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Paterno - Buchi
Reaction
Prof. Harish Chopra, SLIET, Longowal
Background
The Paternò-Büchi reaction is the first photochemical
reaction achieved between a carbonyl compound and an
alkene to produce oxetane was published in 1909 by
Paternò and Chieffi. This experiment was reproduced in
1954 by Büchi et al. and oxetanes were characterized. Yang
et al. were in 1964 the first to determine the
regioselectivity of this reaction.
2
Importance
Oxetane ring is present in many biologically
active scaffolds:
(i) Taxol, Drug for Ovarian Cancer treatment
(ii) Merrilactone A, used for Neurotrophic Activity
(iii) Oxetane containing Antiviral Compounds.
3
Mechanism
4
Mechanisms involving a diradical intermediate
Influence of the amphoteric character of the carbonyl
compound at its nπ* state
Mechanisms involving photoinduced electron transfer (PET)
Paternò-Büchi reaction in the ππ* state of the carbonyl
compound
Mechanism
[Mechanisms involving a diradical intermediate]
5
❏ The carbonyl compound is excited to a
singlet state (usually n*) and then reaches a
triplet state after an ISC
❏ The diradical intermediate 1D,formed by
attack of the singlet excited carbonyl
compound on the alkene, mainly leads to its
triplet counterpart 3D.
❏ After a second ISC,which is controlled by a
spin-orbit coupling, this diradical 3D can
undergo either cyclization to the oxetane
or cleavage of the new C-O covalent bond
to regenerate the substrates in their
ground state.
one or several diradical
intermediates are involved
Mechanism
6
The photoinduced reaction between benzophenone 6 and alkene 7 to give
oxetane derivative 9 nicely prove the existence of the diradical
intermediate. The tetrahydro-oxepine derivative 11 can only be formed by
a rearrangement of the diradical 8 leading to the intermediate 10.
Mechanism
The semi-occupied np orbital of the oxygen atom is electrophilic,
whereas the single electron of the π* orbital induces a nucleophilic
character of the system of the carbonyl compound (3-electron
system)
7
[Influence of the amphoteric character of the carbonyl compound at
its nπ* state]
Mechanism
Mechanisms involving photoinduced electron transfer (PET)
In photochemical reactions between
electron-rich alkenes and carbonyl
compounds, an exciplex can initially
be formed from charge-transfer
interactions. Sometimes, these
excited state charge-transfer complexes
CT lead to an intermolecular electron
transfer process and therefore to the
formation of a radical-ion pair RI
which leads to the formation of
Oxetane.
8
Mechanism
Examples involving PET Mechanism
The reactions involving PET mechanism are more selective than those
involving a diradical intermediate. E.g., naphtaldehyde (A) and the
particular electron rich dihydrofuranderivative (B) under irradiation
form bicycle compound (C) excellent regio- and stereo-selectivities.
9
exo: endo = 93:7(A)
(B) (C)
Mechanism
Examples involving PET Mechanism
The photochemical reaction between 1,4-benzoquinone and
diphenylacetylene gives a radical-ion pair (A) which generates the
unstable oxetene (B) that through ring opening leads to the final product
(C). This latter step is usually observed in Paternò-Büchi reactions
involving an alkyne.
10
(A)
(B) (C)
Mechanism
[Paternò-Büchi reaction in the ππ* state of the carbonyl compound]
The carbonyl compounds which possess a ππ* state lower in energy than
their nπ* state undergo a Paternò-Büchi reaction with higher quantum yields
than those which react in their nπ*state. E.g., photochemical excited acetyl-
selenophene (A) is more stable in its ππ* triplet state. The oxetane (C) is thus
the result of a nucleophilic attack of the π system of the carbonyl
compound on tetramethylethylene (B). The competitive product (D)
results from the [2 + 2] cycloaddition of the alkene to the heterocycle.
11
(A) (B)
(C) (D)
Mechanism
[Solvent Effect]
12
The solvent determines the nature of the mechanism involved. E.g., if the reaction
between benzaldehyde and 2,3-dihydrofuran is performed in
High regioselectivity
Non-polar solvents (benzene) Diradical intermediate
Mechanism
Low regioselectivity
Polar solvents (Acetonitrile) PET Mechanism
Solvent (A) : (B)
Benzene 29 : 01
Acetonitrile 05 : 01
(A) (B)
Selectivity
Site selectivity
describes the preferred
attack of the carbonyl
compound on a
particular double bond,
when the alkene
partner possesses two
or more double bonds
Regioselectivity, which
corresponds, as for all [2 + 2]
cycloadditions, to the “head-to-
head” or “head-to-tail”
connectivity of both reagents.
