D. A. Evans Introduction to Photochemistry Chem 206 Worthwhile General reviews http:/www.courses.fasharvardedu/-chem206/ New insights into an old mechanism: [2+ 2] photocycloaddition of enones to alkenes Schuster, D L; Lem, G; Kaprinidis, N. A. Chem. Rev. 1993, 93, 3 Chemistry 206 Stereoselective intermolecular[2+2-photocycloaddition reactions and their application in synthesis. Bach, T. Synthesis 1998, 683-703 Advanced Organic Chemistry 2+2 photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products. Winkler, J. D; Bowen, C M, Liotta, F. Chem. Rev. 1995, 95, 2003- Lecture number 2020. i "Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions Crimmins, M T.chem.Rev.1988.88.1453 Introduction to Photochemistry The meta photocycloaddition of arenes to alkenes. Cormelisse, Chem. Rev. 1993, 93, Introduction to electronic Excitation 615 a Franck-Condon Principle, Jabolonski Diagram a Photochemistry of Olefins and Dienes a Photochemistry of Carbonyl Compounds a Norrish Type-I& II Processes Paterno Buchi Reaction Chem 206, 1999 Final Exam Question. Provide a mechanism for the followin transfomation that explains the observed stereochemistry ( Marshall, JoC, 1971, 214). I[2+ 2] Photocycloaddition of Olefins Reading Assignment for this Lecture fourth Edition, Chepter 13, "Photochemistry",pp743-789 y Carey, and Sundberg, Advanced Organic Chemistry, Part Stereoselective intermolecular[2+2 -photocycloaddition reactions and their application in synthesis. Bach, T. Synthesis 1998, 683-703. Mechanism?? [2+2 photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural prod Minkler. J. D: Bowen. C. M . Liotta. F. Chem. Rev. 1995. 95, 2003-2020.(handout Matthew d. shair Friday December 20. 2002 The above reaction forms the basis of a photolabile protecting troup strategy for amines and alcohols
D. A. Evans Chem 206 Matthew D. Shair Friday, December 20, 2002 http://www.courses.fas.harvard.edu/~chem206/ Reading Assignment for this Lecture: Introduction to Photochemistry Chemistry 206 Advanced Organic Chemistry Lecture Number 36 Introduction to Photochemistry ■ Introduction to Electronic Excitation ■ Franck-Condon Principle, Jabolonski Diagram ■ Photochemistry of Olefins and Dienes ■ Photochemistry of Carbonyl Compounds ■ Norrish Type-I & II Processes ■ Paterno-Büchi Reaction ■ [2 + 2] Photocycloaddition of Olefins Carey, and Sundberg, Advanced Organic Chemistry, Part A fourth Edition, Chepter 13, "Photochemistry", pp 743-789 "New insights into an old mechanism: [2 + 2] photocycloaddition of enones to alkenes.", Schuster, D. I.; Lem, G.; Kaprinidis, N. A. Chem. Rev. 1993, 93, 3. "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. "[2+2] photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products.", Winkler, J. D.; Bowen, C. M.; Liotta, F. Chem. Rev. 1995, 95, 2003- 2020. "Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions.", Crimmins, M. T. Chem. Rev. 1988, 88, 1453. "The meta photocycloaddition of arenes to alkenes.", Cornelisse,Chem. Rev. 1993, 93, 615. Worthwhile General Reviews Chem 206, 1999 Final Exam Question. Provide a mechanism for the following transformation that explains the observed stereochemistry (Marshall, JOC, 1971, 214). HO Me hn PhH O Me "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. (handout) "[2+2] photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products.", Winkler, J. D.; Bowen, C. M.; Liotta, F. Chem. Rev. 1995, 95, 2003-2020. (handout) O Me hn Me O Mechanism?? NO2 O H N O R hn R –NH2 + CO2 NO CHO The above reaction forms the basis of a photo-labile protecting gtroup strategy for amines and alcohols
B Breit D.A. Evans Introduction to Photochemistry Chem 206 Background Reading Knowing the absorption wavelength in nm, you can calculate the the energ Carey, and Sundberg Advanced Organic Chemistry, Parts A Third Edition, Chepter 13, "Photochemistry, pp 729-765 E( kcal/mol)=286×10 for 1= 200 nm:E=143 kcal Wo a( in nm) forλ New insights into an old mechanism: [2+2 photocycloaddition of enones to alkenes ,I Schuster, D L; Lem, G. Kaprinidis, N. A Chem. Rev. 1993, 93, 3. Stereoselective intermolecular[2+2H-photocycloaddition reactions and their application in synthesis., Bach, T. Synthesis 1998, 683-703 Consider a simple diatomic molecule A-B 2+2 photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products. Winkler, J