Photolysis of ortho-nitrobenzylic derivatives: The importance of the leaving group

Article abstract of DOI:10.1039/c1pp05308f

Quantum yields for the photoinduced release of seven different commonly used leaving groups (LGs) from the o-nitroveratryl protecting group were measured. It was found that these quantum yields depend strongly on the nature of the LGs. We show that the quantum efficiency with which the LGs are released correlates with the stabilization that these LGs provide to o-nitrobenzyl-type radicals because radical stabilizing groups weaken the C-H bond that is cleaved in the photoinduced hydrogen atom transfer step, and hence lower the barrier for this process. At the same time these substituents lower the endothermicity of the thermal hydrogen atom transfer and thus increase the barrier for the reverse process, thereby enhancing the part of the initially formed aci-nitro intermediates which undergo cyclization (which ultimately leads to LG release). Radical stabilization energies computed by DFT methods are thus a useful predictor of the relative efficiency with which LGs are photoreleased from o-nitrobenzyl protecting groups.

Full text of DOI:10.1039/c1pp05308f

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PAPER
Photolysis of ortho-nitrobenzylic derivatives: the importance of the leaving
group†
Tomáš Šolomek,‡ Sébastien Mercier,§ Thomas Bally* and Christian G. Bochet*
Received 22nd September 2011, Accepted 16th November 2011
DOI: 10.1039/c1pp05308f
Quantum yields for the photoinduced release of seven different commonly used leaving groups (LGs)
from the o-nitroveratryl protecting group were measured. It was found that these quantum yields depend
strongly on the nature of the LGs. We show that the quantum efficiency with which the LGs are released
correlates with the stabilization that these LGs provide to o-nitrobenzyl-type radicals because radical
stabilizing groups weaken the C–H bond that is cleaved in the photoinduced hydrogen atom transfer step,
and hence lower the barrier for this process. At the same time these substituents lower the endothermicity
of the thermal hydrogen atom transfer and thus increase the barrier for the reverse process, thereby
enhancing the part of the initially formed aci-nitro intermediates which undergo cyclization (which
ultimately leads to LG release). Radical stabilization energies computed by DFT methods are thus a
useful predictor of the relative efficiency with which LGs are photoreleased from o-nitrobenzyl protecting
groups.
the decay of the 4-acetyl-2-nitrobenzyl protected benzylalcohol
1
Introduction
in methanol was found to be 0.88 while it was only 0.051 for the
Since their introduction in the mid 1960s,1–4 photolabile protect-
ing groups (PPGs) have taken an increasing importance in
organic synthesis,5–7 surface science8–10 and biology,11,12 where
they are also called caging compounds.13 The possibility of
releasing a reactive species at a given time and in specific space
by a non-chemical stimulus – a light beam – is an attractive tool
for many applications, ranging from total synthesis of natural pro-
ducts,14 DNA-chip manufacturing15,16 or neurophysiological
studies.17 ln the growing family of PPGs, the o-nitrobenzyl group
is one of the oldest, but still widely used groups, mainly due to its
robustness and relative photophysical insensitivity to interfering
influences. There have been many studies, both experimental and
computational, on the mechanism of the photoinduced decompo-
sition of this kind of PPGs18–26 but many unanswered questions
remain, such as the impact of substituents in various positions,
and the origin of an intriguing kinetic isotope effect.27,28
corresponding phenylacetic acid.29
In this paper we propose a simple predictive model to explain
a series of quantum yields for the release of seven different com-
monly used leaving groups from o-nitroveratryl PPGs. Based on
a minimal mechanistic scheme, these values are correlated with a
measure that can be evaluated easily using standard quantum
chemical calculations.
2
Results
2.1 Preparation of the substrates
We have investigated seven compounds (1a–g) with different
types of leaving groups at the benzylic position (Fig. 1); they are
all derived from the widely used nitroveratryl core.
For example, it has been shown that the quantum yields ϕ for
the disappearance of nitrobenzyl PPGs, or those for the release
of the leaving groups, depend strongly on the chemical character
of the leaving group (LG). However, it is unclear which feature
of a LG determines its influence on these quantum yields.
Apparently it is not the nucleofugacity, since quantum yields for
Their preparation was achieved using established procedures
(Scheme 1). Ether 1a was prepared by a silver oxide-mediated
etherification of nitroveratrol, whereas amine 1b was prepared by
reductive amination of nitroveratraldehyde with dodecylamine.
Carbamate 1c and amide 1d were both prepared by acylation of
amine 4, which itself was prepared by reduction of the nitrovera-
traldehyde oxime acetate 3 resulting from acylation of 2, with
the corresponding carbonic acid chloride or acid chloride. Carba-
mate 1e and carbonate 1g were prepared by reaction of commer-
cially available nitroveratryl chloroformate with dodecylamine
and decanol, respectively. Finally, the ester 1f was prepared by a
DCC-mediated esterification of tridecanoic acid with nitrovera-
trol. All products were pure, and fully characterized. Details of
the syntheses are given in the Methods section.
Department of Chemistry, University of Fribourg, Chemin du Musée 9,
11700 Fribourg, Switzerland. E-mail: Thomas.Bally@unifr.ch,
Christian.Bochet@unifr.ch
†This paper is part of a themed issue on photoremovable protecting
groups: development and applications.
‡On leave from Masaryk University, Brno, Czech Republic.
§Current address: Syngenta Crop Protection AG, Monthey, Switzerland.
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Table 1 Quantum yields for the disappearance of compounds 1a–g on
photolysis at 360 nm
a
Substrate
Leaving group (LG)
Φ360
1a
1b
1c
1d
1e
1f
O-C8H17
NH-C12H25
NHC(O)O-C9H19
NHC(O)CH2-C10H21
OC(O)NH-C12H25
OC(O)CH2-C11H23
OC(O)O-C10H21
0.22 0.04
0.14 0.01
0.080 0.003
0.072 0.013
0.019 0.005
0.0081 0.0005
0.0091 0.0013
1g
a Acetonitrile solutions (∼1 × 10−3 M) were irradiated at λ = 360 5 nm
(LED-based reactor). was determined by actinometry using
Φ
phenylglyoxylic acid. Standard deviations from two independent
measurements are shown.
by HPLC (see section on Methods). Quantum yields were
obtained from these conversions, against phenylglyoxylic acid
actinometry, and are summarized in Table 1.
