2 Electrocyclic Reactions
Introduction Electrocyclic reactions Reactions in which conjugated polyenes close to form rings or in which rings open to form polyenes simply by having electrons chase to each other’s tails. favor to ring formation favor to ring opening Pericyclic reactions Electrocyclic reactions belong to a broader class of reactions. These reactions proceed via transition state. Usually favor the ring formation because p bond concert to s bond, but strain in cyclobutene causes to open.
Introduction The difference in energy between the open-chain and closed-ring isomers in electrocyclic reactions are usually not great. Thus, partial conversion of the more stable isomers in this reactions to the less stable forms may easily occur as steps. (example) cis,cis-2,4-hexadiene converted to trans,trans-2,4-hexadiene via two electrocyclic steps
Conrotatory and Disrotatory Processes Conrotatory Process In electrocyclic reaction, the terminal carbons of a polyene or the saturated carbons of the cyclized form rotate in the same direction (clockwise or counterclockwise direction)
Conrotatory and Disrotatory Processes In electrocyclic reaction, the terminal carbons of a polyene or the saturated carbons of the cyclized form rotate in the different direction. (one clockwise and other counterclockwise direction)
Conrotatory and Disrotatory Processes Woodward-Hoffmann Rules Stereochemistry of pericyclic reactions are governed by selection rules. (allowed or forbidden) Forbidden reactions are expected to occur with much higher activation energy than allowed reaction. “thermal electrocyclic reactions involving 4n electrons are allowed if they proceed by conrotatory paths: thermal electrocyclic reactions involving 4n+2 electrons are allowed if they proceed by disrotatory paths” Example: Ring closure of 1,3-butadiene and ring opening of cyclobutene (4n electrons) – conrotatory path Ring closure of 1,3,5-hexatriene and ring opening of cyclohexadiene (4n+2 electrons) – disrotatory path
Conrotatory and Disrotatory Processes Conrotatory path (allowed) No detection Disrotatory path (forbidden)
Conrotatory and Disrotatory Processes Woodward-Hoffmann rules should apply to thermal Electrocyclic reactions involving any number of electrons 4n electrons : conrotatory path allowed
Conrotatory and Disrotatory Processes There are two modes of rotation in electrocyclic reaction - clockwise and counterclockwise Electron donating groups (EDG) and weakly electron withdrawing groups (EWG) tend to end up “outside” position, while strongly electron accepting substituents rotate toward to “inside” position Weakly EWG outside position Stongly EWG Inside position
Conrotatory and Disrotatory Processes Woodward-Hoffmann rules can account for the stabilities of some molecules that might be quite reactive. ring strain effect Compound 2 is stable at up to 260 oC even though cyclobutenes normally open to butadienes on heating at steam-bath
Conrotatory and Disrotatory Processes cis-Fused isomer requires a temperature nearly 200 oC than trans-fused isomer to have the central ring open Conrotatory ring opening allowed - Formation of Strained rings from the cis isomer
Conrotatory and Disrotatory Processes Woodward-Hoffmann rules can explain the existence of some extraordinarily high-energy molecules. Aromatic ring (stable) exothermic Strained isomer Compound 3 is stable at room temperature, although it could isomerize to benzene simply by changing the bond angle or by only slightly higher temperature.
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO)
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO)
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO)
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO)
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO)
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO) A : Antisymmtric S : Symmetric
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO) A : Antisymmtric S : Symmetric
Explanations for The Woodward-Hoffmann Rules Symmetries of Molecular Orbitals (MO) HOMO (highest occupied molecular orbital) LUMO (lowest unoccupied molecular orbital)
Explanations for The Woodward-Hoffmann Rules The Frontier Orbital Approach It is common for the chemical properties of atoms to be approximated by considering only the valence orbitals (highest occupied orbital). A similar approach can be employed with molecules (HOMO : the frontier orbitals) Disrotatory reaction
Explanations for The Woodward-Hoffmann Rules The Frontier Orbital Approach It is common for the chemical properties of atoms to be approximated by considering only the valence orbitals (highest occupied orbital). A similar approach can be employed with molecules (HOMO : the frontier orbitals) Conrotatory reaction
Explanations for The Woodward-Hoffmann Rules Correlation Diagrams A chart that follows the molecular orbitals of the starting materials in a reaction and shows how they are converted to the molecular orbitals of the products.
Explanations for The Woodward-Hoffmann Rules Correlation Diagrams The lowest-energy orbital of triene j1 can converted to lowest-energy orbital of cyclohexadiene s. The second lowest-energy orbital of triene j2 can converted to third lowest- energy orbital of cyclo- hexadiene j2 (HOMO). The third lowest-energy orbital of triene j3 can converted to second lowest- hexadiene j1.
Explanations for The Woodward-Hoffmann Rules Correlation Diagrams The lowest-energy orbital of diene j1 can converted to lowest-energy orbital of cyclobutene s. The second lowest-energy orbital of diene j2 can converted to third lowest- energy orbital of cyclo- butene p*. The third lowest-energy orbital of triene j3 can converted to second lowest- butene p. This process would lead to a very high-energy excited state of the product.
Explanations for The Woodward-Hoffmann Rules Correlation Diagrams
Explanations for The Woodward-Hoffmann Rules Correlation Diagrams The lowest-energy orbital of diene j1 can converted to lowest-energy orbital of cyclobutene p. The second lowest-energy orbital of diene j2 can converted to the lowest- energy orbital of cyclo- butene s. The two filled orbitals of 1,3-butadienes can be converted to the two lower-energy orbitals of cyclobutene, so that The conrotatory ring closure is a symmetry-allowed process
Electrocyclic Reactions with Odd Numbers of Atoms Theoretical Predictions If SM is a cation (two electrons), the reaction is allowed because j1 and s orbitals have the same symmtry. However, if the chain is an anion (four electrons), an excited state would be produced and the reaction is forbidden. In contrast, a contotatory process is allowed for the anion but is forbidden for the cation
Electrocyclic Reactions with Odd Numbers of Atoms Reactions of Cations and Anions
Electrocyclic Reactions with Odd Numbers of Atoms Reactions of Cations and Anions The ring opening of cyclopropyl derivatives to allylic cations do proceed in disrotatory fashion. At higher temperature, cation 6a isomerize to 6b, which in turn, as the temperature is raised, isomerizes to the least crowded cation, 6c.
Electrocyclic Reactions with Odd Numbers of Atoms Reactions of Cations and Anions 7
Electrocyclic Reactions with Odd Numbers of Atoms Reactions of Cations and Anions
Electrocyclic Reactions with Odd Numbers of Atoms Reactions of Cations and Anions
Electrocyclic Reactions with Odd Numbers of Atoms Formation and Cyclization of Dipolar Molecules
Electrocyclic Reactions with Odd Numbers of Atoms Formation and Cyclization of Dipolar Molecules Conrotatory path allowed
Electrocyclic Reactions with Odd Numbers of Atoms Formation and Cyclization of Dipolar Molecules
Photochemical Cyclizations Photochemical Reactions
Photochemical Cyclizations Photochemical Reactions Woodward-Hoffmann rules to photochemical cyclizations The stereochemistry of photochemical cyclization should be opposite to that of thermal cyclization : 4n electrons (disrotatory) and 4n+2 electrons (conrotatory)
Photochemical Cyclizations Stereochemistry of Photochemical Electrocyclic Reactions
Photochemical Cyclizations Stereochemistry of Photochemical Electrocyclic Reactions
Photochemical Cyclizations Nonstereospecific Ring Openings 1,3-Butadiene cyclize on photoirradiation with UV light with Wavelengths above 220 nm to form the predicted disrotatory Ring closure products
Photochemical Cyclizations Nonstereospecific Ring Openings