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Supplement A3: The Chemistry of Double-Bonded Functional Groups. Edited by Saul Patai Copyright 1997 John Wiley & Sons, Ltd.

ISBN: 0-471-95956-1

CHAPTER 13

Photochemistry of compounds containing C=C double bonds

NIZAR HADDAD

 

Department of Chemistry, Technion Israel Institute of Technology, Haifa 32000,

Israel.

 

 

Fax: 972-4-8233-735; e-mail: CHR10NH@TX.TECHNION.AC.IL

 

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

641

II. EXCITED STATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

642

III. Z E ISOMERIZATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

643

A. Isomerization of Cyclic Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . .

645

B. Isomerization of Aryl Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . .

646

1.

Two-way (Z E) geometrical isomerization . . . . . . . . . . . . . . .

646

2.

One-way (Z ! E) geometrical isomerization . . . . . . . . . . . . . . .

647

3.

One-way (E ! Z) geometrical isomerization . . . . . . . . . . . . . . .

647

IV. [2 C 2] PHOTODIMERIZATIONS . . . . . . . . . . . . . . . . . . . . . . . . . .

650

A. Dimerizations of Unconjugated Alkenes . . . . . . . . . . . . . . . . . . . . .

650

B. Dimerizations of Vinylaryls . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

652

V. [2 C 2] PHOTOCYCLOADDITIONS OF ALKENES TO ENONES . . . . .

658

A. Regioselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

659

1.

Intermolecular photocycloadditions . . . . . . . . . . . . . . . . . . . . . .

659

2.

Intramolecular photocycloadditions . . . . . . . . . . . . . . . . . . . . . .

664

B. Stereoselectivity and Synthetic Applications . . . . . . . . . . . . . . . . . .

672

1.

Intermolecular photocycloadditions . . . . . . . . . . . . . . . . . . . . . .

672

 

a. Ring fusion selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . .

672

 

b. Diastereofacial selectivity . . . . . . . . . . . . . . . . . . . . . . . . . .

673

2.

Intramolecular photocycloadditions . . . . . . . . . . . . . . . . . . . . . .

682

VI. DI- -METHANE REARRANGEMENT . . . . . . . . . . . . . . . . . . . . . . .

695

VII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

699

I. INTRODUCTION

The photochemistry and photophysics of CDC double bonds have been extensively investigated since the 1960s, especially when new spectroscopic techniques such as

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laser flash spectroscopy, ESR spectroscopy and others have provided powerful tools for precise investigation of photochemical reactions. Much effort is currently focused on the photoisomerization and photodimerization of CDC double bonds conjugated to aromatic substituents, on mechanistic studies and on synthetic applications of stereoselective inter- and intramolecular photocycloadditions of CDC double bonds to enones.

This chapter summarizes and discusses the recent advances in the organic photochemistry of CDC double bonds. Special attention is focused on the photocycloaddition of alkenes to cyclic enones, including the mechanism, regioand stereoselectivity and synthetic applications of the reaction.

II. EXCITED STATES

The ultraviolet spectroscopy and the electronic excited states of CDC and CDO chromophores is well discussed in several recent books and reviews1 6. However, it is necessary to start by presenting some UV properties of these chromophores in unconjugated and conjugated structures. Simple alkenes are known to undergo ! Ł transition to the lowest singlet excited state S1 upon UV absorption at the range of 170 210 nm, which is strongly affected by substituents on the CDC bond. In simple CDO chromophores, two possible excitations take place. The first one is excitation of an electron from the nonbonding orbital on the oxygen to the antibonding orbital (n ! Ł ), with typical UV absorption of unconjugated carbonyls at 290 330 nm, when this forbidden transition is observed with a very small extinction coefficient (ε D 10 102 mol 1 cm 1) and corresponds to excitation to the lowest energy excited state S1 (Figure 1). The next transition occurs by ! Ł excitation upon absorption at 180 220 nm.

Mixed chromophores with both CDC and CDO moieties such as ˛,ˇ-unsaturated enones are one of the most investigated chromophores in organic photochemistry. Conjugation of the two chromophoric moieties results in lowering the n ! Ł and ! Ł energy levels relative to the unconjugated chromophores. UV absorption spectra of selected alkenes, ketones and enones are presented in Table 1.