Fig. 1 Substrates investigated in the present study.
It is immediately apparent that ethers and amines have a
strong beneficial effect on the quantum yields for photodecom-
position, whereas electron-withdrawing substituents on the
oxygen and nitrogen significantly reduce the efficiency of the
reaction.
3
Discussion
In general terms, there are three critical steps in the release mech-
anism of o-nitrobenzyl derived PPGs (5, Scheme 2): (i) the
photoinduced H-atom transfer (HAT*) leading to the primary
aci-nitro intemediates 6, (ii) ground state cyclization of 6 to
benzo[c]isoxazol-1(3H)-ols 7, which competes with (iii) reverse
HAT to re-form 5. In basic media, 7 is deprotonated and cleaves
irreversibly, by concomitant release of X−, to o-nitrosobenzalde-
hydes 8.
Regarding the competition between the ground state reactions
(ii) and (iii), Il’ichev has previously shown that their branching
ratio does not change enough for different leaving groups of the
type considered in our study to explain the observed variations
in quantum yields (it invariably favors the cyclization to 7).30
Thus we will disregard these reactions as determinants for the
Scheme 1 Preparation of compounds 1a–g.
2.2 Quantum yield measurements
The derivatives 1a–g were photolysed in acetonitrile (c ∼ 1 ×
10−3 M) in a LED-based reactor at 360 nm for a given time
enduring a small conversion (<10%), which was then determined
Scheme 2 Simplified mechanism of the photofragmentation of o-nitro-
benzyl compounds.
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Scheme 3 Model reaction to evaluate relative C–H bond strengths.
Table 2 Radical Stabilization Energies (RSEs)
quantum yield of product formation in our set of molecules and
focus on the step (i).
RSEa
Φ360
b
Structure
According to recent experimental and computational evidence
this HAT* step occurs from a (singlet and triplet) excited state,
and it involves a barrier.24,31,32 It would therefore appear reason-
able to assume that the height of this barrier scales with the
strength of the critical C–H bond (marked red in Scheme 2). We
based our computational exploration on the premise that this
bond strength, although it is certainly much smaller in the
(singlet or triplet) excited state, is affected similarly by the pres-
ence of different LGs X as it is in the ground state. LGs which
lower the barrier for photoinduced 1,5-hydrogen atom transfer,
HAT*, should allow this process to compete more efficiently
with other deactivation processes and thus increase the quantum
yield of LG release. A similar approach has been recently
employed by Yamaji et al.33,34
9a
9b
9c
9d
9e
9f
11.5
20.2
9.7
10.4
9.1
10.0
6.6
0.22
0.14
0.08
0.072
0.019
0.0081
0.0091
9g
a Radical stabilization energy in kcal mol−1 calculated with the PCM
method (acetonitrile) by UB2PLYP/6-311+G(3df,2p)//UB3LYP/6-31G
(d) according to Scheme 4 including ZPVE corrections scaled by 0.9806
(i.e. ΔH at 0 K); b Experimental quantum yield for the decomposition of
the corresponding derivative of 1 measured in this work; taken from
Table 1.
Instead of calculating C–H bond dissociation energies, which
are potentially subject to large errors due to the unavoidable
neglect of a part of the correlation energy, we calculated radical
stabilization energies (RSEs), i.e. differences in C–H bond dis-
sociation energies for X = H and X = LG in a series of model
compounds, according to the isodesmic reaction depicted in
Scheme 3. The enthalpy change in the course of this reaction
corresponds to the stabilization of the radical 9˙ offered by X
(relative to H).
intramolecular N–H⋯ONO hydrogen bond exists in 9c and 9d
which is probably no longer present when the photolysis is done
in a polar solvent, especially if residual moisture is present, as is
the case in acetonitrile. Therefore we reoptimized all the struc-
tures in polarizable continuum to simulate the collective effects
of the solvent environment. Now the energy difference between
the conformers of 9c and 9d with and without the intramolecular
H-bond dropped to <1.0 kcal mol−1
.
Thus we decided to present the results calculated for structures
without any internal H-bonding interaction in order to create a
consistent picture based only on the electronic properties of LGs.
The B2PLYP PCM results together with those from quantum
yield measurements are summarized in Table 2 (for details of all
calculations see ESI†).
If our hypothesis is correct, and if the quantum yields of LG
release depend on the barrier of reaction (i), then that barrier
should decrease and the corresponding quantum yield should
increase with increasing RSE. A plot relating the disappearance
quantum yields of 1 on irradiation at 360 nm with calculated
radical stabilization energies is shown in Fig. 2.
Indeed our results suggest that leaving groups in the benzylic
position of the nitroveratryl group which better stabilize the cor-
responding radical are released more efficiently. The correlation
is not (and cannot be) linear, because the maximum value of Φ
must be 1 unless a chain process is present, and the Φ vs. RSE
plot should show a sigmoidal shape as expected for a reaction
with one or more competing processes.
To avoid having to deal with a multitude of conformers we
modified the structures of the nitroveratryl derivatives 1a–g by
“tying up” the two methoxy groups in an acetal moiety, and by
replacing the long alkyl chains in the LGs by methyl groups.
The electronic structure of the compounds 9a–g in Scheme 3
should very closely resemble that of compounds 1a–g with
different LGs as X.
Note that the radical stabilization energies to be used in this
study cannot be replaced by values from tables35 which usually
refer to the methyl radical as a standard, because pairs of substi-
tuents can interact, often to different extents in the closed-shell
precursors and in the radicals, and thus make that RSEs are not
additive.36 Here, one of the radical substituents is the nitrophenyl
moiety, and it is necessary to account for the presence of this
substituent in the evaluation of the relative stabilization that
additional substituents X provide to radicals 9˙.
First we performed the calculations in the gas phase, searching
for the lowest energy conformers of 9a–g and their correspond-
ing radicals. It turned out that a low energy conformer with an
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Fig. 2 Quantum yields for the disappearance of o-nitroveratryl-PPGs
1a–g on irradiation at 360 nm vs. radical stabilization energy (RSE) for
different leaving groups in compounds 9a–g. The outlier is shown in
parentheses (see text for discussion); standard deviation for each point is
included.
