alternative reaction media.pdf

249

11.1 Introduction

In this review, we will give a brief overview of recent (year 2000 or later) examples of the photochemical reactions in alternative media. The word “alternative” is rather ambiguous, but we herein deal with any photochemistry that is conducted in organized, but dimensionally not well-defined, media. The photo-chemistry in well-organized media, such as supramolecular hosts and crystals, will be reviewed in other chapters. The less organized “alternative” media include supercritical fluids, voids in polymer and related films, ionic liquids, liquid crystals, organogels, microemulsions, micelles, vesicles, and passive voids in dendrimers and so on, which have been extensively exploited as unique media for photophysical and pho-tochemical studies.1 We will focus mostly on the photochemistry in such alternative media with only occa-sional reference to the photophysics. Because of the diversity and vagueness of the topics, the chapter will not be comprehensive; rather, we will discuss the current status and characteristics of photochemistry in alternative media by illustrating representative examples.

11.2 Photochemistry in Supercritical Fluids

11.2.1 Supercritical CO2

Dynamically fluctuating supercritical fluids are of special interest for conducting (photo)reactions that are controlled by the diffusion rate of the reactant or intermediate. In particular, supercritical carbon dioxide is desirable not only as a “green” medium but also as a supercritical medium attainable under a relatively mild condition (the critical point at 31°C and 7.38 MPa). It was repeatedly shown that regio- and stereochemical fates of (photo)chemical reactions can be manipulated by changing the medium property,

11Photochemistry in Alternative Media

11.1 Introduction ......................................................................................24911.2 Photochemistry in Supercritical Fluids .........................................249

Supercritical CO2 • Supercritical Rare Gases • Supercritical Methane, Ethane, and Other Hydrocarbons

11.3 Photochemistry in Polymer and Related Films ...........................254Polyethylene and Polypropylene Films • Poly(Vinyl Acetate) Films • Nafion Membranes

11.4 Photochemistry in Ionic Liquids ....................................................25911.5 Photochemistry in Microemulsions, Micelles, Vesicles,

and Dendrimer Voids ...................................................................... 26411.6 Photochemistry in Liquid Crystals and Organogels ...................26711.7 Conclusions........................................................................................269References ......................................................................................................270

Tadashi MoriOsaka University

Yoshihisa InoueOsaka University

250 CRC Handbook of Organic Photochemistry and Photobiology

such as density and dielectric constant, which is readily achieved in supercritical media through altera-tion of pressure and/or temperature in a narrow range. Supercritical fluids have lower viscosity and higher diffusivity than conventional solvents, and these properties enable reactions to proceed beyond the mass transport limitations often found in traditional multiphase systems. From the practical point of view, the easier retrieval of products simply by reducing the pressure is another clear advantage in synthetic (photo)chemistry. Accordingly, the photochemistry in supercritical fluids has been extensively studied and has been recently reviewed2; these (relatively classical) examples will not be repeated in this review.

Since carbon dioxide is totally nonflammable and nontoxic, supercritical CO2 (scCO2) is one of the best solvents for potentially hazardous oxidation and oxygenation processes. Gaseous oxygen is highly soluble in scCO2, where 1O2 is readily generated and lasts longer (5.1 ms at 14.7 MPa and 41°C), although the life-time is slightly dependent on the density of scCO2.3 Consequently, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (1) has been used as a photosensitizer in scCO2. Perfluorination endows the photosensitizer with reasonable solubility in scCO2 under high pressure. Thus, the efficient singlet oxygenations of α-terpinene (2) to ascaridole (3) and of 2,3-dimethylbut-2-ene to 3-hydroperoxy-2,3-dimethyl-1-butene were reported to occur in scCO2. These oxygenation processes have been demonstrated to efficiently proceed in a continuous flow system using a tubular sapphire reactor to afford ca. 1 mL of product in 10 min.4

3O21O2

1O2

scCO2

hν, sensitizer

Sensitizer = C6F5

C6F5

C6F5

C6F5

NH N

N HN

1

2 3

OO

The photoinduced addition of aldehydes to α,β-unsaturated quinones (4) and enones (5) is an effec-tive method for synthesizing 2-acyl-1,4-hydroquinones (6) and 1,4-diketones (7). These reactions were also performed in scCO2, instead of benzene. The yields were improved at higher CO2 pressures or with the addition of 5% t-butyl alcohol as cosolvent under the supercritical conditions.5

O

O

O O

O

O

R

R

OH

OH

RCHO+

RCHO+

scCO2

hν

scCO2

hν

4

5

6

7


The photolysis of anthracene (8) in the presence of appropriate electron and hydrogen donors in scCO2 was reported to give 9,10-dihydro-9-anthracenecarboxylic acid (9) in good yields, which is in sharp contrast to the result in the conventional nonpolar aprotic solvents. In this system, scCO2 acts as both reactant and reaction medium. The reaction involves the photo-induced electron transfer from N,N-dimethylaniline to anthracene to form a radical anion intermediate, which was trapped by carbon dioxide to produce the reduced aromatic carboxylic acid.6

scCO2

hν

H

Electron donor: N,N-dimethylanilineHydrogen donor: 2-PrOH

H8 9

H

CO2H

A photoexcitation of 2,4,6-trimethylbenzoyldiphenylphosphine oxide (10) in the presence of methyl methacrylate (13) was reported to induce free radical polymerization. The rates of propagation in super-critical and liquid CO2, which were determined by the time-resolved electron paramagnetic resonance studies, were comparable to each other. The rates in much viscous liquid CO2 were slower than the diffusion-controlled ones and comparable to those in conventional solvents.7

scCO2

hνO O

O+ P

PPolymers

O

O

CO2Me

CO2Me

Ph

Ph

10 11 12

1213

14

15

P

The photochemistry of uranyl(VI) tributylphosphate complex in scCO2 was also studied. The result was interpreted as the uranyl(VI) complex was reduced to uranium(V) by photoexcitation in the pres-ence of ethanol, which was followed by the disproportionation to give the uranium(IV).8 The photo-catalytic oxidation of 1-octanol on silanized TiO2 suspended in aerated scCO2 was also investigated at a various temperatures and pressures. The maximal photocatalytic production was observed at the CO2 pressure of 10 MPa. These experimental results were explained by differences in mass transfer efficiency of the reactants and products formed on partially desilanized TiO2 powder in scCO2.9