13
Site Selectivity Regioselectivity
Site-Selectivity
14
The photoinduced reaction
between benzaldehyde and 2-
methylfuran is regio- and
stereoselective, but the site
selectivity is not controlled.
45 : 55
The photoinduced reactions
between benzophenone and 2-
methylfuran and thiophene
derivative show a high site
selectivity, because both double
bonds of these heterocycles differ
in energy
Site-Selectivity
15
if one of the double bonds of the furan carries an
electron withdrawing substituent, the site selectivity is
considerably improved.
R (A) : (B)
H 1.3 : 1
Ac 1 : > 10
(A) (B)
Site-Selectivity
16
Steric Hindrance:
The site selectivity of the Paternò-Büchi reaction
between furan derivative and benzaldehyde is
controlled by the presence of a bulky substituent on one
double bond.
Regioselectivity
17
For the diradical mechanism, the initial attack of the electrophilic
oxygen atom of the excited carbonyl compound defines the
structure of the diradical intermediate and therefore the
regioselectivity of the reaction. The relative nucleophilicity of the
carbon atoms of the alkene double bond in the ground state is
thus relevant, since a larger difference in the electronic density of
both carbon atoms will increase the regioselectivity of the reaction
Regioselectivity
18
In the reaction between benzophenone and the uracil
derivatives, the structure of the main regioisomer
depends directly on the position of the methyl on the
double bond
R R’ (A) : (B) Yield [%]
Me H 71 : 29 51
H Me 19 : 81 44
(A) (B)
Regioselectivity
19
The irradiation of 1,3-diacetyl-oxypropan-2-one
(A) with trimethylsilyloxy ethylene (B) produced
regioselective oxetane compound (C).
(A) (B) (C)
Stereoselectivity
20
In the photoinduced reaction
between furan and several
aromatic carbonyl compounds, the
exo/endo stereoselectivity depends
directly on the substituent of the
carbonyl compound
R (exo) : (endo)
H 212 : 1
Me >49 : 1
CN 3.7 : 1
CO2Me 1 : 9
(exo) (endo)
R (A) : (B)
H 89 : 11
Me 50 : 50
(A) (B)
When R = H, irradiation
of compounds [X] and
[Y] selectively yields
oxetane (A), but no
selectivity is observed
when R = Me
(X)
(Y)
Stereoselectivity
21
With benzophenone, chiral 2-furyl methanols form a hydrogen
bond which influences the regio-, diastereo- and site selectivities
of the reaction according to the configuration of the chiral
carbon atom of the furan derivatives
Stereoselectivity
22
The stereoselectivity of the Paternò-Büchi reaction also depends
on the spin multiplicity of the excited carbonyl compound and of the
diradical intermediate. Usually, singlet reactions are more
stereoselective than triplet ones. When the singlet acetone reacts,
the stereochemistry of the alkene is conserved and oxetane is
formed whereas when the triplet acetone reacts, an efficient and
exothermic sensitization process leads to cis/trans isomerization
Stereoselectivity
23
The reaction of the triplet excited benzophenone with alkene (1-
methylthio-2-tert-butylethylene) selectively forms trans-
methylthiooxetane with conservation of the stereochemistry of the
alkene. In this case, bulky substituents reduce the bonds rotation in
the triplet diradical intermediate.
trans product
(>97%)
Stereoselectivity
24
The reaction between aliphatic aldehydes (R = Me, Et, i-Pr) and the
2,3-dihydrofuran showed a strong endo stereoselectivity when the
triplet aldehyde reacts. However, when the mechanism only involves
the singlet aldehyde, the stereochemistry of the oxetane formed is not
controlled and only depends on the initial attack of the carbonyl
compound on the alkene.
endo- selective
product
no- selectivity
Stereoselectivity
Stereoselectivity
25
The temperature has an influence on the selectivity of the Paternò-
Büchi reaction when the mechanism involves a diradical
intermediate and when one of the possible diradical is
thermodynamically favored. E.g., stereoselectivity of the
photoinduced reaction between benzophenone and cyclooctene
depends on temperature.
trans
Stereoselectivity
Effect of Temperature on Selectivity
ciscis / trans
Octene Temp.[°C] (trans) : (cis)
cis - 95 02 : 98
cis + 110 80 : 20
trans - 80 96 : 04
Trans + 110 90 : 10
Stereoselectivity
26
The main side reaction occurring during a Paternò-Büchi reaction is
hydrogen abstraction at the allylic position of the alkene bond by the
oxygen atom of the excited carbonyl compound. This parallel reaction
is dominant when those hydrogen atoms are very labile.
