1. Quick Reference to Pericyclic Reactions and Photochemistry Claude Legault Litterature Meeting December 13 th, 2004

2. 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

3. 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

4. Simplist View of Polyenes systems Ψ1Ψ1 Ψ2Ψ2 Ψ3Ψ3 Ψ4Ψ4 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 This is general... Ψ1Ψ1 Ψ2Ψ2 Ψ3Ψ3 Ψ4Ψ4

5. 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 4n+2 electrons processes (2,6,10,14...) Only SUPRA permitted thermally 4n electrons processes (4,8,12,16...) Only ANTARA permtted thermally When two pi systems reacts in a cycloaddition Examples: [ π 2s+ π 2s] : 4 electrons SUPRA process -> Forbidden [ π 4s+ π 2s] : 6 electrons SUPRA process -> Permitted [ π 6s+ π 4s] : 10 electrons SUPRA process -> Permitted ?

6. DA and HDA and Definitions Intermolecular Intramolecular Level 1 Intramolecular Level 2 Diels-Alder relates only to [ π 4s+ π 2s]

7. An Insight in the DA Process (Correlation Diag) Spino, C. et al. Angew. Chem., Int. Ed. 1998, 37, 3262.

8. The Klopman-Salem Equation Coulombic Interactions Orbitals Interactions Filled Orbitals Repulsion

9. Why the Observed Regio? β ab (c a +m)(c b +n) + β ab (c a )(c b ) = β ab (c a +m)(c b ) + β ab (c a )(c b +n) c a c b + c a n + c b m + mn + c a c b = c a c b + c b m + c a c b + c a n mn > 0 (c a +m)(c b +n) + (c a )(c b ) = (c a +m)(c b ) + (c a )(c b +n)

10. Useful General Values

11. [3+2] Cycloaddition Fu, G. C. et al. J. Amer. Chem. Soc. 2003, 125, 10778. Analoguous to the Diels-Alder Reaction [ π 4s+ π 2s]

12. [3+2] Cycloaddition Stoltz, B. M. et al. J. Amer. Chem. Soc. 2003, 125, 15000.

13. [3+2] Cycloaddition, Bond Strength Importance

14. [3+2] Cycloaddition, Some Useful Values

15. Ene Reaction : Special Case of Cycloaddition Intramolecular Level 1 Intramolecular Level 2 [( π 2s+ σ 2s)+ π 2s] Usually facilitated by a enophile with a low lying LUMO enophile

16. Some Application... Inomata, K. et al. Org. Lett. 2004, 6, 409.

17. 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.

18. 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)

19. 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 Disrotatory (4n+2 e - )Conrotatory (4n e - )

20. 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.

21. Torquoselectivity : Some Useful Values Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5072.

22. Recent Example of a Electrocyclic Rearrangment Hsung, R. P. et al. J. Org. Chem. 2003, 68, 1729.

23. [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.

24. [i,j] Shift, Even More Possibility So... Only 4n+2 (2,6,10,14...) electrons processes (SUPRA) are permitted thermally Migration of a sigma along two pi systems. Only suprafacial-suprafacial migrations are allowed geometrically Some nomenclature... Cope Rearrangment Oxy-Cope Rearrangment Claisen Rearrangment Wittig[2,3] Rearrangment

25. Recent Examples of [i,j] Shifts, Barriault Oxy-cope Claisen Level 2 intramolecular ene Barriault, L. et al. J. Am. Chem. Soc. 2004, 126, 8569.

26. Dyotropic Rearrangments Type 1 Type 2 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. Simultaneous intramolecular migration of two sigma bonds

27. 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: hυhυ hυhυ hυhυ λ =300 nm λ =254 nm λ =180 nm

28. Basics Explained hυhυ Quantum Yield: Φ = # molecules transformed / # photons absorbed The way to measure it : Using an actinometer 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

29. Some Numbers (Activation Energy)

30. First the Practical Stuff... (Absorbance)

31. Jablonsky Diagram Double bondSingle bond hυhυ π π* π σσ CaCa CaCa CbCb CbCb

32. Photodissociation hυhυ Single bondRadical pair σσ σ* CaCa CaCa CbCb CbCb

33. Photosensibilization

34. With acetophenone With benzophenone E *3 norbonene = 309 kJ E *3 benzophenone = 288 kJ E *3 acetophenone = 309 kJ hυhυ hυhυ ISC

35. Classical Reactions (Norrish Fragmentation) Norrish Type 1 fragmentation hυhυ Norrish Type 2 fragmentation hυhυ

36. Photopumping hυhυ low temp.