Substituent and Surfactant Effects on the Photochemical Reaction of Some Aryl Benzoates in Micellar Green Environment?

Article abstract of DOI:10.1111/php.13431

In this study, we carried out preparative and mechanistic studies on the photochemical reaction of a series of p-substituted phenyl benzoates in confined and sustainable micellar environment. The aim of this work is mainly focused to show whether the nature of the surfactant (ionic or nonionic) leads to noticeable selectivity in the photoproduct formation and whether the electronic effects of the substituents affect the chemical yields and the rate of formation of the 5-substituted-2-hydroxybenzophenone derivatives. Application of the Hammett linear free energy relationship (LFER) on the rate of formation of benzophenone derivatives, on the lower energy band of the UV-visible absorption spectra of the aryl benzoates and 5-substituted-2-hydroxybenzophenone derivatives allows a satisfactory quantification of the substituent effects. Furthermore, UV-visible and 2D-NMR (NOESY) spectroscopies have been employed to measure the binding constant K_b and the location of the aryl benzoates within the hydrophobic core of the micelle. Finally, TD-DFT calculations have been carried out to estimate the energies of the absorption bands of p-substituted phenyl benzoates and 5-substituted-2-hydroxybenzophenone derivatives providing good linear correlation with those values measured experimentally.

Full text of DOI:10.1111/php.13431

Photochemistry and Photobiology, 20**, **: *–*
Special Issue Research Article
Substituent and Surfactant Effects on the Photochemical Reaction of
†
Some Aryl Benzoates in Micellar Green Environment
1
,2
3
1,2
Gast o´ n Siano , Stefano Crespi
and Sergio M. Bonesi
*
1
Departamento de Qu ´ı mica Org a´ nica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos
Aires, Argentina
2
3
Centro de Investigaciones en Hidratos de Carbono (CIHIDECAR), CONICET−Universidad de Buenos Aires, Buenos Aires,
Argentina
Stratingh Institute for Chemistry, University of Groningen, Groningen, The Netherlands
Received 7 March 2021, accepted 9 April 2021, DOI: 10.1111/php.13431
ABSTRACT
excitation. In particular, cationic, anionic and neutral surfactant
micelles are confined assemblies that are capable of solubilizing
in water hydrophobic molecules, that is, organic molecules.
Furthermore, they have the ability to: (1) concentrate guest
molecules into relatively small effective volumes (7,13), (2)
exist in a dynamic equilibrium and (3) are capable to organize
organic substrates (14,15). The control of the reactivity of radi-
cal species generated within the micelle and some physical
parameters involved in their reactivity have been previously
studied (14,15).
The hydrophobic cores in micelles can be considered as a
microreactor where (photo)reactions can take place and can be
helpful to direct the selectivity of the (photo)products. Like-
wise, the nature of the surfactant or the counterion effect is a
notable feature that can affect the course of a photoreaction,
modifying significantly the rate of formation of the photoprod-
ucts and perhaps, has influence on the overall chemical and
quantum yields. A striking example is the effect of the nature
of the surfactants on the photochemical reaction of p-benzo-
quinone and duroquinone. Anionic surfactants like sodium lau-
rate and SDS accelerate the photoreaction, the nonionic Tween
In this study, we carried out preparative and mechanistic
studies on the photochemical reaction of a series of p-substi-
tuted phenyl benzoates in confined and sustainable micellar
environment. The aim of this work is mainly focused to show
whether the nature of the surfactant (ionic or nonionic) leads
to noticeable selectivity in the photoproduct formation and
whether the electronic effects of the substituents affect the
chemical yields and the rate of formation of the 5-substi-
tuted-2-hydroxybenzophenone derivatives. Application of the
Hammett linear free energy relationship (LFER) on the rate
of formation of benzophenone derivatives, on the lower
energy band of the UV-visible absorption spectra of the aryl
benzoates and 5-substituted-2-hydroxybenzophenone deriva-
tives allows a satisfactory quantification of the substituent
effects. Furthermore, UV-visible and 2D-NMR (NOESY)
spectroscopies have been employed to measure the binding
b
constant K and the location of the aryl benzoates within the
hydrophobic core of the micelle. Finally, TD-DFT calcula-
tions have been carried out to estimate the energies of the
absorption bands of p-substituted phenyl benzoates and 5-
substituted-2-hydroxybenzophenone derivatives providing
good linear correlation with those values measured experi-
mentally.
8
0 shows no effect, and cationic surfactants such as CTAB
markedly suppressed the photoreaction (16). The photoinduced
reduction of β-arylquinone sulfates was markedly affected by
when the photoreaction was carried out in water containing
cationic surfactants (17). The tetra-O-acyl riboflavin photosensi-
tized monomerization of thymine cyclobutane dimers benefited
from the presence of cationic surfactants, whereas neutral and
anionic surfactants inhibited the photosensitized reaction (18).
The absorbance and fluorescence spectra of chloroquine were
used to probe the effect of the counterions present in different
cationic and anionic surfactants. While the nature of the counte-
rion showed little effect on the chloroquine–micelle interaction,
further evidence revealed that the interaction depends on the
micellar surface charge (19).
INTRODUCTION
Designing photochemical reactions aiming to control the pro-
duct selectivity represents a great challenge in synthetic organic
photochemistry (1,2), especially when radical pairs or radical–-
ion pairs are formed as primary intermediates (3,4). Performing
the photoreactions in water-soluble confined assemblies such as
zeolites (5,6), micelles (7,8), polyolefin films (9), cavitands
(
10,11), or dendrimers (12) affords an intriguing control in the
product distribution, because these confined assemblies impart a
restricted mobility on the reactive intermediates generated after
Among the vast number of photochemical transformations, the
photo-Fries rearrangement reaction represents a prototypical
example to probe the micellar hydrophobic environment, counte-
rion and substrate substituent effects. This photochemical reac-
tion was discovered by Anderson and Reese (20), and it involves
*
Corresponding author email: smbonesi@qo.fcen.uba.ar (Sergio M. Bonesi)
†
This article is part of a Special Issue celebrating the career of Dr. Edward Clen-
nan.
