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Physical Organic Photochemistry andBasic Photochemical Transformations
Group MeetingJan 26th 2011
Scott Simonovich
O O
MeMe
H
H
O
H
H MeMe
OHC
hν (310 nm) Benzene
Electromagnetic Radiation
200
300
400
500
600
700
1,000
5,000
10,000
107
109
1011
λ (nm)
UV
Vis
IR
µW
Radio
143
95
72
57
48
41
29
6
3
3 X 10-2
3 X 10-4
3 X 10-6
E (kcal/mol)
Electromagnetic Radiation
200
300
400
500
600
700
1,000
5,000
10,000
107
109
1011
λ (nm)
UV
Vis
IR
µW
Radio
143
95
72
57
48
41
29
6
3
3 X 10-2
3 X 10-4
3 X 10-6
E (kcal/mol)
C C
C C C C
C O
C C C O
180
220
260
282
280
350
λ (nm)Group
Electromagnetic Radiation
200
300
400
500
600
700
1,000
5,000
10,000
107
109
1011
λ (nm)
UV
Vis
IR
µW
Radio
143
95
72
57
48
41
29
6
3
3 X 10-2
3 X 10-4
3 X 10-6
E (kcal/mol)
83
35
99
111
73
86
146
180
BDE (kcal/mol)
C C
O O
C H
O H
C N
C O
C C
C O
X-rays and gamma-rays lead to ionization
UV/Vis for electronic absorption
IR for nuclear vibrational motion (IR)
µW for electron spin precession (EPR)
Radio for nuclear spin precession (NMR)
Jablonski Diagram
S1
S2
Sn
T1
T2
Tn
hν
S0
Stark-Einstein law - molecule will absorb light to excite a single electron and energy of light must match difference between S0 and S*
High probability transitions usually occur with conservation of spin
Jablonski Diagram
S1
S2
Sn
T1
T2
Tn
hν
S0
Kasha's rule - relaxation to S1 is very rapid compared to other processes
Jablonski Diagram
S1
S2
Sn
T1
T2
Tn
hν
S0
Radiative decay to ground state (fluorescence or phosphorescence)
Non-radiative decay (heat or energy transfer)
Intersystem crossing to different manifold
Jablonski Diagram
S1
S2
Sn
T1
T2
Tn
hν
S0
Photochemistry occurs from S1 (infrequent) or T1 states
Photochemistry must compete with photophysical processes
Jablonski Diagram
S1
S2
Sn
T1
T2
Tn
hν
S0
Photochemistry occurs from S1 (infrequent) or T1 states
Photochemistry must compete with photophysical processes
If excited state is short-lived, there will be less time for chemistry
Photoreaction must generally have Ea less than 20 kcal/mol
S1 reactions will not outcompete fluorescence if Ea is above 10 kcal/mol
Allowed and Forbidden Processes
Spin forbidden transitions alter electron spin angular momentum
spin allowed spin forbidden
(excitation) (excitation)
spin forbidden
(ISC)
S0
S1 T1
S0
T1
S1
Triplet states tend to be more photochemically active b/c they are long-lived
Allowed and Forbidden Processes
Spin forbidden transitions alter electron spin angular momentum
spin allowed spin forbidden
(excitation) (excitation)
spin forbidden
(ISC)
spin allowed
(IC, fluorescence)
spin forbidden
(ISC, phosphorescence)
S0
S1 T1
S0
T1
S1
T1
S0S0
S1
Triplet states tend to be more photochemically active b/c they are long-lived
Bimolecular Photophysical Processes
How could one access T1 exclusively?
S0
S1(n, π*)
T1(n, π*)
slow
Bimolecular Photophysical Processes
How could one access T1 exclusively?
S0
S1(n, π*)
T1(n, π*)
S0
S1(n, π*)
T1(n, π*)
fast
slow
DONOR
Bimolecular Photophysical Processes
How could one access T1 exclusively?
