Thermal rearrangement of aromatic hydrocarbons

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Figure 1. Top) Thermal isomerization of azulene to naphthalene; Bottom) Thermal automerization of naphthalene. Isomerization and Automerization of Azulene and Naphthalene.png
Figure 1. Top) Thermal isomerization of azulene to naphthalene; Bottom) Thermal automerization of naphthalene.

Thermal rearrangements of aromatic hydrocarbons are considered to be unimolecular reactions that directly involve the atoms of an aromatic ring structure and require no other reagent than heat. These reactions can be categorized in two major types: one that involves a complete and permanent skeletal reorganization (isomerization), and one in which the atoms are scrambled but no net change in the aromatic ring occurs (automerization). [1] The general reaction schemes of the two types are illustrated in Figure 1.

Contents

This class of reactions was uncovered through studies on the automerization of naphthalene as well as the isomerization of unsubstituted azulene, to naphthalene. Research on thermal rearrangements of aromatic hydrocarbons has since been expanded to isomerizations and automerizations of benzene and polycyclic aromatic hydrocarbons.

Mechanisms

Automerizations

The first proposed mechanism for a thermal rearrangement of an aromatic compound was for the automerization of naphthalene. It was suggested that the rearrangement of naphthalene occurred due to reversibility of the isomerization of azulene to naphthalene. [2] [3] This mechanism would therefore involve an azulene intermediate and is depicted below:

Proposed mechanism of isomerization of azulene to naphthalene 1.png

Subsequent work showed that the isomerization of azulene to naphthalene is not readily reversible ( the free energy of a naphthalene to azulene isomerization was too high - approximately 90 kcal/mol). [1] A new reaction mechanism was suggested that involved a carbene intermediate and consecutive 1,2-hydrogen and 1,2-carbon shifts across the same C-C bond but in opposite directions. This is currently the preferred mechanism [4] and is as follows:

1,2-Hydrogen shift followed by 1,2-Carbon shift in automatization of naphthalene.png

Isomerizations

The isomerization of unsubstituted azulene to naphthalene was the first reported thermal transformation of an aromatic hydrocarbon, and has consequently been the most widely studied rearrangement. However, the following mechanisms are generalized to all thermal isomerizations of aromatic hydrocarbons. Many mechanisms have been suggested for this isomerization, yet none have been unequivocally determined as the only correct mechanism. Five mechanisms were originally considered: [1] a reversible ring-closure mechanism, which is shown above, a norcaradiene-vinylidene mechanism, a diradical mechanism, a methylene walk mechanism, and a spiran mechanism. It was quickly determined that the reversible ring-closure mechanism was inaccurate, and it was later decided that there must be multiple reaction pathways occurring simultaneously. This was widely accepted, as at such high temperatures, one mechanism would have to be substantially energetically favored over the others to be occurring alone. Energetic studies displayed similar activation energies for all possible mechanisms. [1]

Four mechanisms for thermal isomerizations have been proposed: a dyotropic mechanism, a diradical mechanism, and two benzene ring contraction mechanisms; a 1,2-carbon shift to a carbene preceding a 1,2-hydrogen shift, and a 1-2-hydrogen shift to a carbene followed by a 1,2-carbon shift. [5] [6] The dyotropic mechanism involves concerted 1,2-shifts as displayed below. Electronic studies show this mechanism to be unlikely, but it must still be considered a viable mechanism as it has not yet been disproven.

Diatropic rearrangement of azulene to naphthalene.png

The diradical mechanism has been supported by kinetic studies performed on the reaction, which have revealed that the reaction is not truly unimolecular, as it is most likely initiated by hydrogen addition from another gas-phase species. However, the reaction still obeys first-order kinetics, which is a classical characteristic of radical chain reactions. [7] A mechanistic rational for the thermal rearrangement of azulene to naphthalene is included below. Homolysis of the weakest bond in azulene occurs, followed by a hydrogen shift and ring closure so as to retain the aromaticity of the molecule.

Diradical mechanism for isomerization of azulene to naphthalene.png

Benzene ring contractions are the last two mechanisms that have been suggested, and they are currently the preferred mechanisms. These reaction mechanisms proceed through the lowest free energy transition states compared to the diradical and dyotropic mechanisms. The difference between the two ring contractions is minute however, so it has not been determined which is favored over the other. Both mechanisms are shown as follows for the ring contraction of biphenylene:

Carbon and hydrogen shifts in thermal rearrangements of aromatic hydrocarbons.png

The first involves a 1,2-hydrogen shift to a carbene followed by a 1,2-carbon shift on the same C-C bond but in opposite directions. The second differs from the first only by the order of the 1,2-shifts, with the 1,2-carbon shift preceding the 1,2-hydrogen shift.

