Selectivity in the photo-fries rearrangement of some aryl benzoates in green and sustainable media. Preparative and mechanistic studies

Article abstract of DOI:10.1021/acs.joc.9b00334

Irradiation of a series of p-substituted aryl benzoates under N₂ atmosphere in homogeneous and micellar media was investigated by means of steady-state condition and of time-resolved spectroscopy. A notable selectivity in favor of the 2-hydroxybenzophenone derivatives was observed in micellar media. The benzophenone derivatives were the main photoproduct. On the other hand, in homogeneous media (cyclohexane, acetonitrile, and methanol) the observed product distribution was entirely different, viz. substituted 2-hydroxybenzophenones, p-substituted phenols, benzyl and benzoic acid were found. The binding constants in the surfactant were also measured and NOESY experiments showed that the aryl benzoates were located in the hydrophobic core of the micelle. Laser flash photolysis experiments led to the characterization of both p-substituted phenoxy radical and substituted 2-benzoylcyclohexadienone transients in homogeneous and micellar environment.

Full text of DOI:10.1021/acs.joc.9b00334

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Article
Selectivity in the photo-Fries rearrangement of some aryl benzoates
in green and sustainable media. Preparative and mechanistic studies.
Gastón Siano, Stefano Crespi, Mariella Mella, and Sergio Mauricio Bonesi
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00334 • Publication Date (Web): 15 Mar 2019
Downloaded from http://pubs.acs.org on March 17, 2019
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Selectivity in the photo-Fries rearrangement of some aryl benzoates in green and sustainable
media. Preparative and mechanistic studies.
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Gastón Sianoa, Stefano Crespib, Mariella Mellab and Sergio M. Bonesi*,a,b
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a
Departamento de Química Orgánica, CIHIDECAR – CONICET, 3er Piso, Pabellón 2, Ciudad
Universitaria, Facultad de Ciencias Exactas y Naturales, University of Buenos Aires, Buenos Aires,
1428, Argentina. Phone:+541145763346.
b
Dipartimento di Chimica, Sezione Chimica Organica, v.le Taramelli 12, 27100, University of Pavia,
Pavia, Italy.
E-mail: smbonesi@qo.fcen.uba.ar
Table of content.
O
OH
R
hn
O
O
Photo-Fries
Surfactant
H2O; 25°C
R
rearrangement
Laser Pulse
(266 nm)
Homolysis
Tautomerization
Micellar media
High Selectivity
O
O
O
H
O
R
R
Photo-Fries
Rearrangement
Abstract. Irradiation of a series of p-substituted aryl benzoates under N2 atmosphere in homogeneous
and micellar media was investigated by means of steady-state condition and of time-resolved
spectroscopy. A notable selectivity in favor of the 2-hydroxybenzophenone derivatives was observed in
micellar media. The benzophenone derivatives were the main photoproduct. On the other hand, in
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homogeneous media (cyclohexane, acetonitrile, and methanol) the observed product distribution was
entirely different, viz. substituted 2-hydroxybenzophenones, p-substituted phenols, benzyl and benzoic
acid were found. The binding constants in the surfactant were also measured and NOESY experiments
showed that the aryl benzoates were located in the hydrophobic core of the micelle. Laser flash
photolysis experiments led to the characterization of both p-substituted phenoxy radical and substituted
2-benzoylcyclohexadienone transients in homogeneous and micellar environment.
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Introduction.
Photochemical reactions grant access to a variety of scaffolds difficult, if not impossible, to access
through thermal chemistry. However, the energetic advantage of populating the excited states is often
paid with a lack of selectivity in product formation. For this reason, reactions involving intermediates
such as radical pairs or radical-ion pairs represent a significant challenge regarding product distribution
for the synthetic organic photochemist. Zeolites, micelles, polyolefin films, cavitands, dendrimers, etc.,
as a useful heterogeneous media, can be helpful to direct the selectivity of photoproducts in
photoinduced reactions.1
Surfactants are amphiphilic molecules and aggregation in solution to form micelles can be
achieved when their concentration is 100 times higher than the critical micellar concentration (cmc).
Then, micelles solubilize efficiently hydrophobic compounds in water, albeit micelles are not static
species showing a dynamic equilibrium.2 Also, micelle can concentrate guest molecules into relatively
small effective volumes promoting their re-encounter consequently.1c Inside the homophobic core of
the micelles, significant cage effects are observed when compared to homogeneous media, with
magnitudes impossible to explain considering the sole microviscosity in the constrained environment.
The main reason for this behavior is based on the hydrophobicity of the solutes where inhibition of
their diffusion into the aqueous phase is noteworthy. Thus, the reaction intermediates show a high
lifetime in the restricted hydrophobic core of the micelle. For example, geminate radical pairs that are
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produced photochemically within the micellar core, have their rotational and translational mobility
constrained inside the micelles.2 Indeed, the mobility restricted within the hydrophobic cores of
radicals, radical cations, or other reactive intermediates limits unwanted reactions (e.g. radical–radical
self-quenching reactions) and the access of adventitious reagents (e.g. water and oxygen) that would
cause their collapse in solution. Therefore, micellar solution can induce a product distribution and a
relative chemical yield that can be significantly different when compared with homogeneous
conditions.1,2
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There has been many studies on the control of the reactivity of radical species generated within
the hydrophobic core of the micelle and some physical parameters (e.g. electrostatic, polarity,
hydrophobic interactions, viscosity, as well as hydrogen-bonding solvation) may be involved in
determining their reactivities.3 Also, several studies directed to analyze and quantify the reactivity,
selectivity, and efficiency of micellar cage on photochemical reactions in water have been carried out.
Among the vast number of photochemical transformations, a particular example is represented
by the photo-Fries rearrangement. Anderson and Reese4 have discovered the photoreaction where a
homolytic fragmentation of a carbon–heteroatom bond is involved, i.e., C-O, C-S and C-N, of esters,
thioesters and amides, respectively.5 The photo-Fries rearrangement proceeds via a well-established
radical mechanism, mainly occurring through the excited singlet state.1b,6 Typically, the photoinduced
Fries rearrangement reaction of (hetero)aryl benzoates in homogeneous media affords ortho- and para-
regioisomers as well as the corresponding phenols (Scheme 1).7,1b
O
OH
OH
O
OH
O
Ar
hn
Ar
+
+
Solvent
25ºC
R
O
R
R
R
Ar
Scheme 1. Photochemisytry of aryl benzoates.
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The product distribution of the photo-Fries rearrangement underlines the competition between
in-cage radical recombination versus out-of-cage diffusion and, consequently, aryl esters, as model
substrates, have been chosen to study heterogeneous environments.8 In these studies, SDS (sodium
dodecyl sulfate) was the preferred surfactant to be tested. In the literature, there are examples of photo-
Fries reactions, i.e. irradiation of benzamides in SDS solution and irradiation of aqueous solutions of
acetanilides confined in cyclodextrin.8b,8d Recently, we have studied the photo-Fries rearrangement of a
variety of substituted acetanilides in micellar solution showing the high selectivity of the photoreaction
in favor of the ortho-rearranged photoproducts, viz. substituted 2-aminoacetophenones.9 The
preparation of benzoyloxy benzophenone derivatives requires the use of 2-hydroxybenzophenones as
key compounds, demonstrating biological activity and pharmaceutical properties (e.g. anti-
inflammatory and estrogenic activity).10 Therefore, we carried out the systematic study the photo-Fries
rearrangement of a series of p-substituted phenyl benzoates in surfactant solutions with the aim to
evaluate the selectivity towards the formation of 5-substituted 2-hydroxy benzophenones in constrained
environment. Scheme 2 shows the structures of the surfactants as well as the aryl benzoates employed
in this systematic study.
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(a) Structures of aryl benzoates.
O
1 R = OCH3; 2 R = OPh; 3 R = CH3; 4; R = t-Bu;
5 R = H; 6 R = Ph; 7 R = CN; 8 R = NO2
O
R
(b) Structures of surfactants.
Anionic Surfactant.
Sodium dodecyl sulfate (SDS)
cmc: 8.2 mM
Na
O
O
O
O
S
Non-ionic Surfactants.
Polyoxyethylene(22)lauryl ether (Brij P35)
OH
cmc: 0.09 mM
O
O
22
Scheme 2. Structures of surfactants and aryl benzoates.
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In the present article, we describe the results on the photo-Fries rearrangement of several para-
substituted aryl benzoates in homogeneous and micro heterogeneous media. The binding constants (Kb)
and the location of the substrates within the micelles are measured through spectroscopic methods.
From a preparative point of view, the use of anionic and neutral surfactant micelles shows a high
selectivity in favor of the formation of the 2-hydroxy benzophenone derivatives achieving yields higher
than 90%. From a mechanistic viewpoint, it is furnished the characterization of both p-substituted
phenoxy radical and substituted 2-benzoylcyclohexadienone transients in homogeneous and micellar
environment. The in-cage and out-of-cage rate constants (kR and kE, respectively) of p-substituted
phenoxy radical are measured for the first time by means of laser flash photolysis.
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Results.
Steady-state Photolysis. Photoirradiation of aryl benzoates in homogeneous media. Irradiation of aryl
benzoates (1 – 8) in cyclohexane, MeOH and MeCN which were chosen as representative non-polar,
protic polar and aprotic polar solvents, with lexc = 254 nm under N2 atmosphere provided the expected
photoproducts from the photo-Fries rearrangement, viz. formation of 2-hydroxybenzophenone
derivatives (1a – 8a), the corresponding phenols (1b – 8b) and benzoic acid. The photochemical
reaction is depicted in Scheme 3. The formation of benzoic acid was attributed to the oxidation of
benzaldehyde, which is the primary photoproduct formed, because of the presence of residual
molecular oxygen in the reaction mixture.11 In all the performed reactions, the yield of benzoic acid
was between 5 to 15%.
OH
OH
+
O
hn
O
+
OH
R
solvent
25 °C
R
O
O
R
1 - 8
1a - 8a
1b - 8b
Scheme 3. The photoinduced Fries rearrangement of aryl benzoates (1 – 8).
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When esters are consumed, the chemical yields collected in Table 1 show that benzophenones 1a – 8a
are the main photoproducts in up to 94% yield. Furthermore, the product distribution did not change
significantly with the nature of the solvent and poor selectivity in favor of the benzophenone
derivatives was observed.
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Table 1. Irradiation of aryl benzoates in homogeneous solution. Yield of photoproducts,a reaction
quantum yield (fr)b and fluorescence quantum yield (ff).c
Photoproduct yield (%)
Aryl benzoates Solvent
Benzophenones (a)
Phenols (b)
27
fr
ff
1
2
3
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7
Cyclohexane 46
0.30 0.08
0.36 0.03
0.33 0.10
0.37 0.002
0.71 0.013
0.44 0.002
0.40 0.25
0.62 0.07
0.31 0.11
0.50 0.001
0.35 0.025
0.63 0.002
0.36 0.001
0.14 0.005
0.14 0.002
0.59 0.012
0.32 0.035
0.31 0.033
0.51 0.002
0.82 0.025
MeCN
MeOH
62
66
17
17
28
19
23
22
5
Cyclohexane 55
MeCN
MeOH
69
73
Cyclohexane 42
MeCN
MeOH
94
80
20
27
5
Cyclohexane 59
MeCN
MeOH
95
75
25
30
33
26
23
18
14
19
15
Cyclohexane 40e
MeCN
MeOH
60e
58e
Cyclohexane 41
MeCN
MeOH
73
85
Cyclohexane 38
MeCN 84
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MeOH
75
26
34
40
52
0.93 0.002
0.02 NFd
0.07 NF
0.07 NF
8
Cyclohexane 31
MeCN
MeOH
60
50
a
1
9
Yield of photoproducts determined by H-NMR spectroscopy in the reaction mixture. Concentration
of aryl benzoates: 5.0x10-3 M. Actinometer: KI (0.6 M), KIO3 (0.1 M) and Na2B2O7.10H2O (0.01 M)
b
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solution in water; f(I3 ) = 0.74; lexc = 254 nm.12; Error: ± 0.01. Actinometer: 4-chloroanisole
acetonitrile solution under Ar atmosphere; ff = 0.01913; Error: ±0.002. dNF: non-fluorescent substrate. e
4-Hydroxybenzophenone is also formed: Cyclohexane 30%; MeCN 7%; MeOH 15%.
-
c
The quantum yields of consumption of the aryl benzoates (fr) in polar and non-polar solvents were
measured (see Table 1). The fr values were found to be higher than 0.30, implying that the photo-Fries
rearrangement reaction occurred efficiently. Moreover, in every solvent, a marked increase of the fr
values was observed, moving from esters bearing electron-donor to esters substituted with electron-
acceptor substituents. The only exception found was ester 8. The fr values measured for compound 8
are lower than 0.10 in all the solvents studied. The aryl benzoates are all poorly fluorescent
chromophores (see Table 1) with the exception of benzoate 8 that was found to be non-fluorescent. The
spin coupling effect of the nitro group explains the peculiarity of ester 8.14,15 Indeed, intersystem
crossing pathway competes with both the photo-Fries rearrangement reaction and the fluorescence
emission.
UV-visible and NMR spectroscopies were used to follow the photochemical reaction and aryl
benzoates 1, 2, 3 and 7 have been chosen as representative examples for such spectroscopic studies.
Figure 1(a) showed the time-resolved UV-visible absorption spectrum of the photoreaction of p-
methylphenyl benzoate (3) in cyclohexane showing the growth of a new band located at 352 nm during
the irradiation time. This band was assigned to the n,p* transition of the carbonyl group of
benzophenone 3a14 and was also observed in MeOH and MeCN. However, no significant solvent effect
was observed in the maximum wavelength of the n,p* transition band upon change of the solvent
polarity. A similar solvent effect on the maximum wavelength of the n,p* band of the 2-hydroxy
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benzophenone derivatives (1a, 2a, 4a – 8a) was also observed (see Figure S1 in Supporting
Information). On the other hand, a noticeable substituent effect on the n,p* band of the substituted 2-
hydroxy benzophenones (1a – 8a) was observed. In fact, a hypsochromic effect (Dl = -73 nm) ascribed
to the n,p* band was measured changing the substituent from the electron-donor MeO (lmax = 378 nm)
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to the electron-accepting NO2 (lmax = 305 nm). The course of the photoreaction of ester 1 is detailed in
Figure 1(b), where the benzoyl [1;3]-migration to form benzophenone 1a is shown to be the primary
process. Figures 1(c) and 1(d) show the relative formation of benzophenones 2a and 7a (starting from
the corresponding esters 2 and 7), respectively, in non-polar and polar solvents. The relative rates of
formation of 2a are similar in MeCN and cyclohexane while in MeOH is somewhat lower implying
that in protic polar solvent radiative and non-radiative decay rates compete with the photoreaction
pathway. The rates of formation of benzophenone 7a are quite similar in all the solvents tested. For the
other esters studied the relative rates of formation of the corresponding benzophenone derivatives
showed a similar behavior (see Figure S2 in Supporting Information).
(a)
(b)
0.6
1,0
0.5
0.4
0.3
0.2
0.1
0.0
0,8
0,6
0,4
0,2
0,0
t = 0 s
t = 300 s
250
300
350
l / nm
400
450
0,0
0,5
1,0
1,5
2,0
t / h
(c)
(d)
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0.0
0
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100
150
200
0
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100
150
200
250
t / s
t / s
Figure 1. (a) Time-resolved UV-visible absorption spectrum of 3 in cyclohexane. Blue line: initial time;
red line: 300 s. (b) Relative yield profile vs time of 1 in MeOH: ester 1 (circles); benzophenone 1a
(triangles); phenol 1b (square). (c) Relative absorbance at 358 nm (A/A∞) of formation of 2a in: MeOH
(circles); MeCN (triangles); cyclohexane (square). (d) Relative absorbance at 333 nm (A/A∞) of
formation of 7a in: MeOH (circles); MeCN (triangles); cyclohexane (square).
1
The H-NMR spectra of the photoreaction mixture (6 h irradiation) of 4-cyanophenyl benzoate (7) in
cyclohexane under N2 atmosphere were also recorded (see Figure S3 in Supporting Information). As
expected, the formation of the photoproducts benzophenone 7a and phenol 7b were confirmed through
their diagnostic signals (see Figure S3 in Supporting Information) along with the unreacted ester 7. The
experiments demonstrated poor selectivity of the photoreaction in homogeneous media and this trend
was observed for all the esters (1 – 8) studied (see Table 1).
Photoirradiation of aryl benzoates in micellar media. Irradiation of aryl benzoates (1 – 8) in aqueous
SDS (0.10 M) with lexc = 254 nm under air caused the selective generation of 2-hydroxybenzophenone
derivatives (1a – 8a) in high yields along with lower amounts of the corresponding phenols (1b – 8b)
(for structures, refer to Scheme 3). In these experiments consumption up to 95% of aryl benzoates (1 –
8) was obtained. The chemical yields of benzophenone derivatives (1a – 8a) are collected in Table 2.
Table 2. Irradiation of aryl benzoates in micellar solution. Yield of 5-substituted-2-
hydroxybenzophenonesa and reaction quantum yield (fr)b and fluorescence quantum yield (ff)c of aryl
benzoates.
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Photoproduct yield (%)
fr
Brij-P35
ff
Benzophenones
SDS
94
Brij-P35
95
SDS
0.13
0.79
0.55
0.54
0.50
0.53
0.45
0.13
SDS
1a
2a
3a
4a
5a
6a
7a
8a
0.12
0.21
0.41
0.26
0.21
0.21
0.10
0.01
0.026
0.026
0.024
0.025
0.029
0.062
0.024
NFd
76
85
70
92
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85
89
90e
92e
87
90
80
78
44
56
a
1
Yield of photoproducts determined by H-NMR spectroscopy in the reaction mixture. Concentration
of aryl benzoates: 5.0x10-3 M. b Actinometer: KI (0.6 M), KIO3 (0.1 M) and Na2B2O7.10H2O (0.01 M)
-
c
solution in water; f(I3 ) = 0.74; lexc = 254 nm.12 Error: ± 0.01. Actinometer: 4-chloroanisole
acetonitrile solution under Ar atmosphere; ff = 0.01913; Error: ±0.002. d NF: non-fluorescent substrate.
e 4-Hydroxybenzophenone is also formed in 8 – 10%.
As is apparent from Table 2, SDS and Brij-P35 micellar solutions promoted a high selectivity on the
photo-Fries rearrangement of the aryl benzoates 2 – 8, favoring the formation of the corresponding
substituted benzophenones (2a – 8a) over the p-substituted phenols. The observed selectivity was
attributed to the confinement of the aryl benzoates and the radicals formed after the C-O bond cleavage
within the hydrophobic core provided by the micellar medium. Moreover, the p-substituted phenols
were formed in a minor extent or not formed at all, evidencing the suppression of products arising from
cage escape. It is worth noticing that the irradiation of p-nitrophenyl benzoate (8) gave 2-hydroxy-5-
nitrobenzophenone (8a) only in 44 % yield (the consumption of the starting material was 66 % after 6 h
of irradiation). No p-nitrophenol was detected in the reaction mixture. Competitive deactivation of the
singlet state of p-nitrophenyl benzoate (8) through intersystem crossing accounted for the observed
chemical yield of benzophenone 8a.14 The competitive process populates the triplet excited state, due to
the spin-orbit coupling provided by the nitro group, which is an unproductive excited state of benzoate
8.
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Quantum yields of consumption of the aryl benzoates (fr, see Table 2) were measured in micellar
media, viz. SDS and Brij-P35, and were found to be of the same order of magnitude. However, the
quantum yields measured in SDS solution were larger than in Brij-P35 solutions. This behavior can be
attributed to an enhancement of the non-radiative pathway from the singlet state of the aryl benzoates
in Brij-P35 solutions. The high-consumption quantum yields and low or no fluorescence emission from
aryl benzoates (1 – 8) was consistent with a fast reaction from the singlet state. However, non-radiative
and intersystem crossing pathways from the singlet state compete with the photo-Fries rearrangement.
UV-visible spectroscopy was used for following the photoreaction and p-methoxyphenyl benzoate (1)
was selected as a representative aryl benzoate. Thus, the reaction of 1 in SDS (0.10 M) was followed
by UV-visible absorption spectroscopyand the UV-visible spectral change vs time is shown in Figure
2(a). It is apparent from the UV-visible spectra that the photo-Fries rearrangement of compound 1 to
form 2-hydroxy-5-methoxy benzophenone (1a) is the primary process, according to the appearance of
the characteristic n,p* band of the carbonyl group located at 372 nm. Similar behavior was observed
for compounds 2 – 8 (see Figure S1 in Supporting Information).
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The course of the photoreaction of ester 1 is depicted in Figure 2(b) and clearly shows that the benzoyl
[1;3]-migration to form benzophenone 1a is the main process. On the other hand, we selected aryl
benzoates 7 and 4 as examples of aryl benzoates to show the relative formation of benzophenones 7a
and 4a, respectively, in cyclohexane and 0.10 M SDS and 0.01 M Brij-P35 solutions (see Figures 2(c)
and 2(d)). The relative rates of formation of 7a in surfactant media (see Figure 2(c)) are slightly lower
than in cyclohexane and can be attributed to the radiative and non-radiative decay rates that compete
with the photoreaction pathway. Similar behavior is observed for the rates of formation of
benzophenone 4a (see Figure 2(d)) as well as with for the other benzophenone derivatives (see Figure
S2 in Supporting Information).
(a)
(b)
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1.0
0.8
0.6
0.4
0.2
0.0
1,0
0,8
0,6
0,4
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t = 0 s
0,2
0,0
t = 300 s
400
0,0
0,5
1,0
1,5
2,0
2,5
3,0
200
250
300
350
450
t / h
l / nm
(c)
(d)
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
0
50
100
150
t / s
200
250
0
50
100
t / s
150
200
Figure 2. (a) UV-visible spectral change vs time of 1 in SDS 0.10 M in water. Blue line: initial time;
red line: 300 s. (b) Relative yield profile vs time of 1 in MeOH: ester 1 (circles); benzophenone 1a
(triangles); phenol 1b (square). (c) Relative absorbance at 375 nm (A/A∞) of formation of 7a in:
Cyclohexane (circles); 0.10 M SDS solution (triangles); 0.10 m Brij-P35 solution (square). (d) Relative
absorbance at 355 nm (A/A∞) of formation of 4a in: Cyclohexane (circles); 0.10 M SDS solution
(triangles); 0.10 m Brij-P35 solution (square).
The photoreaction of compound 1 in micellar environment (SDS 0.10 M) was also followed by NMR
1
spectroscopy. The H-NMR spectra of the reaction mixture of benzoate 1 in SDS (0.10 M) irradiated
during 6 h with lexc= 254 nm (see Figure S4 in Supporting Information) showed that benzophenone 1a
was formed in 94 % yield along with p-methoxyphenol and benzoic acid in yields lower than 5%. In
the same spectra, the signals belonging to the surfactant (SDS) were also observed. The consumption of
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benzoate 1 was quantitative. Similar results were obtained with all the aryl benzoates studied (2 – 8)
4
5
and the photoproduct yields are collected in Table 2.
6
7
8
9
Binding constants (Kb) of aryl benzoates in micellar media. When ionic and neutral surfactant solutions
are used as micro reactors to perform photoreactions the knowledge of the reactant’s location in the
micellar system is required. In order to know the reactant’s positioning within micelles, UV-visible and
1H NMR studies of guest molecules (aryl benzoates) in micellar solution were conducted. UV-visible
spectroscopy was used to determine the binding constant (Kb) between micelles and aryl benzoates
applying a methodology that have been reported earlier for aryl acetamide.9 Both bathochromic and
hyperchromic shifts of the lower energy absorption band of the aryl benzoates were observed by
addition of increasing amounts of surfactant, demonstrating the binding of the substrates to the micelle
that took place within the hydrophobic core of the micelle. Indeed, the binding of the aryl benzoates to
the micelle can be described as equilibrium, Kb can be written according to equation 1, where S
represents the benzoates, Surf the surfactants and [S-Surf] the complex formed between benzoates and
the surfactants.
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Kb
[S-Surf]
[Surf]
Kb
[S-Surf]
=
[Surf]
[S]
+
(1)
[S]
Equation 2 was obtained after application of Lambert-Beer law on equation 1. A0 and A are the
absorbances at the maximum wavelength in the absence and presence of surfactant, respectively. The
molar absorptivity of the complex and the benzoates are dubbed accordingly eC and eS. A linear
relationship is observed between (A – A0)-1 and the reciprocal of the concentration of the surfactant in
equation 3 which was obtained after rearranging equation 2.
eC.Kb.[Surf]
eS.(1 + Kb.[Surf])
(A - A0)
A0
(2)
(3)
=
=
eS
eC
eS
A0
1
DA
1
A0
=
+
eC.Kb
[Surf]
(A - A0)
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The experimental data obtained for some aryl benzoates and SDS and Brij-P35 are shown in Figure 3
and the best linear regression curves are also included in the same figure. The plots for the other
systems are collected in Figure S5 (see Supporting Information). The Kb values for 1 – 8 calculated
from the ratio between the slope and the intercept of the regression curve are shown in Table 3. From
these data it is apparent that the benzoates bind to the micelles and the ones possessing hydrophobic
character, i.e. benzoates 1 and 4 in SDS micellar solution, show high Kb values. Likewise, benzoates 1,
4 and 7 possess greater Kb compared to the other substrates in the case of Brij-P35 micellar solution.
Generally, the Kb values obtained for benzoates 1 – 8 are typical of aromatic solutes as it pointed out by
Quina, Treiner and co-workers.16 Estimation of Kb values ≤ 100 M−1 in SDS for weakly hydrophobic
substrates such as phenyl chloroformate have also been reported.17 Likewise, Kb values have been
reported for the binding of Brij-P35 surfactant and a series of benzoyl chloride derivatives and they
were found to be higher than those obtained for other surfactants such as SDS and CTAC.18 Estimation
of a minimum value around 190 M−1 for the binding constant (Kb) of Brij-P35 surfactant was reported
and accounts for a more apolar environment when pyrene was used as a micropolarity probe.18
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(a)
(b)
7
6
5
4
3
2
1
0
25
20
15
10
5
0
0
1000
2000
3000
4000
5000
6000
7000
0
300
600
900
1200
1500
1800
2100
1 / (Surfactant)
1/(Surfactant)
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Figure 3. Reciprocal plotting ((A0/(A-A0) vs concentration of surfactants) in water at room temperature:
(a) SDS (circles: 2; squares: 4; triangles: 7) and (b) Brij-P35 (circles: 2; squares: 4; triangles: 1). In all
the linear fitting regressions: r2 > 0.99.
6
7
8
9
Table 3. Binding constant (Kb) in water of surfactants (SDS and Brij-P35) and aryl benzoates.
Kb / M-1
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Aryl benzoates
SDS
1
2
3
4
5
6
7
8
1402
80
127 539 42 10
69
18
74
Brij-P35
1398 581
423 63 26 1394 181
2D NMR spectroscopy has been recorded to confirm qualitatively the location of the aryl benzoates
within the micelle. NOESY experiments have been often used to determine the localization of
substrates inside the micelle as well as to determine the extent of co-aggregation between two different
kinds of surfactants in water.19 Positive results are achieved when cross-peaks between diagnostic
signals of the substrate and the surfactants, respectively, are observed in the corresponding contour
plots.9 Thus, the NOESY experiments were performed in D2O and Figure 4 shows the 2D NMR
spectrum for a solution of SDS (7 mM) in the presence of benzoate 1 (10 mM). The inset red frames
recognize the NOE (Nuclear Overhauser Effect) between the signals of the surfactant SDS (bulk
hydrogens and a hydrogen) and the signals belonging to the aromatic protons (H-2/H-6, H-3/H-5, H-
10/H-14, H-12 and H-12/H-13) of p-cyanophenyl benzoate 7. Also, Figure 6 shows the labels of the
protons of the surfactant SDS and the aryl benzoate 7, respectively. Similar spectroscopic behavior was
observed for solutions of SDS and Brij-P35 in D2O in the presence of aryl benzoates 1 and 3 (see
Figures S6, S7 and S8 in Supporting Information). The cross-peaks of diagnostic signals observed in
the 2D NMR contour plots are in agreement with and reinforce the UV-visible spectroscopic analyses.
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H-3/H-5
H-2/H-6
H-11-H-13
H-10/H-14
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H-12
w
Bulk
b
a
Figure 4. 2D NOESY contour plot of a solution of SDS (7 mM) and 7 (10 mM) in D2O at room
temperature.
However, we cannot estimate the location of the benzoates with accuracy but we can suggest that the
benzoates are located inside the hydrophobic core of the micelle because the proton nuclei of the aryl
benzoates correlate nicely with the proton nuclei of the surfactants as can be seen through the cross-
peaks of the contour plots.
Laser Flash photolysis of aryl benzoates. Irradiation of p-methoxyphenyl benzoate (1) in acetonitrile
and cyclohexane solutions with a laser pulse (266 nm) under nitrogen atmosphere gives the transient
absorption spectra shown in Figures 5(a) and (b). Four absorption bands with maximum wavelength at
290, 340, 400 and 720 nm were observed in the transient spectra. According to the data reported in the
literature, we attributed the bands located at 290 nm and 400 nm to the p-methoxyphenoxyl radical
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while those bands centred at 340 nm and 720 nm were attributed to cyclohexadienone transient.20 Two
consecutive pathways from the singlet excited state of ester 1 are involved in the formation of both
transients after the pulse (266 nm): (i) homolytic fragmentation of the C-O bond of the ester group
affording p-methoxyphenoxy and benzoyloxy radical species in the solvent cage (intermediates A and
B, respectively, in Scheme 4), and then, (ii) coupling of both radical species to give the 4-methoxy-2-
benzoylcyclohexadienone intermediate (C in Scheme 4). Also, in Figures 5(c) and (d) are reported the
transient absorption spectra of compounds 6 and 2 in acetonitrile after the laser pulse (266 nm). Two
characteristic bands located around 340 and 400 nm are observed which were assigned to the
substituted 2-benzoylcyclohexadienone and substituted phenoxy radical intermediates, respectively.
Noteworthy similar results are obtained for the other aryl benzoates studied (see Figure S9 in
Supporting Information).
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O
O
kE
kR
+
R
O
Laser Pulse
(266 nm)
O
O
R
O
H
O
O
R
R
A
B
C
Scheme 4. Formation of transient species after the laser pulse (266 nm).
(a) (b)
0.04
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.03
0.02
0.01
0.00
300
400
500
600
700
300
400
500
l / nm
600
700
l / nm
(c)
(d)
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0.010
0.005
0.015
0.010
0.005
0.000
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58
59
60
0.000
300
350
400
450
300
320
340
360
380
400
420
440
460
l / nm
l / nm
Figure 5. Transient absorption spectra obtained after a laser pulse (100 µs; lexc: 266 nm) of solutions
(5.1x10-4 M) of: (a) 1 in cyclohexane; (b) 1 in acetonitrile; (c) 6 in acetonitrile and (d) 2 in acetonitrile
under nitrogen atmosphere.
The decay traces of the transient signal assigned to 4-substituted phenoxyl radicals were also measured
at 400 nm in N2-saturated cyclohexane and acetonitrile solutions (after a laser pulse at 266 nm). The
experiment was done with the aim of determining both the rate constants of radical out-of-cage escape
(kE) and ortho coupling reaction (kR) (see Scheme 4). Some selected examples of the decay traces are
shown in Figure 6. The decay traces of 4-substituted phenoxyl radicals show biexponential decay
fitting with r2 values > 0.998 independent of the solvent used. Two half lifetime values were obtained
from the nonlinear fittings, tE and tR: the short lifetime (tR) was assigned to in-cage coupling process
while the large lifetime (tE) was assigned to the phenoxyl radical out-of-cage process by comparison
with earlier reported data regarding thiyl radicals.20e,21 This biexponential behaviour can be interpreted
considering the two competitive pathways the 4-substituted phenoxyl radical can undergo, viz. out-of-
cage escape and in-cage coupling pathways, (see Scheme 4). This behavior can be described according
•
to equation 4 where ArO represents the substituted phenoxy radical and PhCO• represents the benzoyl
radical.
d[ArO ]
(4)
-
k
[ArO ] [PhCO]
kR
=
E [ArO ]
+
dt
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The out-of-cage escape of the substituted phenoxy radical is a unimolecular pathway. Therefore, the
rate constant (kE) of this process can be calculated from the reciprocal of the lifetime, kE = 1 / tE (see
Table 4). The in-cage coupling rate constants (kR) were obtained by plotting the reciprocal of the
concentration of the substituted phenoxy radicals against time and excellent linear correlations were
observed (see Figure S11 in Supporting Information for the linear correlations). Then, after applying a
linear regression fitting the in-cage coupling rate constants (kR) were obtained from the slopes and
these data are also shown in Table 4. As can be seen in Table 4, it is apparent that the out-of-cage rate
constants (kE) for the unimolecular escape process of 4-substituted phenoxyl radicals are quite similar
in all the solvents studied (1.5 to 8.3 x105 s-1) and somewhat independent of the substituent. On the
other hand, the bimolecular in-cage coupling of 4-substituted phenoxyl and benzoyl radicals was found
to be a second-order rate constant (kR) of 109 – 1010 M-1.s-1 in N2-saturated solvents with no significant
substituent effect associated to it.
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(a)
(b)
0.08
l = 400 nm
MeCN
0.03
l = 400 nm
Cyclohexane
0.06
0.04
0.02
0.00
0.02
0.01
0.00
0
20
40
t / µs
60
80
100
0
20
40
t / µs
60
80
100
(c)
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0.015
0.010
0.005
0.000
0.04
l = 400 nm
Cyclohexane
l = 410 nm
MeCN
0.03
0.02
0.01
0.00
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0
20
40
t / µs
60
80
100
0
20
40
t / us
60
80
100
Figure 6. Decay traces of 4-substituted phenoxyl radical recorded at 400 nm after the laser pulse (lexc:
266 nm) of solutions (5.1x10-4 M) of: (a) 4-methoxyphenyl benzoate (1) in cyclohexane; (b) 4-
methoxyphenyl benzoate (1) in acetonitrile; (c) 4-tert-butylphenyl benzoate (4) in cyclohexane; (d) 4-
phenoxyphenyl benzoate (2) in acetonitrile under N2 atmosphere.
Table 4. Out-of-cage (kE ) and in-cage coupling (kR) rate constants of 4-substituted phenoxyl radicals
measured by laser flash photolysis (266 nm) in different solvents under N2 atmosphere.a
Rate constants
O
MeCN
MeOH
Cyclohexane
SDS (0.10 M)
O
R
R
kEx10-5 / s-1
2.2±0.1
8.3±0.2
3.0±0.1
1.5±0.1
5.0±0.2
4.8±0.2
kRx10-9 / M-1.s-1 kEx10-5 / s-1 kRx10-9 / M-1.s-1 kEx10-5 / s-1 kRx10-9 / M-1.s-1 kRx10-9 / M-1.s-1
OMe (1)
OPh (2)
Me (3)
t-Bu (4)
Ph (6)
CN (7)
5.5±0.1
14±1
2.8±0.1
1.0±0.2
6.7±0.1
3.4±0.1
1.0±0.2
2.3±0.2
15±1
4.5±0.1
2.2±0.2
2.5±0.1
3.4±0.1
8.9±0.1
46±1
3.9±0.1
25±1
24±1
1.4±0.1
28±1
1.7±0.1
12±1
7.3±0.1
3.4±0.1
3.0±0.1
26±1
4.6±0.1
9.5±0.1
6.1±0.1
9.9±0.1
Insoluble
36±1
1.2±0.1
17±1
1.3±0.2
a Concentration of aryl benzoates: 5.0x10-4 M.
A similar spectroscopic analysis was carried out with aryl benzoates in aqueous SDS (0.10 M)
solutions after irradiation with a pulse at 266 nm. Two characteristic bands (340 – 360 nm and 400 nm)
are observed in the transient absorption spectra and 4-substituted phenoxyl radicals showed second
order kinetic decay traces. The in-cage coupling rate constants (kR) were obtained by plotting the
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reciprocal of the concentration of the 4-substituted phenoxyl radicals against time and, after applying a
linear regression fitting, the rate constants (kR) were obtained from the slopes (see Table 4). No
significant substituent effect on the rate constants (kR) was observed when the solvent was a micellar
solution (SDS 0.10 M) which gave a similar trend that was observed in homogeneous media.
Because phenyl benzoate (5) has no substituent in para position, two possible ortho- and para-
benzoylcyclohexadienone regioisomers can be proposed (see Scheme 5). Therefore, we have recorded
the transient absorption spectra and transient decay traces of compound 5 in different N2-saturated
solvents with a laser pulse at 266 nm systematically. Figure 7(a) shows both the transient absorption
spectra and decay trace recorded at 400 nm in MeCN. The band located at 400 nm in the transient
absorption spectra was assigned to the phenoxyl radical while the large structured band located
between 330 and 385 nm was assigned to ortho-benzoylcyclohexadienone and para-
benzoylcyclohexadienone transients (see Scheme 5).
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O
kE
O
+
O
Laser Pulse
(266 nm)
O
O
O
H
O
kortho
O
l
abs = 400 nm
kpara
ortho-benzoylcyclohexadienone
para-benzoylcyclohexadienone
O
H
O
Scheme 5. Formation of transient species after the laser pulse (266 nm) from phenyl benzoate.
On the other hand, Figure 7(b) shows the decay trace of phenoxyl radical measured at 400 nm in N2-
saturated acetonitrile solution after the laser pulse (266 nm). Biexponential decay was observed after a
non-linear fitting. This biexponential behaviour can be interpreted considering the competitive
pathways the phenoxyl radical can take viz. out-of-cage escape and ortho and para in-cage coupling
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pathways (see Scheme 5). The rate constant (kE) of the out-of-cage escape pathway was calculated
from the reciprocal of the lifetime, kE = 1/tE. The rate constants thus obtained are shown in Table 5. In
addition, the ortho and para in-cage coupling constants, kortho and kpara, were obtained by plotting the
reciprocal of the concentration of the phenoxy radical against time and two nice linear correlations
were observed (see Figure S12 Supporting Information for the linear correlations). Then, after applying
a linear regression fitting to the linear correlations, the in-cage coupling constants were obtained from
the slopes and these data are also shown in Table 5. No significant solvent effect on the rate constants
was observed. However, the in-cage para-coupling pathway is rather lower than the ortho-coupling
pathway.
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(a)
(b)
0.03
0.02
0.02
0.01
0.00
0.01
0.00
300
320
340
360
380
400
420
440
460
0
20
40
t / us
60
80
100
l / nm
Figure 7. (a) Transient absorption spectra of phenyl benzoate (5) recorded in N2-saturated acetonitrile
solution (5.1x10-4 M) and (b) decay trace of phenoxyl radical recorded at 400 nm after the laser pulse
(100 µs; lexc: 266 nm) of N2-saturated acetonitrile solution (5.1x10-4 M) of phenyl benzoate (5).
Table 5. Out-of-cage (kE) and in-cage coupling (kortho and kpara) rate constants of phenoxyl radicals
measured by laser flash photolysis (266 nm) in different solvents under N2 atmosphere.a
Rate constants
Solvents
kEx10-5 / s-1 korthox10-9 / M-1.s-1 kparax10-9 / M-1.s-1
Cyclohexane
4.8±0.1 8.7±0.1 4.1±0.1
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MeCN
MeOH
4.8±0.1
3.1±0.1
8.4±0.1
3.2±0.1
1.4±0.1
1.5±0.2
2.7±0.1
SDS (0.10 M) ---
5.5±0.2
a Concentration of phenyl benzoates: 5.0x10-4 M.
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Discussion.
As it was described above, direct irradiation (254 nm) of aryl benzoates in homogeneous media
(cyclohexane, MeCN and MeOH) as well as in micro heterogeneous media (SDS and Brij-P35 aqueous
solutions) under N2 atmosphere took place efficiently (see Figures 1 and 2). During the irradiation of
aryl benzoates a noticeable selectivity in favour of the benzophenone derivatives was observed in
micellar media. In this case, the formation of the corresponding substituted phenols was lower than 2 %
because of the confined hydrophobic core of the micelle (compare the data shown in Tables 1 and 2).
The reaction mechanism depicted in Scheme 6 was sustained by the results obtained under steady-state
conditions and laser flash photolysis experiments. When aryl benzoates are irradiated at 254 nm,
population of the singlet state is achieved efficiently. This excited state is the photo reactive state of the
photoreaction as it was reported in the literature.5 Two pathways are involved in the deactivation of the
singlet state, (i) homolytic fragmentation of the C-O bond (path (a); Scheme 6) affording aryl phenoxy
(ArO•) and benzoyl (PhCO•) radicals that evolves to 5-substituted-2-hydroxybenzophenone and 4-
substituted phenol and (ii) photophysical deactivation (kd; Scheme 3) of the singlet state involving
fluorescence emission and internal conversion pathways that give the aryl benzoates in their ground
state. In the case of ester 8, intersystem crossing pathway must be considered as a process involved in
the physical deactivation pathway estimating a fT value around 0.50.14,15 Irradiation of aryl benzoates
in N2-saturated solutions with a laser pulse (lexc = 266 nm) gave the transient absorption of 4-
substituted phenoxyl radical and 5-substituted-2-benzoylcyclohexadienone and these transients were
formed immediately at 10 µs after the incident light (see, for example, Figure 5 for compounds 1, 2 and
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6) demonstrating that C-O homolytic fragmentation (path (a) in Scheme 6) and in-cage coupling
pathway (path (d) in Scheme 6) occurred efficiently.
6
7
8
9
The decay traces of the phenoxyl radical transients were obtained at 400 nm in homogeneous media
under inert atmosphere and biexponential decay was observed. Second-order kinetics (kR) of 109 to 1010
M-1.s-1 were obtained (see Table 4 and Figure 6) and were attributed to the in-cage coupling of the
substituted phenoxy radical and benzoyl radical (path (d) in Scheme 6) providing the substituted 2-
benzoylcyclohexadienone intermediates C. These last intermediates, i.e. intermediates C, are long-
lived species showing lifetime values higher than 200 µs. The [1;3]-hydrogen migration and
aromatization reaction pathway (path (e) in Scheme 6) were estimated to occur with first-order kinetics
(kH) lower than 5x103 s-1 providing the substituted 2-hydroxybenzophenone derivatives as the main
photoproducts (see Table 1). On the other hand, first-order kinetics (kE) of 105 s-1 were also obtained
(see Table 4) which were assigned to the escape of the phenoxy radical from the solvent cage (path (b)
in Scheme 6) that in turn evolved to the 4-substituted phenol by abstraction of hydrogen from the
reaction solvent (path (c) in Scheme 6; SH: solvent).
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SH
S
O
fr
kE
1ArOC(O)Ph*
ArO +
+
ArOH PhC(O)H
PhCO
O
path (c)
Homolytic
fragmentation
R
Out-cage
escape
path (b)
hn
O2
Solvent
N2; 25°C
kd
(ArOH)
(PhCO)
path (a)
PhCOOH
In-cage
coupling
path (d) kR
ArOC(O)Ph
OH
R
O
O
O
H
kH
path (e)
C
R
Scheme 6. The reaction mechanism for the irradiation of aryl benzoates with lexc = 254 nm.
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Second-order rate constants (kR) in the range of 109 to 1010 M-1.s-1 were obtained (see Table 4) from the
fitting analysis of the decay traces of the phenoxyl radicals measured at 400 nm in micellar media (SDS
and Brij-P35) under N2 atmosphere. These values describe the in-cage coupling within the core of the
micelle of the substituted phenoxy radical (ArO•) and benzoyl radical (PhC(O)•) (see path (d) in
Scheme 6). This pathway afforded the substituted 2-benzoylcyclohexadienone intermediates C, also
formed within the hydrophobic core of the micelle, that evolved through a [1;3]-hydrogen migration
and aromatization reactions to substituted 2-hydroxybenzophenone. Because intermediates C are also
long-lived species in micellar media (t > 200 µs) the rate constants kH were estimated to be lower than
5 x 103 s-1 as it was observed in homogeneous media.
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It is worth to mention that the only photoproducts detected in micellar media were the 2-
hydroxybenzophenone derivatives 1a – 8a formed with chemical yields up to 95 % (see Table 2) the
yield of substituted phenols were lower than 5 %. Therefore, we suggest that the escape of the phenoxy
radical from the hydrophobic core of the micelle (path (b) in Scheme 6) is not a productive pathway.
The decay traces of phenoxy radicals in micellar media did not show biexponential decay traces as it
was observed in homogeneous media but second-order kinetics that is in agreement with the in-cage
coupling pathway (path (d) in Scheme 6).
Additional comments about the irradiation of phenyl benzoate (5) in homogeneous and micellar media
are needed. The results obtained under steady-state conditions and time-resolved spectroscopy on
phenyl benzoate led us to advance the different reaction pathways depicted in Scheme 7. Irradiation of
phenyl benzoate populated the singlet excited state efficiently and competitive physical deactivation
(kd) and homolytic fragmentation (path (a) in Scheme 7) pathways occurred.
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8
Out-cage
escape
SH
S
O
kE
fr
1PhOC(O)Ph*
ArO +
+
PhCO
ArOH
PhC(O)H
Homolytic
fragmentation
path (a)
O
path (c)
path (b)
hn
O2
(PhCO)
(ArO)
Solvent
N2; 25°C
kd
PhCOOH
path (d)
path (d')
9
In-cage
coupling
kortho
kpara
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PhOC(O)Ph
OH
O
O
O
O
O
kH
kH
H
H
path (f)
path (e)
O
HO
D
C
Scheme 7. Proposed reaction mechanism for direct irradiation of phenyl benzoate (5).
After fragmentation phenoxy and benzoyl radical species were formed in the solvent cage, then, escape
of the radical species from the solvent cage (path (b)) and hydrogen abstraction from the solvent (path
(c)) gave the conventional products. Laser flash photolysis experiments provided rate constants (kE) in
cyclohexane, MeCN and MeOH (see Table 5) following the decay trace of phenoxy radical at 400 nm.
No escape of the radical species was detected in micellar solution. Because phenyl benzoate has no
substituent in para position, two possible ortho- and para-benzoylcyclohexadienone regioisomers viz.
intermediates C and D were formed through the in-cage coupling of the radical species (path (d) and
path (e) in Scheme 7). Both intermediates C and D were observed in the absorption transient spectra
(see Figure 7) with characteristic bands located in the range of 340 – 380 nm. Besides, the kortho and
kpara values in the order of 109 M-1.s-1 belonging to in-cage coupling pathways (paths (d) and (d’) in
Scheme 7) implied that the coupling reaction occurred efficiently in all the solvents studied. Then,
intermediates C and D formed in the solvent cage evolved to the regioisomers 2-hydroxybenzophenone
and 4-hydroxybenzophenone through the sequence [1;3]-hydrogen migration and aromatization
pathways (path (e) and path (f) in Scheme 7). Again, intermediates C and D showed lifetimes higher
than 200 µs and the rate constants kH was estimated to be lower than 103 s-1. These photoproducts were
formed with 30 – 60 % yields together with the corresponding phenol in homogeneous media (see
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Table 1) while in micellar solution were formed in up to 95 % yield and no phenol was detected in the
4
5
micellar reaction mixture.
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Conclusions.
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The photochemical reaction of aryl benzoates examined in this paper takes place efficiently in
homogeneous and micellar media. High selectivity in the formation of the 5-substituted-2-
hydroxybenzophenone derivatives was observed in micellar media providing these photoproducts in
yields up to 95 % without the formation of the corresponding phenols. Location of the aryl benzoates
with 2D NOESY NMR spectroscopy in the shell or in the hydrophobic core of the micelle and
measurement of the binding constants (Kb) between the benzoates and the surfactants account for the
selective behaviour 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 homogeneous media such as cyclohexane,
MeCN and MeOH but no selectivity was observed.
Laser flash photolysis led to characterize two intermediates viz. the substituted phenoxy radical and the
5-substituted-2-benzoylcyclohexadienone transients. These intermediates were formed in the cage
solvent within 10 µs after the laser pulse. Also, the phenoxy radical escapes from the solvent cage with
first-order rate constants (kE) of 105 s-1 that in turn evolve to the corresponding phenols by hydrogen
abstraction from the reaction solvent (see Schemes 6 and 7). The kinetic parameters (kR) for in-cage
coupling pathways of the radical species viz. substituted phenoxy and benzoyl radicals were also
measured in all the solvents studied, providing the corresponding 5-substituted-2-
benzoylcyclohexadienone intermediates (intermediate C in Scheme 6 and intermediates C and D in
Scheme 7). These species which are formed in the solvent cage evolved to the regioisomeric 5-
substituted 2-hydroxybenzophenone derivatives through the sequence [1;3]-hydrogen migration and
aromatization pathways (path (e) in Scheme 6 and paths (f) in Scheme 7). Because, these intermediates
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showed lifetimes higher than 200 µs the rate constants kH were estimated to be lower than 103 s-1 in all
the solvents studied.
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9
Finally, the finding that the selectivity observed in the photo-Fries rearrangement of some aryl
benzoates in green and sustainable micellar media gives 5-substituted-2-hydroxybenzophenone
derivatives in yields up to 95 % could be applied in the preparation of a new wide variety of
substituted-2-hydroxybenzophenoe derivatives.
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Experimental.
Materials and equipment. p-Substituted phenols, benzoyl chloride, pyridine, sodium dodecyl sulfonate
and Brij-P35 were obtained from commercial sources. Spectroscopic grade solvents were used as
received. Pyridine was distilled and stored over KOH pellets. Melting Points were determined with a
1
13
Fisher Jones apparatus and are not corrected. H and C NMR spectra were recorded in CDCl3 on a
300 MHz spectrometer; chemical shifts (d) are reported in part per million (ppm), relative to signal of
tetramethylsilane, used as internal standard. 2D NOESY spectra were recorded in D2O on a 500 MHz
spectrometer, using NOESY-ph pulse sequence with a 600 ms mixing time and a recovery delay of 1.5
s. 2K data points were collected for 512 increments of 16 scans, using TPPI f1quadrature detection;
chemical shifts (d) are reported in part 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 ºC, 2 minutes;
gradient rate: 10 ºC.min-1; final temperature: 250 ºC, 10 minutes. The UV-visible spectra were
measured with a Shimadzu UV-1203 spectrophotometer using two-faced stoppered quartz cuvettes (1
mm x 1 mm) at 298 K.
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Determination of the binding constants (Kb) of phenyl benzoates in micellar media. Solutions of phenyl
benzoates were prepared in deionized water (MilliQ) and their concentration varied between 5.5x10-5 M
and 1.0x10-4 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 (A0) was read. Subsequently, aliquots
of concentrated surfactant solution (10 µL) were added. The UV-visible spectra were registered,
recording for each solution the A value at the maximum absorption wavelength. After each addition of
surfactant solution was stirred for 20 minutes before measuring the absorbance. With the values of A0
and A in hands, the values of (A0/(A - A0) versus the reciprocal of the concentration of the micellar
surfactant were plotted and the data were fitted with a linear regression program. The Kb values were
obtained calculating the ratio of the slope and the origin.
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Laser Flash Photolysis. The laser pulse photolysis apparatus consisted of a Flash lamp-pumped Q-
switched SpitLight-100 Nd:YAG laser from InnoLas, used at the fourth harmonic of its fundamental
wavelength. The LP920-K monitor system (supplied by Edinburgh Instruments), arranged in a cross-
beam configuration, consisted of a high-intensity 450 W ozone free Xe arc lamp (operating in pulsed
wave), a Czerny-Turner with Triple Grating Turret monochromator, and a five-stage dynode
photomultiplier. The signals were captured by means of a Tektronix TDS 3012C digital phosphor
oscilloscope, and the data was processed with the L900 software supplied by Edinburgh Instruments.
The solutions to be analysed were placed in a fluorescence cuvette (d = 10 mm).
Synthesis of phenyl benzoates 1 – 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) were added dropwise in 10 minutes under
stirring. Subsequently, the reaction mixture was kept under stirring for 60 minutes. 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
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