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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
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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

    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

    S0

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

  • 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

  • Jablonski Diagram

    S1

    S2

    Sn

    T1

    T2

    Tn

    S0

    Photochemistry occurs from S1 (infrequent) or T1 states

    Photochemistry must compete with photophysical processes

  • 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

  • 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 P2P3 P4

    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 P2P3 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

  • 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

    photostationary state

  • 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

  • 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

  • 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

    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

    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+

  • 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%

  • 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

    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 PhOPh

    MeMei-PrOH

  • 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

  • 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

  • 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

  • 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

  • Norrish Type I Fragmentation

    OMeMe

    OMeMe

    OMeMe

    Me

    Me

    O

    Ohν

    O O

    H

    O

    +Me Me

  • Norrish Type I Fragmentation

    OMeMe

    OMeMe

    OMeMe

    Me

    Me

    O

    Ohν

    O O

    H

    O

    +

    O

    OCO2+

    Me Me

    O

    O

  • Electrocyclic Reactions

    Reversal of Pericyclic Selection Rules

    2468

    D CΔ

    Δ

    Δ

    Δhνhν

    hνhν

    Me

    Me

    Me

    Me

    ΔMe

    Me

    Me

    Me

    dis.

    con.

  • 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.

  • 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

  • Sigmatropic Rearrangements

    Orbital Symmetry Rules control what rearrangements are allowed

    H

    Me Me

    H

    Δ

    Me

    H

    Me

  • Sigmatropic Rearrangements

    Orbital Symmetry Rules control what rearrangements are allowed

    H

    Me Me

    H

    Δ

    Me

    H+

    HOMO

    LUMO

    nodes

    0

    1

    2

    3

  • 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

  • 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

  • Sigmatropic Rearrangements

    Me

    H+

    HOMO

    LUMO

    nodes

    0

    1

    2

    3

    Me

    H

    Me

    4

    5

  • Sigmatropic Rearrangements

    Me

    H+

    HOMO

    LUMO

    nodes

    0

    1

    2

    3

    Me

    H

    Me

    4

    5

  • 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

  • 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]

  • 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

    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

    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

  • 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

  • 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

  • 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

  • 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 CN NC

    CN

    CNMe

  • 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 CN NC

    CN

    CNMe

  • 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 CN NC

    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

    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.