However, the Φ of 1b is smaller than that measured for 1a,
although, the amino substituent offers significantly more stabiliz-
ation to the benzyl radical. Corrie et al. have shown that nitro-
benzyl based chelators with –NH–CH2–COO− substituents in
the benzylic position undergo photoinduced intramolecular elec-
tron transfer from the amine to the nitro group, which leads to
decarboxylation next to intramolecular hydrogen atom transfer
and thus diminishes the yield of LG release.37
In our case decarboxylation is impossible, but non-productive
electron transfer may compete with hydrogen atom transfer and
thus lead to the surprisingly small quantum yield observed for
1b. Conjugation of the amine nitrogen with a carbonyl function,
as it is the case in 1c and 1d, prevents this photoinduced charge
transfer process from occurring, because the ionization energies
of monoalkyl amides are approximately 1 eV higher than those
of dialkylamines.38
Scheme 5 Scheme including the pivotal steps in the photoreactions of
o-nitrobenzyl PPGs. Note that the size of the blue vertical arrow is inver-
sely proportional to that of the green one by virtue of the Bell-Evans-
Polanyi principle. The size of the red and the green vertical arrows
diminishes with increasing RSE, while that of the blue one increases.
relative to the nitrobenzyl compounds 9, i.e. decrease ΔrH(HAT)
and thus increase the barrier for the thermal back HAT, Ea(HAT).
Thereby the productive cyclization process (ii) is favored over
thermal back HAT (iii). Naturally, the outcome of the reaction is
also influenced by the barrier for process (iii), which we did not
consider in the present study. As shown by Il’ichev,30 the (ii)/
We also calculated the enthalpy change ΔrH for the thermal
HAT (9 → 10, Scheme 4), using the same methods as for the
RSEs (for details, see ESI†), because these values should corre-
late, by virtue of the Bell-Evans-Polanyi principle, with the bar-
riers for the reverse HAT process (iii) which competes with the
cyclization (ii), as shown in Scheme 5.
Scheme 4
It turned out that the same substituents which stabilize the
benzyl radicals 9˙ (and thus weaken the critical C–H bond and
lower Ea(HAT*)) stabilize also the aci-nitro compounds 10
Fig. 3 Linear correlation of ΔrH(9 →10) with the RSEs for radicals 9˙
(Scheme 3). Correlation coefficient is shown.
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(iii) branching ratio varies very little for LGs of the type pre-
sently considered. However, the (ii)/(iii) branching ratio may
differ substantially for other substituents X than those in
Scheme 3. In such case the barrier for the reverse HAT really
comes to bear.
In fact, the ΔrH(HAT) and the RSEs correlate quite well, as
shown in Fig. 3, which may be explained by the fact that a di-
radical resonance structure contributes significantly to 6, as
shown in Scheme 5.
column chromatography was carried out with silica gel (32–63,
60 Å) from Brunschwig. The 1H and 13C NMR spectra were
recorded on a Brucker 360 or 500 MHz in CDCl3. Chemical
shifts are expressed in parts per million (δ, ppm) using residual
solvent protons as internal standards: chloroform (δ 7.26 ppm
1
1
for H, δ 77.16 ppm for 13C), acetonitrile (δ 1.94 ppm for H),
1
CD3OD (δ 3.31 ppm for H). Coupling constant (J) are reported
in Hz. Splitting patterns are designated as s (singlet), d (doublet),
t (triplet), q (quartet), br s (broad singlet), m (multiplet).
Finally, we also considered the possibility that deprotonation
of the aci-nitro compounds 6 may compete with cyclization
(which is unlikely under our experimental conditions, but must
be accounted for in proton accepting solvents). The relative rates
of this process will depend on the relative pKA of compounds 6.
We calculated the relative deprotonation energies of 10 and
found that these, too, correlate quite well with the RSEs (for
details, see Scheme S1, Table S4 and Fig. S1 in the ESI†), i.e.
substituents X which stabilize radicals 9˙ decrease the acidity of
aci-nitro compounds 10 and therefore decrease the rate of depro-
tonation at given experimental conditions, hence favoring again
the irreversible cyclization which precedes the LG release.
Thus, the radical stabilization energy (RSE), which can be
readily calculated by the methods we have used in this study,
appears to be a good measure of the capacity of different leaving
groups (LGs) to affect the efficiency with which they are photo-
released from o-nitrobenzyl PPGs.
High performance liquid chromatography (HPLC) was done
using a Dionex Acclaim® 120 C18 3 mm 120 Å, 2.1 × 100 mm
column equipped with an ASI-100 Automated Sample Injector,
a P580 Pump and a PDA-100 Photodiode Array Detector.
ESI-HRMS mass spectra were obtained by Brucker FTMS 4.7T
BioAPEX II. IR spectroscopy was done on Mattson Galaxy
Series 5000 FT-IR in CHCl3 or neat on a NaCl cell. UV spectra
were recorded with a Perkin Elmer Lambda 25 UV/VIS Spec-
trometer. The photolyses were carried out in a LED-based photo-
reactor (model LUMOS, Atlas Photonics).
Photon flux determination for the LEDs (254 nm) of the
LUMOS photoreactor. In a square quartz cell (1 cm), a solution
of phenylglyoxylic acid (2.5 × 10−2 M, A > 3) in MeCN : H2O
(3 : 1) was irradiated and UV spectra of the solution were
recorded every 30 min. The absorbance of the solution was fol-
lowed at 390 nm (A < 1) to convert it to a conversion rate.
Photon flux determination for the LEDs (360 nm) of the
LUMOS photoreactor. In a square quartz cell (1 cm), a solution
of phenylglyoxylic acid (5 × 10−2 M, A ∼ 3) in MeCN : H2O
(3 : 1) was irradiated and UV spectra of the solution were
recorded every 5 min. The absorbance of the solution was fol-
lowed at 390 nm (A < 1) to convert it to a conversion rate.