The lowest vibrational level of the S1 state is the origin in most of the known photoreactions. The average lifetime of the S1 excited state before fluorescence radiative decay is 107 1010 s 1. Competing processes, chemical reactions or intersystem crossing (ISC) to the triplet excited state T1 must take place at a faster rate than the S1 S0 decay. The T1 lifetime is significantly longer (ca 106 s 1) than the S1 state.

Direct excitation of CDC bonds to the T1 state can be achieved via transfer of triplet excitation from electronically excited sensitizers. Selected singlet and triplet energies

ISC

S1

Fluorescence

Absorption

T1

Phosphorescence

Radiationless

Decay

Radiationless Decay

S0

FIGURE 1. Modified Jablonski diagram. Reproduced by permission of Academic Press, Inc. from Ref. 3

13. Photochemistry of compounds containing CDC double bonds

643

TABLE 1. UV absorption spectra of selected CDC and CDO chromophores1

 

Chromophore

max(nm)

εmax

Transition type

 

C

D

C

 

 

 

180

10,000

 

, Ł

 

C

D

C

 

C

D

220

20,000

 

, Ł

 

 

 

C

 

 

Benzene

 

260

200

 

, Ł

 

Naphthalene

310

200

 

, Ł

 

C

D

O

C

O

280

20

 

n, Ł

 

C

D

C

 

350

30

 

n, Ł

 

C

C

C

D

220

20,000

 

, Ł

 

D

 

D

 

 

 

 

O

 

 

 

 

 

 

 

 

TABLE 2. S1 and T1 energies of CDC bonds and

 

 

 

 

 

 

 

related chromophores1

 

 

 

 

 

 

 

 

 

 

Compound

S1

T1

 

 

 

 

 

 

 

CH2DCH2

120

82

 

 

 

 

 

 

 

 

CH2DCMe2

95

81

 

 

 

 

 

 

 

 

Acetone

84

78

 

 

 

 

 

 

 

 

Acetophenone

80

74

 

 

 

 

 

 

 

 

Benzophenone

76

69

 

 

 

 

 

 

 

 

CH2DCH CHO

74

70

 

 

 

 

 

 

 

 

Cyclopentenone

83

74

 

 

 

 

 

 

 

 

Cyclohexenone

80

75(n, Ł )

 

 

 

 

 

 

 

 

 

74( , Ł )

 

 

 

 

 

 

 

2-Methylcyclohexenone

ca 76

76(n, Ł )

 

 

 

 

 

 

 

 

 

74( , Ł )

 

 

 

 

 

 

 

Benzene

115

85

 

 

 

 

 

 

 

 

Naphthalene

90

61

 

 

 

 

 

 

 

 

 

 

 

 

 

(kcal mol 1) of CDC bonds and related chromophores are summarized in Table 2.

III. Z E ISOMERIZATIONS

Photochemical Z E isomerization of CDC double bonds is one of the typical reactions of acyclic alkenes which can occur by direct or sensitized excitation. The mechanism and the potential energy surfaces of this isomerization have been extensively investigated since the sensitized isomerization studies of stilbenes, first reported by Hammond, Saltiel and coworkers in the 1960s7 9. In most cases the isomerization process takes place by excitation of either Z or E alkene to its first excited singlet state (S1), generally agreed to be responsible for the isomerization, followed by rapid decay to the ground state (S0), which is faster in most alkenes than the alternative intersystem crossing to the corresponding excited triplet state (T1). The isomerization process can proceed at either the S1 or T1 excited state via twisting motion on the CDC bond until it reaches a corresponding geometry, crossed at the potential energy surface with radiationless decay, after which it decays to the S0 ground state as could be generally described by the typical potential energy surfaces in Figure 2.

