Physical Organic Photochemistry and Basic Photochemical...

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Physical Organic Photochemistry and Basic Photochemical Transformations Group Meeting Jan 26th 2011 Scott Simonovich O O Me Me H H O H H Me Me OHC hν (310 nm) Benzene

Transcript of Physical Organic Photochemistry and Basic Photochemical...

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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

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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)

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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

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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)

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Jablonski Diagram

S1

S2

Sn

T1

T2

Tn

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

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Jablonski Diagram

S1

S2

Sn

T1

T2

Tn

S0

Kasha's rule - relaxation to S1 is very rapid compared to other processes

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Jablonski Diagram

S1

S2

Sn

T1

T2

Tn

S0

Radiative decay to ground state (fluorescence or phosphorescence)

Non-radiative decay (heat or energy transfer)

Intersystem crossing to different manifold

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Jablonski Diagram

S1

S2

Sn

T1

T2

Tn

S0

Photochemistry occurs from S1 (infrequent) or T1 states

Photochemistry must compete with photophysical processes

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Jablonski Diagram

S1

S2

Sn

T1

T2

Tn

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

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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

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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

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Bimolecular Photophysical Processes

How could one access T1 exclusively?

S0

S1(n, π*)

T1(n, π*)

slow

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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

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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*)

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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

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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

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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

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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

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Photochemistry

S S* IP1 P2

P3 P4

photophysics photochemistryradical/ion chemistry

Φ = quantum yield =# molecules that undergo a process

# of photons absorbed by starting material

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Photochemistry

Photophysical rates must provide an excited species that persists long enough for photochemistry to occur

S S* IP1 P2

P3 P4

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

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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

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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

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Photochemistry

Adiabatic photoreaction

E

Nuclear Movement

S0

S1

Transition to excited state, which has smaller energy barrierInitially generates product in excited state

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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

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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

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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

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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

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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

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Olefin Isomerization

Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added

313 nm

photostationary state

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Olefin Isomerization

Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added

313 nm

Complete conversion to one isomer is difficult b/c both isomers absorb

photostationary state

Ratio controlled by absorption and quantum yield for isomerization

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Olefin Isomerization

Simple olefins do not absorb at easily-generated wavelengths, so additional chromophores are added

313 nm

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

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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

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Olefin Isomerization

Olefin isomerization is a powerful feature of photochemistry

t-But-Bu

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

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Olefin Isomerization

Olefin isomerization is a powerful feature of photochemistry

t-But-Bu

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

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Olefin Isomerization

Ph hν, ROH Ph

isolated

Ph hν, ROH Ph Ph

HOR

Ph

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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

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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

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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+

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Norrish Type II

Intramolecular H abstraction

O

R

HO

R

H OH

R

R

OH

HO

R

O

R

H+

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Norrish Type II

Intramolecular H abstraction

O

R

HO

R

H OH

R

R

OH

HO

R

O

R

H+

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%

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Norrish Type II

O

Me

Me

O

Me

Me

t-Bu t-Buhν+

OH

Me

Me

t-Bu

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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

hνOH

REt

Me

OH

REt

Me

fast

slow

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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

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Alkene Hydrogen Abstraction

Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)

Ph Ph PhO

Ph

MeMe

Me3xylene

MeMe Me

MeMe

OHMe

Me

i-PrOH

i-PrOH

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Alkene Hydrogen Abstraction

Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)

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

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Alkene Hydrogen Abstraction

Alkenes abstract hydrogens when excited to T1, but act as zwitterions if excited to S1 (and cannot isomerize)

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+

hν3xylene

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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

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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

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Norrish Type I Fragmentation

OMeMe

OMeMe

OMeMe

Me

Me

O

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Norrish Type I Fragmentation

OMeMe

OMeMe

OMeMe

Me

Me

O

Ohν

O O

H

O

+Me Me

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Norrish Type I Fragmentation

OMeMe

OMeMe

OMeMe

Me

Me

O

Ohν

O O

H

O

+

O

OCO2+

Me Me

O

O

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Electrocyclic Reactions

Reversal of Pericyclic Selection Rules

2468

D CΔ

Δ

Δ

Δhνhν

hνhν

Me

Me

Me

Me

ΔMe

Me

Me

Me

dis.

con.