4
Conclusions
We have measured quantum yields for the photoinduced release
of seven different commonly used leaving groups (LGs) from o-
nitroveratryl protecting groups, and found that these quantum
yields depend strongly on the nature of the LG. We have shown
that the quantum efficiency with which LGs are released corre-
lates with the stabilization that the LGs provide to o-nitrobenzyl-
type radicals. We rationalize this by arguing that radical stabiliz-
ing groups weaken the C–H bond that is cleaved in the photoin-
duced hydrogen atom transfer step, and hence lower the barrier
for this process. At the same time these substituents lower the
endothermicity of the thermal hydrogen atom transfer and thus
increase the barrier for the reverse process, thereby augmenting
the chances for the competing cyclization reaction (which ulti-
mately leads to LG release). Radical stabilization energies are
thus a useful predictor of the relative efficiency with which LGs
are photoreleased from o-nitrobenzyl protecting groups.
Determination of quantum yields at 254 nm and 360 nm. In a
square quartz cell (1 cm), an acetonitrile solution of compound
1a–g (1 × 10−3 M, A > 5) was irradiated for the required time to
obtain a conversion rate lower than 10%. The conversion rate
was determined by HPLC, after addition of an internal standard
of known concentration (valerophenone).
5.2 Syntheses
(4,5-Dimethoxy-2-nitrophenyl)methanol. To a solution of 6-
nitroveratraldehyde (2 g, 9.5 mmol) in 40 mL of methanol
NaBH4 (180 mg, 4.7 mmol) was slowly added. The mixture was
stirred at room temperature for 2 h. The solvent was then evapor-
ated, and the residue was partitioned between AcOEt and water.
After extraction with AcOEt, the combined organic layers were
washed with brine, dried with MgSO4, filtered, and evaporated.
The desired product was obtained as a yellow powder without
5
Methods
5.1 General procedures
1
Unless otherwise indicated, all starting materials were obtained
from commercial suppliers (Fluka, Sigma, Across) and were
used without further purification. Deuterated solvents such as
acetonitrile, chloroform, and water were obtained from Cam-
bridge Isotope Laboratories. Solvents were dried on Alox and
degassed with argon. Analytical thin layer chromatography was
performed on Kieselgel F-254 pre-coated aluminium sheets TLC
plates from Merck. Visualization was performed with, either a
254 nm ultraviolet lamp, or a KMnO4 solution. Silica gel
further purification (1.94 g, 96%). H NMR (CDCl3): δ (ppm)
2.34 (1H, br s), 3.95 (3H, s), 3.99 (3H, s), 4.95 (2H, s), 7.17
(1H, s), 7.70 (1H, s).
1-(Bromomethyl)-4,5-dimethoxy-2-nitrobenzene. To a stirred
solution of (4,5-dimethoxy-2-nitrophenyl)methanol (1 g,
4.7 mmol) in dry toluene (120 mL) and 8 drops of pyridine at
0 °C, phosphorus tribromide (725 mg, 2.7 mmol) was added
dropwise. The mixture was allowed to warm to r.t. and stirred
552 | Photochem. Photobiol. Sci., 2012, 11, 548–555
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overnight. Then, water was added and the mixture was extracted
with Et2O. The organic layer was dried with MgSO4, filtered and
the solvent was removed to obtain the crude residue which was
purified by flash chromatography (SiO2, CH2Cl2 : pentane, 1 : 1)
to give the desired product as a yellow solid (780 mg, 60%). H
NMR (CDCl3): δ (ppm) 3.96 (3H, s), 4.00 (3H, s), 4.87 (2H, s),
1584, 1523, 1499, 1462, 1440, 1332, 1275, 1216, 1143, 1063,
929 cm−1. HRMS (ESI): (M-Na+) 405.2002, found 405.1990.
N-(4,5-Dimethoxy-2-nitrobenzyl)dodecanamide 1d. To
a
1
stirred mixture of lauroyl chloride (125 mg, 0.57 mmol) in
CH2Cl2/sat. NaHCO3 (1 : 1, 2 mL), 4 (145 mg, 0.68 mmol) was
added and the mixture was stirred overnight. Then, the biphasic
solution was partitioned between CH2Cl2 and sat. NaHCO3. The
organic layer was washed with brine, dried over MgSO4, filtered
and evaporated to afford the desired product as a yellow solid
(224 mg, 99%). 1H NMR (CDCl3): δ (ppm) 0.87 (3H, m),
1.20–1.33 (16H, m), 1.60 (2H, m), 2.16 (2H, m), 3.94 (3H, s),
3.98 (3H, s), 4.62 (2H, d, J = 6.3 Hz), 6.41 (1H, br t, J = 6.1
Hz), 7.10 (1H, s), 7.65 (1H, s). 13C NMR (CDCl3): δ (ppm)
14.3, 22.8, 25.8, 29.4, 29.5, 29.5, 29.6, 29.7, 29.7, 32.0, 36.9,
41.8, 56.5, 56.8, 108.2, 114.5, 129.1, 140.8, 148.4, 153.8, 173.4.
IR (CHCl3): 3452, 3067, 3020, 2951, 2929, 2855, 1668, 1583,
1523, 1498, 1465, 1439, 1424, 1332, 1275, 1213, 1219, 1062,
929 cm−1. HRMS (ESI): (M-Na+) 417.2365, found 417.2357.
6.94 (1H, s), 7.68 (1H, s).
1,2-Dimethoxy-4-nitro-5-(octyloxymethyl)benzene 1a. To
a
stirred solution of 1-octanol (50 mg, 0.385 mmol) and nitrovera-
tryl bromide (317 mg, 1.15 mmol) in dry CH2Cl2 (2.5 mL) in a
Schlenk tube, silver oxide (266.5 mg, 1.15 mmol) was added
and the mixture was stirred at 45 °C under argon for 9 days pro-
tected from light. Then the mixture was filtered and the solvent
was evaporated. Purification by flash chromatography (SiO2,
CH2Cl2 : pentane, 2 : 1) gave the desired product as a yellow
solid (43.3 mg, 35%). 1H NMR (CDCl3): δ (ppm) 0.88 (3H, m),
1.27–1.44 (10H, m), 1.68 (2H, m), 3.59 (2H, t, J = 6.6 Hz), 3.95
(3H, s), 3.99 (3H, s), 4.89 (2H, s), 7.32 (1H, s), 7.70 (1H, s).
