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In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction. The reaction is electrocyclic if the result is one pi bond becoming one sigma bond or one sigma bond becoming a pi bond. Electrocyclic reactions share the following properties:
- electrocyclic reactions are driven by light (photoinduced) or heat (thermal)
- the reaction mode is determined by the number of pi electrons in the part with more pi bonds
- an electrocyclic reaction can close a ring (electrocyclization) or open a ring
- the stereospecifity is determined by a conrotatory or a disrotatory transition state formation as predicted by the Woodward–Hoffmann rules.
The torquoselectivity in an electrocyclic reaction refers to the direction that the substituents rotate. For example, the substituents in a reaction that is conrotatory can still rotate in two directions. It produces a mixture of two products that are the mirror image of each other (enantiomeric products). A reaction that is torquoselective restricts one of these directions of rotation (partially or completely) to produce a product in enantiomeric excess (where one stereoisomer is produced much more than the other).
Chemists are interested in electrocyclic reactions because the geometry of the molecules confirm a number of predictions made by theoretical chemists. They confirm the conservation of molecular orbital symmetry.
The frontier-orbital method explains how this reaction works. The sigma bond in the reactant will open in a way that the resulting p-orbitals will have the same symmetry as the highest occupied molecular orbital (HOMO) of the product (a butadiene). This can only happen with a conrotatory ring-opening that results in opposite signs for the two lobes at the broken ends of the ring. (A disrotatory ring-opening would form an anti-bond.) The following diagram shows this:
|system||Thermally Induced (ground state)||Photochemically Induced (excited state)|
|"4n + 2" e-||Disrotatory||Conrotatory|
The stereospecificity of the result depends on whether the reaction proceeds through a conrotatory or disrotatory process.
Woodward-Hoffman rules[change | change source]
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.
Frontier molecular orbital theory[change | change source]
The above diagram shows two examples. For the 5,6-dimethylcyclohexa-1,3-diene (top row of diagram), only a disrotatory mode would result in p-orbitals having the same symmetry as the HOMO of hexatriene. The two p-orbitals rotate in opposite directions. For the 3,4-dimethylcyclobutene (bottom row of diagram), only a conrotatory mode would result in p-orbitals having the same symmetry as the HOMO of butadiene. The p-oribtals rotate in the same direction.
Excited state electrocyclizations[change | change source]
Light can move an electron up to an excited state that occupies a higher orbital. The excited electron will occupy the LUMO, which has a higher energy level than the electron's old orbital. If light opens the ring of 3,4-dimethylcyclobutene, the resulting electrocyclization would be occur by a disrotatory mode instead of a conrotatory mode. The correlation diagram for the allowed excited state ring opening reaction shows why:
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[change | change source]
The first step involves light opening the ring of 7-dehydrocholesterol to form pre vitamin D3. This is a photochemically induced conrotatory electrocyclic reaction. The second step is a [1,7]-hydride shift to make vitamin D3.
Phenylalanine is used to make diketopiperazine (not shown). Then enzymes epoxidate diketopiperazine to make the arene oxide. This 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 thought of as a disrotatory electrocyclic reaction, followed by two suprafacial 1,5-sigmatropic hydrogen shifts, as shown below:
Scope[change | change source]
An example of an electrocyclic reaction is the conrotatory thermal ring-opening of benzocyclobutane. The reaction product is a very unstable ortho-quinodimethane. This molecule can be trapped in an endo addition with a strong dienophile such as maleic anhydride to the Diels-Alder adduct. The chemical yield for the ring opening of the benzocyclobutane depicted in scheme 2 is found to depend on the nature of the substituent R. With a reaction solvent such as toluene and a reaction temperature of 110 °C, the yield increases going from methyl to isobutylmethyl to trimethylsilylmethyl. The increased reaction rate for the trimethylsilyl compound can be explained by silicon hyperconjugation as the βC-Si bond weakens the cyclobutane C-C bond by donating electrons.
References[change | change source]
- "electrocyclic reaction" (PDF). IUPAC Compendium of Chemical Terminology (Second ed.). 1997. Retrieved 2011-09-21.
- 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
- 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
- Accelerated Electrocyclic Ring-Opening of Benzocyclobutenes under the Influence of a -Silicon Atom Yuji Matsuya, Noriko Ohsawa, and Hideo Nemoto J. Am. Chem. Soc.; 2006; 128(2) pp 412 - 413; (Communication) DOI: 10.1021/ja055505+ Abstract
- The endiandric acid cascade. Electrocyclizations in organic synthesis. 4. Biomimetic approach to endiandric acids A-G. Total synthesis and thermal studies K. C. Nicolaou, N. A. Petasis, R. E. Zipkin J. Am. Chem. Soc., 1982, 104 (20), pp 5560–5562 doi:10.1021/ja00384a080
- Inspirations, Discoveries, and Future Perspectives in Total Synthesis K. C. Nicolaou J. Org. Chem., 2009 Article ASAP doi:10.1021/jo802351b