Photoisomerization of alkenes via the triplet excited state is known to be possible by triplet sensitization, usually efficient in conjugated CDC bonds that fulfill the requirement of possessing triplet excited energies below those of the typical triplet sensitizers such as acetone, acetophenone, benzophenone, etc. (Table 2). Sensitization with the opposite order of triplet excited energies is possible in cases with strong electronic or strong

644

Nizar Haddad

S1

T1

ENERGY

S 0

90˚

180°

 

TORSIONAL ANGLE

 

FIGURE 2. Potential energy surfaces of S1, T1 and S0 states of alkenes and their dependence on the torsional angle. Reprinted with permission of Academic Press, Inc. from Ref. 3

H

hν

H

H H

(1)

(2)

 

 

 

 

 

 

 

 

14

[cis]s

[trans]s 7

0

40

45

50

55

60

65

70

75

ET (kcal/mole)

SCHEME 1. The effect of the triplet excitation energy of the sensitizer on the Z/E isomerization of stilbene at the photostationary state. Reprinted with permission from Ref. 7. Copyright (1964) American Chemical Society

13. Photochemistry of compounds containing CDC double bonds

645

spin orbit interactions between the sensitizer and the alkene10. The ratio of the isomers at the photostationary state depends on the triplet excitation energy of the sensitizer (Scheme 1). This was examined by Hammond and coworkers and best demonstrated by the Z E isomerization of stilbene with various triplet sensitizers7.

A. Isomerization of Cyclic Alkenes

Cyclic alkenes undergo geometrical photoisomerization in medium and large ring systems upon either sensitized or direct irradiation. Cyclooctene was the smallest cycloalkene possessing an E double bond which has been isolated. Isomerization studies on cyclooctene using different triplet sensitizers (Et 74 kcal mol 1) revealed the Z- alkene as the major product in a mixture with the corresponding E-isomer11, and a similar result was obtained in the isomerization of cyclododecene (5 6)12. However, direct irradiation (185 nm) of cyclooctene11b affords a photostationary state mixture with an E/Z ratio of approximately unity11b,13,14 and provides a convenient method for the preparation of the corresponding E-isomer 4 (Scheme 2). However, it should be noted that among the best yields, the E-isomer was obtained13 in about 37% yield in a mixture with methylenecycloheptane and bicyclo[5.1.0]octane.

hν

(3)

(4)

hν

(5)

(6)

SCHEME 2

E-Cyclooctene has two enantiomeric forms 7 and 8 (Scheme 3). Interestingly, sensitized photoisomerization of cyclooctene using chiral sensitizers afforded (R)-( )-E-cyclooctene 7 with ca 4% enantiomeric excess15. Studies on the asymmetric photoisomerization of Z-cyclooctene with simple ˇ-cyclodextrins via direct irradiation at 185 nm in the solid state or in water suspension afforded in the photostationary state E/Z mixtures with negligible optical activity (<1%) in the E-isomer16. However, enantioselective photoisomerization of cyclooctene has been achieved17 in high optical purity (64%) upon irradiation via sensitization with optically active benzopolycarboxylates 9 affording E-7. Recently Inoue and coworkers18 have employed ˇ-cyclodextrin 6-O-monobenzoate 10 as a novel photosensitizer host molecule carrying a chiral cavity. Irradiation of 1:1 complex of Z-cyclooctene with cyclodextrin 11 in water methanol solutions afforded the best ratios of E/Z (0.8) and enantiomeric excess (6%) in 50% MeOH solution. The enantiomeric

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Nizar Haddad

 

 

hν

+

 

 

cis-(3)

() - (R) - E - (7)

(+) - (S) - E - (8)

RO2 C

CO2 R

 

RO2 C

CO2 R R =

 

 

(9)

 

 

 

O

O

 

O

O

 

+

(10)

(11)

 

hν

O

O

O

O

+

SCHEME 3

excess (ee) was found to be affected by the solvent composition and the irradiation period with preferential formation of the (R)-E-isomer 7; the best ee was obtained in ca 11% ee in very low Z ! E conversions.

Photoisomerizations of cycloheptenes and cyclohexenes are known as well, and photosensitized excitation and isomerization of these have been directly detected using nanosecond flash photolysis techniques19 and time-resolved photoacoustic calorimetry20.