13C NMR (CDCl3): δ (ppm) 14.2, 22.8, 26.5, 29.4, 29.6, 29.9,
32.0, 56.5, 56.5, 69.6, 71.6, 108.0, 109.5, 131.7, 139.3, 147.6,
153.9. IR (CHCl3): 3023, 2960, 2931, 2855, 1582, 1520, 1464,
1328, 1275, 1216, 1105, 1064, 876 cm−1. HRMS (ESI):
(M-Na+) 348.1787, found 348.1783.
4,5-Dimethoxy-2-nitrobenzyl dodecylcarbamate 1e. To
a
stirred solution of 1-dodecylamine (185 mg, 1.0 mmol) and
Et3N (121 mg, 1.2 mmol) in dry CH2Cl2 (1 mL) at 0 °C under
argon, nitroveratryl chloroformate (331 mg, 1.2 mmol) was
added. Then the mixture was stirred at r.t. overnight and the
solvent was evaporated. Flash chromatography (SiO2, CH2Cl2)
of the residue gave the desired product as a off-white solid
(270 mg, 64%). 1H NMR (CDCl3): δ (ppm) 0.88 (3H, m),
1.20–1.36 (18H, m), 1.51 (2H, m), 3.21 (2H, dt, J = 6.9 Hz),
3.95 (3H, s), 3.97 (3H, s), 4.83 (1H, br m), 5.50 (2H, s), 7.00
(1H, s), 7.70 (1H, s). 13C NMR (CDCl3): δ (ppm) 14.3, 22.8,
26.9, 29.4, 29.5, 29.7, 29.7, 29.8, 29.8, 30.1, 32.0, 41.4, 56.5,
56.5, 63.6, 108.3, 110.4, 128.4, 140.1, 148.2, 153.6, 156.0. IR
(CHCl3): 3451, 3065, 3020, 2951, 2928, 2855, 1722, 1583,
1525, 1465, 1440, 1425, 1379, 1331, 1278, 1219, 1211, 1134,
1068, 1033, 928 cm−1. HRMS (ESI): (M-Na+) 447.2471, found
447.2468.
N-(4,5-Dimethoxy-2-nitrobenzyl)dodecan-1-amine 1b. A stirred
mixture of 6-nitroveratraldehyde (2.1 g, 10 mmol), 1-dodecyl-
amine (2.04 g, 11 mmol) and Na2SO4 (1.56 g, 11 mmol)
in dry toluene (20 mL) under N2 was stirred overnight at reflux
with a Dean–Stark apparatus. Then, the solvent was removed
and the residue was dissolved in dry MeOH (200 mL). NaBH4
(378 mg, 10 mmol) was added and the mixture was stirred for
2 h at r.t. Then the solvent was removed, the residue was dis-
solved in CH2Cl2 and washed with H2O. Flash chromatography
(SiO2, hexane : AcOEt, 9 : 1) gave the desired product as a
1
yellow solid (2.70 g, 71%). H NMR (CDCl3): δ (ppm) 0.85
(3H, m), 1.18–1.37 (18H, m), 1.51 (2H, m), 1.74 (1H, br s),
2.63 (2H, t, J = 7.0 Hz), 3.92 (3H, s), 3.97 (3H, s), 4.02 (2H, s),
7.13 (1H, s), 7.62 (1H, s). 13C NMR (CDCl3): δ (ppm) 14.2,
22.8, 27.5, 29.7, 29.7, 29.7, 29.8, 29.8, 29.8, 30.3, 32.0, 49.9,
51.6, 56.4, 56.5, 108.2, 112.5, 131.6, 141.0, 147.6, 153.4. IR
(CHCl3): 3065, 3019, 2951, 2928, 2855, 1582, 1520, 1464,
1440, 1427, 1361, 1332, 1273, 1215, 1210, 1175, 1063,
929 cm−1. HRMS (ESI): (M − H+) 381.2753, found 381.2755.
4,5-Dimethoxy-2-nitrobenzyl tridecanoate 1f. To a stirred sol-
ution of tridecanoic acid (214.3 mg, 1.0 mmol), o-nitroveratrol
(1.06 g, 5.0 mmol) and DMAP (24.5 mg, 0.2 mmol) in dry
CH2Cl2 (3.4 mL) at 0 °C under argon, DCC (247.7 mg,
1.2 mmol) was added and the mixture was stirred for 3 h at r.t.
Then the reaction mixture was filtered through a pad of Celite
and the filtrate was evaporated. Purification of the residue by
flash column chromatography (SiO2, CH2Cl2 : pentane, 2 : 1)
gave the desired product as a slightly yellow solid (334 mg,
82%). 1H NMR (CDCl3): δ (ppm) 0.88 (3H, m), 1.20–1.37
(18H, m), 1.68 (2H, m), 2.41 (2H, t, J = 7.6 Hz), 3.96 (3H, s),
3.98 (3H, s), 5.51 (2H, s), 7.00 (1H, s), 7.72 (1H, s). 13C NMR
(CDCl3): δ (ppm) 14.3, 22.8, 25.2, 29.3, 29.4, 29.5, 29.6, 29.7,
29.8, 29.8, 32.1, 34.5, 56.5, 56.6, 63.2, 108.4, 110.6, 127.4,
140.2, 148.4, 153.6, 173.3. IR (CHCl3): 3065, 3020, 2956,
2929, 2855, 1737, 1584, 1524, 1465, 1440, 1426, 1382, 1333,
1278, 1217, 1175, 1068, 1013, 932 cm−1. HRMS (ESI):
(M-Na+) 432.2362, found 432.2353.
Nonyl 4,5-dimethoxy-2-nitrobenzylcarbamate 1c. To a stirred
solution of 4 (140 mg, 0.66 mmol) and Et3N (80 mg,
0.80 mmol) in dry CH2Cl2 (1 mL) at 0 °C under argon, nonyl
carbonochloridate (164 mg, 0.79 mmol) was added. Then the
mixture was stirred at r.t. overnight and the solvent was evapor-
ated. Flash chromatography (SiO2, CH2Cl2) of the residue gave
the desired product in the presence of residual 1-nonanol.