Recombination
Side Reactions
DimerPinacol
Stereoselectivity
27
Another side reaction of the Paternò-Büchi reaction is the Norrish
type-II reaction (𝜸-hydrogen atom abstraction). E.g., 𝝰-ketoester
[Ethyl(phenyl)glyoxylate] on reaction with electron-rich alkenes
such as cyclohexa-1,3-diene yields oxetane under irradiation.
However, with electron-poor alkenes such as allyl bromide,
ethyl(phenyl)glyoxylate is fragmented according to a Norrish type-II
reaction via the typical intermediate.
Side Reactions
28
Applications
Synthesis of Oxetanes from enol ethers
29
Applications Synthesis of Oxetanes from silyl enol ethers
The reaction of benzaldehyde with trimethylsilyl derivative of
cinnamyl alcohol gave trans-oxetane with high stereoselectivity.
The reaction of benzaldehyde with trimethylsilyl derivative having a
stereogenic centre in the 𝛃-alkyl group (A) gave oxetanes with high
diastereoisomeric ratio (92:8).
(A)
92 : 08
30
Applications
Synthesis of Oxetanes from Enamines
N-Acyl enamines (A) and
(B) gave the
corresponding oxetane
derivatives with high
regio- and
stereoselectivities. The
thermodynamically more
stable isomer is formed in
these reactions.
(A)
(B)
90 : 10
31
Applications
Synthesis of Oxetanes from Furans
Furan or Benzofuran on
reaction with benzophenone
gave regioselective oxetanes
with high exo-selectivity.
The photochemical
irradiation of benzofuran
derivative (A) with
benzaldehyde gave endo-
oxetane derivative (B) .
(A) (B)
32
Intramolecular Paterno-Buchi Reaction
Synthesis of Diquinanes and Triquinanes
33
Intramolecular Paterno-Buchi Reaction
Synthesis of 1,13-Herbertenediol
1,13-Herbertenediol
Synthesis of Merrilactone A Scaffold
Merrilactone A Scaffold
34
References
The Paternò-Büchi reaction—Mechanisms and application to organic
synthesis Maxime Fréneau∗, Norbert Hoffmann*, Journal of
Photochemistry and Photobiology C: Photochemistry Reviews, 33 (2017)
83–108.
Paternò-Büchi reaction, CRC Handbook of Organic Photochemistry and
Photobiology, Maurizio D Auria, Vol 1, 653-681, CRC Press (Taylor &
Francis), (2012)
Thank You !
35

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Paterno buchi reaction

  • 1. Paterno - Buchi Reaction Prof. Harish Chopra, SLIET, Longowal
  • 2. Background The Paternò-Büchi reaction is the first photochemical reaction achieved between a carbonyl compound and an alkene to produce oxetane was published in 1909 by Paternò and Chieffi. This experiment was reproduced in 1954 by Büchi et al. and oxetanes were characterized. Yang et al. were in 1964 the first to determine the regioselectivity of this reaction. 2
  • 3. Importance Oxetane ring is present in many biologically active scaffolds: (i) Taxol, Drug for Ovarian Cancer treatment (ii) Merrilactone A, used for Neurotrophic Activity (iii) Oxetane containing Antiviral Compounds. 3
  • 4. Mechanism 4 Mechanisms involving a diradical intermediate Influence of the amphoteric character of the carbonyl compound at its nπ* state Mechanisms involving photoinduced electron transfer (PET) Paternò-Büchi reaction in the ππ* state of the carbonyl compound
  • 5. Mechanism [Mechanisms involving a diradical intermediate] 5 ❏ The carbonyl compound is excited to a singlet state (usually n*) and then reaches a triplet state after an ISC ❏ The diradical intermediate 1D,formed by attack of the singlet excited carbonyl compound on the alkene, mainly leads to its triplet counterpart 3D. ❏ After a second ISC,which is controlled by a spin-orbit coupling, this diradical 3D can undergo either cyclization to the oxetane or cleavage of the new C-O covalent bond to regenerate the substrates in their ground state. one or several diradical intermediates are involved
  • 6. Mechanism 6 The photoinduced reaction between benzophenone 6 and alkene 7 to give oxetane derivative 9 nicely prove the existence of the diradical intermediate. The tetrahydro-oxepine derivative 11 can only be formed by a rearrangement of the diradical 8 leading to the intermediate 10.