Enantiodifferentiating photoaddition of methanol to 1,1-diphenylpropene (16) has been extensively studied using the chiral 1,4-naphthalenedicarboxylate (17) as the sensitizer. The enantiomeric excess (ee) of photoadduct (18) obtained in the reaction was critically affected by applied pressure and/or temperature. The product’s ee was enhanced by increasing alcohol size and pressure, thus affording a best ee of 43% for the photoaddition of 2-propanol in scCO2 at 18 MPa.10 However, the dependence of ee was discontinuous at the critical density, accompanying a big jump caused most probably by enhanced clustering of the alcohol. Interestingly, such sudden leap in the ee near the critical CO2 density was observed at any given temperature, but at different pressures.11 The differential activa-tion volumes, | |ΔΔVR S−

‡ , obtained for the supercritical and particularly the subcritical regions, were


much larger than those obtained with the same photoreaction in conventional solvents, indicating the more selective solvation to one of the diastereomeric exciplexes particularly in the subcritical region. The ΔΔVR S−

‡ values obtained at the near-critical region (9–10 MPa) were anomalously large, which was attributed to the dynamic nature of scCO2 in the subcritical region, as independently demonstrated physicochemically by the density fluctuation studies. These explanations were also supported by the florescence studies.12

CO2R*

CO2R*

Sensitizer =

17

R*:O O

OO O

scCO2

hν, sensitizerROH+

R = Me, Et,i-Pr, t-Bu

16 18

OR*

The photolysis of 5,5-diphenyl-4-penten-1-ol (19) in the presence of the same chiral sensitizer (17) was performed in scCO2 to give the intramolecular cyclization product (20) in good to modest yields. The ee of product (20) was again highly pressure dependent, displaying a dramatic leap from nearly 0 in near-critical CO2 (8–10 MPa) to the maximum value of 29% in purely scCO2 at 12 MPa. More surprisingly, it was revealed that the dramatic increase of the ee of photoproduct is achieved by simply adding an entrainer such as diethyl ether to scCO2, although the detailed mechanism was not fully elucidated.13

scCO2

hν, sensitizer

19 20

Sensitizer = 17

OH

O*

As another example of the unimolecular reactions, the sensitized enantiodifferentiating photoi-somerization of (Z)-cyclooctene (21) to (R)- or (S)-(E)-cyclooctene (23) with chiral benzenepolycar-boxylates (such as 22) was studied in scCO2. A similar jump of the product’s enantioselectivity in the near-critical region (8–10 MPa of CO2) was also observed in this system.14

scCO2

Sensitizer =

21

22

(R)-(?)-23 (S)-(?)-23

hν, sensitizer

*RO2C CO2R*

CO2R**RO2CR*:

O O O

+

OO


11.2.2 Supercritical Rare Gases

The photolysis of iron pentacarbonyl, Fe(CO)5 (24), has been widely investigated as a representative compound in organometallic photochemistry, using various techniques such as matrix isolation, flash photolysis, time-resolved infrared spectroscopy, and electron diffraction. Consequently, the addition reactions of the triplet fragments 3Fe(CO)4 and 3Fe(CO)3 (25–27) formed upon photolysis of Fe(CO)5 (24) in supercritical rare gases (xenon and argon) have been investigated in detail.15 The photobehavior was compared with that in supercritical methane, as well as in conventional solvents. A theoretical study on these systems revealed that these spin-forbidden reactions are caused by the spin crossover followed by ligand addition via the nonadiabatic transition state.16 The photochemistry of related molybdenum species, trans-[CpMo(CO)3]2 (Cp = η5-C5H5), in scCO2 has been also studied by the time-resolved infra-red spectroscopy.17

1Fe(CO)4(L)

3Fe(CO)3(L)

3Fe(CO)3Ar

3Fe(CO)4

Fe(CO)5

CO

L

L

28

25

26

27

24

hνscXe/scAr

L = H2 or solvent

The photolysis of asymmetrically substituted diazene (29) in supercritical xenon and CO2 and compressed krypton was investigated by nanosecond laser flash photolysis.18 The cage effects on the radical intermediates (30 and 31) in different fluids (supercritical CO2, Xe, and Kr) were examined to disclose the absence of the critical clustering or so-called enhanced cage effect under the near-critical conditions.

hνscXe/scCO2

N N

In-cage/out-of-cageproducts

+

29 30 31

32

11.2.3 Supercritical Methane, Ethane, and Other Hydrocarbons

Photoreaction of a homogeneous mixture of ethane, carbon monooxide, and rhodium catalyst, [Rh(CO)(PMe3)2Cl] (33), was reported, and the formation of propionaldehyde was confirmed by mass spectrom-etry. During the course of photolysis, the catalyst was decomposed to [Rh(CO)2(PMe3)Cl] (34), and this species was shown to be responsible for the carbonylation of ethane. The photochemical carbonylation of ethane and other hydrocarbons by rhodium catalysts was also reported in scCO2. The similar carbon-ylation of methane did not proceed in supercritical methane.19


3433

35 36

PMe3

Me3P Me3P

Cl CO CO+

+ CO

CO

CO

O

H

Cl RhRh

H

hνscCO2

34

scCO2

Upon laser irradiation of near-critical benzene at 266 nm, carbon onions were produced in both subcritical (200°C) and supercritical (290°C) benzene, whereas carbon coils were obtained on an alloy catalyst composed of Fe, Cr, and Ni in supercritical benzene. The use of supercritical fluids for preparation of structured carbons is attractive, since the method requires much lower temperature than that used in conventional methods.20

11.3 Photochemistry in Polymer and Related Films

11.3.1 Polyethylene and Polypropylene Films

A number of investigations demonstrate that the cavities in polyethylene (PE) films can be subtly “tuned” by temperature and the induced microscopic changes in polymer structure and morphology are valuable for altering or tuning the reactivity and selectivity of a wide variety of photoreactions. The passive voids of PE cavities can be easily modulated by their degree of crystallinities and by unidirectionally stretching the films. The voids can also be templated by incorporating some guest molecules and the template shape is retained for some time even after the removal of the guest template. As such, the passive PE cavities are positively employed for controlling the photochemical reactions, as shown in the following examples.