Heating at 60 °C for several hours gave the desired product as a
1
yellow solid (163 mg, 65%). H NMR (CDCl3): δ (ppm) 0.87
(3H, m), 1.20–1.35 (12H, m), 1.58 (2H, m), 3.94 (3H, s), 3.98
(3H, s), 4.03 (2H, t, J = 6.6 Hz), 4.56 (2H, d, J = 6.9 Hz), 5.60
(1H, br t), 7.05 (1H, s), 7.67 (1H, s). 13C NMR (CDCl3): δ
(ppm) 14.2, 22.8, 26.0, 29.1, 29.4, 29.4, 29.6, 32.0, 43.5, 56.5,
56.7, 65.5, 108.3, 113.9, 129.4, 140.7, 148.4, 153.7, 157.0. IR
(CHCl3): 3452, 3065, 3017, 2962, 2952, 2931, 2856, 1711,
4,5-Dimethoxy-2-nitrobenzyl decyl carbonate 1g. To a stirred
solution of 1-decanol (63.6 mg, 0.40 mmol) and DMAP
(98.3 mg, 0.80 mmol) in dry CH2Cl2 (1 mL) at 0 °C under
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Photochem. Photobiol. Sci., 2012, 11, 548–555 | 553

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argon, nitroveratryl chloroformate (133 mg, 0.48 mmol) was
added. Then the mixture was stirred at r.t. overnight and the
solvent was evaporated. Flash chromatography (SiO2, CH2Cl2)
of the residue gave the desired product as a yellow solid
(148 mg, 93%). 1H NMR (CDCl3): δ (ppm) 0.87 (3H, m),
1.20–1.40 (14H, m), 1.69 (2H, m), 3.96 (3H, s), 3.98 (3H, s),
4.19 (2H, t, J = 6.8 Hz), 5.58 (2H, s), 7.06 (1H, s), 7.73 (1H, s).
13C NMR (CDCl3): δ (ppm) 14.2, 22.8, 25.8, 28.8, 29.3, 29.4,
29.6, 29.6, 32.0, 56.6, 56.6, 66.3, 68.9, 108.3, 109.9, 127.2,
139.8, 148.4, 153.8, 155.0. IR (CHCl3): 3105, 3064, 3019,
2962, 2928, 2856, 1751, 1583, 1524, 1458, 1440, 1428, 1402,
0.88 (3H, s), 1.20–1.40 (12H, m), 1.72 (2H, m), 4.31 (2H, t, J =
6.7 Hz).
5.3 Quantum chemical calculations
All quantum chemical calculations were performed with the
Gaussian 09 (revision A.02) package of programs.39 Geometries
were fully optimized at the RB3LYP/6-31G(d) level of theory in
the case of closed shell species. Unrestricted versions of the
functionals were employed in calculations of radicals. For all
stationary points, harmonic vibrational frequencies were com-
puted to obtain the ZPVE correction, which was scaled by
0.9806.40 Single point energies at the B3LYP geometries were
computed with the B2PLYP41 and M06-2X42 functionals using
the 6-311+G(3df,2p) basis set. These functionals have proven to
perform satisfactorily well in computations of thermodynamic
properties of both closed- and open-shell compounds.35,43–46
The resulting enthalpies are given at 0 K (E + scaled ZPVE). All
the calculations were performed in gas phase as well as in a
polarizable continuum to simulate the solvation effect of the sur-
rounding solvent. The polarizable continuum model (PCM)
using integral equation formalism variant (IEF-PCM), as
implemented in Gaussian 09, was employed.
1379, 1332, 1278, 1254, 1220, 1212, 1169, 1066, 929 cm−1
HRMS (ESI): (M-Na+) 420.1998, found 420.1994.
.
4,5-Dimethoxy-2-nitrobenzaldehyde oxime 2. 2-nitroveratral-
dehyde (5 g, 23.7 mmol) and hydroxylamine hydro–chloride
(1.72 g, 24.9 mmol) were added to a 1 : 1 solution of pyridine :
EtOH (150 mL). The mixture was refluxed for 8 h and solvent
was then removed. The residue was suspended in CH2Cl2 and
sat. NaHCO3 was added. Partition between organic and aqueous
layers followed by washings with sat. NaHCO3 and H2O, drying
with MgSO4, filtration and evaporation of the organic phase
1
gave the desired product as a yellow solid (5.16 g, 96%). H
NMR (CDCl3): δ (ppm) 3.98 (3H, s), 4.00 (3H, s), 7.35 (1H, s),
7.64 (1H, s), 8.80 (1H, s).
Acknowledgements
4,5-Dimethoxy-2-nitrobenzaldehyde O-acetyl oxime 3. To a
solution of 2 (5 g, 22.1 mmol) in dry CH2Cl2 (130 mL) at r.t.
under argon, Et3N (4 mL, 28.7 mmol) was slowly added. Then
after stirring for 5 min, Ac2O (2.7 mL, 28.7 mmol) was added
dropwise. The mixture was stirred for 3h, then CH2Cl2 was
added and the resulting solution was washed with H2O. The
organic layer was dried with MgSO4, filtered and evaporated.
The residue was purified by filtration (SiO2, CH2Cl2) to give the
desired product as a yellow solid (3.18 g, 54%). 1H NMR
(CDCl3): δ (ppm) 2.25 (3H, s), 4.00 (3H, s), 4.04 (3H, s), 7.44
(1H, s), 7.70 (1H, s), 9.04 (1H, s).
This work is part of project No. 200020-109316 and project No.
200020-132005 of the Swiss National Science Foundation. T.S.
profited from a stipend of the SCIEX (Scientific Exchange Pro-
gramme between Switzerland and the New Member States of the
EU) program of the Swiss Confederation and of the Brno Ph.D.
Talent program sponsored by Brno City Municipality.
References
1 R. B. Merrifield, Solid Phase Peptide Synthesis. 1. Synthesis of a Tetra-
peptide, J. Am. Chem. Soc., 1963, 85, 2149–2154.