S0
S1(n, π*)
T1(n, π*)
S0
S1(n, π*)
T1(n, π*)
fast
slow
DONORACCEPTOR
S0
S1(n, π*)
T1(n, π*)
Sensitization
D* + A D + A*
acetophenonebenzophenoneanthraquinone
biacetyl
73.668.562.254.9
sensitizer triplet energy(kcal/mol)
Sensitizer should absorb at a different wavelength than acceptor molecule
Triplet energy of senitizer must be greater than that of acceptor
Two mechanisms for photosensitization
Bimolecular Photophysical Processes
Stern-Volmer quenching kinetics
A* Ak1
Q + A* Akq
1/τ2 = 1/τ1 + kq[Q]
τ1 = lifetime of A* without Q
τ2 = lifetime of A* with Q
Description of quenching efficiency
Quenching requires close contact between A* and Q
Olefins and dienes are often used as quenchers
Bimolecular Photophysical Processes
Stern-Volmer quenching kinetics
A* Ak1
Q + A* Akq
1/τ2 = 1/τ1 + kq[Q]
τ1 = lifetime of A* without Q
τ2 = lifetime of A* with Q
Description of quenching efficiency
Quenching requires close contact between A* and Q
Olefins and dienes are often used as quenchers
Photoinduced electron transfer - leads to quenching of fluorescence
LUMO
HOMO
LUMO
HOMO
Bimolecular Photophysical Processes
Stern-Volmer quenching kinetics
A* Ak1
Q + A* Akq
1/τ2 = 1/τ1 + kq[Q]
τ1 = lifetime of A* without Q
τ2 = lifetime of A* with Q
Description of quenching efficiency
Quenching requires close contact between A* and Q
Olefins and dienes are often used as quenchers
Photoinduced electron transfer - leads to quenching of fluorescence
LUMO
HOMO
LUMO
HOMO
Photochemistry
S S* IP1 P2
P3 P4
hν
photophysics photochemistryradical/ion chemistry
Φ = quantum yield =# molecules that undergo a process
# of photons absorbed by starting material
Photochemistry
Photophysical rates must provide an excited species that persists long enough for photochemistry to occur
S S* IP1 P2
P3 P4
hν
photophysics photochemistryradical/ion chemistry
Φ = quantum yield =# molecules that undergo a process
# of photons absorbed by starting material
Rate of photochemistry from S1 must outcompete fluorescence
Phosphorescence from T1 to S0 must be slow compared to photochemistry
Two or three energy surfaces are involved due to excited states
Three types of photochemical reaction diagrams
Photochemistry
E
Nuclear Movement
SM
SM*I
P
Diabatic photoreaction
Small gap between surfaces promotes crossingMinimum on upper surface matches maximum on ground state surface
Frequent return back to SM results in low quantum yield
Photochemistry
E
Nuclear Movement
SM
SM*I
P
S0 (SM)
S1 (SM)
S0 (pdt)
Diabatic photoreaction
Small gap between surfaces promotes crossingMinimum on upper surface matches maximum on ground state surface
Frequent return back to SM results in low quantum yield
Photochemistry
Adiabatic photoreaction
E
Nuclear Movement
S0
S1
Transition to excited state, which has smaller energy barrierInitially generates product in excited state
Photochemistry
Adiabatic photoreaction
E
Nuclear Movement
S0 (SM)
S1 (SM)
S0 (pdt)
S1 (pdt)
S0
S1
Transition to excited state, which has smaller energy barrierInitially generates product in excited state
Photochemistry
Hot Ground State reaction (thermal)
E
Nuclear Movement
S0
S1