The four described mechanisms would all result in the isomerization from azulene to naphthalene. Kinetic data and 13C-labeling have been used to elucidate the correct mechanism, and have led organic chemists to believe that one of the benzene ring contractions is the most likely mechanism through which these isomerizations of aromatic hydrocarbons occur. [5] [8]

History

Indications of thermal rearrangements of aromatic hydrocarbons were first noted in the early 20th century by natural products chemists who were working with sesquiterpenes. At the time, they noticed the automerization of a substituted azulene shown below, but no further structural or mechanistic investigations were made.

Automerization of sesquiterpene-esque azulene.png

The oldest characterized thermal rearrangement of an aromatic compound was that of the isomerization of azulene to naphthalene by Heilbronner et al. in 1947. [9] Since then, many other isomerizations have been recorded, however the rearrangement of azulene to naphthalene has received the most attention. Likewise, since the characterization of the automerization of naphthalene by Scott in 1977, [2] similar atom scramblings of other aromatic hydrocarbons such as pyrene, [10] azulene, [3] [11] benz[a]anthracene [12] and even benzene have been described. [13] While the existence of these reactions has been confirmed, the isomerization and automerization mechanisms remain unknown.

Reaction conditions and flash vacuum pyrolysis

Thermal rearrangements of aromatic hydrocarbons are generally carried out through flash vacuum pyrolysis (FVP). [14] In a typical FVP apparatus, a sample is sublimed under high vacuum (0.1-1.0 mmHg), heated in the range of 500-1100 °C by an electric furnace as it passes through a horizontal quartz tube, and collected in a cold trap. Sample is carried through the apparatus by nitrogen carrier gas.

FVP has numerous limitations:

Possible applications

Thermal rearrangements of aromatic hydrocarbons have been shown to be important in areas of chemical research and industry including fullerene synthesis, materials applications, and the formation of soot in combustion. [5] Thermal rearrangements of aceanthrylene and acephenanthrylene can yield fluoranthene, an important species in syntheses of corannulene and fullerenes that proceed through additional internal rearrangements. [8] [16]

Fluoranthene conversions by thermal rearrangement.png
Scott's flash vacuum pyrolysis syntheses of corannulene. Synthesis of corannulene.png
Scott's flash vacuum pyrolysis syntheses of corannulene.

Many of the polycyclic aromatic hydrocarbons known to be tumorigenic or mutagenic are found in atmospheric aerosols, which is connected to the thermal rearrangement of polycyclic aromatic hydrocarbons in fast soot formation during combustion. [16]

Related Research Articles

Aromatic compounds are organic compounds also known as "mono- and polycyclic aromatic hydrocarbons". The parent member is benzene. Heteroarenes are closely related, since at least one carbon atom of CH group is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.

Naphthalene Chemical compound

Naphthalene is an organic compound with formula C
10H
8. It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings. It is best known as the main ingredient of traditional mothballs.

Aromaticity Phenomenon providing chemical stability in resonating hybrids of cyclic organic compounds

In chemistry, aromaticity is a property of cyclic (ring-shaped), typically planar (flat) molecular structures with pi bonds in resonance that gives increased stability compared with other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability.

Silabenzene Chemical compound

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.

A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular reaction. The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon–carbon bonds and the Greek word tropos, meaning turn. In this type of rearrangement reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions are usually uncatalyzed, although Lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement, Carroll rearrangement, and the Fischer indole synthesis.

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule, hence these reactions are usually intramolecular. In the example below the substituent R moves from carbon atom 1 to carbon atom 2:

A 1,2-rearrangement or 1,2-migration or 1,2-shift or Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. In the example below the substituent R moves from carbon atom C2 to C3.

An alkyne trimerisation reaction is a [2+2+2] cycloaddition reaction in which three alkyne units react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. Being a cycloaddition reaction, it has high atom economy. Many variations have been developed including cyclisation of mixtures of alkynes and alkenes as well as alkynes and nitriles.

Aromatization is a chemical reaction in which an aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cyclohexane into benzene. Aromatization includes the formation of heterocyclic systems.

