Cationic and radical intermediates in the acid photorelease from aryl sulfonates and phosphates

Article abstract of DOI:10.1039/c0pp00284d

The irradiation of a series of phenyl sulfonates and phosphates leads to the quantitative release of acidity with a reasonable quantum yield (≈0.2). Products characterization, ion chromatography analysis and potentiometric titration are consistent with the intervening of two different paths in this reaction, viz. cationic with phosphates and (mainly) radical with sulfonates. The Royal Society of Chemistry and Owner Societies 2011.

Full text of DOI:10.1039/c0pp00284d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Photochemical &
Photobiological Sciences
Cite this: Photochem. Photobiol. Sci., 2011, 10, 123
www.rsc.org/pps
PAPER
Cationic and radical intermediates in the acid photorelease from aryl
sulfonates and phosphates†
Marco Terpolilli,a Daniele Merli,b Stefano Protti,a Valentina Dichiarante,a Maurizio Fagnoni*a and
Angelo Albinia
Received 20th September 2010, Accepted 15th October 2010
DOI: 10.1039/c0pp00284d
The irradiation of a series of phenyl sulfonates and phosphates leads to the quantitative release of
acidity with a reasonable quantum yield (ª0.2). Products characterization, ion chromatography analysis
and potentiometric titration are consistent with the intervening of two different paths in this reaction,
viz. cationic with phosphates and (mainly) radical with sulfonates.
case, the initial step leads either to (non-acid-generating) photo-
Fries rearrangement or to hydrogen abstraction from the solvent,
depending on the structure.4 Polysulfonates, e.g. the tris(mesylate)
Introduction
Molecules capable of generating acidity upon irradiation (Photo
of pyrogallol (MeSB), release more acidity than monosulfonates,5
and indeed MeSB-based resists were found to be more photosensi-
tive than those incorporating a sulfonium salt.6 In recent years, our
own and other groups have been studying the photochemistry of
electron-rich aromatic halides and esters (1 in Scheme 1) in polar
solvents and found that heterolysis of the Ar–X bond occurs, most
effectively under acetone sensitization, and leads to triplet phenyl
cations.7–9 These are synthetically useful intermediates that have
been exploited for a variety of arylation reactions.
Organic preparations by this method are usually carried out
under buffered conditions, because otherwise the solution pH
strongly decreases (Scheme 1). The latter characteristics is appeal-
ing because, if the reaction occurs according to Scheme 1, it should
be possible not only to generate acidity from such compounds, but
also to tune the strength of the photogenerated acid by changing
the group X in the precursor.
As an example, triflic (pKa = -13), hydrochloric (pKa = -6),
methanesulfonic (pKa = -1.54), diethylphosphoric (pKa = 0.71)
and hydrofluoric acid (pKa = 3.18) could be photoreleased as
preferred from compounds of formula 1.
In the following, a study aimed to explore the potentialities of
this class of compounds for the photorelease of acids is reported.
Some families of aromatic derivatives were examined, of which the
amount and nature of the acid liberated had not been previously
Acid Generators, PAGs), are used as initiators in cationic polymer-
ization, in chemically amplified photoresists,1a–c as well as in bioor-
ganic synthesis.1d–e The most widely used class of these compounds
is that of aromatic onium salts ArnX+MYm-, such as iodonium and
sulfonium salts.2 These are a convenient choice because of their
thermal stability (>200 ◦C) and the easy chemical modification
that allows tuning the absorption characteristics. Some drawbacks,
such as the limited solubility in organic solvents that may cause
phase separation from spun-on resist polymer matrices, make the
development of new “caged protons” desirable, with particular
regard to non-ionic molecules. The most important example
of the latter class are the diazonaphthoquinone derivatives
(DNQ), where a carboxylic acid is liberated after a photo-Wolff
rearrangement.1a The photorelease of other acids (e.g. sulfonic) has
been more elusive and involves substituted esters. Mechanisms that
have been considered include the photoinduced intramolecular hy-
drogen abstraction in 2-nitrobenzyl sulfonates3 and the homolytic
photocleavage of the ArO–S bond in aryl sulfonates. In the latter
aDepartment of Organic Chemistry, University of Pavia, Pavia, 27100, Italy.
E-mail: fagnoni@unipv.it
bDepartment of General Chemistry, University of Pavia, Pavia, 27100, Italy
† Electronic supplementary information (ESI) available: Potentiometric
titration of 1b–1e irradiated solutions and 1H and 13C NMR spectra of
compounds 1d–f. See DOI: 10.1039/c0pp00284d
Scheme 1 Photo acid generation via phenyl cations.
The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 123–127 | 123
This journal is
©

View Article Online
determined. In the choice of the potential PAGs to explore this
issue, those liberating volatile acids such as HF and HCl that can
cause pinholes in the resist films were excluded. Sulfonates were
thus preferred, on the basis of the potential release of strong acids,
including triflic acid, a somewhat volatile but strong acid10 that
is known for inducing acid amplification in photoresists.11 It was
appealing also to investigate phosphate esters, at they are presently
scarcely considered.12,13
photoproducts from 1a,f have been quantified on the basis of cali-
bration curves from commercial samples. The presence of gaseous
fluoroform in the photolyzed solution has been revealed by means
of GC-MS and IR analysis (gas, n/cm-1 3035, 1374, 115820). In
the last case, the analysis was performed on the gas released in a
10 cm cell by using sodium chloride windows, internal volume 100
ml, purged with nitrogen before the introduction of the sample.
The acidity liberated has been determined on 10 mL of the
photolysed solutions by dilution with water (50 mL) and titration
with aqueous 0.1 M NaOH. Potentiometric acid titrations (NaOH
0.1 M) of 1b–e have been followed by using an Orion mod.
250 potentiometer equipped with an Orion pH glass combined
electrode mod 91–56. The titrations by using 0.1 M NaOH have
been performed on 10 mL photolyzed solutions in MeOH diluted
with 100 mL water.
Ion chromatography analyses have been performed by means
of a Dionex GP40 instrument equipped with a conductimetric
detector (Dionex 20 CD20) and an electrochemical suppressor
(ASRS Ultra II, 4 mm) by using the following conditions:
chromatographic column IONPAC AS23 (4 mm ¥ 250 mm),
guard column IONPAC AG12 (4 mm ¥ 50 mm), eluent: NaHCO3
0.8 mM + Na2CO3 4.5 mM, flux: 1 mL min-1; current imposed
at detector: 50 mA. Commercially available tetrabutylammonium
mesylate, trifluoromethanesulfonic acid and diethyl phosphate
were used as standards.
Materials and methods
Previous studies have shown that phenyl cations are conveniently
photogenerated starting from various electron-donating substi-
tuted aromatics.10 However, aminophenol and hydroxyphenol
esters were not considered for this work due to their poor thermal
stability and easy oxidation. Thus, sulfonate and phosphate esters
of p-methoxy (1a,c) and p-t-butylphenol (1d,f) were chosen.
Experimental details: general
Unless otherwise stated, all reagents were commercially available
1
and used without purification. H and 13C NMR spectra were
recorded on a 300 MHz (Bruker) spectrometer. The attributions
1
were made on the basis of H and 13C NMR; chemical shifts
are reported in ppm downfield from TMS. GC-MS analyses
have been carried out with a Thermo Scientific DSQ II Single
Quadrupole instrument (Column Rtx-5 MS (Restek), Detection
mode: Electronic Impact).
Results and discussion
Photophysical and photochemical properties of esters 1a,f have
been resumed in Table 1. Methoxyphenyl esters 1a–c exhibited
two absorption maxima in the UV around 276 (e > 103 L cm-1
mol-1) and 224 nm (e ª 104 L cm-1 mol-1), while t-butylphenyl
esters 1d–f absorbed somewhat blue-shifted and less intensively
(260, e > 102 L cm-1 mol-1, and 212 nm, e ª 104 L cm-1 mol-1 as
the maxima, see Table 1 and Fig. 1).
In this study, 0.01 M methanol solutions of esters 1a–f were de-
aerated and irradiated, resulting in complete consumption after
6 h in almost all cases. The quantum yield of disappearance of 1
(UR), at 254 nm ranged from 0.11 to 0.2, except for the case of less
reactive phosphate 1f (UR = 0.02).
The percent yield of organic products and the mmol of inorganic
products formed upon 6 h irradiation (24 h for 1f) of solutions
containing 0.3 mmol of starting compound 1, as well as the acidity
generated are reported in Table 1.
The pH of the photolyzed solutions was around 1 and we
were happy to find that the amount of acid released was close
to quantitative (>90%) as titrated by NaOH.
More precisely, potentiometric titrations evidenced a single pKa
value for 1c and 1d, but two for 1b and 1e (ratio between the
stronger and the weaker acid 4 : 1 and 2 : 1 respectively, see Fig. 2).
As for the organic moiety, the photoprocesses occurring are
reduction (giving 2), formal hydrolysis (forming deprotected 3)
and solvolysis (4), since no photo-Fries adducts were detected by
GC analysis. Triflates 1b, 1e gave the corresponding phenol 3 as the
exclusive (from 1b) or the main photoproduct (1e gave also a 5%
amount of t-butylbenzene 2b). In the case of p-methoxyphenyl me-
sylate (1a), deprotection and reduction took place in a comparable
yield (25% vs. 19%), but some dimethoxybenzene was formed too
(4, 9%), while the other mesylate (1e) was exclusively deprotected
to give p-t-butylphenol 3b.
Synthesis of esters 1a–f
p-Methoxy esters 1a–c were obtained as previously described.8
Mesylate 1d, triflate 1e and phosphate 1f were obtained by
known procedures from the corresponding phenols. 4-tert-
Butylphenylmethanesulfonate (1d) was obtained from the cor-
responding phenol by a known procedure14 in 56% yield as a
colorless solid, mp 49–50 ◦C (from ethanol, lit: 50–52 C15).
◦
Spectroscopic data of 1d in accordance with literature data.15 Anal.
calcd for C11H16O3S: C, 57.87; H, 7.06%. Found: C, 57.9; H, 7.1%.
4-tert-Butylphenyltrifluoromethanesulfonate (1e) was synthesized
from p-(t-butyl)phenol in 43% yield as an oil.16 Spectroscopic
data of 1e in accordance with literature data.17 Anal. calcd for
C11H13F3O3S: C, 46.80; H, 4.64%. Found: C, 46.9; H, 4.6%. 4-
tert-Butylphenyldiethylphosphate (1f) was obtained by a known
procedure18 in 69% yield as an oil. H NMR (d, CDCl3):19 1.30
1
(s, 9H), 1.35–1.40 (t, 6H, J = 6 Hz), 4.20–4.30 (qui, J = 6 Hz),
7.10–7.35 (AA¢BB¢, 4H). 13C NMR (d, CDCl3): 15.9 (CH3), 31.3
(CH3), 34.2, 64.3 (CH2), 119.2 (CH), 126.4 (CH), 147.7, 148.3. IR
(neat, n/cm-1: 2965, 1513, 1272, 1031, 964, 840. Anal. calcd for
C14H13O4P: C, 58.73; H, 8.10%. Found: C, 58.7; H, 8.0%.
Photolysis of esters 1a–f
The photochemical reactions were performed by using nitrogen-
purged solutions (except where indicated) of the chosen ester
(0.01 M) in quartz tubes. HPLC grade purity methanol (0.05%
water content as determined by Karl-Fischer titration) was used
as solvent for all the photolysis experiments. Irradiations were
performed in a multilamp reactor fitted with 4 low pressure
Hg lamps (emission maximum: l = 254 nm), and the reaction
course was followed by GC analysis. As described below, organic
124 | Photochem. Photobiol. Sci., 2011, 10, 123–127
This journal is
The Royal Society of Chemistry and Owner Societies 2011
©

View Article Online
Table 1 Photolysis of esters 1 in methanol
Ester
1a
l
max/nm (e/L cm-1 mol-1)
tir/ha
UR
Photoproducts, yield (%)b
2a, 19; 3a, 25; 4, 9
3a, 51
H+ yield (%)c
95.0
Anions, yield (%)c,d
CH3SO3-, 46.7
276 (1210)
224 (7530)
275 (1855)
225 (12170)
279 (2395)
224 (8934)
261 (240)
214 (8112)
258 (214)
212 (9643)
265 (530)
6
0.13
0.2
1b
6
95.0
CF3SO3-, 1.2e (31.7)f;
F-, 36.7 (8.0)f
1c
6
0.16g
0.17
0.11
0.02
2a, 58; 4, 25
3b, 68
98.3
(EtO)2PO2-, 100
1d
6
95.0
CH3SO3-, 93.3
1e
6
2b, 5; 3b, 60
2b, 36
93.3
CF3SO3-, 0h (56.7)f;
F-, 56.7 (25.7)f
1f
24
90.0
(EtO)2PO2-, 93.3
211 (7750)
a 0.01 M solution of 1a–f in degassed methanol, irradiated at 254 nm until complete consumption of the ester. b GC yield. c The analysis is based on a
0.03 mmol amount of starting 1. d Variable amounts of formate anion have been detected in all the photolyzed solutions. e Fluoroform was detected by
IR analysis. f Reaction carried out in oxygen saturated solution. g See ref. 14. h Fluoroform was detected by GC-MS analysis.
Fig. 2 Potentiometric titration of a 10-2 M solution of 1c ( , 0.1 mmol)
᭺
and 1e (ꢀ, 0.1 mmol) after irradiation in MeOH.
the mesylate anion, respectively in a close to quantitative yield
(>92%). On the contrary, only traces of triflate anion were present
in photolyzed solutions of 1b,1e, where the main anion was
fluoride (up to 56% yield). In this case, a gas was liberated, as
apparent from the fizz on opening the reaction tubes, that was
shown to be fluoroform, CHF3 by IR20 and GC-MS analysis.
It is noteworthy that, when the photolysis of 1b,1e was
Fig. 1 Absorption spectra of (a) 1 ¥ 10-4 M MeOH solution of 1a (dotted
performedinoxygensaturated methanol, triflate anionwas formed
line), 1b (dashed line), 1c (solid line) and (b) 5 ¥ 10-4 M MeOH solution
in a good yield at the expense of fluoride (Table 1).
In the case of 1e, small amounts of sulfur dioxide, sulfurous
of 1d (dotted line), 1e (dashed line) and 1f (solid line).
acid dimethyl ester as well as dimethoxymethane, ethandiol and
With phosphates 1c and 1f photoreduction was the main process
giving anisole 2a (58%, along with ether 4 in 25% yield) and
alkylbenzene 2b (36%), respectively.
According to ion chromatography analyses, phosphates and
mesylates (except 1a) released the O,O-diethylphosphate and
4-t-butylanisole were also detected. Moreover, variable amounts of
formate anion have been detected in all of the irradiated solutions.
The data above demonstrate that all of the aryl esters release
acidity with a reasonable quantum yields (except for 1f). The
thermal stability, good miscibility with resins and convenient (and
This journal is
The Royal Society of Chemistry and Owner Societies 2011
Photochem. Photobiol. Sci., 2011, 10, 123–127 | 125
©

View Article Online
tunable) absorption in the UV-C make these compounds suitable
candidates as PAGs. It is noteworthy that two different mecha-
nisms operate involving a cationic and a radical intermediates,
as shown in Schemes 2 and 3. With phosphates 1c,f, intersystem
crossing (ISC) is efficient,21 and product analysis supports that the
reaction proceeds from the triplet state and involves heterolysis of
the aryl-oxygen bond (cleavage a, Scheme 2). Triplet phenyl cation
(31+) and the corresponding counterion (EtO)2PO2- are formed. As
previously demonstrated,22 the cation abstracts a hydrogen atom
from the medium and gives anisole (2a) or t-butylbenzene (2b) via
radical cation 5 along with formaldehyde that under acidic condi-
tions is transformed into dimethoxymethane or (photo)oxidized23
to formic acid (Scheme 2).
Scheme 3 Radical intermediates in the photolysis of 1.
and gives the corresponding phenol 3 along with a ∑CH2OH radical
that ultimately is converted into formic acid or dimerized to form
∑
ethandiol. As for the sulfonyl radical CR¢3SO2 , its fate depends on
∑
the nature of group CR¢3. The CH3SO2 radical gives methanesul-
Scheme 2 Cationic intermediates in the photolysis of 1.
fonic acid,26 while in the case of trifluoromethanesulfonyl radical
In part, the triplet cation intersystem crosses to the singlet
(11+, R = OMe)9 that in turn adds the solvent and gives p-
dimethoxybenzene (4).24 The last pathway scarcely contributes in
the case of triplet 4-t-butylphenyl cation, which rather abstracts
hydrogen and thus gives 2b with only a trace of the solvolysis
product, 4-t-butylanisole.23 The heterolytic path accounts also for
part of the reaction of mesylate 1a, as witnessed by the formation
of products 2a and 4.
In the aim of confirming the proposed mechanism, irradiations
experiments in deuterated solvents have been carried out. Thus,
photolysis of phosphate 1c in CH3OD afforded non-deuterated
anisole 2a exclusively. On the contrary, in CD3OD complete mon-
odeuterated anisole 2a-d1, was formed along with trideuterated
4-d3.‡ On the other hand, among sulfonates homolytic cleavage
of the oxygen-sulfur bond competes (in the case of 1a) or
becomes the exclusive path (with mesylate 1d and triflates 1b,e).
Since heterolytic cleavage is the exclusive reaction in the acetone
sensitized irradiation of 1a and 1b,8 homolytic cleavage proceeds
from the singlet. A phenoxyl (6) and a sulfonyl radical are formed
(cleavage b, Scheme 3)25 and escape from the solvent cage (no
photo-Fries products are detected). Radical 6 abstracts hydrogen
such a path is overcome by the known a cleavage to trifluoromethyl
∑
radical CF3 and SO2.27,28 The sulfur dioxide formed is either
liberated as a gas or reacts with the solvent giving MeOSO2H29
(and in part the dimethyl ester30). This accounts for the formation
of a certain amount of weak sulfurous acid, as evidenced in the
potentiometric titration. The acid is formed upon dilution of the
photolyzed solution with water (see ESI†). As for the CF3∑ radical,
this abstracts hydrogen from the solvent giving inert fluoroform
as the final product31 or CF3OH.31,32 The last compound is known
33
to fragment thermally to HF and COF2 that liberates a further
amount of HF by reaction with MeOH.34
The case with oxygenated solutions is different, where the
∑
35
radical CF3SO2 is trapped by O2 and gives triflic acid via the
∑
peroxidic intermediate CF3SO2–O2 .
Conclusions
The promising characteristics of a number of aryl esters for
acid photorelease have been demonstrated. Besides the homolytic
cleavage of the oxygen-sulfur bond, presumably operating also in
some known PAGs as pyrogallol trimesylate, heterolytic cleavage
of the aryl-oxygen bond has been demonstrated to contribute. The
latter path had not been previously invoked among PAGs and its
participation depends on the nature of the leaving group and of
the aromatic moiety. Such a path can be made more important,
‡ Compound 2a, MS (m/z): 108 (M+, 100), 79 (15), 78 (60), 77 (20), 65
(35), 51 (10); 2a-d1, MS (m/z): 109 (M+), 80 (20), 79 (65), 78 (25), 66 (45),
52 (10); 4-d3, MS (m/z): 141 (M+), 126 (60), 123 (55), 98 (20), 95 (15).
126 | Photochem. Photobiol. Sci., 2011, 10, 123–127
This journal is
The Royal Society of Chemistry and Owner Societies 2011
©

View Article Online
at least with methoxyphenyl sulfonate esters, by passing to triplet
sensitized conditions.8 In contrast, the heterolytic path is exclusive
with phosphates and does not require sensitization. When the
homolytic path is followed with triflates, the role of oxygen is
determining for forming a strong acid and avoiding the release of
volatile substances (HF and CHF3) that can have an adverse effect
in a photoresist.
13 M. Shi, K. Yamamoto, Y. Okamoto and S. Takamuku, Photolysis
of aryl esters of tri- and tetracoordinate phosphorus compounds,
Phosphorus, Sulfur Silicon Relat. Elem., 1991, 60, 1–14.
14 R. K. Crossland and K. L. Servis, Facile synthesis of methanesulfonate
esters, J. Org. Chem., 1970, 35, 3195–3196.
15 R. H. Munday, J. R. Martinelli and S. L. Buchwald, Palladium-
catalyzed carbonylation of aryl tosylates and mesylates, J. Am. Chem.
Soc., 2008, 130, 2754–2755.
16 D. E. Frantz, D. G. Weaver, J. P. Carey, M. H. Kress and U. H. Dolling,
Practical synthesis of aryl triflates under aqueous conditions, Org. Lett.,
2002, 4, 4717–4718.
17 (a) F. Y. Kwong, C. W. Lai, M. Yu, Y. Tian and K. S. Chan, Palladium-
catalyzed phosphination of functionalized aryl triflates, Tetrahedron,
2003, 59, 10295–10305; (b) W. M. Seganish and P. DeShong, Prepara-
tion and palladium-catalyzed cross-coupling of aryl triethylammonium
bis(catechol) silicates with aryl triflates, J. Org. Chem., 2004, 69, 1137–
1143.
Acknowledgements
We are grateful to Dr Barbara Mannucci and Dr Federica Corana
of Centro Grandi Strumenti (University of Pavia) for technical
support.
18 K. Genkina, A. E. Shipov, T. A. Mastryukova and M. I. Kabachnik,
Synthesis of unsymmetrical triaryl phosphates under phase-transfer
catalysis, Russ. J. Gen. Chem., 1996, 66, 1742–1744.
19 R. Tacke, M. Strecker and R. Niedner, Sila-Pharmaka, 22.
Cholinesterase-hemmende organophosphorsa¨ureester und ihre sila-
analoga, Liebigs Ann. Chem., 1981, 1981, 387–395.
20 R. A. Levine and W. B. Person, Absolute infrared intensity measure-
ments of fluoroform (CHF3) fundamentals, J. Phys. Chem., 1977, 81,
1118–1119.
21 V. Dichiarante, D. Dondi, S. Protti, M. Fagnoni and A. Albini, A
meta effect in organic photochemistry?The case of SN1 reactions in
methoxyphenyl derivatives, J. Am. Chem. Soc., 2007, 129, 5605–5611,
correction: 11662.
Notes and references
1 (a) S.-Y. Moon and J.-M. Kim, Chemistry of photolithographic
imaging materials based on the chemical amplification concept, J.
Photochem. Photobiol. C, 2007, 8, 157–173; (b) J. M. Freche´t, The
photogeneration of acid and base within polymer coatings: Approaches
to polymer curing and imaging, Pure Appl. Chem., 1992, 64, 1239–1248;
(c) M. Shirai and M. Tsunooka, Photoacid and photobase generators:
Chemistry and applications to polymeric materials, Prog. Polym. Sci.,
1996, 21, 1–45; (d) P. J. Serafinowski and P. B. Garland, Novel photoacid
generators for photodirected oligonucleotide synthesis, J. Am. Chem.
Soc., 2003, 125, 962–965; (e) X. Gao, E. Gulari and X. Zhou, In situ
synthesis of oligonucleotide microarrays, Biopolymers, 2004, 73, 579–
596.
2 J. V. Crivello, The discovery and development of onium salt cationic
photoinitiators, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 4241–
4254.
3 F. M. Houlihan, O. Nalamasu, J. Kometani and E. Reichmanis, A
retrospective on 2-nitrobenzyl sulfonate photoacid generators, J. Imag.
Sci. Technol., 1997, 41, 35–40.
4 Y.-S. Chang, J.-S. Jang and M.-L. Deinzer, Photochemistry of irgasan-
triflate: A simple conversion of an aromatic hydroxyl group to chlorine
in the synthesis of polychlorinated diphenyl ethers and polychlorinated
dibenzofurans, Tetrahedron, 1990, 46, 4161–4164.
5 (a) T. Ueno, L. Schlegel, N. Hayashi, H. Shiraishi and T. Iwayanagi,
Acid formation from various sulfonates in a chemical amplification
positive resist, Polym. Eng. Sci., 1992, 32, 1511–1515; (b) L. Schlegel, T.
Ueno, H. Shiraishi, N. Hayashi and T. Iwayanagi, Acid formation and
deprotection reaction by novel sulfonates in a chemical amplification
positive photoresist, Chem. Mater., 1990, 2, 299–305.
6 L. Schlegel, T. Ueno, H. Shiraishi, N. Hayashi and T. Iwayanagi, Highly
sensitive positive deep UV resist utilizing a sulfonate acid generator and
a tetrahydropyranyl inhibitor, Microelectron. Eng., 1991, 13, 33–36.
7 (a) V. Dichiarante and M. Fagnoni, Aryl cation chemistry as an
emerging versatile tool for metal-free arylations, Synlett, 2008, 787–
800; (b) M. Fagnoni and A. Albini, Arylation reactions: the photo- SN1
path via phenyl cation as an alternative to metal catalysis, Acc. Chem.
Res., 2005, 38, 713–721; (c) M. Slegt, S. Hermen, H. S. Overkleeft and
G. Lodder, Fingerprints of singlet and triplet phenyl cations, Eur. J.
Org. Chem., 2007, 5364–5375.
8 M. De Carolis, S. Protti, M. Fagnoni and A. Albini, Metal-free
cross-coupling reactions of aryl sulfonates and phosphates through
photoeterolysis of aryl-oxygen bonds, Angew. Chem., Int. Ed., 2005,
44, 1232–1236.
22 V. Dichiarante, M. Fagnoni and A. Albini, Eco-friendly hydrodehalo-
genation of electron-rich aryl chlorides and fluorides by photochemical
reaction, Green Chem., 2009, 11, 942–945.
23 O. A. Snytnikova, A. N. Simonov, O. P. Pestunova, V. N. Parnom and Y.
P. Tsentalovich, Study of the photoinduced formose reaction by flash
and stationary photolysis, Mendeleev Commun., 2006, 16, 9–11.
24 Studies on the photodecomposition of phenyldiazonium ions demon-
strated that singet phenyl cations add non-selectively even to weak
nucleophiles. See: S. Milanesi, M. Fagnoni and A. Albini, Cationic
arylation through photo(sensitised) decomposition of diazonium salts.
Chemoselectivity of triplet phenyl cations, Chem. Commun., 2003, 216–
217.
25 See, for example: J. Andraos, G. G. Barclay, D. R. Medeiros, M.
V. Baldovi, J. C. Scaiano and R. Sinta, Model studies on the
photochemistry of phenolic sulfonate photoacid generators, Chem.