B. Isomerization of Aryl Alkenes

1. Two-way (Z E ) geometrical isomerization

Extensive investigations on the Z E isomerization of stilbenes have revealed that stilbene undergoes mutual isomerization in the singlet or triplet manifold on direct or

13. Photochemistry of compounds containing CDC double bonds

647

triplet sensitization, respectively7 9,21. Polar substituents on the phenyl rings of stilbene have shown no considerable effect on the mutual isomerization as found by Gorner and coworkers22. A typical mechanistic scheme for the Z E isomerization of 4-nitrostilbene is shown in Scheme 4.

1t*

1C*

 

1p*

3t*

3p*

 

1p

1t

1C

SCHEME 4. Reprinted with permission of VCH from Ref. 22g

On excitation of the E and Z ground states (1t and 1c respectively) into their corresponding first excited singlet states, the lowest triplet states are populated by the intersystem crossing (ISC) steps 1tŁ 3tŁ and 1cŁ 1pŁ 3pŁ . The planar and twisted (3pŁ ) triplet configurations form a rapidly established 3tŁ 3pŁ equilibrium23. Radiationless decay of 3tŁ to 1t is important in highly viscous solutions but does not play a role at room temperature. However, decay of the triplet occurs predominantly via the twisted configuration, followed by conversion to the ground state. In solutions, ISC of the twisted triplet 3PŁ is the main process. In accordance with these results, along with Mulliken’s calculations of the potential energy surface of the excited and ground state of ethylenes along the rotation of the CDC bond linkage, it had long been accepted that the olefins generally would undergo isomerization mutually between their corresponding Z E configurations. However, in the last decade a one-way isomerization of aryl olefins was extensively investigated by Tokumaru and coworkers9.

2. One-way (Z ! E ) geometrical isomerization

In comparison to the conventional two-way isomerization (Z E) of stilbene and other aryl alkenes 12 (Scheme 5), a novel one-way isomerization (Z ! E) of CDC double bonds was achieved upon replacing a phenyl group of stilbene by a 2-anthryl group9a,c. Tokumaru and coworkers found in isomerization studies on substituted anthracenes9 that substitution at the CDC bond resulted in complete isomerization of the Z-isomer to the corresponding E-isomer upon irradiation, via a quantum chain process. Interestingly, the isomerization takes place as an adiabatic process in the triplet manifold on both direct and triplet sensitized irradiations.

Typical potential energy surfaces proposed for one-way (a,c) and two-way (b) isomerizations of olefins are described in Figure 3.

3. One-way (E ! Z ) geometrical isomerization

Photoisomerization of E-CDC double bonds substituted with heteroaromatic groups that allow intramolecular hydrogen bonding were found to afford one-way E ! Z

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Ar

R

Ar

H

 

 

hν

 

H

H

H

R

 

(12)

(13)

 

Ar =

,

,

; R=t-Bu, Ph

H H

(14)

hν

 

 

 

H

 

 

 

 

 

H

 

 

 

H

(15)

 

R

 

 

R

 

 

 

H

H

9

1

H

9

1

 

 

 

 

2

hν

 

2

 

 

 

 

 

(16)

 

 

(17)

 

 

R=Ph, t-Bu

 

 

 

 

SCHEME 5

13. Photochemistry of compounds containing CDC double bonds

649

Energy (kcal mol 1)

60

(a)

 

 

3c*

 

 

3p*

 

3t*

40

 

 

 

 

 

 

20

 

 

 

 

0

1

t

 

1c

 

 

 

 

 

0

90

180

 

Angle of Twist (deg)

Energy (kcal mol 1)

60

(b)

 

 

3c*

 

3t*

3p*

 

40

 

 

 

 

20

 

 

 

 

0

1

t

 

1c

 

 

 

 

 

0

90

180

 

Angle of Twist (deg)

Energy (kcal mol 1)

60

(c)

 

 

 

 

 

 

 

40

3t*

3p*

3c*

 

 

 

 

20

 

 

 

 

0

1

t

 

1c

 

 

 

 

 

0

90

180

 

Angle of Twist (deg)

FIGURE 3. Potential energy surfaces proposed for one-way (a,c) and two-way (b) isomerization of alkenes. Reprinted with permission from Ref. 9c. Copyright (1993) American Chemical Society