Occurs when rate of internal conversion is much faster than photochemistry excited surface
Before energy is lost to collisions with solvent, chemistry in ground state occurs
Photochemistry
Hot Ground State reaction (thermal)
E
Nuclear Movement
S0 (SM)
S1 (SM)
S0 (pdt)
S1 (pdt)
S0
S1
Occurs when rate of internal conversion is much faster than photochemistry excited surface
Before energy is lost to collisions with solvent, chemistry in ground state occurs
Tendencies of S and T states
Singlet and triplet excited states often show different reactivity
Singlet
tend to have considerable zwitterionic character
undergo rapid, concerted reactions
stereospecific
electrocyclic rearrangements
cycloadditions
sigmatropic rearrangements
must compete with facile photophysical processes
Tendencies of S and T states
Singlet and triplet excited states often show different reactivity
Singlet
tend to have considerable zwitterionic character
undergo rapid, concerted reactions
stereospecific
electrocyclic rearrangements
cycloadditions
sigmatropic rearrangements
must compete with facile photophysical processes
Triplet
tend to have biradical character
triplet state is longer lived
no stereospecificity
stepwise reactions
hydrogen abstraction
radical rearrangements
additions to unsaturation
homolytic fragmentation
Different products due to the fact that the ground state is a singlet state
Olefin Isomerization
Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added
313 nm
hν
photostationary state
Olefin Isomerization
Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added
313 nm
hν
Complete conversion to one isomer is difficult b/c both isomers absorb
photostationary state
Ratio controlled by absorption and quantum yield for isomerization
Olefin Isomerization
Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added
313 nm
hν
Complete conversion to one isomer is difficult b/c both isomers absorb
photostationary state
Ratio controlled by absorption and quantum yield for isomerization
trans and cis stilbene absorb with the same efficiency at 313 nm
lifetime of S1(trans) = 68 ps
lifetime of S1(cis) = 1 ps
Φ (trans to cis) = 0.50
Φ (cis to trans) = 0.35
Olefin Isomerization
Olefin isomerization is a powerful feature of photochemistry
The relative energy of two minima on So surface does not matter
Key issue is how system exits from S1 or T1 onto S0
Olefin Isomerization
Olefin isomerization is a powerful feature of photochemistry
t-But-Bu
hν
5 % 95 %
Ohν
O
isolated in matrix
The relative energy of two minima on So surface does not matter
Key issue is how system exits from S1 or T1 onto S0
Olefin Isomerization
Olefin isomerization is a powerful feature of photochemistry
t-But-Bu
hν
5 % 95 %
Ohν
O
isolated in matrix
3-4 (impossible); 5 (difficult, reactive); 6-7 (facile, reactive); 8 or above (stable)
Cyclic trans olefins are used in situ for functionalization
The relative energy of two minima on So surface does not matter
Key issue is how system exits from S1 or T1 onto S0
Olefin Isomerization
Ph hν, ROH Ph
isolated
Ph hν, ROH Ph Ph
HOR
Ph
Olefin Isomerization
Ph hν, ROH Ph
isolated
Ph hν, ROH Ph Ph
HOR
Ph
Me
MeMe
hν, H+
xylene
Me
MeMe
Me
MeMe
Me
MeMe
Photoadditions/substitutions
Hydrogen abstration is frequent in these reactions:
Strength of bond being broken
Strength of bond being formed
Steric effects of approach
Solvent effects on reagent or transition state
Photoadditions/substitutions
Hydrogen abstration is frequent in these reactions:
Strength of bond being broken
Strength of bond being formed
Steric effects of approach
Solvent effects on reagent or transition state
O
Ph Ph Ph Ph
OH+ hν
OH
PhPh
Ph
OHPh
O
Ph Ph Ph Ph
OH
+H
OH
Ph Ph
OH
Ph Ph+
Norrish Type II
Intramolecular H abstraction
O
R
HO
R
H OH
R
R
OH
HO
R
O
R
H+
hν
Norrish Type II
Intramolecular H abstraction
O
R
HO
R
H OH
R
R
OH
HO
R
O
R
H+
hν
Six-membered H abstracted preferntially
Seven- and five-membered H abstraction possible if six-membered position is blocked or disfavored
Ph
O OH HOPh Me
Me
60%
hν
Norrish Type II
O
Me
Me
O
Me
Me
t-Bu t-Buhν+
OH
Me
Me
t-Bu
Norrish Type II
Stereospecificity of singlet and triplet states - Example: disproportionation in Type II reactions
O
Me
Me
O
Me
Me
t-Bu t-Buhν+
OH
Me
Me
t-Bu
O
REt
Me
O
REt
Me
O
REt
Me3sens
hν
hνOH
REt
Me
OH
REt
Me
fast
slow
Alkene Hydrogen Abstraction
Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)
hνPh Ph PhO
Ph
MeMei-PrOH
Alkene Hydrogen Abstraction
Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)
hν
hν
Ph Ph PhO
Ph
MeMe
Me3xylene
MeMe Me
MeMe
OHMe
Me
i-PrOH
i-PrOH
Alkene Hydrogen Abstraction
Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)
hν
hν
Ph Ph PhO
Ph
MeMe
Me3xylene
MeMe Me
MeMe
OHMe
Me
i-PrOH
i-PrOH
Ph
HO Ph Ph
HO PhMe
Ph
O PhO
Ph
PhMe
hν
Alkene Hydrogen Abstraction
Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)
hν
hν
Ph Ph PhO
Ph
MeMe
Me3xylene
MeMe Me
MeMe
OHMe
Me
i-PrOH
i-PrOH
Ph
HO Ph Ph
HO PhMe
Ph
O PhO
Ph
PhMe
Ph
HO PhMe
Ph
HO Ph HO Ph
Ph
Me+
hν
hν3xylene
Norrish Type I Fragmentation
C=O bond is a very strong bond, so there is a considerable driving force to reform it
O
R1 R2
hν(n, π*)
O
R1 R2
OR1
R2
Several factor control rate of this α-cleavage reaction
Norrish Type I Fragmentation
C=O bond is a very strong bond, so there is a considerable driving force to reform it
O
R1 R2
hν(n, π*)
O
R1 R2
OR1
R2
Several factor control rate of this α-cleavage reaction
Rate of Type II reaction, phosphorescence, or ISC
Nature of R1 and R2
Stability of resulting radical
Me = 103 s-1
1o alkyl = 107 s-1
Benzyl = 1010 s-1
Ring strain
Norrish Type I Fragmentation
OMeMe
OMeMe
OMeMe
Me
Me
O
hν
Norrish Type I Fragmentation
OMeMe
OMeMe
OMeMe
Me
Me
O
hν
Ohν
O O
H
O
+Me Me
Norrish Type I Fragmentation
OMeMe
OMeMe
OMeMe
Me
Me
O
hν
Ohν
O O
H
O
+
O
OCO2+
hν
Me Me
O
O
Electrocyclic Reactions
Reversal of Pericyclic Selection Rules
2468
D CΔ
Δ
Δ
Δhνhν
hνhν
Me
Me
Me
Me
hν
ΔMe
Me
Me
Me
dis.
con.
Electrocyclic Reactions
Reversal of Pericyclic Selection Rules
2468
D CΔ
Δ
Δ
Δhνhν
hνhν
Me
Me
Me
Me
hν
ΔMe
Me
Me
Me
dis.
con.
Me R
HOH
Me
Me R
Me
HO
hνcon.
Electrocyclic Reactions
Reversal of Pericyclic Selection Rules
2468
D CΔ
Δ
Δ
Δhνhν
hνhν
Me
Me
Me
Me
hν
ΔMe
Me
Me
Me
dis.
con.
Me R
HOH
Me
Me R
Me
HO
hνcon.
O
O
O
O
O
O
H
H
H
H
hνdis.
Pb(OAc)4
Sigmatropic Rearrangements
Orbital Symmetry Rules control what rearrangements are allowed
H
Me Me
H
Δ
hν
Me
H
Me
HΔ
hν
Sigmatropic Rearrangements
Orbital Symmetry Rules control what rearrangements are allowed
H
Me Me
H
Δ
hν
Me
H+
HOMO
LUMO
nodes
0
1
2
3
Sigmatropic Rearrangements
Orbital Symmetry Rules control what rearrangements are allowed
H
Me Me
H
Δ
hν
Me
H+
HOMO
LUMO
nodes
0
1
2
3
X
Thermally disallowed
Sigmatropic Rearrangements
Orbital Symmetry Rules control what rearrangements are allowed
H
Me Me
H
Δ
hν
Me
H+
HOMO
LUMO
nodes
0
1
2
3
X
Thermally disallowed
HOMO
LUMO
nodes
0
1
2
3
Photochemically allowed
Sigmatropic Rearrangements
Me
H+
HOMO
LUMO
nodes
0
1
2
3
Me
H
Me
HΔ
hν
4
5
Sigmatropic Rearrangements
Me
H+
HOMO
LUMO
nodes
0
1
2
3
Me
H
Me
HΔ
hν
4
5
Sigmatropic Rearrangements
Me
H+
HOMO
LUMO
nodes
0
1
2
3 HOMO
LUMO
nodes
0
1
2
3
Me
H
Me
HΔ
hν
4
5
4
5
rare for antarafacial shift to occur
Cycloadditions
Photochemical [2 + 2] cycloaddition
HOMO
LUMO
nodes
0
1 X
HOMO
LUMO
nodes
0
1
Cycloadditions
Photochemical [2 + 2] cycloaddition
HOMO
LUMO
nodes
0
1 X
HOMO
LUMO
nodes
0
1
HOMO
nodes
0
1
HOMO
LUMO
nodes
0
1
Cycloadditions
Acyclic olefins tend to undergo isomerization rather than [2 + 2]
R
R
R R
R
R
R
R
R
R
R
R
R
RR
R R
R
R
R
hνΦ = 0.04
Φ = 0.5
hνΦ = 0.04
Cycloadditions
Acyclic olefins tend to undergo isomerization rather than [2 + 2]
R
R
R R
R
R
R
R
R
R
R
R
R
RR
R R
R
R
R
hνΦ = 0.04
Φ = 0.5
hνΦ = 0.04
Small and medium ring cyclic olefins do, however, generally undergo facile [2 + 2]
hν
Cycloadditions
O
Me
OHhν O
Me OHretro-aldol
O
MeO
base
O
DeMayo reaction - [2 + 2] with 1,3-diketone followed by retro-aldol
Cycloadditions
S1 [4 + 2] is not allowed by orbital symmetry, but radical T1 undergoes stepwise addition
hν3sens
O
Me
OHhν O
Me OHretro-aldol
O
MeO
base
O
DeMayo reaction - [2 + 2] with 1,3-diketone followed by retro-aldol
Cycloadditions
Paterno-Buchi reaction - stereospecificity
O
H
Me
hν
S1
OMe
Me
Me
95 %
O
Mehν
T1
OMe
Me
90 %
PhMe
Note that carbonyl/alkene [2 + 2]occurs from both S1 and T1 states
Cycloadditions
Paterno-Buchi reaction - stereospecificity
O
H
Me
hν
S1
OMe
Me
Me
95 %
O
Mehν
T1
OMe
Me
90 %
PhMe
Paterno-Buchi reaction - regioselectivity with electron rich olefins
O
Ph Ph
hν
Me
Me
O
PhPh
MeMe
O
PhPh
MeMe
90 % 10 %
Note that carbonyl/alkene [2 + 2]occurs from both S1 and T1 states
Cycloadditions
Paterno-Buchi reaction - regioselectivity with electron rich olefins
O
Ph Ph
hν
Me
Me
O
PhPh
MeMe
O
PhPh
MeMe
90 % 10 %
O
Ph Ph Me Me
O
Ph PhMe
Me
Φ = 0.5
O
Me Me
OR
hν O
MeMe
OR
O
MeMe
OR
7 : 3
Oxygen radical is more electrophilic than carbon radical, so it adds first to form the more stable biradical
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
O
Me Me
hν
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
S1(n, π*)NC CNO
MeMe
CN
CN
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
O
Me Me
hν
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
S1(n, π*)NC CNO
MeMe
CN
CN
O
Me Me
hν
S1(n, π*)Me
O
MeMe
CN CNMe
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
O
Me Me
hν
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
S1(n, π*)NC CNO
MeMe
CN
CN
O
Me Me
hν
S1(n, π*)Me
O
MeMe
CN
O
Me Me
hν
T1(n, π*)NC CNNC
CN
CNMe
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
O
Me Me
hν
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
S1(n, π*)NC CNO
MeMe
CN
CN
O
Me Me
hν
S1(n, π*)Me
O
MeMe
CN
O
Me Me
hν
T1(n, π*)Me MeNC
CN
MeCN
O
Me Me
hν
T1(n, π*)NC CNNC
CN
CNMe
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
O
Me Me
hν
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
S1(n, π*)NC CNO
MeMe
CN
CN
O
Me Me
hν
S1(n, π*)Me
O
MeMe
CN
O
Me Me
hν
T1(n, π*)Me MeNC
CN
MeCN
O
Me Me
hν
T1(n, π*)NC CNNC
CN
CNMe
Cycloadditions
Paterno-Buchi reaction - electron poor olefins
Only occurs from S1 state, so reactions are complete stereospecificProducts are not the expected adducts based on electron-rich alkene mechanism
O
Me Me
hν
S1(n, π*)Me
O
MeMe
CN
O
Me Me
MeCN
Me CN O
MeMe
Me
CN
Carbon radical is more nucleophilic than oxygen, so it adds first to make the more stable biradical
Cycloadditions
Cycloadditions of alkenes with enones are efficient reactions as well
O
Me Me
hνO
MeMe
90 %
O
δ+
δ-Me Meδ+
δ-
Examples in Synthesis
HN OO
N
NH
NH
air, MeCN
HN
NH
N
NH
OOHN
NH
N
NH
OO
H H
(con.)
sunlight
91%
Berlinck, R. G., et al. J. Org. Chem. 1998, 63, 9850.
AcN O
NNAc
Br
O
OTsTsO
AcN
NAc
N
O
O
OTsTsO
hν, iPr2NEt, CH2Cl2
96%
(con.)
Kobayashi, Y., Fujimoto, T., Fukuyama, T. J. Am. Chem. Soc. 1999, 121, 6501.
Examples in Synthesis
O
O
O
OO
O
O
O
O
O
O
OHH
O
O
hν, benzene
O
O
O
OHO
O
O
68%
Kraus, G. A., Chen, L. J. Am. Chem. Soc. 1990, 112, 3464.
Examples in Synthesis
O
O
O
OO
O
O
O
O
O
O
OHH
O
O
hν, benzene
O
O
O
OHO
O
O
68%
Kraus, G. A., Chen, L. J. Am. Chem. Soc. 1990, 112, 3464.
O OMe
Me
O
H
H Me Me
CHO
hν (310 nm)cyclohexane
92%
OO
MeMe
H
Lin, C. -H., Su, Y. -L., Tai, H. -M. Heterocycles, 2006, 68, 771.
Examples in Synthesis
2. hν, benzeneHO
Br H
H O
MeMe HO
Me
Me
O
H
H
HOMe
Me
O
1. Bu3SnHAIBN
O
O
Me
Me
Cotterill, I. C., et al. J. Chem. Soc. Perkin Trans. 1, 1991, 2505.
48%
Examples in Synthesis
1. hν (350 nm)
2. H2, Pd/C, PhH
O
O
OBn
OBn
OBn
OO
OBn
OH
H
Me
O
O
O
O
OH
OH
OH
O
OH
H
Me
O
OOH
OH
84%
(−)-littoralisone
Mangion, I. K., MacMillan, D. W. C. J. Am. Chem. Soc. 2005, 127, 3696.