  • 7. Mechanism The semi-occupied np orbital of the oxygen atom is electrophilic, whereas the single electron of the π* orbital induces a nucleophilic character of the system of the carbonyl compound (3-electron system) 7 [Influence of the amphoteric character of the carbonyl compound at its nπ* state]
  • 8. Mechanism Mechanisms involving photoinduced electron transfer (PET) In photochemical reactions between electron-rich alkenes and carbonyl compounds, an exciplex can initially be formed from charge-transfer interactions. Sometimes, these excited state charge-transfer complexes CT lead to an intermolecular electron transfer process and therefore to the formation of a radical-ion pair RI which leads to the formation of Oxetane. 8
  • 9. Mechanism Examples involving PET Mechanism The reactions involving PET mechanism are more selective than those involving a diradical intermediate. E.g., naphtaldehyde (A) and the particular electron rich dihydrofuranderivative (B) under irradiation form bicycle compound (C) excellent regio- and stereo-selectivities. 9 exo: endo = 93:7(A) (B) (C)
  • 10. Mechanism Examples involving PET Mechanism The photochemical reaction between 1,4-benzoquinone and diphenylacetylene gives a radical-ion pair (A) which generates the unstable oxetene (B) that through ring opening leads to the final product (C). This latter step is usually observed in Paternò-Büchi reactions involving an alkyne. 10 (A) (B) (C)
  • 11. Mechanism [Paternò-Büchi reaction in the ππ* state of the carbonyl compound] The carbonyl compounds which possess a ππ* state lower in energy than their nπ* state undergo a Paternò-Büchi reaction with higher quantum yields than those which react in their nπ*state. E.g., photochemical excited acetyl- selenophene (A) is more stable in its ππ* triplet state. The oxetane (C) is thus the result of a nucleophilic attack of the π system of the carbonyl compound on tetramethylethylene (B). The competitive product (D) results from the [2 + 2] cycloaddition of the alkene to the heterocycle. 11 (A) (B) (C) (D)
  • 12. Mechanism [Solvent Effect] 12 The solvent determines the nature of the mechanism involved. E.g., if the reaction between benzaldehyde and 2,3-dihydrofuran is performed in High regioselectivity Non-polar solvents (benzene) Diradical intermediate Mechanism Low regioselectivity Polar solvents (Acetonitrile) PET Mechanism Solvent (A) : (B) Benzene 29 : 01 Acetonitrile 05 : 01 (A) (B)
  • 13. Selectivity Site selectivity describes the preferred attack of the carbonyl compound on a particular double bond, when the alkene partner possesses two or more double bonds Regioselectivity, which corresponds, as for all [2 + 2] cycloadditions, to the “head-to- head” or “head-to-tail” connectivity of both reagents. 13 Site Selectivity Regioselectivity
  • 14. Site-Selectivity 14 The photoinduced reaction between benzaldehyde and 2- methylfuran is regio- and stereoselective, but the site selectivity is not controlled. 45 : 55 The photoinduced reactions between benzophenone and 2- methylfuran and thiophene derivative show a high site selectivity, because both double bonds of these heterocycles differ in energy
  • 15. Site-Selectivity 15 if one of the double bonds of the furan carries an electron withdrawing substituent, the site selectivity is considerably improved. R (A) : (B) H 1.3 : 1 Ac 1 : > 10 (A) (B)
  • 16. Site-Selectivity 16 Steric Hindrance: The site selectivity of the Paternò-Büchi reaction between furan derivative and benzaldehyde is controlled by the presence of a bulky substituent on one double bond.
  • 17. Regioselectivity 17 For the diradical mechanism, the initial attack of the electrophilic oxygen atom of the excited carbonyl compound defines the structure of the diradical intermediate and therefore the regioselectivity of the reaction. The relative nucleophilicity of the carbon atoms of the alkene double bond in the ground state is thus relevant, since a larger difference in the electronic density of both carbon atoms will increase the regioselectivity of the reaction
  • 18. Regioselectivity 18 In the reaction between benzophenone and the uracil derivatives, the structure of the main regioisomer depends directly on the position of the methyl on the double bond R R’ (A) : (B) Yield [%] Me H 71 : 29 51 H Me 19 : 81 44 (A) (B)
  • 19. Regioselectivity 19 The irradiation of 1,3-diacetyl-oxypropan-2-one (A) with trimethylsilyloxy ethylene (B) produced regioselective oxetane compound (C). (A) (B) (C)