Prismane Chemical compound

Prismane or 'Ladenburg benzene' is a polycyclic hydrocarbon with the formula C6H6. It is an isomer of benzene, specifically a valence isomer. Prismane is far less stable than benzene. The carbon (and hydrogen) atoms of the prismane molecule are arranged in the shape of a six-atom triangular prism—this compound is the parent and simplest member of the prismanes class of molecules. Albert Ladenburg proposed this structure for the compound now known as benzene. The compound was not synthesized until 1973.

Cyclophane Ring molecule with two nonadjacent atoms linked by a chain

In organic chemistry, a cyclophane is a hydrocarbon consisting of an aromatic unit and a chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known. Cyclophanes are well-studied in organic chemistry because they adopt unusual chemical conformations due to build-up of strain.

Cyclic compound Molecule with a ring of bonded atoms

A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.

Bergman cyclization

The Masamune-Bergman cyclization or Masamune-Bergman reaction or Masamune-Bergman cycloaromatization is an organic reaction and more specifically a rearrangement reaction taking place when an enediyne is heated in presence of a suitable hydrogen donor. It is the most famous and well-studied member of the general class of cycloaromatization reactions. It is named for Japanese-American chemist Satoru Masamune and American chemist Robert G. Bergman. The reaction product is a derivative of benzene.

Wolff rearrangement

The Wolff rearrangement is a reaction in organic chemistry in which an α-diazocarbonyl compound is converted into a ketene by loss of dinitrogen with accompanying 1,2-rearrangement. The Wolff rearrangement yields a ketene as an intermediate product, which can undergo nucleophilic attack with weakly acidic nucleophiles such as water, alcohols, and amines, to generate carboxylic acid derivatives or undergo [2+2] cycloaddition reactions to form four-membered rings. The mechanism of the Wolff rearrangement has been the subject of debate since its first use. No single mechanism sufficiently describes the reaction, and there are often competing concerted and carbene-mediated pathways; for simplicity, only the textbook, concerted mechanism is shown below. The reaction was discovered by Ludwig Wolff in 1902. The Wolff rearrangement has great synthetic utility due to the accessibility of α-diazocarbonyl compounds, variety of reactions from the ketene intermediate, and stereochemical retention of the migrating group. However, the Wolff rearrangement has limitations due to the highly reactive nature of α-diazocarbonyl compounds, which can undergo a variety of competing reactions.

Geodesic polyarene

A geodesic polyarene in organic chemistry is a polycyclic aromatic hydrocarbon with curved convex or concave surfaces. Examples include fullerenes, nanotubes, corannulenes, helicenes and sumanene. The molecular orbitals of the carbon atoms in these systems are to some extent pyramidalized resulting a different pi electron density on either side of the molecule with consequences for reactivity.

The Wulff–Dötz reaction (also known as the Dötz reaction or the benzannulation reaction of the Fischer carbene complexes) is the chemical reaction of an aromatic or vinylic alkoxy pentacarbonyl chromium carbene complex with an alkyne and carbon monoxide to give a Cr(CO)3-coordinated substituted phenol. Several reviews have been published. It is named after the German chemist Karl Heinz Dötz (b. 1943) and the American chemist William D. Wulff (b. 1949) at Michigan State University. The reaction was first discovered by Karl Dötz and was extensively developed by his group and W. Wulff's group. They subsequently share the name of the reaction.

In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions.

The vinylcyclopropane rearrangement or vinylcyclopropane-cyclopentene rearrangement is a ring expansion reaction, converting a vinyl-substituted cyclopropane ring into a cyclopentene ring.

Benzvalene Chemical compound

Benzvalene is an organic compound and one of several isomers of benzene. It was first synthesized in 1971 by Thomas J. Katz et al.

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring.

References

  1. 1 2 3 4 Scott, Lawrence T. (1982). "Thermal rearrangements of aromatic compounds". Accounts of Chemical Research. 15 (2): 52–58. doi:10.1021/ar00074a004.
  2. 1 2 Scott, Lawrence T.; Agopian, Garabed K. (1977). "Automerization of naphthalene". Journal of the American Chemical Society. 99 (13): 4506–4507. doi:10.1021/ja00455a053.
  3. 1 2 Scott, Lawrence T.; Kirms, Mark A. (1981). "Azulene thermal rearrangements. Carbon-13 labeling studies of automerization and isomerization to naphthalene". Journal of the American Chemical Society. 103 (19): 5875–5879. doi:10.1021/ja00409a042.
  4. ↑ Scott, Lawrence T.; Hashemi, Mohammed M.; Schultz, Thomas H.; Wallace, Michael B. (1991). "Thermal rearrangements of aromatic compounds. 15. Automerization of naphthalene. New evidence consistent with the intermediacy of benzofulvene". Journal of the American Chemical Society. 113 (25): 9692–9693. doi:10.1021/ja00025a055.
  5. 1 2 3 Pastor, Michael B.; Kuhn, Ariel J.; Nguyen, Phuong T.; Santander, Mitchell V.; Castro, Claire; Karney, William L. (2013). "Hydrogen shifts and benzene ring contractions in phenylenes". Journal of Physical Organic Chemistry. 26 (9): 750–754. doi:10.1002/poc.3126.
  6. ↑ Cioslowski, Jerzy; Schimeczek, Michael; Piskorz, Pawel; Moncrieff, David (1999). "Thermal Rearrangement of Ethynylarenes to Cyclopentafused Polycyclic Aromatic Hydrocarbons: An Electronic Structure Study". Journal of the American Chemical Society. 121 (15): 3773–3778. doi:10.1021/ja9836601.
  7. ↑ Scott, Lawrence T. (1984). "Thermal rearrangements of aromatic compounds, part 8. Azulene-to-naphthalene rearrangement. A comment on the kinetics". The Journal of Organic Chemistry. 49 (16): 3021–3022. doi:10.1021/jo00190a030.
  8. 1 2 Scott, Lawrence T.; Roelofs, Nicolas H. (1987). "Thermal rearrangements of aromatic compounds. 11. Benzene ring contractions at high temperatures. Evidence from the thermal interconversions of aceanthrylene, acephenanthrylene, and fluoranthene". Journal of the American Chemical Society. 109 (18): 5461–5465. doi:10.1021/ja00252a025.
  9. ↑ Heilbronner, E.; Plattner, P. A.; Wieland, K. Rearrangement of Azulene to Naphthalene. Experientia1947, 3, 70–71.
  10. ↑ Scott, L. T.; Kirms, M. A.; Berg, A.; Hansen, P. E. Automerization of Pyrene a Test for the Mechanism of Naphthalene Automerization. Tetrahedron Letters1982, 23 (18), 1859–1862. DOI: 10.1016/S0040-4039(00)87204-4
  11. ↑ Becker, Juergen; Wentrup, Curt; Katz, Ellen; Zeller, Klaus Peter (1980). "Azulene-naphthalene rearrangement. Involvement of 1-phenylbuten-3-ynes and 4-phenyl-1,3-butadienylidene". Journal of the American Chemical Society. 102 (15): 5110–5112. doi:10.1021/ja00535a056.
  12. ↑ Scott, L. T.; Tsang, T.-H.; Levy, L. A. Automerizations in Benzenoid Hydrocarbons. New Mechanistic Insights from the Thermal Rearrangement of Benz[a]anthracene-5-13C. Tetrahedron Letters1984, 25 (16), 1661–1664. DOI: 10.1016/S0040-4039(01)81138-2
  13. ↑ Scott, Lawrence T.; Roelofs, Nicolas H.; Tsang, Tsze Hong (1987). "Thermal rearrangements of aromatic compounds. 10. Automerization of benzene". Journal of the American Chemical Society. 109 (18): 5456–5461. doi:10.1021/ja00252a024.
  14. 1 2 Tsefrikas, Vikki M.; Scott, Lawrence T. (2006). "Geodesic Polyarenes by Flash Vacuum Pyrolysis". Chemical Reviews. 106 (12): 4868–4884. doi:10.1021/cr050553y. PMID   17165678.
  15. 1 2 Sygula, Andrzej; Rabideau, Peter W. (2006). "Synthesis and Chemistry of Polycyclic Aromatic Hydrocarbons with Curved Surfaces: Buckybowls". Carbon-Rich Compounds. pp. 529–565. doi:10.1002/3527607994.ch12. ISBN   9783527607990.
  16. 1 2 Richter, H.; Grieco, W. J.; Howard, J. B. Formation Mechanism of Polycyclic Aromatic Hydrocarbons and Fullerenes in Premixed Benzene Flames. Combustion and Flame1999, 119 (1–2), 1–22. DOI: 10.1016/S0010-2180(99)00032-2
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