Mater., 1998, 10, 1694–1699.
26 E. Robert-Banchereau, S. Lacombe and J. Ollivier, Unsensitized pho-
tooxidation of sulfur compounds with molecular oxygen in solution,
Tetrahedron, 1997, 53, 2087–2102.
27 F. Bertrand, F. Leguyader, L. Liguori, G. Ouvry, B. Quiclet-Sire, S.
Seguin and S. Z. Zard, a-Scission of sulfonyl radicals: a versatile process
for organic synthesis, C. R. Acad. Sci. II C, 2001, 4, 547–555.
28 D. Gonbeau, M. F. Guimon, S. Duplantier, J. Ollivier and G. Pfister-
Guillouzo, Sulfochloration of trifluoromethane. Theoretical study of
the trifluoromethane sulfonyl radical, Chem. Phys., 1989, 135, 85–89.
29 L. S. Guss and I. M. Kolthoff, The behavior of sulfur dioxide as an
acid in methanol, J. Am. Chem. Soc., 1944, 66, 1484–1488.
30 A. Seubert and G. Wu¨nsch, Nebenreaktionen in Karl-Fischer-
reagentien, Fresenius’ Z. Anal. Chem., 1989, 334, 256–260.
31 E. R. Morris and J. C. J. Thynne, Reactions of radicals containing
fluorine. Part 1. Hydrogen and deuterium atom abstraction from
trideuteromethanol by trifluoromethyl radicals, Trans. Faraday Soc.,
1968, 64, 414–421.
9 S. Lazzaroni, D. Dondi, M. Fagnoni and A. Albini, Geometry and
energy of substituted phenyl cations, J. Org. Chem., 2008, 73, 206–211.
10 R. D. Allen, J. Opitz, C. E. Larson, T. I. Wallow, R. A. DiPietro,
G. Breyta, R. Sooriyakumaran and D. C. Hofer, The influence of
photoacid structure on the design and performance of 193 nm resists,
J. Photopolym. Sci. Technol., 1997, 10, 503–510.
11 (a) S. Lee, K. Arimitsu, S.-W. Park and K. Ichimura, Synthesis and
evaluation of novel acid amplifiers liberating fluoroalkanesulfonic
acids, J. Photopolym. Sci. Technol., 2000, 13, 215–216; (b) S. Kruger,
S. Revuru, C. Higgins, S. Gibbons, D. A. Freedman, W. Yueh, T. R.
Younkin and R. L. Brainard, Fluorinated acid amplifiers for EUV
lithography, J. Am. Chem. Soc., 2009, 131, 9862–9863.
32 J. Lillie, D. Behar, R. J. Sujdak and R. H. Schuler, Lifetime of
trifluoromethyl radical in aqueous solution, J. Phys. Chem., 1972, 76,
2517–2520.
33 K. Seppelt, Trifluoromethanol, CF3OH, Angew. Chem., Int. Ed. Engl.,
1977, 16, 322–323.
34 M. Avataneo, U. De Patto, M. Galimberti and G. Marchionni,
Synthesis of a,w-dimethoxyfluoropolyethers: reaction mechanism and
kinetics, J. Fluorine Chem., 2005, 126, 631–637.
35 R. Flyunt, O. Makogon, M. N. Schuchmann, K.-D. Asmus and C. von
Sonntag, OH-Radical-induced oxidation of methanesulfinic acid. The
reactions of the methanesulfonyl radical in the absence and presence of
dioxygen, J. Chem. Soc., Perkin Trans. 2, 2001, 787–792.
12 S. Protti and M. Fagnoni, Phosphate esters as “tunable” reagents in
organic synthesis, Chem. Commun., 2008, 3611–3621.
This journal is
The Royal Society of Chemistry and Owner Societies 2011
Photochem. Photobiol. Sci., 2011, 10, 123–127 | 127
©