  • 20. Stereoselectivity 20 In the photoinduced reaction between furan and several aromatic carbonyl compounds, the exo/endo stereoselectivity depends directly on the substituent of the carbonyl compound R (exo) : (endo) H 212 : 1 Me >49 : 1 CN 3.7 : 1 CO2Me 1 : 9 (exo) (endo) R (A) : (B) H 89 : 11 Me 50 : 50 (A) (B) When R = H, irradiation of compounds [X] and [Y] selectively yields oxetane (A), but no selectivity is observed when R = Me (X) (Y)
  • 21. Stereoselectivity 21 With benzophenone, chiral 2-furyl methanols form a hydrogen bond which influences the regio-, diastereo- and site selectivities of the reaction according to the configuration of the chiral carbon atom of the furan derivatives
  • 22. Stereoselectivity 22 The stereoselectivity of the Paternò-Büchi reaction also depends on the spin multiplicity of the excited carbonyl compound and of the diradical intermediate. Usually, singlet reactions are more stereoselective than triplet ones. When the singlet acetone reacts, the stereochemistry of the alkene is conserved and oxetane is formed whereas when the triplet acetone reacts, an efficient and exothermic sensitization process leads to cis/trans isomerization
  • 23. Stereoselectivity 23 The reaction of the triplet excited benzophenone with alkene (1- methylthio-2-tert-butylethylene) selectively forms trans- methylthiooxetane with conservation of the stereochemistry of the alkene. In this case, bulky substituents reduce the bonds rotation in the triplet diradical intermediate. trans product (>97%)
  • 24. Stereoselectivity 24 The reaction between aliphatic aldehydes (R = Me, Et, i-Pr) and the 2,3-dihydrofuran showed a strong endo stereoselectivity when the triplet aldehyde reacts. However, when the mechanism only involves the singlet aldehyde, the stereochemistry of the oxetane formed is not controlled and only depends on the initial attack of the carbonyl compound on the alkene. endo- selective product no- selectivity Stereoselectivity
  • 25. Stereoselectivity 25 The temperature has an influence on the selectivity of the Paternò- Büchi reaction when the mechanism involves a diradical intermediate and when one of the possible diradical is thermodynamically favored. E.g., stereoselectivity of the photoinduced reaction between benzophenone and cyclooctene depends on temperature. trans Stereoselectivity Effect of Temperature on Selectivity ciscis / trans Octene Temp.[°C] (trans) : (cis) cis - 95 02 : 98 cis + 110 80 : 20 trans - 80 96 : 04 Trans + 110 90 : 10
  • 26. Stereoselectivity 26 The main side reaction occurring during a Paternò-Büchi reaction is hydrogen abstraction at the allylic position of the alkene bond by the oxygen atom of the excited carbonyl compound. This parallel reaction is dominant when those hydrogen atoms are very labile. Recombination Side Reactions DimerPinacol
  • 27. Stereoselectivity 27 Another side reaction of the Paternò-Büchi reaction is the Norrish type-II reaction (𝜸-hydrogen atom abstraction). E.g., 𝝰-ketoester [Ethyl(phenyl)glyoxylate] on reaction with electron-rich alkenes such as cyclohexa-1,3-diene yields oxetane under irradiation. However, with electron-poor alkenes such as allyl bromide, ethyl(phenyl)glyoxylate is fragmented according to a Norrish type-II reaction via the typical intermediate. Side Reactions
  • 29. 29 Applications Synthesis of Oxetanes from silyl enol ethers The reaction of benzaldehyde with trimethylsilyl derivative of cinnamyl alcohol gave trans-oxetane with high stereoselectivity. The reaction of benzaldehyde with trimethylsilyl derivative having a stereogenic centre in the 𝛃-alkyl group (A) gave oxetanes with high diastereoisomeric ratio (92:8). (A) 92 : 08
  • 30. 30 Applications Synthesis of Oxetanes from Enamines N-Acyl enamines (A) and (B) gave the corresponding oxetane derivatives with high regio- and stereoselectivities. The thermodynamically more stable isomer is formed in these reactions. (A) (B) 90 : 10
  • 31. 31 Applications Synthesis of Oxetanes from Furans Furan or Benzofuran on reaction with benzophenone gave regioselective oxetanes with high exo-selectivity. The photochemical irradiation of benzofuran derivative (A) with benzaldehyde gave endo- oxetane derivative (B) . (A) (B)
  • 32. 32 Intramolecular Paterno-Buchi Reaction Synthesis of Diquinanes and Triquinanes
  • 33. 33 Intramolecular Paterno-Buchi Reaction Synthesis of 1,13-Herbertenediol 1,13-Herbertenediol Synthesis of Merrilactone A Scaffold Merrilactone A Scaffold
  • 34. 34 References The Paternò-Büchi reaction—Mechanisms and application to organic synthesis Maxime Fréneau∗, Norbert Hoffmann*, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 33 (2017) 83–108. Paternò-Büchi reaction, CRC Handbook of Organic Photochemistry and Photobiology, Maurizio D Auria, Vol 1, 653-681, CRC Press (Taylor & Francis), (2012)