D; Bowen, C. M: Liotta, F. Chem. Rev. 1995, 95, 2003- ·A-B A-B Synthetic Applications of Intramolecular Enone-olefin Photocycloadditions Crimmins, M T.chem.Rev198888,1453 The meta photocycloaddition of arenes to alkenes Cornelisse, J. Chem. Rev. 1993, 93, internuclear distance B A electronic ■ Morse curve a Important Regions of the Electromagnetic Spectrum gA-B Basic Types Information Energy Range excited state Ultraviolet-Visible Electronic States 40-140 kcal/mol Infrared Functional Groups 2-12 kcal/mol NMR H&c Connectivity 1010a Light-induced electron excitation ground state Electrons are excited to higher energy levels when a molecule absorbs 二 E vibration Time scale: electron excitation: 10-15s a photon of energy equal to the energy difference between the ground- state electronic level and the excited state electronic level Time scale: nuclei movement: 10-12s E= h for 2= 200 nm E= 143 kcal a Franck-Condon Principle: Upon light-induced electronic excitation, only the electrons are reorganized; the heavier nuclei stay in their ground-state for 1=700 nm: E =40.9 kcal geometry. (Vertical Transitions)
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Background Reading Carey, and Sundberg, Advanced Organic Chemistry, Parts A Third Edition, Chepter 13, "Photochemistry", pp 729-765 "New insights into an old mechanism: [2 + 2] photocycloaddition of enones to alkenes.", Schuster, D. I.; Lem, G.; Kaprinidis, N. A. Chem. Rev. 1993, 93, 3. "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. "[2+2] photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products.", Winkler, J. D.; Bowen, C. M.; Liotta, F. Chem. Rev. 1995, 95, 2003- 2020. "Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions.", Crimmins, M. T. Chem. Rev. 1988, 88, 1453. "The meta photocycloaddition of arenes to alkenes.", Cornelisse, J. Chem. Rev. 1993, 93, 615. Worthwhile General Reviews 10-5–10-6 kcal/mol 2-12 kcal/mol Electronic States 40-140 kcal/mol Functional Groups NMR H & C Connectivity Infrared Ultraviolet-Visible Basic Types Information Energy Range ■ Important Regions of the Electromagnetic Spectrum Electrons are excited to higher energy levels when a molecule absorbs a photon of energy equal to the energy difference between the groundstate electronic level and the excited state electronic level. E = hn = h c l ■ Light-induced electron excitation E (kcal/mol) = 2.86 x 10+4 Knowing the absorption wavelength in nm, you can calculate the the energy l( in nm) for l = 200 nm: E = 143 kcal for l = 700 nm: E = 40.9 kcal for l = 200 nm: E = 143 kcal for l = 700 nm: E = 40.9 kcal DE electronic s * A–B s A–B r 0 internuclear distance Consider a simple diatomic molecule A–B A B ■ Morse Curve Potential Energy r 0 (internuclear distance) E electronic DE vibration excited state ground state ■ Franck-Condon Principle: Upon light-induced electronic excitation, only the electrons are reorganized; the heavier nuclei stay in their ground-state geometry. (Vertical Transitions) Time scale: electron excitation: 10-15 s Time scale: nuclei movement: 10-12 s B A
B. Breit. D. A. Evans Introduction to Photochemistry Chem 206 Summary of Photochemical Processes Selection rules Not all excitations/transitions are allowed(have high probability) (a)Spin-forbidden: Transitions between states of different multiplicity M nonradiative decay M=2s+1 s=∑s ersystem S: total spin Spins Paired: Singlet State: S= ES=(+1/2-12)=0 2 Excitation FLuorescence Spins Unpaired: Triplet State: S= ES=(+12+12)=1 S, or S2 Phosphorescence forbidden (hv3 (b )Space forbidden: transitions between orbitals which do not overlap vacuum UV only(△ E large) probability for that ro(internuclear distance) transition is low.) Absorption max(nm) Simple alkenes 190200 I Sensitizer: e.g. acetophenone, benzophenone 220-250 Small AE between So& S, Facile excitation into S, followed by IsC intoT1 Cyclic dienes 250-270 Interaction with substrate 270-300 Saturated ketones 270-280 Substrate(So)+ Sens. T1) Substrate(1)+ Sens(So) Unsaturated ketones 310-330 Aromatic ketones (aldehydes) 280-300 Aromatic compounds 250-280 Substrate reacts via T, excited state transmits absorbs Pyrex Glass 400nm 3Triplet-Quencher. e.g.O2, piperylene Me transmits Quartz Glass Reacts immediately with molecules in T, excited state, depopulate T1 400nm Quencher(So)+ Substrate (1)- Quencher(T)+ Substrate(So) a ubstrate reacts via S, excited state