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Electrocyclic Reactions

Reversal of Pericyclic Selection Rules

2468

D CΔ

Δ

Δ

Δhνhν

hνhν

Me

Me

Me

Me

ΔMe

Me

Me

Me

dis.

con.

Me R

HOH

Me

Me R

Me

HO

hνcon.

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Electrocyclic Reactions

Reversal of Pericyclic Selection Rules

2468

D CΔ

Δ

Δ

Δhνhν

hνhν

Me

Me

Me

Me

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

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Sigmatropic Rearrangements

Orbital Symmetry Rules control what rearrangements are allowed

H

Me Me

H

Δ

Me

H

Me

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Sigmatropic Rearrangements

Orbital Symmetry Rules control what rearrangements are allowed

H

Me Me

H

Δ

Me

H+

HOMO

LUMO

nodes

0

1

2

3

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Sigmatropic Rearrangements

Orbital Symmetry Rules control what rearrangements are allowed

H

Me Me

H

Δ

Me

H+

HOMO

LUMO

nodes

0

1

2

3

X

Thermally disallowed

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Sigmatropic Rearrangements

Orbital Symmetry Rules control what rearrangements are allowed

H

Me Me

H

Δ

Me

H+

HOMO

LUMO

nodes

0

1

2

3

X

Thermally disallowed

HOMO

LUMO

nodes

0

1

2

3

Photochemically allowed

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Sigmatropic Rearrangements

Me

H+

HOMO

LUMO

nodes

0

1

2

3

Me

H

Me

4

5

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Sigmatropic Rearrangements

Me

H+

HOMO

LUMO

nodes

0

1

2

3

Me

H

Me

4

5

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Sigmatropic Rearrangements

Me

H+

HOMO

LUMO

nodes

0

1

2

3 HOMO

LUMO

nodes

0

1

2

3

Me

H

Me

4

5

4

5

rare for antarafacial shift to occur

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Cycloadditions

Photochemical [2 + 2] cycloaddition

HOMO

LUMO

nodes

0

1 X

HOMO

LUMO

nodes

0

1

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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

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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

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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]

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Cycloadditions

O

Me

OHhν O

Me OHretro-aldol

O

MeO

base

O

DeMayo reaction - [2 + 2] with 1,3-diketone followed by retro-aldol

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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

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Cycloadditions

Paterno-Buchi reaction - stereospecificity

O

H

Me

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

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Cycloadditions

Paterno-Buchi reaction - stereospecificity

O

H

Me

S1

OMe

Me

Me

95 %

O

Mehν

T1

OMe

Me

90 %

PhMe

Paterno-Buchi reaction - regioselectivity with electron rich olefins

O

Ph Ph

Me

Me

O

PhPh

MeMe

O

PhPh

MeMe

90 % 10 %

Note that carbonyl/alkene [2 + 2]occurs from both S1 and T1 states

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Cycloadditions

Paterno-Buchi reaction - regioselectivity with electron rich olefins

O

Ph Ph

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

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Cycloadditions

Paterno-Buchi reaction - electron poor olefins

O

Me Me

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

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Cycloadditions

Paterno-Buchi reaction - electron poor olefins

O

Me Me

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

S1(n, π*)Me

O

MeMe

CN CNMe

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Cycloadditions

Paterno-Buchi reaction - electron poor olefins

O

Me Me

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

S1(n, π*)Me

O

MeMe

CN

O

Me Me

T1(n, π*)NC CNNC

CN

CNMe

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Cycloadditions

Paterno-Buchi reaction - electron poor olefins

O

Me Me

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

S1(n, π*)Me

O

MeMe

CN

O

Me Me

T1(n, π*)Me MeNC

CN

MeCN

O

Me Me

T1(n, π*)NC CNNC

CN

CNMe

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Cycloadditions

Paterno-Buchi reaction - electron poor olefins

O

Me Me

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

S1(n, π*)Me

O

MeMe

CN

O

Me Me

T1(n, π*)Me MeNC

CN

MeCN

O

Me Me

T1(n, π*)NC CNNC

CN

CNMe

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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

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

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Cycloadditions

Cycloadditions of alkenes with enones are efficient reactions as well

O

Me Me

hνO

MeMe

90 %

O

δ+

δ-Me Meδ+

δ-

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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.

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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.

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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.

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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%

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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.