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Electrocyclic Reactions
[edit]Introduction: What is an Electrocyclic Reaction?
[edit]Electrocyclic Reactions are intramolecular ring closing/opening reactions of conjugated polyenes/cycloalkenes. They involve a cyclic transition state. A ring closing electrocyclic reaction typically results in the conversion of a π bond to a σ bond whereas the reverse ring opening reactions converts a σ bond to a π bond. [1]
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The transition states arising in electrocyclic reactions can either be formed via a conrotatory or a disrotatory mode of transition state formation as dictated by symmetry requirements. These requirements are predicted by the Woodward-Hoffmann rules.
Whether the reaction is allowed to proceed in a conrotatory or disrotatory fashion depends on the number of electrons participating in the reaction and on whether or not the cyclization is induced thermally or photochemically.
| system | Thermally Induced (ground state) | Photochemically Induced (excited state) |
|---|---|---|
| "4n" e- | Conrotatory | Disrotatory |
| "4n + 2" e- | Disrotatory | Conrotatory |
The stereospecificity is then determined by whether the reaction proceeds through a conrotatory or disrotatory process.
Theory Behind Electrocyclic Reactions: Allowed or Forbidden Processes
[edit]A classic example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene to 2,4-hexadiene[2] compared to the thermal ring opening of 5,6-dimethylcyclohexa-1,3-diene to 2,4,6-octatriene.[3]
The stereospecificity of these reactions can be explained by (1) the Woodward-Hoffman Rules and (2) Frontier Molecular Orbital Theory.
- 1. The Woodward-Hoffman Rules
Correlation diagrams, which connect the molecular orbitals of the reactant to those of the product having the same symmetry, can then be constructed for the two processes.[4]
These correlation diagrams indicate that only a conrotatory ring opening of 3,4-dimethylcyclobutene is symmetry allowed whereas only a disrotatory ring opening of 5,6-dimethylcyclohexa-1,3-diene is symmetry allowed. This is because only in these cases would maximum orbital overlap occur in the transition state. Also, the formed product would be in a ground state rather than an excited state.
- 2. Frontier Molecular Orbital Theory
According to the Frontier Molecular Orbital Theory, the sigma bond in the ring will open in such a way that the resulting p-orbitals will have the same symmetry as the HOMO of the product.[5]
For the 5,6-dimethylcyclohexa-1,3-diene, only a disrotatory mode would result in p-orbitals having the same symmetry as the HOMO of hexatriene. For the 3,4-dimethylcyclobutene, on the other hand, only a conrotatory mode would result in p-orbitals having the same symmetry as the HOMO of butadiene.
- Excited State Electrocyclizations
If the ring opening of 3,4-dimethylcyclobutene were carried out under photochemical conditions the resulting electrocyclization would be occur via a disrotatory mode instead of a conrotatory mode as can be seen by the correlation diagram for the allowed excited state ring opening reaction.
Only a disrotatory mode, in which symmetry about a reflection plane is maintained throughout the reaction, would result in maximum orbital overlap in the transition state. Also, once again, this would result in the formation of a product that is in an excited state of comparable stability to the excited state of the reactant compound.
Electrocyclic Reactions in Biological Systems
[edit]Electrocyclic reactions occur frequently in nature.[6] One of the most common such electrocyclizations is the biosynthesis of Vitamin D3.
The first step involves a photochemically induced conrotatory ring opening of 7-dehydrocholesterol to form pre vitamin D3. A [1,7]-hydride shift then forms Vitamin D3.
Another example is in the proposed biosynthesis of aranotin, a naturally occurring oxepine, and its related compounds.
Enzymatic epoxidation of phenylalanine-derived diketopiperazine forms the arene oxide, which undergoes a 6π disrotatory ring opening electrocyclization reaction to produce the uncyclized oxepine. After a second epoxidation of the ring, the nearby nucleophilic nitrogen attacks the electrophilic carbon, forming a five membered ring. The resulting ring system is a common ring system found in aranotin and its related compounds.
The benzonorcaradiene diterpenoid (A) was rearranged into the benzocycloheptatriene diterpenoid isosalvipuberlin (B) by boiling a methylene chloride solution. This transformation can be envisaged as a disrotatory electrocyclic reaction, followed by two suprafacial 1,5-simatropic hydrogen shifts, as shown bellow.[7]
References
[edit]- ^ Miller, Bernard. Advanced Organic Chemistry (2nd edition). 2004 (Pearson Education, Inc) ISBN 0-13-065588-0
- ^ The preparation and isomerization of - and -3,4-dimethylcyclobutene. Tetrahedron Letters, Volume 6, Issue 17, 1965, Pages 1207-1212 Rudolph Ernst K. Winter doi:10.1016/S0040-4039(01)83997-6
- ^ Electrocyclic reactions: stereochemistry of the triene electrocyclization. Tetrahedron (journal), Volume 29, Issue 23, 1973, Pages 3781-3789 E. N. Marvell, G. Caple, B. Schatz, and W. Pippin doi:10.1016/0040-4020(73)80195-4
- ^ 2. The conservation of orbital symmetry. Acc. Chem. Res., Volume 1, Issue 1, 1968, Pages 17–22 Roald Hoffmann and Robert B. Woodward doi:10.1021/ar50001a003
- ^ Fleming, Ian. Frontier Orbitals and Organic Chemical Reactions. 1976 (John Wiley & Sons, Ltd.) ISBN 0-471-01820-1
- ^ Biosynthetic and Biomimetic Electrocyclizations. Chem. Rev., Volume 105, Issue 12, 2005, Pages 4757-4778 Christopher M. Beaudry, Jeremiah P. Malerich, and Dirk Trauner doi:10.1021/cr0406110
- ^ J. T. Arnason, Rachel Mata, John T. Romeo. Phytochemistry of Medicinal Plant(2nd Edition).1995 (Springer) ISBN:0306451816, 9780306451812