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Summary of Photochemical Processes Energy r 0 (internuclear distance) S0 S1 T1 Excitation (+ hn1 ) Fluorescence (– hn2 ) Intersystem crossing Phosphorescence (– hn3 ) nonradiative decay (–D) Substrates Absorption max (nm) 190–200 220–250 250–270 270–300 270–280 310–330 280–300 250–280 Simple alkenes Acyclic dienes Cyclioc dienes Styrenes Saturated ketones Unsaturated ketones Aromatic ketones (aldehydes) Aromatic compounds Pyrex Glass 300 nm absorbs Quartz Glass 200 nm absorbs transmits transmits Hg lamp 254 nm, 313 nm, 366 nm M = 2S + 1 S = S s M: multiplicity S: total spin s: spin of a single electron (±1/2) S0 S1 or S2 T1 allowed S0 forbidden s s * p p * n p * vacuum UV only (DE large) UV(VIS) allowed forbidden (Does not mean impossible. Implies only that probability for that particular transition is low.) Selection Rules Not all excitations/transitions are allowed (have high probability): (a) Spin-forbidden: Transitions between states of different multiplicity M (b) Space-forbidden: transitions between orbitals which do not overlap. ■ Sensitizer: e.g. acetophenone, benzophenone Small DE between S0 & S1 . Facile excitation into S1 followed by ISC into T1 . Interaction with substrate: Substrate (S0 ) + Sens. (T1 ) Substrate (T1 ) + Sens. (S0 ) ■ Triplet-Quencher: e.g. O2 , piperylene Me Reacts immediately with molecules in T1 excited state, depopulate T1 Quencher (S0 ) + Substrate (T1 ) Quencher (T1 ) + Substrate (S0 ) 400 nm 400 nm Spins Paired: Singlet State: S = S s = (+1/2 –1/2) = 0 Spins Unpaired: Triplet State: S = S s = (+1/2 +1/2) = 1 Substrate reacts via S1 excited state. Substrate reacts via T1 excited state. Substrate reacts via S1 excited state
B Breit. D. A. Evans Introduction to Photochemistry Chem 206 Jablonski Diagram conversio ≥10 conversion 106-102sec crossing 10-10se ≥10 Figure 12.2 Jablonski diagram Energy levels of excited states of a polyatomic molecule. The west vibrational energy levels of a state are indicated by thick horizontal lin other horizontal lines represent associated vibrational levels. Vertical straight lines represent radiative transitions, wavy lines nonradiative transitions. The orders of magnitude of the first-order rate constants for the various processes are indicated From Cundall, R. B; Gilbert, A. Photochemistry. "Thomas Nelson: London 1970. Reproduced by permission of Thomas Nelson and Sons Limited
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Jablonski Diagram
D A. Evans Introduction to Photochemistry Chem 206 Photochemical reactions Hexatriene. Frontier MO Description derotation Fleming I. Frontier Orbitals and Organic Chemical Reactions", Chapter 6. " Photochemical reactio Reactions of olefins Consider [2+ 2] cycloaddition: Photochemical activation MO HOMO 中1e 中3 199 The rotatory motion of ning closure may be reversed by photo-activation One of the first cases where heat and light induced electrocyclization t concerted followed different pathways energy Havinga, Tetrahedron, 1961, 16, 1 The(2+ 2 cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state Excited State Geometry disputation connotation 84 xcited state('s and T
D. A. Evans Introduction to Photochemistry Chem 206 Photochemical Reactions Frontier MO Description Fleming, I. "Frontier Orbitals and Organic Chemical Reactions", Chapter 6, "Photochemical Reactions" Reactions of Olefins p* p concerted The [2 + 2] cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state. ✻ bonding bonding HOMO + energy ✻ + light ✻ p p* new HOMO light Consider [2 + 2] cycloaddition: Photochemical activation LUMO C C C C C C C C C C C C C C C C C C The rotatory motion of ring closure may be reversed by photo-activation controtation 1eY4 (hexatriene HOMO) 2e- 2e- 1e- 2e- 2e- 2elight Hexatriene: disrotation Y3 (hexatriene HOMO) H Me H Me H Me H Me Me H H Me Me H Me H Y4 Excited State Geometry p* p p p* new HOMO light C R H R H Excited state (1S and 3T) Rotation Me Me RO R H Me Me RO R heat disrotation light controtation Me Me RO R H One of the first cases where heat and light induced electrocyclizations followed different pathways. Havinga, Tetrahedron, 1961, 16, 146. Me Me RO R H Me Me RO R H Y4 Y1 Y3 Y3 Y1