The selectivity of photo-Fries rearrangements has been widely investigated in isotropic solutions. Accordingly, the photolysis of 1-naphthyl and 2-naphthyl acylates (37 and 43) was conducted in unstretched and stretched PE films of different crystallinities and the results were compared with those obtained in isotropic solutions.21 The influence of variables such as size and position of the aryl groups of the esters, degree of crystallinity, free volume, and unstretched/stretched state of the films has been explored. The void’s free volumes were much smaller than the van der Waals volumes of the naphthyl molecules under investigation and the naphthyl esters inside PE cavities acted as templates for the for-mation of photoproducts. Stretching of the film enhanced the templating effect and strengthened the van der Waals attractions with cavity walls in the case of naphthyl myristates, thus inducing an increase in reaction selectivities in PE films. There was, on the contrary, no direct correlation between the void’s free volume of PE films and the selectivity of photoreactions.

+ + +

40 41 42

R

O O

OH

Ph Ph+ + +

O

O37

38 39R

R = Me, tridecyl, PhR

+

R

O

O

OH OH

hν

PE �lms


46

47

48

++ +

O

O

R

R

OHOH OH

43 45

44

R

O

O

O

O

O+

R

R

O

OH

R

hν

PE films

Photo-Fries rearrangements of phenyl phenylacylates (49) were investigated in isotropic solutions and in PE films and the results were compared with those obtained for the corresponding naphthyl esters. Unlike the naphthyl esters, the product distribution and the rate of radical recombination were much less sensitive to film stretching or the type of PE employed in the case of phenyl esters. Photoirradiation of 5,6,7,8-tetrahydro-1-naphthyl phenylacetate, which is comparable in size to 1-naphthyl phenylacyl-ates, but electronically equivalent to phenyl phenylacylates, revealed that the electronic properties of the two aryloxy radicals are principally responsible for the differences in the overall rates of recombinations of the intermediate radical pairs.22

52 53 54

55 56 57

+ + +

+++

R

R

R

R

R R

OOH OH

OH

49 50 51

RO

O

O

O

+

R

R

OH OH

R = CH2Ph, CH(Me)Ph

hν

PE �lms

Recombination of prochiral radical pairs derived photochemically from 1-naphthyl (R)-2-phenylpropanoate (58) in PE films was reported to occur with significant enantioselectivities due to the confining effect in PE cavities. The regioselectivities of photo-Fries (59 and 60) and of decarbonylation products (61–63) obtained upon photolysis of this chiral ester were very different from each other, and the enantioselectivities of the decarbonylation products were heavily dependent on the properties of the PE cavities, such as wall stiffness and mean free volumes.23


OH OH O

61 62 63

+ + +

Me

Me

Me

Ph

Ph

Ph

OH OHO

O

O

5859 60O

HMePh

H+Me

Ph

HMePh

hνPE �lms

The photochemistry of 1-(4-methylphenyl)-3-phenyl-2-propanone (64) has been investigated in PE films of varying crystallinities at different temperatures.24 The recombination selectivity of the triplet pairs of benzylic radicals derived from the photolysis of ketone 64 has been used as a tool for assess-ing the nature of the PE cavities and its influence on the motion of the radicals. It was shown that the in-cage recombination is favored at low temperatures and in PE of high crystallinities. These results were interpreted from the structural point of view, by considering the shape, free volume, wall stiff-ness, and permeability of the reaction cavities, and also from the dynamic point of view, by taking into account of the kinetic competition of the in-cage combination versus cage escape of the radical pairs. Recent examinations by the steady-state and laser-pulse irradiations of dibenzyl ketone derivatives with p-methyl or p-hexadecyl chain in PE films with 0%, 46%, and 68% crystallinities revealed that the separate components of reaction, affected by the (in-cage) “cage effect” as well as the (out-of-cage) “persistent radical effect,” are both responsible for the formation and relative contributions of decar-bonylated products (65–67).25

hνPE �lms

R = H, Me, hexadecyl

PhPh Ph

++

Ph

R R

R

O

R

64 65 66 67

Photo-Claisen rearrangement of benzyl phenyl and benzyl 1-naphthyl ethers (68) has been also examined in PE films of different crystallinities. The ratios of the principal rearrangement products, benzyl arylol positional isomers (69–71), indicated that the reaction is less selective in the PE films than in the zeolite cavities. The results were explained in terms of the passive walls of the PE cages and the limited free volume of cavities.26


Ph

65

71 72OH OH

+ + +

Ph Ph

Ph

O

68 69 70

OH

+

OH

hν

PE films

The radical pairs produced directly from the lysis of the first excited singlet state of enantiomerically pure 1-naphthyl (R)-1-phenylethyl ether (73), or indirectly from the decarbonylation of the aroyl radical formed in the photo-Fries reaction of 1-naphthyl (R)-2-phenylpropanoate (58), have been shown to (par-tially) lose their stereochemistry in the PE films, which is in contrast to the photo-Fries rearrangement in PE films, where the stereochemistry is fully preserved.27 Quite interestingly, these results are better interpreted by assuming that the PE cavities are templated by the guest molecules and the templated shapes are retained to some degree for a period required for decarbonylation. Comparison of the fates of the directly and indirectly formed radical pairs has been employed as a tool for elucidating the nature of the reaction cavities in the polyethylene films and how the combination of the radical pairs is influenced by the initial location within a cavity.

hνPE �lms

HO Me

73

74

61 62

Ph

Ph+ +

Ph

The photodecarboxylation of 2,4,6-trimethylphenyl (S)-2-methylbutanoate (75) in unstretched high-density polyethylene (HDPE) films afforded (S)-1-(2-methylpropyl)-2,4,6-trimethylbenzene (76) as the sole detectable product.28 In other PE films, organic solvents, and cyclodextrin cavities, the cage-escaped products derived from the Fries-type bond scission were concomitantly obtained, but the decarboxyl-ation product was obtained with stereoretention without exception, indicating that the decarboxylation is a completely concerted process.29 A recent investigation on the photoreaction of o-cresyl acylates in PE films confirms that the importance of the shape of substrate and thus the ability to interact with their environment to the stereoselectivity of the photochemical outcomes.30 These results reveal the impor-tance of the media in controlling the conformation and photoreaction of aryl esters.

hνHDPE �lms

O

OO

O

75 76

>98% (>98% ee)