N

H

 

 

 

H

 

CH3

 

 

 

 

 

 

 

 

 

 

 

N

 

 

 

 

 

N

 

 

N

 

a

 

 

 

 

 

 

 

 

 

b

 

 

 

 

O

 

CH3

 

 

 

 

 

 

 

 

 

 

 

 

O

 

 

 

 

 

 

(18)

(a) hν, EtOH or DMSO

 

(19)

 

 

H

(b) hν, CH2 Cl2

 

 

 

 

O

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

 

N

 

 

 

 

 

 

CH3

N

(20)

hν

N

H

N

(21)

SCHEME 6

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isomerization. For example24, direct irradiation of 18 affords complete isomerization to the corresponding Z-isomer 19 in methylene chloride as solvent, while irradiation in ethanol or DMSO provides two-way isomerization (Scheme 6). Another example was found25 in the irradiation of 20 to 21. In general, such isomerizations take place upon direct irradiation in nonpolar solvents presumably via the first excited singlet state.

The effect of intramolecular hydrogen bonding plays an important role in the Z ! E photoisomerization, e.g. in the (4Z, 15Z) equilibrium of 23 (Scheme 7), the final metabolic product of hemoglobin in living bodies26. Isomerization studies in solvents with different polarity27 revealed that the more the intramolecular hydrogen bonds are weakened, the more efficient the Z ! E isomerization at C-4 becomes, and this isomerization is necessary for the cyclization of 26 to 24. In human serum albumin (HSA) 23 undergoes specific photoisomerization to the ZE-isomer 22 induced by intermolecular hydrogen bonding or salt bridges with the amino acid residues of HSA.

The geometric isomerization of olefins via photochemical electron transfer is well known28,29 and can be divided into two categories: (a) isomerization via the radical cation, in which case the olefin is the donor in the presence of an excited electron acceptor; (b) isomerization via the radical ion pair, which leads to the triplet-excited olefin, and in this mechanism the olefin is the acceptor. This subject is not discussed in this chapter because of space limitations. However, several reviews30 can be consulted in this regard.

IV. [2 + 2] PHOTODIMERIZATIONS

The intermolecular photodimerization of alkenes is known to take place via addition of the triplet or singlet excited state of the CDC double bond to the ground state alkene. The dimerization in the former mechanism is a competing reaction with the photoisomerizations of acyclic or large cycloalkenes (n 8) discussed above. Acyclic, exocyclic and large-ring alkenes do not undergo photosensitized dimerization, presumably due to the fast relaxation of the triplet excited state via its orthogonal geometry2. However, direct irradiation of acyclic and large-ring cyclic alkenes provided photodimerization.

A. Dimerizations of Unconjugated Alkenes

Direct irradiation of tetramethylethylene 27 afforded octamethylcyclobutane 28 (Scheme 8) in good yield31. Stereospecific dimerization was obtained in the irradiation of concentrated solutions of Z- and E-2-butenes 29 and 33, respectively32. Irradiation of 29 afforded a mixture of the all-cis product 30 and the cis anti cis 31 along with double bond migration (32) and Z E isomerization (33). Irradiation of E-2-butene 33 afforded 31 and trans anti trans 34 as the dimeric products.

Photosensitized dimerization of small-ring alkenes using acetone and/or acetophenone as triplet sensitizers is an efficient reaction. Typical examples are summarized in Scheme 9. Dimerization of cyclopropene 35 afforded head-to-head (H,H) 36 and head-to-tail (H,T) 37 isomers in good yield33. Much lower yields were obtained in the dimerization of acyclic and larger cyclic alkenes34,35 38 and 40. Interestingly, cyclohexene36,37 undergoes an efficient sensitized photodimerization affording three isomeric dimers 43, 44 and 45 in 92% total yield. Bridged bicyclic alkenes that could not undergo efficient Z E isomerization provide the corresponding dimers in acceptable to good yields. The dimerization of 2-norbornene 46, which was first reported by Sharf and Korte38 and later examined in a large number of solvents and sensitizers39, afforded in all cases isomeric mixtures of 47 and 48 in good yields.

Соседние файлы в папке Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups