounds with three different types of silver (I) salts is examined in the solid state. The assembly of 1-(2,2-dicyanovinyl)naphthalene with silver hexafluoroantimonate resulted in a cationic 3D network. Complexation of 4-(2,2-dicyanovinyl)biphenyl with silver tetrafluoroborate and hexafluoroantimonate from benzene generated two similar structures. While the former displays a cationic 3D network, the latter is defined by cationic 2D sheets. Complexation of 9-(2,2-dicyanovinyl)anthracene with silver hexafluoroantimonate from toluene afforded a cationic 2D ribbon, and from benzene, it yielded cationic 2D sheets. These complexes contained solvent bonded to their structures. However, the hexafluoroantimonate ion is nonbonding. The crystal association of 1,4-bis(cyanovinyl)benzene with silver triflate from benzene yielded neutral 2D sheets whose imperfect-rectangular macrocyclic arrangements are interconnected on both sides by bridges of benzene. On the other hand, the assembly of 1,3- bis(cyanovinyl)benzene with silver triflate from benzene afforded a neutral 3D network formed by two interconnected rings.
    [Show full text]
  • Anti‐Aromatic Versus Induced Paratropicity ... -.:. Michael Pittelkow
    Angewandte Forschungsartikel Chemie Deutsche Ausgabe:DOI:10.1002/ange.201913552 Circulenes Internationale Ausgabe:DOI:10.1002/anie.201913552 Anti-Aromatic versus Induced Paratropicity:Synthesis and InterrogationofaDihydro-diazatrioxa[9]circulene with aProton Placed Directly above the Central Ring Stephan K. Pedersen, Kristina Eriksen, Nataliya N. Karaush-Karmazin, Boris Minaev, Hans gren, Gleb V. Baryshnikov* und Michael Pittelkow* Abstract: We present ahigh-yielding intramolecular oxidative coupling within adiazadioxa[10]helicene to give adihydro- diazatrioxa[9]circulene.This is the first [n]circulene contain- ing more than eight ortho-annulated rings (n > 8). The single- crystal X-raystructure reveals atight columnar packing, with aproton from apendant naphthalene moiety centred directly abovethe central nine-membered ring. This distinct environ- ment induces asignificant magnetic deshielding effect on that particular proton as determined by 1HNMR spectroscopy. The origin of the deshielding effect was investigated computation- ally in terms of the NICS values.Itisestablished that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure,and is not due to the nine-membered ring being antiaromatic.UV/Vis spectroscopyreveals more efficient Figure 1. Simplified illustration of the influence of diatropic and para- conjugation in the prepared diazatrioxa[9]circulene compared tropic ring currents on the 1Hchemical shift inside and outside of to the parent helical azaoxa[10]helicenes,and
    [Show full text]
  • Π-Conjugated Carbocycles and Heterocycles Via Annulation Through C-H and X-Y Activation Across CC Triple Bonds
    Issue in Honor of Dr. Jhillu S. Yadav ARKIVOC 2016 (ii) 9-41 π-Conjugated carbocycles and heterocycles via annulation through C-H and X-Y activation across CC triple bonds Yoshinori Yamamoto,a,b Abdulrahman I. Almansour,c Natarajan Arumugam,c and Raju Suresh Kumar c a WPI-AIMR, Tohoku University, Sendai 980-8577, Japan b State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116 023, China c Department of Chemistry, College of Science, King Saud University, P.O.Box 2455, Riyadh 11451, Saudi Arabia E-mail: [email protected], [email protected] This account is dedicated to Professor J. S. Yadav, in honor of his distinguished career in synthetic organic chemistry, and of his 65th birthday DOI: http://dx.doi.org/10.3998/ark.5550190.p009.282 Abstract In this account our own research results on the methods of annulation and hetero-annulation across alkynes are summarized, especially emphasizing the annulation and hetero-annulation through C-H and X-Y activation. A wide range of quinolines, isoquinolines, indoles, benzofurans, indenes, and benzothiophenes can be synthesized via mono-annulation of ortho-alkynylaryl derivatives using transition metal catalysts and/or iodine reagents. Benzocarbazoles, benzonaphthothiophenes, and indenochromenes can be synthesized via cascade annulation of 1,2-dialkynyl substituted benzene derivatives. Palladium-catalyzed crossover annulation of compounds bearing two vicinal CC triple bonds attached to an unsaturated ring gives dibenzopentalenes through C-H activation, and intramolecular cross annulation of bis- biarylalkynes affords 9,9-bifluorenylidenes via dual C-H activation. Keywords: Optoelectronic materials, quinolines, isoquinolines, indoles, benzofurans, indenes, benzothiophenes, C-H activation, palladium catalysis, gold catalysis Table of Contents 1.
    [Show full text]
  • Synthesis of Functionalized Benzenoid Macrocycles: an Approach to Accessing Functionalized Cycloparaphenylenes
    Synthesis of Functionalized Benzenoid Macrocycles: An Approach to Accessing Functionalized Cycloparaphenylenes by Rolande Meudom A thesis submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Master of Science Auburn, Alabama May 8, 2016 Keywords: Carbon nanotubes, Cycloparaphenylenes, p-terphenylophanes, Bent p-phenylene Macrocycles, Copyright 2016 by Rolande Meudom Approved by Bradley L. Merner, Chair, Assistant Professor of Chemistry and Biochemistry Peter Livant, Associate Professor of Chemistry and Biochemistry Anne E. V. Gorden, Associate Professor of Chemistry and Biochemistry Steven Mansoorabadi, Assistant Professor of Chemistry and Biochemistry Abstract Carbon nanotubes (CNTs) are cylindrically shaped allotropes of carbon with exceptionally good electronic and optical properties. The current state of art for making these CNTs involves heating graphite in a high temperature furnace (above 600 °C). The use of harsh temperatures makes the process unselective and produces CNTs as an inseparable mixture of products. Currently, there is no efficient purification technique for isolating pure CNTs. In order to fully exploit the electronic properties of CNTs, they need to be made as homogeneous and monodispersed structures. Thus, synthetic chemists envisioned using chemical synthesis from the ground-up via a template strategy as a more viable approach to achieve the selective synthesis of uniform CNTs structures with a defined chirality and diameter. This is because the selective synthesis of curved PAHs like [n]ciruclenes and CNT substructures could only be accomplished through chemical synthesis. The work described herein focuses on a new approach to accessing highly distorted p-phenylenes, which does not rely on cross coupling reactions and the use of the Burgess reagent as a mild dehydrative aromatization agent, giving access to highly strained benzenoid macrocycles.
    [Show full text]
  • Cycloosmathioborane Compounds: Other Manifestations of the Hückel
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Repositorio Universidad de Zaragoza Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX pubs.acs.org/IC Cycloosmathioborane Compounds: Other Manifestations of the Hückel Aromaticity † ‡ † † Miguel A. Esteruelas,*, Israel Fernandez,́ Cristina García-Yebra, Jaime Martín, † and Enrique Oñate † Departamento de Química Inorganica,́ Instituto de Síntesis Química y Catalisiś Homogenea,́ Centro de Innovacioń en Química Avanzada (ORFEO−CINQA), Universidad de Zaragoza, CSIC, 50009 Zaragoza, Spain ‡ Departamento de Química Organicá I, Facultad de Ciencias Químicas, ORFEO−CINQA, Universidad Complutense de Madrid, 28040 Madrid, Spain *S Supporting Information have reported EP2 triangles (E = Ge, Sn, Pb), which are ABSTRACT: The discovery of cycloosmathioborane stabilized within the coordination sphere of a sterically protected compounds is reported. These species, which are prepared diniobium unit,13 whereas Guha’s group has computationally by the simultaneous dehydrogenation of a trihydride predicted that the substitution of a B atom in the triangle fi 2− ff hydrogensul de osmium(IV) complex and a BH3NHR2 [B3H3] by a group 15 element should a ord neutral aromatic − − − 14 amine borane, bear an Os S B three-membered ring, H2B2XH rings (X = N, P). Herein, we take one step forward in being a manifestation of the 4n +2Hückel aromaticity in this fascinating field by reporting the preparation and full which n = 0 and where the two π electrons of the ring are characterization of the first aromatic triangles having three provided by the S atom. different vertexes, namely, two main-group elements, S and B, and a transition metal with its associated ligands.
    [Show full text]
  • [7]Helicene to a Planar Hetero[8]Circulene Bodil Lousen,[A] Stephan K
    Communication Chemistry—A European Journal doi.org/10.1002/chem.201905339 & Non-Planar Aromatics Compressing aNon-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene Bodil Lousen,[a] Stephan K. Pedersen,[a] Pernille Bols,[a] Kasper H. Hansen,[a] MichelleR.Pedersen,[a] Ole Hammerich,[a] Sergey Bondarchuk,[b] Boris Minaev,[b] Glib V. Baryshnikov,[c] Hans gren,[c, d] and MichaelPittelkow*[a] cally chiral moleculeshave found vast applications in such di- Abstract: This work describes asyntheticapproachwhere verse fields as molecular recognition,catalysis anddetection or anon-planar aromatic heterocyclic [7]helicene is com- emissionofcircularlypolarized light.[2–5] Structurally related pressedtoyield ahetero[8]circulene containing an inner motifstothe [n]helicenes are the [n]circulenes. These struc- antiaromatic cyclooctatetraene (COT) core. This [8]circu- tures also consist of ortho-annulatedaromatic rings, but in- lene consists of four benzene rings and four heterocyclic stead forms acyclic structure, having acentral ring with n rings, and it is the first heterocyclic[8]circulenecontaining sides.Itisillustrative to view the all-benzene series of [n]circu- three differentheteroatoms. The synthetic pathway pro- lenes,which consistsofthe bowl-shaped carba[5]circulene ceeds via athe flattened dehydro-hetero[7]helicene, (corannulene), the planar carba[6]circulene(coronene) andthe which is partially ahelicene and partially acirculene:itis saddle-shaped[7]- and [8]circulenes. Hetero[8]circulenes incor- non-planar and helically chiral as helicenes, and contains a poratingfour five-membered heterocycles(furans,pyrroles, COT motif like [8]circulenes. The antiaromaticity of the thiophenes)are planar,[6,7] and these structures can be viewed COT core is confirmed by nucleusindependent chemical as containingantiaromaticcyclooctatetraene(COT) cores.
    [Show full text]
  • FULL PAPER Hückel's Rule of Aromaticity Categorizes
    FULL PAPER Hückel’s Rule of Aromaticity Categorizes Aromatic closo Boron Hydride Clusters Jordi Poater,[a,b] Miquel Solà,*[c] Clara Viñas,[d] and Francesc Teixidor*[d] Abstract: A direct connection is established between tridimensional P, N, or other incorporating transition or lanthanide metal aromatic closo boron hydride clusters and planar aromatic fragments. Consequently, the icosahedron or bicapped square antiprism are very relevant geometrical features in BHs and, by [n]annulenes for medium and large size boron clusters. In particular, extension, to other areas in which clusters are significant, e.g. our results prove the existence of a link between the two-dimensional Zintl phases. Hückel rule followed by aromatic [n]-annulenes and Wade-Mingos’ 2- In the last few years, we have given evidence that the concept rule of three-dimensional aromaticity applied to the aromatic [BnHn] of aromaticity, in terms of planar or tridimensional aromaticity, was closo boron hydride clusters. Our results show that closo boron not too distant, and that most probably -aromaticity and 3D hydride clusters can be categorized into different series according to aromaticity were based on the same grounds.[4] This is why we the n value of the Hückel (4n+2) rule. The distinct categories studied named them as the two sides of the same coin.[4b] To reach such in this work correspond to values of n = 1, 2, and 3. Each category conclusion we compared the conventional (4n+2) planar organic increases in geometrical difficulty but, more importantly, it is possible cyclic compounds with closo BHs. The aromaticity properties of to associate each category with the number of pentagonal layers in planar organic cyclic compounds had been demonstrated by their stability, hydrogenation experimental studies, substitution but not the structure perpendicular to the main axis.
    [Show full text]