©
2021 American Society for Photobiology
1

2
Gast o´ n Siano et al.
a homolytic cleavage of a carbon–heteroatom bond, that is, C-O,
C-S and C-N (21,22), Hence, the photochemical reaction under-
goes an in-cage radical mechanism. It is well-established that the
reaction proceeds mainly through the singlet state (9,22),
Scheme 1 depicts the photochemical reaction of (hetero)aryl ben-
zoates displaying the ortho- and para-rearranged photoproducts
as well as the corresponding phenol (21).
The competition between in-cage radical recombination versus
out-of-cage diffusion dictates the product distribution of the
photo-Fries rearrangement reaction. The characteristics of the
environment influence the choice between these two paths, and
consequently aryl esters are ideal candidates to study micellar
environments (23–25). Indeed, the photo-Fries reaction of benza-
mides is documented in sodium dodecyl sulfate (SDS) micellar
solution (26). Also, the photorearrangement of a variety of sub-
stituted acetanilides showed high selectivity in favor of the 5-
substituted-2-aminoacetophenones in micellar solution (27). Fur-
thermore, cyclodextrines as confined environments was found to
influence the product distribution of the photo-Fries rearrange-
ment of acetanilides (28) and phenyl esters and phenyl amides,
respectively (29).
MATERIALS AND METHODS
Materials. p-Chloro phenol, ethyl p-hydroxyphenylcarboxylate, benzoyl
chloride, pyridine, sodium dodecyl sulfonate, cetyltrimethylammonium
chloride and Brij-P35 were obtained from commercial sources. Aryl
benzoates 1–6, 9 and 10 and the corresponding benzophenone derivatives
have been prepared according to the methodology reported previously in
the literature (34). Spectroscopic grade solvents were used as received.
Pyridine was distilled and stored over KOH pellets. Melting points were
1
determined with a Fisher-Johns apparatus and are not corrected. H and
1
3
3
C NMR spectra were recorded in CDCl on a 300 MHz spectrometer;
chemical shifts (δ) are reported in parts per million (ppm), relative to
signal of tetramethylsilane, used as internal standard. 2D NOESY spectra
were recorded in D
pulse sequence with a 600 ms mixing time and a recovery delay of 1.5 s.
K data points were collected for 512 increments of 16 scans, using
2
O on a 500 MHz spectrometer, using a NOESY-ph
2
TPPI f1 quadrature detection; chemical shifts (δ) are reported in parts per
million (ppm), relative to the signal of trimethylsilylpropionic acid, used
as internal standard. Coupling constant (J) values are given in Hz. The
measurements were carried out using standard pulse sequences. GC
analysis was carried out on a Hewlett Packard 5890 gas chromatograph
using an Ultra 2 capillary chromatographic column. The chromatograms
were recorded with the following program: initial temperature: 100°C,
2 min; gradient rate: 10°C min− ; final temperature: 250°C, 10 min. The
1
UV-visible spectra were measured with
spectrophotometer using two-faced stoppered quartz cuvettes
a
Shimadzu UV-1203
The preparation of benzophenone derivatives is an elegant
example of the application of photochemistry in organic synthe-
sis. Benzoyloxy benzophenones and their derivatives have
demonstrated biological activity and pharmaceutical properties
such as estrogenic activity and anti-inflammatory property, and
their preparation involves the use of benzophenones as key syn-
thons (30–33). Therefore, we performed a systematic study of
the photo-Fries rearrangement of a series of p-substituted phenyl
benzoates in micellar solution in order to evaluate: (1) the effect
of the nature of the surfactant, viz. cationic, anionic or neutral
surfactants on the consumption of the aryl benzoates and the for-
mation of the photoproducts, (2) the effect of the substituents at
para-position of the benzoates on the photoreaction and, (3) the
constrained environment of the micelles to provide a high selec-
tivity toward the formation of 5-substituted-2-hydroxy benzophe-
nones. The structures of the aryl benzoates as well as the
structures of the surfactants employed in this systematic study
are shown in Scheme 2.
Herein, we describe the results obtained on the direct irradia-
tion of several para-substituted aryl benzoates in confined and
sustainable micellar solutions. From a preparative viewpoint, the
photoreaction shows high selectivity toward the formation of 5-
substituted-2-hydroxybenzophenone derivatives in good to excel-
lent yields (higher than 90%). The location of the aryl benzoates
within surfactant micelles (ionic and nonionic micelles) behaving
as microreactors was demonstrated using NOESY NMR spec-
(
1 mm × 1 mm) at 298 K.
Determination of the binding constants (K ) of aryl benzoates in
b
micellar media. Solutions of aryl benzoates were prepared in deionized
−
5
water (MilliQ), and their concentration varied between 5.5ꢀ10 M and
−
4
1
.0ꢀ10 M. An aliquot (2 mL) of the phenyl benzoate solution was
placed in fluorescence stoppered quartz cuvette provided with a stirring
bar, and the UV-visible spectrum was recorded. The initial absorbance
value at the maximum absorption wavelength (A
Subsequently, aliquots of concentrated surfactant solution (10 µL) were
added. The A value at the maximum absorption wavelength for each
solution was consequently recorded. After each addition of surfactant, the
solution was stirred for 20 min before measuring the absorbance. With
0
)
was read.
the values of A
0
and A in hands, the values of (A
0
0
/(A–A )) versus the
reciprocal of the concentration of the micellar surfactant were plotted and
the data were fitted with a linear regression program. The K values were
obtained calculating the ratio of the slope and the origin.
Synthesis of aryl benzoates 7 and 8. To a solution of the substituted
phenols (0.010 mol) in pyridine (10 mL) cooled in an ice-bath, benzoyl
chloride (0.012 mol) was added dropwise in 10 min under stirring.
Subsequently, the reaction mixture was kept under stirring for 60 min.
After total consumption of the starting material was confirmed by TLC,
the reaction mixture was extracted with dichloromethane (10 mL) and
washed with a solution of diluted HCl (10 mL). The organic phase was
b
2 4
then washed with water, dried on Na SO , filtrated and evaporated under
pressure. The phenyl benzoates were purified from the solid residue by
recrystallization using ethanol–water mixtures giving the corresponding
phenyl benzoates in excellent yields (>90%). The aryl benzoates 7 and 8
were characterized comparing the melting points (m.p.) and spectroscopic
data ( H-NMR and C-NMR) with the ones reported in the literature
(
1
13
see Supporting Information).
Photoirradiations of aryl benzoates in cyclohexane. A stock solution
troscopy while the binding constant K
measured by UV-visible spectroscopy.
b
of the substrates was
of a given benzoate (1–10, 0.1 mmol in 200 mL cyclohexane) was
placed in a stoppered Erlenmeyer quartz flask and degassed with argon
Scheme 1. The photo-Fries rearrangement of aryl benzoates.

Photochemistry and Photobiology
3
Scheme 2. Structures of the aryl benzoates and the surfactants studied.
for 30 min. The flask was placed in a homemade optical bench able to
switch between a four or eight lamp irradiation setup. The solution was
stirred during the entire irradiation. Irradiations with λexc = 254 nm
were carried with four germicide lamps (Philips, each of 20 Watts).
The reaction progress was monitored by TLC [eluent: hexane–ethyl
acetate (8:2 v/v); spots were visualized with UV light (254 and
RESULTS AND DISCUSSION
Photoirradiation of aryl benzoates in CTAC micellar media
In order to analyze if cationic, anionic and neutral micelles can
function as photochemical microreactors and are able to induce
photoproduct selectivity from aryl benzoates, the photo-Fries
rearrangement reaction of a series of p-substituted phenyl ben-
zoates was investigated in CTAC micellar media and was com-
pared with those results that have been previously obtained (34).
The general photochemical reaction of aryl benzoates is depicted
in Scheme 3.
Irradiation of aryl benzoates (1–10) in CTAC micellar solu-
tions with light of 254 nm under air provided 5-substituted-2-hy-
droxybenzophenone (1a–10a) with noticeable selectivity and in
high yields (for structures, refer to Scheme 2). The photochemi-
cal reactions were carried out until 95% consumption of the aryl
benzoates (1–10). The chemical yields of benzophenone deriva-
tives (1a–10a) are collected in Table 1.
As is apparent from Table 1, the CTAC micro-heterogeneous
media induced a high selectivity on the product distribution of
the photoreaction, favoring the formation of benzophenone
derivatives. The clean and high-yield (up to >99%) one-pot
photo-Fries rearrangement of the aryl benzoates into 5-substi-
tuted-2-hydroxybenzophenones in micelles stands out as an effi-
cient synthetic transformation. Indeed, phenolic and benzoic acid
by-products were detected only to a minor extent or not detected
at all in the micellar reaction mixture, while irradiation of aryl
benzoates in cyclohexane provided a distinct product distribution
366 nm)] and by GC analysis (Ultra 2 capillary column, vide supra).
When the conversion of the starting material was higher than 90%, the
photolyzed solution was carefully evaporated to dryness under reduced
pressure. The yellowish solid residue obtained was purified by silica gel
column chromatography (eluent: hexane 100% followed by
hexane–ethyl acetate mixtures). From the eluted fractions, the
photoproducts were isolated and characterized by means of physical
and spectroscopic methods.
Photoirradiations of aryl benzoates in micellar media. Stock
solutions of surfactants in deionized water (SDS 0.10 M, CTAC 0.02 M
and Brij-P35 0.05 M) were freshly prepared before each experiment.
The aryl benzoate (5 mg) was placed in
provided with a stirring bar (3 mL), and the surfactant stock solution
2 mL) was added. Then, the solution was vigorously stirred for one
a stoppered quartz cell
(
hour and degassed with argon for 20 min. The quartz cell was placed
in a homemade optical bench provided with two germicide lamps (each
of 20 W). The progress of the photoreaction was monitored by two
different methods: (i) UV-visible spectroscopy and GC analysis (Ultra 2
capillary column). The conversion of the benzoates was kept below
2
0% to avoid secondary reactions and the formation of by-products.
Previously to the injection into the GC apparatus, the micellar solutions
were treated as follows. The photolyzed solutions were diluted with
2
mL of an aqueous solution of NaCl and then extracted with ethyl
acetate (3×2 mL) while the system was carefully shaken to avoid the
formation of emulsions. The organic layer was separated, dried over
Na SO and evaporated to dryness under vacuum. The yellowish solid
2 4
residue was diluted in dichloromethane (2 mL), and this solution was
injected into the GC for chromatographic analysis. The products were
characterized by comparison with the ones reported in the literature
(
see Supporting Information).
Computational analysis. All the structures were prescreened using the
(see Table 1), where the benzophenone derivatives were formed
CREST driver in the xTB software (35), using the GFN2-xTB level. In
this way, the most stable conformers for each structure were picked via
the default series of metadynamics and dynamics runs implemented in
the driver, and reoptimized at the PW6B95D3/def2-SVP level, modeling
the molecule in cyclohexane and methanol via the SMD implicit solvent
method. The UV-vis spectra were calculated simulating the first 30
singlet transitions at the TD-PBE0/6-311 + g(2d,p)//PW6B95D3/def2-
SVP and TD-CAM-B3LYP/6-311 + g(2d,p)//PW6B95D3/def2-SVP. All
structures were confirmed to be minima by the absence of imaginary
vibrational modes. All the DFT and TD-DFT calculations were carried
out via Gaussian 16, Rev. B.01 (36).
only in low yields. On the other hand, the high selectivity for
the formation of benzophenone derivatives in CTAC micellar
media (Table 1) is similar to the high selectivity obtained in
SDS and Brij-P35 micellar solutions (34). Therefore, these
results led to conclude that the selectivity and the high yield of
benzophenone derivatives observed during the photochemical
reaction of aryl benzoates under micellar condition seem to be
unaffected by the nature of the surfactant, viz. anionic surfactant
SDS, neutral surfactant Brij-P35 and cationic surfactant CTAC.

4
Gast o´ n Siano et al.
Scheme 3. The photochemical Fries rearrangement of aryl benzoates.
Table 1. Yields of photoproducts* and reaction quantum yield (ϕ
r
)† measured in cyclohexane and CTAC micellar media.