11.3.2 Poly(Vinyl Acetate) Films

The photo-Fries and associated photoreactions of 1-naphthyl acylates (37) have been also examined in two types of poly(vinyl acetate) or PVA films above and below their glass transition temperatures.31 Comparison of the results in PVA films with those in low-viscosity solvents (ethyl acetate and hexane) and low-polarity polymer films (PE and polypropylene) indicated that interactions of the radicals pro-duced in the photolysis of esters with the acetate pendant groups of PVA films, as well as the mode of PVA’s chain motions, enormously influence the course of the photo-Fries rearrangements. The distribu-tions of photo-Fries products were reasonably explained by the initial conformation of the guest mol-ecules accommodated in the film cavities.

hνPVA �lms

+ + + +37 38 39 40 41 42

11.3.3 Nafion Membranes

Methanol-swollen Nafion beads also provide the microenvironment for controlling photochemical reaction pathways. Thus, the product selectivities have been examined in photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propanone, Norrish Type I and Type II reactions of benzoin alkyl ethers, and photodimerization of acenaphthylene and N-benzyl maleimide.32 For example, the photoirradiation of naphthyl benzoate (37) in Na+, Tl+, and Cs+ ion exchanged Nafion beads affords single product (2-benzoyl-1-naphthol, 38), while a pair of rearranged products as well as the cage-escaped products are concomitantly formed in isotropic solvents such as hexane, methanol, and benzene.

hν

Na�on37 38

The ion-exchanged Nafion interior was also used to investigate the inf luence of chiral auxil-iaries in photochemical reactions. Thus, the asymmetric photoisomerization of trans,trans-2,3-diphenylcyclopropane-1-carboxylic acids, electrocyclization of 2-oxo-1,2-dihydropyridine acetic acids, and oxa-di-π-methane rearrangement of 1,2-dihydronapthalenones have been reported. For example, irradiation of pyridone derivative 77 in Na+ or Li+ Nafion yielded the bicyclic products (78 and 79) in 13%–21% diastereomeric excess (de), whereas the same reaction in solution afforded only 0%–4% de.33

NNH

77 78 79

H Na�on

hν

CO2Me

OO R

COX COX

N NNH

NH

HHCO2Me CO2Me

OO

+

OOR R

R = PhCH2, i-Pr

Water- and methanol-swollen Nafion membranes were used as the microenvironment to effectively control the photochemical reaction pathway of stilbazole derivatives (80). Azaphenanthrene (82) was obtained in high yield upon irradiation of 80 in the membrane, while only the cyclodimerization prod-ucts (83 and 84) were formed in homogeneous solutions.34


N N

NN

RR

RR83 84

R

N N N

80 81 82

R = H, OC8H15

R R

hνNa�on

11.4 Photochemistry in Ionic Liquids

The photochemistry in ionic liquids (ILs) has been extensively studied and thus repeatedly reviewed.35 Therefore, we deal with only the recent development and will not refer to the work cited in the chap-ter “Photochemistry in Ionic Liquids” of the previous edition of this book.36 The photochemistry of ILs that contain photoreactive groups will also be disregarded, and we will focus on the systems that use ILs as reaction media for photochemical reactions. Typical room-temperature ILs, such as 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4) and hexafluorophosphate (bmim-PF6), are most frequently employed as the media for photoreactions, but the ILs that contain weakly coordinat-ing bis(trifluoromethanesulfonyl)amide moiety, such as 1-butyl- and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amides (bmim-NTf2 and emim-NTf2), are also used. The photoreactions can be conducted also in the microemulsions formed by mixing IL with organic solvent and/or aqueous solution. In this section, we will outline the recent examples of photoreactions performed in ILs and in the microemulsions using ILs.

The IL bmim-PF6 has been employed as a solvent for a series of representative photochemical reac-tions involving energy transfer, hydrogen transfer, and electron transfer.37 ILs show unique characteris-tics such as low oxygen solubility, slow molecular diffusion, elongation of triplet excited state and radical ion lifetimes, and weakening of the donor–acceptor (or charge transfer) interactions. The photoinduced electron transfer of anthraquinone38 and duroquinone39 in mixed binary solutions of IL (bmim-PF6 or bmim-BF4) and organic solvent has been studied by laser flash photolysis. The absorption of the triplet excited state of the quinone was blueshifted in the IL–acetonitrile mixtures, and the triplet excited state of the quinone abstracted hydrogen from the IL. In a binary mixture of IL and acetonitrile, the rate of electron transfer depended primarily on the concentration of IL at lower fractions of IL. In contrast, at higher fractions of IL, the network structures due to the hydrogen bond and viscosity became domi-nant and the decay rate constant decreased with increasing fraction of IL. A critical point was observed at a volume fraction of ∼0.3 in IL. Photoinduced electron transfer from a series of electron donors to 9,10-dicyanoanthracene has been studied in bmin-PF6 and omim-PF6 (1-octyl-3-methylimidazolium hexafluorophosphate).40 The fluorescence quenching occurred at rates 1–2 orders of magnitude greater than the diffusion limit. This and related theoretical studies revealed that the electron transfer between the excited 9,10-dicyanoanthracene and the donor molecule occurs in the hydrophobic, nonpolar alkyl chain region of the ILs. A detailed study of the photoinduced electron transfer between pyrene


and N,N-dimethylaniline was performed recently in four different room-temperature ILs (bmim-BF4, bmim-PF6, bmim-NTf2, and emim-NTf2), by using the steady-state and time-resolved fluorescence and laser flash photolysis techniques.41 Because of the slow diffusion in ILs, quenching occurs at a slower rate in ILs than in conventional organic solvents. In sharp contrast to the well-established observation of exciplex fluorescence in conventional media, no exciplex emission was observed in ILs. However, the quenching rate constant, much larger than the diffusion-controlled rate, was explained by the microen-vironmental viscosity around the electron donor and acceptor, which is different from the bulk viscosity of the IL. Furthermore, the solvent-mediated electron transfer in a pyridinium IL, N-butylpyridinium bis(trifluoromethanesulfonyl)amide, was reported recently.42 The electron transfer between Ru(bpy)32+ complex and methyl viologen, coupled with the reduction of the carbonyl group in ILs, was reported.43 Intramolecular electron transfer was also studied in IL, and the photolysis of 4-(N-pyrrolidino)naph-thalene-1,8-imide-pyromellitimide was examined in emim-NTf2.44