2 J. A. Barltrop and P. Schofield, Organic photochemistry. II. Some photo-
sensitive protecting groups, J. Chem. Soc., 1965, 4758–4765.
3 J. A. Barltrop, P. J. Plant and P. Schofield, Photosensitive protective
groups, J. Chem. Soc., Chem. Commun., 1966, 822–823.
4 A. Patchornik, B. Amit and R. B. Woodward, Photosensitive protecting
groups, J. Am. Chem. Soc., 1970, 92, 6333–6335.
5 V. N. R. Pillai, Photoremovable protecting groups in organic synthesis,
Synthesis, 1980, 1–26.
6 C. G. Bochet, Photolabile protecting groups and linkers, J. Chem. Soc.,
Perkin. Trans. 1, 2002, 125–142.
7 A. P. Pelliccioli and J. Wirz, Photoremovable protecting groups: reaction
mechanisms and applications, Photochem. Photobiol. Sci., 2002, 1, 441–
458.
8 U. Jonas, A. del Campo, C. Kruger, G. Glasser and D. Boos, Colloidal
assemblies on patterned silane layers, Proc. Natl. Acad. Sci. U. S. A.,
2002, 99, 5034–5039.
9 A. del Campo, D. Boos, H. W. Spiess and U. Jonas, Surface modification
with orthogonal photosensitive silanes for sequential chemical lithogra-
phy and site-selective particle deposition, Angew. Chem., Int. Ed., 2005,
44, 4707–4712.
10 V. San Miguel, C. G. Bochet and A. del Campo, Wavelength-Selective
Caged Surfaces: How Many Functional Levels Are Possible?, J. Am.
Chem. Soc., 2011, 133, 5380–5388.
(4,5-Dimethoxy-2-nitrophenyl)methanamine 4. To a stirred
suspension of 3 (3 g, 11.2 mmol) in dry THF (12 mL) at 0 °C
under argon, BH3·THF (1 M, 67 mL) was added dropwise. The
solution was then refluxed for 24h. After cooling to −20 °C,
H2O (5 mL) was added followed by addition of aq. KOH (6.8
M, 3 mL). The mixture was stirred at r.t. for 3h. CH2Cl2 was
added (500 mL) and the organic layer was washed with brine
and H2O. After drying with MgSO4, filtration and evaporation of
the solvent, the residue was purified by flash chromatography
(SiO2, CH2Cl2 : MeOH : Et3N, 95 : 5 : 0.5) to give the desired
1
product as a reddish solid (285 mg, 12%). H NMR (CDCl3): δ
(ppm) 1.83 (2H, br s), 3.95 (3H, s), 4.00 (3H, s), 4.12 (2H, s),
7.07 (1H, s), 7.66 (1H, s).
Nonyl carbonochloridate. To a stirred solution of 1-nonanol
(433 mg, 3 mmol) in dry THF (2.1 mL) under argon, a solution
of COCl2 in toluene (5.2 mL) was added at 0 °C. Then, the
mixture was stirred for 3 h at 0 °C and then overnight at r.t. Then
a flux of argon was passed through the solution to remove the
excess of COCl2. Then the solvent was removed to afford the
desired product (572 mg, 92%). 1H NMR (CDCl3): δ (ppm)
11 J. H. Kaplan, B. Forbush, III and J. F. Hoffman, Rapid photolytic release
of adenosine 5′-triphosphate from a protected analog: utilization by the
sodium:potassium pump of human red blood cell ghosts, Biochemistry,
1978, 17, 1929–1935.
554 | Photochem. Photobiol. Sci., 2012, 11, 548–555
This journal is © The Royal Society of Chemistry and Owner Societies 2012

View Article Online
12 S. R. Adams and R. Y. Tsien, Controlling cell chemistry with caged com-
pounds, Annu. Rev. Physiol., 1993, 55, 755–784.
13 G. Mayer and A. Heckel, Biologically active molecules with a “light
switch”, Angew. Chem., Int. Ed., 2006, 45, 4900–4921.
14 K. C. Nicolaou, C. W. Hummel, E. N. Pitsinos, M. Nakada, A. L. Smith,
K. Shibayama and H. Saimoto, Total synthesis of calicheamicin gamma-1
(I), J. Am. Chem. Soc., 1992, 114, 10082–10084.
15 S. P. A. Fodor, R. P. Rava, X. H. C. Huang, A. C. Pease, C. P. Holmes
and C. L. Adams, Multiplexed biochemical assays with biological chips,
Nature, 1993, 364, 555–556.
32 A. Migani, V. Leyva, F. Feixas, T. Schmierer, P. Gilch, I. Corral,
L. Gonzalez and L. Blancafort, Ultrafast irreversible phototautomerization
of o-nitrobenzaldehyde, Chem. Commun., 2011, 47, 6383–6385.
33 M. Yamaji, X. C. Cai, M. Sakamoto, M. Fujitsuka and T. Majima, Photo-
decomposition Profiles of beta-Bond Cleavage of Phenylphenacyl
Derivatives in the Higher Triplet Excited States during Stepwise Two-
Color Two-Laser Flash Photolysis, J. Phys. Chem. A, 2008, 112, 11306–
11311.
34 M. Yamaji, X. Cai, M. Sakamoto, M. Fujitsuka and T. Majima, alpha-
Bond Dissociation of p-Phenylbenzoyl Derivatives in the Higher Triplet
Excited State Studied by Two-Color Two-Laser Flash Photolysis, J. Phys.
Chem. A, 2009, 113, 1696–1703.
16 M. C. Pirrung, How to make a DNA chip, Angew. Chem., Int. Ed., 2002,
41, 1276–1289.
17 G. C. R. Ellis-Davies, Caged compounds: photorelease technology for
control of cellular chemistry and physiology, Nat. Methods, 2007, 4,
619–628.
18 M. Schwörer and J. Wirz, Photochemical reaction mechanisms of 2-nitro-
benzyl compounds in solution I. 2-nitrotoluene: Thermodynamic and
kinetic parameters of the aci-nitro tautomer, Helv. Chim. Acta, 2001, 84,
1441–1458.
