Quick Reference toPericyclic Reactions and Photochemistry Claude Legault Litterature Meeting December 13th, 2004
Goal of the Presentation Not a course, not a litterature meeting... Small review of orbital symmetry rules Simple tricks to determine permitted and forbidden processes Classify Pericyclic Reactions into main classes Explain the basics of photochemistry
Definition of the Pericyclic Reactions Definition Concerted reactions going through a cyclic transition state Types 1. Cycloaddition (Diels-Alder, [3+2]-cycloaddition, ene reactions) Reactions with the formation of two sigma bonds between the extremity of two pi systems. (ene impliquates a sigma system) 2. Cheletropic reactions Formation or breakage of two sigma bond between the extremity of a pi system and one atom 3. Electrocyclic rearrangments Formation or breakage of a sigma bond between the extremity of one pi system 4. Sigmatropic rearrangments [1,j] : Migration of an atom along a pi system [i,j] : Migration of a sigma bond between two pi systems 5. Dyotropic rearrangments Simultaneous intramolecular migration of two sigma bonds
This is general... Ψ4 So a first simple thing to remember: 4n+2 (2,6,10,...) electrons polyenes: The HOMO will be symmetric 4n (4,8,12,...) electrons polyenes: The HOMO will be antisymmetric Ψ3 Ψ2 Ψ1 Simplist View of Polyenes systems Ψ4 Ψ3 Ψ2 Ψ1
4n+2 electrons processes (2,6,10,14...) Only SUPRA permitted thermally 4n electrons processes (4,8,12,16...) Only ANTARA permtted thermally [π6s+π4s] : 10 electrons SUPRA process -> Permitted First Trick : SUPRA and ANTARA Nomenclature Suprafacial Rxn happening on the same side of the pi system Antarafacial Rxn happening on the opposite sides of the pi system Suprafacial + Suprafacial = SUPRA Suprafacial + Antarafacial = ANTARA Antarafacial + Suprafacial = ANTARA Antarafacial + Antarafacial = SUPRA When two pi systems reacts in a cycloaddition Examples: [π2s+π2s] : 4 electrons SUPRA process -> Forbidden [π4s+π2s] : 6 electrons SUPRA process -> Permitted ?
DA and HDA and Definitions Diels-Alder relates only to [π4s+π2s] Intermolecular Intramolecular Level 1 Intramolecular Level 2
An Insight in the DA Process (Correlation Diag) Spino, C. et al. Angew. Chem., Int. Ed. 1998, 37, 3262.
The Klopman-Salem Equation Filled Orbitals Repulsion Coulombic Interactions Orbitals Interactions
(ca+m)(cb+n) + (ca)(cb) = (ca+m)(cb) + (ca)(cb+n) mn > 0 Why the Observed Regio? βab(ca+m)(cb+n) + βab(ca)(cb) = βab(ca+m)(cb) + βab(ca)(cb+n) cacb + can + cbm + mn + cacb = cacb + cbm + cacb+ can
[3+2] Cycloaddition Analoguous to the Diels-Alder Reaction [π4s+π2s] Fu, G. C. et al. J. Amer. Chem. Soc. 2003, 125, 10778.
[3+2] Cycloaddition Stoltz, B. M. et al. J. Amer. Chem. Soc. 2003, 125, 15000.
Ene Reaction : Special Case of Cycloaddition [(π2s+σ2s)+π2s] Usually facilitated by a enophile with a low lying LUMO enophile Intramolecular Level 1 Intramolecular Level 2
Some Application... Inomata, K. et al. Org. Lett. 2004, 6, 409.
Recent Use of a Ene/Retro-Ene Process Corey, E. J. et al. Org. Lett. 2003, 5, 1999. Halls, D. G. et al. J. Org. Chem. 2004, 69, 8429.
Electrocyclic Rearrangments Why? Driving forces can be: - Geometric tension - Aromatic stabilization (formation of an aromatic moiety) - Delocalization (opening or closing permits delocalization in an other pi system)
The Same Trick for Opening/Closing How to know how it closes or open? The simple trick: Take the open system (polyene) and apply the same principle as seen earlier by simply considering the HOMO of the polyene 4n+2 electrons HOMO (2,6,10,14...) Only Disrotatory opening/closing permitted thermally 4n electrons HOMO (4,8,12,16...) Only Conrotatory opening/closing permtted thermally Conrotatory (4n e-) Disrotatory (4n+2 e-)
Torquoselectivity If R is electrondonating, then this filled orbital would unfavorably interact with the breaking sigma bond, so outward opening is favored. If R is electronwithdrawing, then this empty orbital would favorably interact with the breaking sigma bond, so inward opening is favored. Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5072.
Torquoselectivity : Some Useful Values Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5072.
Recent Example of a Electrocyclic Rearrangment Hsung, R. P. et al. J. Org. Chem. 2003, 68, 1729.
[1,j] Shift, Definition and possibility Suprafacial + retention = SUPRA Suprafacial + inversion = ANTARA Antarafacial + retention = ANTARA Antarafacial + inversion = SUPRA 4n+2 electrons processes (2,6,10,14...) Only SUPRA permitted thermally 4n electrons process (4,8,12,16...) Only ANTARA permtted thermally Hydrogen shift: The valence orbital of H being an 1S (spherical) orbital), inversion is impossible with this atom. An antarafacial migration is only possible with a polyene of at least 6 carbons. In cyclic systems, antarafacial migrations are impossible in smaller than 10 membered rings.
[i,j] Shift, Even More Possibility Migration of a sigma along two pi systems. Only suprafacial-suprafacial migrations are allowed geometrically So... Only 4n+2 (2,6,10,14...) electrons processes (SUPRA) are permitted thermally Some nomenclature... Cope Rearrangment Claisen Rearrangment Oxy-Cope Rearrangment Wittig[2,3] Rearrangment
Level 2 intramolecular ene Recent Examples of [i,j] Shifts, Barriault Oxy-cope Claisen Barriault, L. et al. J. Am. Chem. Soc. 2004, 126, 8569.
Simultaneous intramolecular migration of two sigma bonds Dyotropic Rearrangments Type 2 Type 1 Recent example of a type 2 dyotropic shift used in a total synthesis Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5111.
Now for Photochemistry Advantages: Non thermic activation (low temperature) Give access to thermally forbidden processes Photopumping (synthesis of high energy molecules) Selective activation of chromophores: hυ λ =300 nm hυ λ =254 nm hυ λ =180 nm
Basics Explained hυ The LUMO of the compound becomes a SOMO, the highest occupied orbital, so the reaction occurs through that orbital: A thermally fobidden process now becomes permitted Quantum Yield: Φ = # molecules transformed / # photons absorbed The way to measure it : Using an actinometer
Jablonsky Diagram π* π* hυ Ca Cb Ca Cb π π σ σ Double bond Single bond
Photodissociation σ* σ* hυ Ca Cb Ca Cb σ σ Single bond Radical pair
E*3norbonene =309 kJ Photosensibilization With benzophenone E*3benzophenone =288 kJ hυ ISC With acetophenone E*3acetophenone =309 kJ hυ ISC
Norrish Type 1 fragmentation Classical Reactions (Norrish Fragmentation) hυ Norrish Type 2 fragmentation hυ
Photopumping hυ low temp.