The photoisomerization of 3,30-diethyloxadicarbocyanine iodide was investigated in bmim-PF6 and in a binary mixture of the IL and water, by using the steady-state and time-resolved fluorescence spec-troscopy. The photoisomerization rate was dramatically retarded in the IL, compared to that in an iso-tropic solvent of comparable polarity. The activation energy for photoisomerization was also determined in various media.45 Photoisomerizations of 4′-hydroxyflavylium46 and 4′,7-dihydroxyflavylium47 were studied in bmim-PF6, and their multistate, multifunctional switching properties in a biphasic IL–water mixture were shown to be quite valuable. The photochromic properties of N-methyl nitrobenzospiro-pyran–merocyanine system (85 and 86) were studied in ILs containing bis(trifluoromethanesulfonyl)amide anion, and the kinetics and thermodynamics of the process were shown to be sensitive to the nature of the cation.48 Recently, a variety of ILs containing carboxylate anions were prepared and N-substituted photochromic spiropyran derivatives were employed to determine the polarities of these protic or fluorine-containing ILs.49

NN+NO2 bmim-NTf2, etc.

hν

OO–

NO2

85 86

The formation of 1O2 was suggested upon photoirradiation of indigo carmine in bmim-BF4, and the quantum yield of 1O2 production was estimated to be 0.53.50 The oxidation of thioanisole photosensitized by methylene blue has recently been studied in a series of anhydrous imidazolium- and pyrrolidinium-contain-ing ILs.51 It was shown that the ILs can be used as convenient solvents for oxidation of thioanisole, although thioanisole is practically unreactive in conventional aprotic solvents. Upon oxidation of thioanisole by 1O2, a small amount of the corresponding sulfone was formed in pyrrolidinium-based ILs, but the sulfoxide was exclusively produced in imidazolium-based ILs. This was explained as the persulfoxide is converted to a hydroperoxysulfonium ylide intermediate in pyrrolidinium-type ILs (as it occurs in conventional aprotic solvents), while in imidazolium-type ILs hydrogen-bonded persulfoxide is probably formed. The photo-chemical oxidation of alcohols with molecular oxygen was efficiently performed in high conversion and selectivity in several ILs by using 12-tungstophosphoric acid immobilized in mesoporous MCM-41.52

Millimeter-long nanowires of charge transfer silver tetracyanoquinodimethane materials were pre-pared by the photochemical reduction of tetracyanoquinodimethane in bmim-BF4 containing benzyl alcohol as a sacrificial electron donor. The nanowires were also prepared in this IL by using the oxida-tion of water as a counterreaction (at a slower rate), indicating that the ILs may provide a favorable environment for photochemical water splitting. It is to note that the adventitious water as impurity may have affected the photochemical behavior in ILs.53

Intra- and intermolecular copper(I)-catalyzed [2 + 2] photocycloaddition reactions of nonconjugated alkenes were recently achieved efficiently in an ammonium salt-based room-temperature ILs such as


trimethyl(butyl)ammonium bis(trifluoromethylsulfonyl)imide (tmba-NTf2). The IL, tmba-NTf2, is transparent at the irradiation wavelengths and was quite stable under the irradiation conditions, and was easily recovered and recycled. In contrast, no cycloadduct could be isolated when the same reaction was carried out under the identical irradiation conditions in bmim-NTf2, which absorbs the UV light to suffer extensive photodecomposition.54 Intramolecular photocycloaddition of 1,6-diene (87) smoothly proceeded in bmim-NTf2 in yields comparable to those obtained in diethyl ether. Intermolecular [2 + 2] photocycloaddition in tmba-NTf2 was found to proceed in an even better yield than in conventional organic solvents. Thus, the photoirradiation of bicyclopentadiene and norbornene in IL produced the corresponding dimers in 71% and 90% yield, respectively. The irradiation of cyclopentene derivative (89) under the same conditions gave a 8:1 mixture of head-to-tail cis-anti-cis cycloadduct (90) and its diastereomer in 56% combined yield.

H

X

R R΄

CuOTf, hν

tmba-NTf2

X = NCO2Et, NCO2Bn, O, C(CO2Me)2

R , R΄ = H, Me, Ph

88

X

R R΄

87

H H

H H

CH2CO2MeMeO2CH2C

CH2CO2Me

CuOTf, hν

tmba-NTf2

9089

Arenediazonium tetrafluoroborate salts (91) underwent the nucleophilic substitution followed by the dediazoniation upon photolysis in bmin-NTf2 to predominantly give the oxy anion quenching products (92) in good yields, together with small amounts of Schiemann reaction product (93) and bis(trifluoromethylsulfonyl)imide (94).55 Despite the nonnucleophilic and noncoordinating nature of NTf2 anion, the nucleophilic attack occurred predominantly at the O-atom of NTf2 anion, while the Schiemann reaction was a minor pathway in this IL. On the contrary, the Schiemann reaction of imid-azole derivatives (96) was reported to occur efficiently in bmim-BF4. The use of bmim-BF4 dramatically improved the yield of the fluorodediazoniation product (97). Careful temperature control at 0°C was shown to be essential to minimize the photodecomposition of the IL.56

N2+BF4

–

O

+ +

O F

S NSO2CF3

N(SO2CF3)2

F3C

R R R RR = H, 4-CI, 4-Br, 3-NO2,

4-OMe, 4-tBu, 2,4,6-Me391 92 93 94

hν

bmin-NTf2

FN2NO+

HN HNHN

H2N

N NN

RR+R

R = H, CO2Et bmim-BF2

95 96 97

hν


The photochemistry of chiral (salen)aluminum(III) chloride complex (98) has been studied in imid-azolium ILs (bmim-BF4 or PF6). The ILs particularly stabilized the (saren)Al(II) radicaloid species (99) generated by the homolysis of the Al–Cl bond. The half-life of this species varied depending on the sol-vent, indicating that the counteranion of IL played an important role by changing the IL’s polarity and viscosity to affect the lifetimes of the singlet excited state of pentacoordinated salen Al(III) and of the Al(II) radicaloid intermediate.57