1 9 Y. V. I l ’ichev, M. A. Schwörer and J. Wirz, Photochemical Reaction
Mechanisms of 2-Nitrobenzyl Compounds: Methyl Ethers and Caged
AT P, J. Am. Chem. Soc., 2004, 126, 4581–4595.
20 M. Gaplovsky, Y. V. Il’ichev, Y. Kamdzhilov, S. V. Kombarova, M. Mac,
M. A. Schworer and J. Wirz, Photochemical reaction mechanisms
of 2-nitrobenzyl compounds: 2-Nitrobenzyl alcohols form 2-nitroso
hydrates by dual proton transfer, Photochem. Photobiol. Sci., 2005, 4,
33–42.
21 B. Hellrung, Y. Kamdzhilov, M. Schworer and J. Wirz, Photorelease of
alcohols from 2-nitrobenzyl ethers proceeds via hemiacetals and may be
further retarded by buffers intercepting the primary aci-nitro intermedi-
ates, J. Am. Chem. Soc., 2005, 127, 8934–8935.
22 J. E. T. Corrie, A. Barth, V. R. N. Munasinghe, D. R. Trentham and
M. C. Hutter, Photolytic Cleavage of 1-(2-Nitrophenyl)ethyl Ethers
Involves Two Parallel Pathways and Product Release Is Rate-Limited by
Decomposition of a Common Hemiacetal Intermediate, J. Am. Chem.
Soc., 2003, 125, 8546–8554.
23 F. Bley, K. Schaper and H. Gorner, Photoprocesses of molecules with 2-
nitrobenzyl protecting groups and caged organic acids, Photochem.
Photobiol., 2008, 84, 162–171.
24 T. Schmierer, S. Laimgruber, K. Haiser, K. Kiewisch, J. Neugebauer and
P. Gilch, Femtosecond spectroscopy on the photochemistry of ortho-nitro-
toluene, Phys. Chem. Chem. Phys., 2010, 12, 15653–15664.
25 Y. V. Il’ichev and J. Wirz, Rearrangements of 2-nitrobenzyl
compounds. 1. Potential energy surface of 2-nitrotoluene and its isomers
explored with ab initio and density functional theory methods, J. Phys.
Chem. A, 2000, 104, 7856–7870.
35 A. S. Menon, G. P. F. Wood, D. Moran and L. Radom, Bond dissociation
energies and radical stabilization energies: An assessment of contempor-
ary theoretical procedures, J. Phys. Chem. A, 2007, 111, 13638–13644.
36 A. S. Menon, D. J. Henry, T. Bally and L. Radom, Effect of substituents
on the stabilities of multiply-substituted carbon-centered radicals, Org.
Biomol. Chem., 2011, 9, 3636–3657.
37 A. Barth, S. R. Martin and J. E. T. Corrie, Decarboxylation is a significant
reaction pathway for photolabile calcium chelators and related com-
pounds, Photochem. Photobiol. Sci., 2006, 5, 107–115.
38 K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Ioniz-
ation energies, Ab initio assignments, and valence electronic structure for
200 molecules, in Handbook of HeI Photoelectron Spectra of Fundamen-
tal Organic Compounds, Japan Scientific Soc. Press, Tokyo, 1981.
39 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov,
J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,
R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09
(Revision A.02), Gaussian, Inc., Wallingford, CT, 2009.
40 J. P. Merrick, D. Moran and L. Radom, An evaluation of harmonic
vibrational frequency scale factors, J. Phys. Chem. A, 2007, 111, 11683–
11700.
41 S. Grimme, Semiempirical hybrid density functional with perturbative
second-order correlation, J. Chem. Phys., 2006, 124, 034108.
42 Y. Zhao and D. G. Truhlar, The M06 suite of density functionals for main
group thermochemistry, thermochemical kinetics, noncovalent inter-
actions, excited states, and transition elements: two new functionals and
systematic testing of four M06-class functionals and 12 other functionals,
Theor. Chem. Acc., 2007, 120, 215–241.
43 T. Schwabe and S. Grimme, Theoretical thermodynamics for large mol-
ecules: Walking the thin line between accuracy and computational cost,
Acc. Chem. Res., 2008, 41, 569–579.
26 P. Sebej, T. Šolomek, L. Hroudna, P. Brancova and P. Klan, Photo-
chemistry of 2-Nitrobenzylidene Acetals, J. Org. Chem., 2009, 74, 8647–
8658.
27 A. Blanc and C. G. Bochet, Isotope effects in photochemistry. 1. o-Nitro-
benzyl alcohol derivatives, J. Am. Chem. Soc., 2004, 126, 7174–7175.
28 A. Blanc and C. G. Bochet, Isotope effects in photochemistry: Appli-
cation to chromatic orthogonality, Org. Lett., 2007, 9, 2649–2651.
29 L. Kammari, T. Šolomek, B. P. Ngoy, D. Heger and P. Klan, Orthogonal
Photocleavage of a Monochromophoric Linker, J. Am. Chem. Soc., 2010,
132, 11431–11433.
44 A. S. Menon, G. P. F. Wood, D. Moran and L. Radom, Bond dissociation
energies and radical stabilization energies: An assessment of contempor-
ary theoretical procedures (vol 111A, pg 13638, 2007), J. Phys. Chem. A,
2008, 112, 5554–5554.
45 Y. Zhao and D. G. Truhlar, Density functionals with broad applicability
in chemistry, Acc. Chem. Res., 2008, 41, 157–167.
3 0 Y. V. I l ’ichev, Rearrangements of 2-nitrobenzyl compounds. 2. Substitu-
ent effects on the reactions of the quinonoid intermediates, J. Phys.
Chem. A, 2003, 107, 10159–10170.
31 T. Schmierer, F. Bley, K. Schaper and P. Gilch, The early processes in the
photochemistry of ortho-nitrobenzyl acetate, J. Photochem. Photobiol., A,
2011, 217, 363–368.
46 Y. Zhao and D. G. Truhlar, How well can new-generation density func-
tionals describe the energetics of bond-dissociation reactions producing
radicals?, J. Phys. Chem. A, 2008, 112, 1095–1099.
This journal is © The Royal Society of Chemistry and Owner Societies 2012
Photochem. Photobiol. Sci., 2012, 11, 548–555 | 555