Photoproduct yields (%)
r
Reaction quantum yield (ϕ )
Cyclohexane‡
CTAC
Aryl benzoates; R
Benzophenones (a)
Phenols (b)
Benzophenones (a)
Cyclohexane‡
CTAC
1
2
3
4
5
6
7
8
9
1
; OMe
; OPh
; Me
; t-Bu
; H
; Ph
; Cl
46
55
42
59
27
28
22
27
30
23
21
24
19
34
93
89
88
87
90
84
97
>99
>99
54
0.30
0.37
0.40
0.50
0.35
0.59
0.47
0.42
0.51
0.02
0.29
0.17
0.14
0.27
0.41
0.50
0.30
0.28
0.14
0.05
§
¶
40
41
52
48
38
31
; COOEt
; CN
0; NO
2
†
*
1
−3
Yield of photoproducts determined by H NMR spectroscopy in the reaction mixture. Concentration of aryl benzoates: 5.0 × 10 M. Actinometer: KI
‡
§
(
0.6 M), KIO
3
(0.1 M) and Na
2
B
2
O
7
ꢀ10H
2
O (0.01 M) solution in water; ϕ(I
3
−) = 0.74; λexc = 254 nm.(37) Error: ꢁ 0.01. Data taken from Ref. (34). 4-
¶
Hydroxybenzophenone is also formed in 27%. 4-Hydroxybenzophenone is also formed in 8−10%.
Furthermore, this methodology gains relevance in organic syn-
thesis because it is possible to carry out the photochemical reac-
tion in aqueous solution with surfactants, despite of their charge.
Also, this photochemical methodology shows a good tolerance
of a wide variety of electron-donor and electron-acceptor groups
attached to the aryl moiety of the aryl benzoates.
deactivation pathway of the singlet excited state competes effi-
ciently with the photo-Fries rearrangement reaction (38,39). A
similar behavior regarding the spin-coupling effect of the nitro
group has been reported with nitro(hetero)arene derivatives
(40,41).
The observed selectivity toward the benzophenone derivatives
over the corresponding phenols was attributed to the confinement
of the esters and the radical species formed after the C-O bond
cleavage within the micellar hydrophobic core. Furthermore, the
micelle reduces the mobility of the primary radical species sup-
pressing the formation of photoproducts that arise from the out-
of-cage diffusion to the bulk solution.
Effect of the nature of the surfactants and the substituents on
the photoreaction of aryl benzoates
UV-visible spectroscopy was used to follow the photochemical
reaction of the aryl benzoates (1–10) in homogeneous (cyclohex-
−3
ane) and heterogeneous media (SDS 0.10 mol dm , Brij-P35
0.10 mol dm− and CTAC 0.02 mol dm solutions). The solu-
3
−3
−5
−3
The quantum yields of consumption of the aryl benzoates (ϕ
r
)
tions of the aryl benzoates (5.0 × 10 mol dm ) were irradi-
ated with light of 254 nm under inert atmosphere. For example,
Fig. 1 shows the time-resolved UV-vis spectra of benzoates 1
and 3 in a micellar solution of CTAC 0.02 mol dm− where new
absorption bands located at 374 nm and 350 nm for both ben-
zoates grow-in with irradiation time. These new bands were
assigned to the n,π* electronic transition of the carbonyl group
of 2-hydroxybenzophenone derivatives 1a and 3a, respectively
(38,39). Moreover, these new bands were also observed in the
time-resolved UV-vis spectra of benzoates 1 and 3 recorded in
cyclohexane as well as in SDS and Brij-P35 micellar solutions
(see Figure S1). A similar spectroscopic behavior was also
observed for all the other aryl benzoates (2, 4–10) in all the sol-
vents studied.
in CTAC solution and in cyclohexane are also collected in
Table 1, showing comparable order of magnitude. However, the
ϕr values measured in CTAC solutions were smaller than in
cyclohexane. This distinct behavior accounts for a significant
increase in the nonradiative and intersystem crossing pathways
from the singlet excited state of aryl benzoates (1–9) in CTAC
micellar solutions which competes efficiently with the photo-
Fries rearrangement reaction pathway, whereas in cyclohexane
such a competition was not observed. Additionally, it was found
3
r
that the ϕ of aryl benzoates measured in CTAC solution are
similar and in the same order of magnitude as it was obtained
previously in SDS and Brij-P35 (34).
In the case of p-nitrophenyl benzoate (10), the ϕ values were
r
found to be one order of magnitude lower than the other aryl
benzoates in all the solvents studied. The spin-coupling effect of
the nitro group attached at para-position of the phenyl moiety
favors the intersystem crossing pathway. The fast nonradiative
In order to analyze the solvent and the substituent effects on
the UV-visible absorption spectra of the aryl benzoates (1–10)
and 5-substituted-2-hydroxybenzophenones (1a–10a), the absorp-
tion spectra were recorded in homogeneous and micro-

Photochemistry and Photobiology
5
(
a)
(b)
0.8
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
0.6
0.4
0.2
300 s
0 s
6
00 s
s
0
0.0
200
250
300
350
400
450
200
250
300
350
400
450
λ / nm
λ / nm
Figure 1. Time-resolved UV-visible absorption spectrum of (a) p-methoxyphenyl benzoate 1 and (b) p-methylphenyl benzoate 3 in solution of CTAC
−
3
0
.02 mol dm .
heterogeneous media. Table 2 collects the maximum absorption
wavelengths (λabs) and the associated energy (E) values for the
aryl benzoates and the corresponding benzophenone derivatives
in cyclohexane, methanol and micellar solutions of SDS, Brij-
P35 and CTAC.
values of the aryl benzoates as well as the 2-hydroxybenzophenone
derivatives. Furthermore, no change of the λabs was observed when
the hydrophobic core of the micelles as the solvent medium was
compared with the data recorded in cyclohexane. However, a
noticeable substituent effect was observed on the energy values of
the aryl benzoates and the corresponding benzophenone derivatives
(see Table 2), and this effect was quantified by means of the
Table 2 shows that there is no significant effect of the solvent on
the maximum absorption wavelengths and hence on the energy
Table 2. Energy (E) and maximum absorption wavelengths (λabs) values of the UV-visible lower energy band of aryl benzoates (1–10) and 5-substi-
tuted-2-hydroxybenzophenone (1a–10a) in different solvents.
Cyclohexane
Methanol
abs (nm)
SDS (0.10 M)
Brij (0.10 M)
CTAC (0.02 M)
λ
abs (nm)
E (ev)
λ
E (ev)
λ
abs (nm)
E (ev)
λ
abs (nm)
E (ev)
λ
abs (nm)
E (ev)
O
O
R
OMe (1)
OPh (2)
Me (3)
t-Bu (4)
H (5)
Ph (6)
Cl (7)
COOEt (8)
CN (9)
2
NO (10)
273
269
266
262
266
250
231
238
238
266
4.53
4.59
4.64
4.72
4.64
4.94
5.35
5.19
5.19
4.64
275
272
270
267
258
247
233
239
238
269
4.49
4.54
4.57
4.62
4.79
5.00
5.30
5.18
5.19
4.59
274
272
273
269
263
254
236
243
240
276
4.51
4.54
4.53
4.59
4.70
4.87
5.24
5.09
5.15
4.48
274
272
272
268
268
253
238
240
239
273
4.51
4.54
4.54
4.61
4.61
4.89
5.19
5.15
5.17
4.53
275
274
271
268
264
255
235
241
241
275
4.49
4.51
4.53
4.61
4.68
4.85
5.26
5.13
5.13
4.49
OH
R
O
OMe (1a)
OPh (2a)
Me (3a)
t-Bu (4a)
H (5a)
Ph (6a)
Cl (7a)
COOEt (8a)
CN (9a)
2
NO (10a)
375
357
353
341
340
355
353
330
331
328
3.30
3.46
3.50
3.62
3.64
3.48
3.50
3.74
3.73
3.77
374
357
353
348
334
340
344
323
324
306
3.30
3.46
3.50
3.55
3.70
3.63
3.59
3.82
3.81
4.04
373
357
352
348
336
350
354
329
329
350
3.31
3.46
3.51
3.55
3.68
3.53
3.50
3.76
3.76
3.53
372
360
353
348
345
346
350
332
331
324
3.32
3.43
3.50
3.55
3.58
3.57
3.69
3.73
3.73
3.81
374
352
355
335
340
348
350
325
330
329
3.30
3.51
3.48
3.69
3.64
3.55
3.69
3.80
3.75
3.76

6
Gast o´ n Siano et al.
(a)
(b)
7
6
5
4
3
2
5
Cl
CN
COOEt
4
3
2
Ph
Ph
t-Bu
Cl
t-Bu Me
NO
2
COOEt
H
MeO Me PhO
NO
PhO
CN
2
MeO
H
Cyclohexane
SDS (0.10 M)
Brij (0.10 M)
MeOH
Cyclohexane
SDS (0.10 M)
Brij-P35 (0.10 M)
MeOH
CTAC (0.02 M)
CTAC (0.02 M)
-
0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
-
0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
σ
p
σ
p
Figure 2. Hammett Correlation of the UV-visible lower Energy band (E) of (a) aryl benzoates and (b) 5-substituted-2-hydroxy-benzophenones in differ-
ent solvents.
Hammett linear free energy relationships (LFER) (42,43). Thus, Eq.
describes the Hammett LFER where the energy values E(R = X)
at the maximum absorption wavelengths are correlated with Ham-
mett substituent constants (σ ) (44).
p
that were obtained with the σ Hammett parameters and the
absorption energies (E) of substituted aryl benzoates and 2-hy-
droxybenzophenones by applying Eq. 1.
1
p
A nice linear correlation was obtained for the case of the aryl
benzoates as can be seen in Fig. 2a displaying a slope ρ value of
EðR ¼ XÞ ¼ r:sp þEðR ¼ HÞ
2
0
.70 (r = 0.90) and an E(R = H) value of 4.69 eV. Then, we
please change to
can say that the energy of the absorption band under study is
mostly affected by the electronic ability of the substituent itself
in promoting hypsochromic shifts of the spectroscopic band.
Indeed, the linear regression shows that as the electron-withdraw-
ing character of the substituents increases, the absorption wave-
length decreases noticeably. It is worth noticing that the linear
regression was performed using the whole data obtained in all
the solvents studied due to the absence of solvent effect. Like-
wise, Fig. 2b shows a good linear correlation for the family of 5-
(1)
EðR ¼ XÞ ¼ ρ:σp þEðR ¼ HÞ
The ρ value is a constant reflecting the sensitivity of the spec-
troscopic parameter values to the substituent and E(R = H) is the
energy value of the phenyl benzoate or 2-hydroxybenzophenone,
respectively. Then, Fig. 2 shows the linear correlation fittings
(
a)
(b)
1
.0
.8
.6
.4
.2
.0
1.0
0
0
0
0
0
0.8
0.6
0.4
Cyclohexane
SDS (0.10 M)
Brij-P35 (0.05 M)
CTAC (0.02 M)
Cyclohexane
SDS (0.10 M)
Brij-P35 (0.05 M)
CTAC (0.02 M)
0.2
0.0
0
100
200
300
400
500
600
700
0
50
100
150
t / s
200
250
300
t / s
∞
Figure 3. Relative absorbance (A/A ) of formation of: (a) benzophenone 1a at 374 nm and (b) benzophenone 9a at 331 nm, in cyclohexane and micel-
lar solutions.

Photochemistry and Photobiology
7
substituted-2-hydroxybenzophenones giving a slope ρ value of
effectively the substituent effect employing the linear regression
depicted in Eq. 2,
2
0
.40 (r = 0.91) and an E(R = H) value of 3.54 eV. Therefore, a
hypsochromic shift of the absorption wavelength was observed
as the electron-withdrawing character of the substituents
increases.
logðk=k0Þ ¼ ρ:σp
(2)
where k represents the rate constants for p-substituted phenyl
On the other hand, we have selected aryl benzoates 1 and 9
in order to analyze the solvent effect on the relative formation of
benzoates while k represents the rate constant for phenyl ben-
zoate. The rate constants (k) were easily gathered from the rela-
tive grow-in (A/A ) of the benzophenones profiles shown in
0
2
-hydroxybenzophenones 1a and 9a, respectively, through the
0
measuring of the relative absorption values (A/A ) as depicted
∞
Fig. 4a for each p-substituted aryl benzoates (1a–9a) recorded in
CTAC (0.02 M) aqueous solution. The same kinetic data were
also raised involving a first order kinetic analysis for the con-
sumption of each p-substituted aryl benzoates in micellar solu-
tion as well as in cyclohexane (see Figure S4). Then, with the
rate constants measured in CTAC (0.02 M) aqueous solution in
hand, a Hammett plot was constructed and a linear correlation
was obtained providing a slope ρ of 0.69 and R-square of 0.836
(Fig. 4b). The substituent effect does not play an important role
on the homolytic fragmentation of the C-O bond of the ester dur-
ing the photo-Fries rearrangement reaction of the p-substituted
phenyl benzoates as can be judged from the magnitude of the ρ
value. Furthermore, the positive sign of ρ shows that as the elec-
tron-withdrawing character of the substituents increases, the rate
constants increase noticeably. The electron-donor substituents
have the opposite effect.
in Fig. 3. The relative rates of formation of 1a in surfactant
media (see Fig. 3a) are slightly lower than in cyclohexane and
radiative, and nonradiative decay rates account for the observed
trend indicating a significant competition with the photoreaction
pathway. Furthermore, the relative rate of compounds 1a in
CTAC solution is significantly lower than in SDS or Brij-P35
solutions attributing this behavior to the appreciable effect of the
nature of the surfactant on the photoreaction. Indeed, CTAC is a
tetraalkylammonium chloride and the chloride anion has the abil-
ity to quench the singlet excited state of the aryl benzoate pro-
moting the deactivation through the intersystem crossing
pathway. This quenching process became efficiently because the
chloride anion operates like an external heavy atom quencher
(
38,39). A similar behavior was observed for the other aryl ben-
zoates and the relative rates of formation (A/A ) of the corre-
∞
sponding benzophenones in cyclohexane and in different
surfactant solutions are depicted in Figure S2.
Linear regression on Hammett’s plot was also obtained with
the rate constants measured in micellar solutions (SDS 0.10 M
and Brij-P35 0.10 M) and in cyclohexane providing a ρ value of
0.74 which is similar to that obtained for the CTAC solution
(see Figure S5). This linear trend led to conclude that electron-
withdrawing substituents increase whereas electron-donor sub-
stituents diminish the rate constants, as it was described previ-
ously in the case of the measurements performed in CTAC
solution.
However, for aryl benzoate 9 the relative absorbance (A/A∞)
of formation of benzophenone 9a in CTAC solutions shows a
similar profile as in cyclohexane and SDS solution but are higher
than in Brij-P35 solution. This peculiar behavior is due to an
intramolecular quenching of the singlet excited state of benzoates
9
through a spin–orbit coupling process because this benzoate
bears a cyano group which is a pseudo-halogen group. Therefore,
no external heavy atom effect of the chloride anion on the ben-
zophenone relative rates could be observed.
Location of the aryl benzoates in the micellar and
measurement of the binding constants (K )
b
Next, we decided to study the effect of the substituents on the
photochemical reaction of the aryl benzoates in micellar solution
and in cyclohexane through the photoreaction rate constants (k).
Once again, we applied the Hammett correlation to quantify
We applied 2D NMR (NOESY experiments) and UV-visible
spectroscopies to estimate the location of the aryl benzoates and
(
a)
1
(b)
0
0
0
0
.8
.6
.4
.2
.0
.8
.6
.4
.2
.0
CN
0
0
0
0
0
PhO
p-Me
p-H
Cl
0.0
-0.2
-0.4
MeO
Me
t-Bu
H
p-MeO
p-CN
p-t-Bu
p-Cl
Ph
p-Ph
p-PhO
-0.6
-0.8
0
100
200
300
400
500
600
700
-
0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
t / s
σp
Figure 4. (a) Plots of grow-in of benzophenones derivatives in CTAC (0.020 M) solution. (b) Hammett Correlation of the log (k/k
stituent parameters for the irradiation of aryl benzoates in CTAC (0.020 M) solution.
0
) with the σ
p
sub-

8
Gast o´ n Siano et al.
2
Figure 5. 2D NOESY contour plot of a solution of SDS (0.10 M) and benzoate 3 (5 mM) in D O at room temperature.
to measure the binding constants, respectively. The NOESY
experiments have been often employed to determine the extent
of coaggregation between two different kinds of surfactants in
water and also to estimate qualitatively the localization of sub-
strates inside the micelle (29,34). Indeed, the presence of cross-
peaks between the diagnostic signals of the substrate and the sur-
factants in the contour plots indicates such interaction to occur
Table 3. Binding constants (K
and CTAC) and aryl benzoates.
b
) in water of surfactants (SDS, Brij-P35
(
29,34). In Fig. 5 is shown the contour plot obtained after a
NOESY experiment that was performed for p-methylphenyl ben-
zoate 3 (5 mM) in a solution of SDS (0.10 M) in D O at room
K
b
/M−
1
2
temperature. The protons of the surfactant SDS and those of
methyl group of benzoate 3 are also highlighted in the same fig-
ure. The black framed insets underline the NOE (nuclear Over-
hauser effect) between the signals belonging to the α, β, ω and
the bulk protons of the surfactant SDS and the methyl protons of
p-methylphenyl benzoate 3 confirming the location of the ben-
zoate within the hydrophobic core. NOESY experiments have
been also carried out for benzoate 1 in solutions of SDS and
Aryl benzoates; R
SDS*
Brij-P35*
CTAC
1
2
3
4
5
6
7
8
9
1
; OMe
; OPh
; Me
; t-Bu
; H
; Ph
; Cl
1402
80
127
539
42
10
10
102
18
74
1398
581
69
423
63
70
31
24
26
16
22
33
157
41
32
26
162
1774
1394
181
; COOEt
; CN
2 2
Brij-P35 in D O and for benzoate 9 in solution of SDS in D O
(
see Figures S7–S9). Cross-peaks of diagnostic signals were
0; NO
2
observed in the 2D NMR contour plots also in these cases. How-
*
ever, all the NOESY experiments performed with benzoates 1, 3
Data taken from Reference (34) except for compounds 7 and 8.

Photochemistry and Photobiology
9
(
a)8
(b)
0
0
0
0
0
40
30
20
10
0
p-MeO
p-PhO
p-Me
p-t-Bu
p-MeO
p-CN
6
4
2
p-tBu
p-Ph
p-CN
p-COOEt
p-Cl
p-Ph
40
60
80
100
/ [Micelle]
/(A-A ) vs reciprocal of concentration of surfactants in water at room temperature: (a) CTAC and (b) SDS.
120
140
160
180
200
0
300
600
900
1200
1500
1800
2100
1
1/ [Micelle]
Figure 6. Plots of reciprocal of (A
0
0
Table 4. Energy (E) of the lowest allowed transition obtained for the
optimized geometries of the aryl benzoates (1–10) and 5-substituted-2-hy-
droxybenzophenone (1a–10a) in cyclohexane or methanol (SMD) at the
TD-PBE0/6-311 + g(2d,p)//PW6B95D3/def2-SVP and TD-CAM-B3LYP/
of the micelle because the proton nuclei of the benzoates corre-
late nicely with the proton nuclei of the surfactants as can be
seen through the cross-peaks of the contour plots.
Next, we have focused in the measurement of the binding
constants Kb between the surfactants and the aryl benzoates
using UV-visible spectroscopy applying the methodology
reported earlier in the literature for the case of aryl acetamide
6-311 + g(2d,p)//PW6B95D3/def2-SVP
Cyclohexane
Methanol
E(CAM-
E
E(CAM-
E
(PBE0)
(eV)
−1
(
29). A linear relationship is observed between (A–A0) and the
reciprocal of the concentration of the surfactant according to
equation 3 where A and A are the absorbance at the maxima
wavelength in the presence and absence of surfactant, respec-
tively, and ϵ and ϵ are the molar absorptivity of the aryl ben-
(
(
PBE0)
eV)
B3LYP)
(eV)
B3LYP)
(eV)
0
O
O
S
C
R
zoates and the complex.
OMe (1)
OPh (2)
Me (3)
t-Bu (4)
H (5)
Ph (6)
Cl (7)
COOEt (8)
CN (9)
2
NO (10)
4.08
4.94
4.90
5.08
5.08
5.07
4.77
5.06
5.04
5.02
4.10
4.13
4.22
4.51
4.52
4.76
4.26
4.61
4.78
4.79
4.04
4.99
4.95
5.07
5.07
5.06
4.83
5.05
5.03
5.02
4.11
A0
ɛS
ɛS
1
4.12
4.43
4.44
4.66
4.17
4.52
4.69
4.73
4.03
¼
þ
(3)
ðAꢂA0Þ ɛC ɛC ꢀKb ½Surfꢃ
The ratio between the intercept and the slope of the linear
relationship provides the binding constants (K ) of the aryl ben-
zoates which are collected in Table 3. On the other hand, Fig. 6
shows the experimental data and the best linear regression curves
obtained for some aryl benzoates and surfactants such as CTAC
and SDS, respectively. The plots for the other system (Brij-P35
as the surfactant) are collected in Figure S6.
b
OH
R
The data collected in Table 3 show that the benzoates bind to
the micelles but it is apparent that the K values obtained in
b
O
OMe (1a)
OPh (2a)
Me (3a)
t-Bu (4a)
H (5a)
Ph (6a)
Cl (7a)
3.27
3.39
3.59
3.56
3.74
3.39
3.54
3.82
3.57
3.77
3.87
3.88
4.03
3.78
3.88
4.12
3.26
3.26
3.59
3.62
3.74
3.36
3.54
3.83
3.58
3.67
3.88
3.93
4.06
3.78
3.92
4.15
Brij-P35 were found to be higher than those obtained in SDS
and CTAC. An estimation of a minimum value around 190 M for
the binding constant (K ) of Brij-P35 surfactant have been also
b
reported (17). The behavior observed between Brij-P35 and the
benzoates can be ascribed to a more nonpolar environment of
the micelle core which was previously measured using pyrene as
COOEt
a
micropolarity probe (45). Indeed, benzoates possessing
hydrophobic character such as 1, 2, 4, 8 and 9 in Brij-P35 micel-
lar solution, show high K values. Generally, the binding con-
stants (K ) obtained for aryl benzoates 1–10 are typical of
(
8a)
CN (9a)
NO (10a)
3.78
3.89
4.11
4.14
3.77
3.89
4.16
4.16
b
2
b
aromatic solutes as it was previously reported in the literature
(46–49). In fact, substrates like benzonitrile, methyl and ethyl
benzoates, acetanilide and benzamide among others show bind-
ing constants ranging between 8 to 100 M (47). Furthermore, it
and 9 demonstrate that it is challenging to estimate precisely the
location of the aryl benzoates with accuracy but we can suggest
that the aryl benzoates are located within the hydrophobic core

1
0
Gast o´ n Siano et al.
6
5
4
3
2
1
0
5.0
-
3
(
a)
I=-9.910
S=1.09
r ²=0.8079
(b)
4.5
4.0
3.5
3.0
Cyclohexane
SDS (0.10 M)
Cyclohexane
SDS (0.10 M)
Brij-P35 (0.10 M)
CTAC (0.02 M)
MeOH
I=1.67
Brij-P35 (0.10 M)
CTAC (0.02 M)
MeOH
S=0.57
r ²=0.4085
4.4
4.6
4.8
5.0
5.2
5.4
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
E(exp) / eV
E(exp) / eV
Figure 7. Correlation of the theoretical and experimental energies of the lower absorption band of: (a) p-substituted phenyl benzoates and (b) 5-substi-
tuted-2-hydroxybenzophenones.
was found for a series of substituted benzoyl chloride that the
binding constants in SDS and in CTAC are in the same order of
magnitude (50), and this trend is observed for our experimental
data with the exception of benzoates 1 and 4.
Finally, NOESY experiments demonstrated that aryl benzoates
are located within the hydrophobic core of the micelle because
cross-peaks between proton nuclei of the benzoates correlate
nicely with the proton nuclei of the surfactants as can be seen in
the contour plots. Furthermore, it was found that aryl benzoates
the photoreaction. No significant effect of the solvent on the low
absorption band energy (E) values of aryl benzoates and 2-hy-
doxybenzophenone derivatives was observed when moving from
cyclohexane to polarity brought the hydrophobic core of the
micelles. However, a noticeable substituent effect was observed
on the energy (E) values of both the aryl benzoates and ben-
zophenone derivatives displaying a hypsochromic shift after
quantification by means of the Hammett linear free energy rela-
tionships (LFER). The substituent effect was also analyzed on
the photoreaction of aryl benzoates, and a low role of the sub-
stituents on the homolytic fragmentation of the C-O bond of the
ester group was observed in cyclohexane and micellar solutions.
In fact, the ρ values indicated that electron-withdrawing sub-
stituents accelerate the photoreaction while electron-donor sub-
stituents deaccelerate the photoreaction.
The nature of surfactants has also displayed a noticeable effect
on the photoreaction, and it was found that the photoreaction
performed in CTAC solution is significantly lower than in SDS
or Brij-P35 solutions. This behavior was attributed to the chlo-
ride anion of CTAC surfactant because it operates like an exter-
nal heavy atom quencher quenching the singlet excited state of
the aryl benzoate and promoting the deactivation through a com-
petitive intersystem crossing pathway.
1-10 bind efficiently to the SDS, CTAC and Brij-P35 micelles,
respectively, as can be judge from the binding constants (Kb)
which are typical constants observed for aromatic compounds
measured using UV-visible spectroscopy. Both spectroscopic
methods confirm and reinforce the idea that the photochemical
reaction takes place within the hydrophobic core of the ionic and
nonionic micelles which can be considered like mini-photochem-
ical reactors.
We also computed the UV-Vis spectra of the species involved
in the study using two different functionals, namely PBE0 and
CAM-B3LYP. Both functionals afforded similar results. Interest-
ingly, the computed spectra followed more closely the experi-
mental values for the benzophenones, while the aryl benzoates
were not affected by the substitution (see Table 4). However, a
nice fit could be plotted comparing the theoretical and experi-
mental values using the PBE0 functional for aryl benzoates
whereas CAM-B3LYP functional was used for substituted ben-
zophenones, as depicted in Fig. 7.
Location of the aryl benzoates with 2D NOESY NMR spec-
troscopy in the shell or in the hydrophobic core of the micelle
and measurement of the binding constants (K ) between the ben-
b
zoates and the surfactants account for the selective behavior
observed where diffusion of the radical species from the micelle
is inhibited. On the other hand, benzophenone derivatives, as the
main photoproducts, and the p-substituted phenols were formed
when the irradiations were carried out in cyclohexane but no
selectivity was observed.
The calculated spectra were used to compare the theoretical
UV-Vis values with the experimental ones, showing the same
trend observed.
CONCLUSION
The photo-Fries rearrangement reaction of aryl benzoates exam-
ined in this paper takes place efficiently in cyclohexane and with
noticeable selectivity in micellar media. Indeed, high selectivity
was observed in favor of the formation of 5-substituted-2-hy-
droxybenzophenone derivatives in micellar media using ionic
(
SDS and CTAC) and neutral (Brij-P35) surfactants. These ben-
Finally, the finding that the selectivity observed in the photo-
Fries rearrangement of some aryl benzoates in green and sustain-
able micellar media gives 5-substituted-2-hydroxybenzophenone
derivatives in yields up to 95 % could be applied in the
zophenone derivatives were obtained in yields up to 95 % while
no substituted phenols were detected. The substituent effect was
also studied on the UV-visible absorption spectroscopy and on

Photochemistry and Photobiology 11
preparation of a new wide variety of substituted-2-hydroxyben-
zophenone derivatives and the results will be independent from
the nature of the surfactant employed to perform the photoreac-
tions.
phenylacylates and aryl benzyl ethers in media comprised of poly-
olefinic films. J. Photochem. Photobiol. C: Photochemistry Reviews
2
, 117–137.
0. Kaanumalle, L. S., C. L. D. Gibb, B. C. Gibb and V. Ramamurthy
2007) Photo-Fries reaction in water made selective with a capsule.
1
1
(
Org. Biomol. Chem. 5, 236–238.
1. Kulasekharan, R., R. Choudhurry, R. Prabhakar and V. Ramamurthy
Acknowledgements—SMB is research member of CONICET, Argentina.
(
2011) Restricted rotation due to the lack of free space within a cap-
sule translates into product selectivity: Photochemistry of cyclohexyl
phenyl ketones within a water-soluble organic capsule. Chem. Com-
mun. 47, 2841–2843.
SUPPORTING INFORMATION
1
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Additional supporting information may be found online in the
Supporting Information section at the end of the article:
Appendix S1. Additional supporting information may be
found online in the Supporting Information section at the end of
the article
Data S1. Time-resolved UV-visible absorption spectroscopy
under steady-state.
Data S2. Relative absorption (A/A0) versus time in homoge-
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1
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1
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chemistry in micelles. In Topics in Current Chemistry, Photochem-
istry and Organic Synthesis (Edited by F. L. Boschke). pp 57.
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Data S3. Measure of the aryl benzoate consumption rate con-
stants (k).
Data S4. Hammett Linear correlations.
Data S5. Determination of the binding constants Kb in micel-
lar media.
Data S6. 2D NMR (NOESY Experiments) spectra in micellar
media.
Data S7. Physical and spectroscopic characterization of com-
pounds 7, 7a, 8 and 8a.
Data S8. References.
Data S9. Copy of the 1H and 13C spectra of aryl benzoates 7
and 8.
16. Kano, K. and T. Matsuo (1973) Photochemical reaction of the p-ben-
zoquinones in micellar system. Chem. Lett. 2, 1127–1132.
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flavins in micelles: specific effects of anionic surfactants on the
monomerization of thymine cyclobutane dimers photosensitized by
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Z. G. Naal (2014) Binding of chloroquine to ionic micelles: Effect
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Data S10. Copy of the 1H and 13C spectra of 2-hydroxy-5-
substituted benzophenones 7a and 8a.
2
0. Anderson, J. C. and C. B. Reese (1960) Photoinduced Fries Rear-
rangement, p. 217. Proc. Chem. Soc, London.
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