O

O O

O

t-Bu t-Bu

t-Bu

t-Bu

t-Bu

t-Bu

t-Bu

t-Bu

t-Bu t-Bu

98 99 100

t-Bu

t-Bu

hν

bmim-BF4 or PF6CIAI AI O

O

O

OAI

The 1-methylimidazole-containing triplet photosensitizers (103 and 104) were designed and synthe-sized in order to enhance the solubility in room-temperature ILs. The photosensitized isomerization of trans- to cis-β-ionol (101 and 102) was efficiently carried out in bmim-BF4 and the sensitizer/IL mixture was reusable. These sensitizers remained in the IL layer when the solution was extracted with organic solvent.58

O O O ON N

+ + +N NN N

103 104

OH

OH

Sensitizers =

hν

bmim-BF4

101 102

Chiral ILs are particularly attractive in view of their potential ability of chiral recognition and dis-crimination, which is applicable to asymmetric synthesis and optical resolution. Recently, several chiral ILs (107–110) have been used as chiral solvents for the enantiodifferentiating photoisomerization of dibenzobicyclo[2.2.2]octatriene diacid (105). The enantioselectivities obtained (3%–12%), though mod-est in an absolute sense, are the highest values reported so far for a unimolecular photochemical reac-tion performed in chiral IL. The chiral induction was attributed to the ion-pairing interaction of the deprotonated diacid with the IL cation.59


HO

OH

NC

N

N+

+

NPh Ph

N

N+Me3Tf2N– N+Me3Tf2N–

Tf2N–

Tf2N–

Chiral ionic liquids:

107 108 109 110

CO2H HO2C HO2C

+

CO2H CO2H

ent-106

hν

Chiral ionic liquid

HO2C

105 106

A large magnetic field effect on the photoinduced hydrogen abstraction reaction of benzophenone with thiophenol was reported in the ammonium, pyrrolidinium, and piperidinium ILs.60 Interestingly, the effect of magnetic field was not straightforward61; the yield of escaped benzophenone ketyl radical gradually decreased by applying the magnetic field of up to 2 T, then reached a terrace at 2–10 T, and again decreased at 10–28 T to eventually afford a 25% reduced yield at 28 T, indicating the operation of more than one mechanism for the ketyl radical.

An IL–organic solvent–water ternary system, consisting of 1-tetradecyl-3-methylimidazolium bro-mide (tmim-Br), p-xylene, and water, was employed for the photodimerization of 9-substituted anthra-cenes (111). The ternary system forms hexagonal and lamellar liquid crystals as well as microemulsions. These organized media were effective in solubilizing and preorientating the anthracene derivatives with a polar 9-substituent, and thus enhanced the formation of head-to-head cyclodimer (112).62 Fluorescence resonance energy transfer study from coumarin 480 to rhodamine was also reported in microemulsions comprised of pmim-BF4 (1-pentyl-3-methylimidazolium tetrafluoroborate) in a mixture of TX-100 and benzene with and without added water.63

R

R

R

R

R

hν

tmim-BrLiquid crystals ormicroemulsions

R = CH2N+Me3Br–, CH2CO2Na, CH2OH,

CO2H, CHO, COMe, Me

111 112, head-to-head 113, head-to-tail

Choosing the proper IL for specific photochemical reaction is encouraged, since the IL used as a solvent is frequently involved in the photoreaction. For instance, the photochemistry of IL investigated by femtosecond pump-probe absorption spectroscopy revealed that, in addition to the generation of solvated electron, di- and trihalide ions are formed upon continuous irradiation of halide-containing ILs (bmim-I or hmim-Cl).64 It is also to note that the diffusion coefficient of diiodide anion radical in ILs becomes much larger than that in conventional organic solvents, due to the Coulomb interaction between diiodide and IL cation.65


11.5 Photochemistry in Microemulsions, Micelles, Vesicles, and Dendrimer Voids

Reverse micelles and water-in-oil microemulsions are microscopic spherical pools of water surrounded by a monolayer of surfactant separating the water pool from the hydrophobic bulk solution. The sen-sitized photooxidation via the 1O2 has been extensively investigated in such media. Thus, the photo-oxidation of glycyl-glycine sensitized by water-soluble anthraquinone derivatives was investigated in the micelles and microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate.66 The formation of semi-quinone radicals in the photosensitized oxidation of hydroquinone substrates was also studied in the reverse micelle.67 The photooxidation of γ,δ-unsaturated ketone was investigated in microemulsion and in homogeneous solution and the results were compared. The photooxidation of homoallylic sub-strate 114 in acetonitrile or acetone gave tertiary hydroperoxide (116) and 3-hydroxy-1,2-dioxane (117) as the major products, while dioxane 117 and epoxide (118) were obtained in sodium dodecyl sulfate microemulsion, both of which were the secondary products derived from the allylic hydroperoxides (115 and 116)68 Diastereoselectivity of the singlet photooxidation of mesitylol was also examined in microemulsions.69

OOHMicroemulsions

hν OOH+

+

OO

O

OO

OOH

O

116115114

117118

The photosensitized oxidation of several olefins was also investigated in mixed surfactant vesicles. In the photosensitized oxidation of (E)-stilbene and (E,E)-1,4-diphenyl-1,3-butadiene in vesicles, the corresponding 1,2-dioxetanes were formed in quantitative yields, which is in sharp contrast to the oxidation in homogeneous solution where the [4 + 2] cycloaddition product of sin-glet oxygen to the diene was the sole product. It is interesting to note that 1O2 generated in the bilayer or the inner water pool of a vesicle is able to diffuse out and enter into the bilayer of another vesicle through the outer aqueous phase and reacts with the target molecules.70 The singlet oxy-genation of α-pinene was also investigated in vesicles.71 The selectivity of oxygenation in mixed surfactant vesicles was controlled by changing the location of the substrate and sensitizer molecules in the reaction media.72 The quantum yield of 1O2 upon photosensitization with palladium bacte-riopheophorbide, a potential reagent for vascular targeted photodynamic therapy, was almost unity in organic solvents, but only ca. 0.5 in micelles and vesicles, where superoxide and hydroxyl radicals are formed in a minimal quantum yield of 0.1%. Analysis of the photoproducts suggests that the formation of oxygen radicals involves both electron and proton transfer from the pheophorbide at the membrane/water interface to a colliding oxygen molecule, consequently forming superoxide, then hydrogen peroxide, and finally hydroxyl radicals.73

The photoisomerization of (E)-stilbene (119) was studied in microemulsions composed of Triton X-100, 1-pentanol, and water in different ratios. The yield of (Z)-stilbene (120) increased with the increase of water content or with the decrease of Triton X-100 content, and the oil-in-water structure


was found most suitable for the photoisomerization.74 (E)-1-(4-Methoxyphenyl)propene (anethole, 121) was employed as a substrate in the study of photochemical behavior in microemulsions, as this sub-strate undergoes spontaneous emulsification without adding external surfactants.75 Thus, the pho-tochemical behavior of microemulsions obtained upon dilution of ethanolic solutions of anethole (121) with water was compared with that of homogeneous ethanolic solutions of the substrate. The photolysis of 121 afforded the (Z)-isomer (122) and cyclodimers (123–125), along with the oxida-tion and/or solvolysis products (126–128). Significant differences in reactivity were observed in both media, as the yields of isomerization (122) and dimerization (123–125) were considerably reduced in microemulsions. In contrast to the photoreaction in homogeneous solutions, where the (Z)-rich photostationary state (ca. 80%) was reached rapidly, the proportion of (Z)-isomer upon irradiation of anethole microemulsions remained below 15%. In the presence of oxygen, the formation of anethole oxide was observed, which further underwent polymerization in the aggregated microemulsion of anethole. The lower photoreactivity of anethole in microemulsions was interpreted in terms of the faster nonradiative decay due to faster internal conversion.

Microemulsions

hν

119 120

OMe

OMe OMe OMe OMe OMe

OMe

OMeOMeOMe

O RO RO OR

R = H, OEtH

+ +

+ +

+

OMe

hνhν

121

123 124

126 127 128

125

122

Microemulsions

Photosensitizing surfactants (132 and 136) have been employed for the polar addition of indene (129) and 1,1-diphenylethene (133) in oil-in-water emulsion. These photosensitizing surfactants were shown to work more efficiently in microemulsion (in water) than in homogeneous organic solvent.76 Although the reaction of indene in an oil-in-water emulsion proceeded efficiently to give the corresponding alcohol as a major product, the yield reported was strongly influenced by the size of oil droplet.77


Microemulsions

Microemulsions

Photosensitive surfactant, 132

Photosensitive surfactant, 136

hν

hν

CO2H

N+Et3Br–

OH +

+OH

129

133 134 135

131130

The photocycloaddition of 9-substituted anthracenes (111) has been investigated in water-in-oil microemulsions prepared from sodium bis(2-ethylhexyl) sulfosuccinate, dichloromethane, and water. While irradiation of the aforementioned substrates in isotropic media afforded the corresponding head-to-tail photocyclodimer (113) as the major product, photoirradiation in the microemulsion almost exclusively yielded the head-to-head dimer (112). These observations were interpreted in terms of the preorientation of the substrate molecules at the interface of the water pool in the microemulsions.78 The reductive photodebromination of polybrominated diphenyl ethers was investigated in nonionic surfactant micelles.79 Photoinduced electron transfer from zinc tetraphenylporphyrin was studied in nonaqueous microemulsions composed of n-heptane, sodium bis(2-ethylhexyl) sulfosuccinate, and ethylene glycol.80 The thermodynamics and kinetics of photoinduced electron transfer in bacterial pho-tosynthetic reaction centers were also investigated in micelles and vesicles formed from different phos-pholipids of physiological importance.81

Microemulsions(+

hν111 112 113)

Water-soluble poly(alkyl aryl ether) dendrimers (137) have been extensively explored for their use as hosts of organic substrates in aqueous media. The photo-Fries reactions of 1-naphthyl benzoate and phenyl ester as well as the α-cleavage reactions of dibenzyl ketones and benzoin alkyl ethers in voids of dendrimers have been examined.82 The photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether as well as the photodimerization of acenaphthylene was investigated in voids of different types of dendrimers.83 It was demonstrated that the dendrimer can encapsulate the substrate, interme-diates, and products in its voids and restricts the mobility of radical intermediates. Comparative stud-ies of the same photoreaction in micellar media demonstrated that dendritic media offers much better constraint than the micelles.


RR

R

RR

R

O

O

OO

O O

O

OO

O

O

O

O R

R

R

R

O

O

O

O

O

O

OO

O

OO

O

O

O

O

O

O

O

O O

O

O

RR

RR

R

R

O

O

O

O

R R

R

R

R

R

R

R

137

Typical dendrimers for microenvironment: R = OH, CO2H, CO2Me

O

O

O

11.6 Photochemistry in Liquid Crystals and Organogels

A number of investigations have been carried out to induce the chirality into liquid crystals by means of photochemical procedures and the representative results have recently been reviewed.84 In this chapter, we focus on the photochemistry of molecules inside the liquid crystals as media, but will not refer to the photochemistry and photophysics of the liquid crystal itself.

The charge separation and recombination processes in a carotenoid–porphyrin–fullerene triad, as a mimic of the photosynthetic reaction center, have been studied in the different phases of two uniaxial liquid crystals.85 The effects of orientation on the electronic coupling element of the molecular triad in liquid crystal were interpreted by the simulations of the EPR spectra of the carotenoid triplet state, which can be useful in designing a better model system with an optimized charge separation efficiency.

Lyotropic liquid crystals have been used as media for the photochemical inter- and intramolecular hydrogen abstraction of cyclohexyl phenyl ketone (138) to afford α-cyclohexylbenzyl alcohol (139) and 1-phenyl-6-hepten-1-one (140). The efficiency of the photoreaction was much higher in the liquid crys-tals than in hexane solution. The ratio of intra- and intermolecular products (140/139) was found to be 30:1 in 1-propanol but less than 3:1 in both lamellar and hexagonal liquid crystals, showing that the intramolecular reaction is greatly hampered in the liquid crystals. Studies on the product distribution


in the absence and presence of electron donors revealed that liquid crystals not only restrict the motion of the substrate and intermediates but also keep the substrate and electron donor together during the photoreaction. A comparison with the same reaction in sodium dodecyl sulfate micelle indicated that the liquid crystal provides a better constraint for the reaction.86 The chiral induction in 139 obtained upon irradiation with prolinol, ephedrine, and their derivatives added as chiral inductor turned out to be inefficient, affording as low as 4%–5% ee.

Hhν

O

138

Liquid crystals

OH

+

139

O

140

+

H OH

ent-139

Photochemistry of tropolone methyl ether 141 (R = Me) has been examined in lyotropic liquid crystals in the presence of chiral inductor. Anisotropic hexagonal liquid crystals significantly induced the chirality in the photoelectrocyclization of the tropolone methyl ether better than lamellar liquid crystals. Thus, norephedrine was able to give the major product 142 (R = Me) in 40% ee, when the reaction was performed in the hexagonal liquid crystals. The photoreaction of chiral tropolone ether 141 (R = 2-methylbutyl) was also performed in the presence of an additional chiral inductor in liquid crystals. It was shown that norephedorin and prolinol are effective in the hexagonal liquid crystals, both affording 142 in 35% de.87 The partially rigid environment of liquid crystals was considered to prevent the dissociation of the substrate–inductor complex, enhancing the stereoselectivity of the photoproducts.

R = Me,

OOR

hν

Liquid crystals

O O

+ + +

OOR

OR OR

ent-143

OOR

142 143ent-142141

The photodimerization of 2-anthracenecarboxylic acid (144) has been studied in the presence of the amphiphilic amino alcohol (149) in the liquid-crystalline medium. The photoreaction was comparatively studied in two smectic and isotropic phases. The two smectic phases were found to show acceptable reactivity to give a mixture of the corresponding dimers (145–148) in 23%–55% yield upon photoirradiation, while 87% of the starting material was consumed in the isotropic phase under the similar conditions except for the phase and temperature. These and related observations indicated that the liquid-crystalline phases are superior to the crystalline phase in view of reaction probability. Quite interestingly, the regioselectivity (head-to-head versus head-to-tail ratio) of the photodimerization changed dramatically depending on the phase, and the head-to-head dimers were obtained exclusively in both of the smectic phases, demonstrating the peculiar property of liquid crystals as constrained reaction media. The liquid-crystalline media also offer better environments for chiral induction.88 The photodimerization of 1-anthracenecarboxylic acid (150) was also investi-gated in the liquid-crystalline medium. The reaction proceeded with an excellent regioselectivity to give the head-to-head dimers exclusively. The moderate diastereoselectivity (anti/syn ratio) of 24% was reported for the dimers 151 and 152, but the ee of the anti head-to-head product 151 was quite unsatisfactory (2% ee).


CO2H

CO2H CO2H CO2H

CO2HHO2C

CO2H

CO2HCO2H

CO2H

CO2H

CO2H

HO2C HO2C

+

+

+

+

hν

hν

144

149

C12H25O

C12H25OOC12H25

NH2

OH

Amphiphilic amino alcohol, 149

149150

145, anti head-to-tail

147, anti head-to-head 148, syn head-to-head

151, anti head-to-head 152, syn head-to-head

146, syn head-to-tail

The d-alanine derivative possessing a 3,4,5-tris(n-dodecyloxy)benzamide moiety (153) was found to be an efficient gelator for nonpolar solvents such as cyclohexane. The photocyclodimerization of 2-anthracenecarboxylic acid (144) in the gel matrix was highly stereoselective, exclusively afford-ing the head-to-head photodimers (147 and 148). This is in contrast to the results in isotropic media. Unfortunately, the enantioselectivity induced by the chiral gelator was poor (10% ee).89 The several other gelators containing a 3,4,5-tris(n-dodecyloxy)benzoylamide backbone have been developed and their photochemistry was explored.90

O

OC12H25O

C12H25O

Geletor:

Gel

hν

OC12H25

153

144 147 148

Me+

HH

+

NHN NH3

The organogels have been also employed as media for photochromic systems, and the photochromic behavior of a variety of benzopyranindolines was studied in organogels.91 The lifetime of the colored photomerocyanine with a succinyl ester functionality was found to be increased by a factor of >300 in the organogel derived from 4-tert-butyl-1-phenylcyclohexanol.

11.7 Conclusions

In this concise review of the photochemistry in alternative media, we overview the recent exploration of a variety of photoreactions in the nonconventional media.

Supercritical fluids are used as media that provide much efficient diffusion of the reactant and inter-mediate than in the conventional organic solvents. This usually facilitates the efficiency of bimolecular


reaction, but the most striking features of the supercritical fluids are found in the near-critical region, that is, the transition region from gas phase to supercritical phase. The effect was explained by the spe-cific clustering to the solute of medium molecules in the near-critical region. Although the usefulness of such peculiar phenomena on the control of stereo- and regioselective photoreaction is evident, the detailed mechanism remains to be elucidated.

Polymer films can be employed to provide the passive voids for organic molecules. In contrast to the well-defined, solid cages of zeolites, for example, the passive voids usually restrict the motion and con-formational changes of the reactant and intermediate within the voids, as the initial free volume of the cavity tends to be smaller than the species considered. This, of course, affects the reactivity and selectiv-ity of the photoreaction. Therefore, the choice of the proper polymer films, in terms of the free volume, shape, wall stiffness, and permeability of the void modulated by the stretching of the films, may provide the best (or at least better) cavities for the desired photoreactions. It is rather unanticipated and hence emphasized that the cavities are templated or memorized for some period of time, even after releasing the initial substrate, and the secondary reactions within the cavities are shown to be controlled by the templated or memorized shape of the voids.

ILs are distinctive and valuable media for a variety of reactions, as they possess charges and are relatively viscous. The typical room-temperature ILs, such as alkylmethylimidazolium salts, have been employed as media for several photoreactions, but precaution should be excersized to avoid the photo-decomposition of the ILs themselves by choosing the irradiation wavelength. The photoinduced elec-tron transfer in ILs has been most extensively studied, as the ionic character as well as the viscosity of the media stabilize the radical ion pairs and alter the rates of electron transfer and recombination processes. The control of other types of photoreactions has been also explored.

Other types of less-defined cavities such as microemulsions, micelles, vesicles, and voids in den-drimers, liquid crystals, and organogels have been also exploited for a range of photochemical reactions. Although none of them becomes an exclusive general approach for controlling photochemical outcomes, the alternative media can be fairly attractive when chosen properly for the specific photoreactions.

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