Triplet phenacylimidazoliums-catalyzed photocycloaddition of 1,4-dihydropyridines: An experimental and theoretical study

Article abstract of DOI:10.1016/j.jphotochem.2012.05.013

The photocycloadditions of 1,4-dihydropyridines (DHPs) were achieved by using phenacylimidazoliums (PIms) as photosensitizers. Irradiation of DHPs 3a-g in the presence of PIms 1a-e and 2 performed an efficient formation of 3,9-diazatetraasteranes in shorter times under a lower power lamp. The mechanism of photocycloaddition catalyzed by PIm was studied by laser flash photolysis and theoretical DFT computation. These time-resolved results showed that the triplet excited states of PIms were generated with high efficiency and detected by their characteristic ultraviolet absorptions, which were quenched by DHP at almost diffusion controlled rate. Theoretical studies suggest that PIm is involved in the photocycloaddition process through the ³(DHP? PIm)* triplet complexes and assists the stabilization of intermediates. All subsequent steps are predicted to be favorable and exothermic, leading to the cage dimers.

Full text of DOI:10.1016/j.jphotochem.2012.05.013

Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Triplet phenacylimidazoliums-catalyzed photocycloaddition of
1,4-dihydropyridines: An experimental and theoretical study
Xiaohe Zhu, Weipeng Li, Hong Yan∗, Rugang Zhong
College of Life Science and Bio-Engineering, Beijing University of Technology, Pingleyuan Street No. 100, Chaoyang District, Beijing 100124, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocycloadditions of 1,4-dihydropyridines (DHPs) were achieved by using phenacylimidazoliums
(PIms) as photosensitizers. Irradiation of DHPs 3a–g in the presence of PIms 1a–e and 2 performed an effi-
cient formation of 3,9-diazatetraasteranes in shorter times under a lower power lamp. The mechanism of
photocycloaddition catalyzed by PIm was studied by laser flash photolysis and theoretical DFT computa-
tion. These time-resolved results showed that the triplet excited states of PIms were generated with high
efficiency and detected by their characteristic ultraviolet absorptions, which were quenched by DHP at
almost diffusion controlled rate. Theoretical studies suggest that PIm is involved in the photocycloaddi-
tion process through the 3(DHP· · ·PIm)* triplet complexes and assists the stabilization of intermediates.
All subsequent steps are predicted to be favorable and exothermic, leading to the cage dimers.
© 2012 Elsevier B.V. All rights reserved.
Received 28 October 2011
Received in revised form 18 April 2012
Accepted 14 May 2012
Available online 19 May 2012
Keywords:
Photocycloaddition
Triplet photosensitizer
3,9-Diazatetraasterane
Laser flash photolysis
DFT calculation
1. Introduction
chromophore is characterized by low triplet energy and can be gen-
erated with a high fluorescence quantum yield. Its triplet state is
The photochemical [2 + 2] cycloaddition is a well-established
methodology for the synthesis of cyclobutane compounds with
high structural complexity [1], and key intermediates in the
total syntheses of natural products [2]. The high levels of regio-
and stereoselectivity usually achieved and the synthetic versa-
tility of adducts render this method great attractive [3], such as
coumarin [4], C60-fullerene [5], and dibenz[b,f]azepines [6]. In
particular, our group has extensively studied the [2 + 2] photocy-
cloadditions of 1,4-dihydropyridines [7] and 4-aryl-4H-pyrans [8],
and subsequently exploited these highly symmetrically products,
3,9-diazatetraasteranes and 3,9-dioxatetraasteranes as novel non-
peptidic HIV protease inhibitors [9,10].
In many cases, photosensitizers are used as an effective way
to accelerate photocycloaddition reactions. Electron transfer cat-
alysts have been employed, such as (thia)pyrylium salts [11],
naphthalenes [12], and anthracenes [13]. The mechanism involves
exchange of one electron between the excited state of the sen-
sitizer (singlet or triplet) and the ground state of the substrate.
Alternatively, energy transfer photosensitizer is another general
type of sensitized photochemical reactions, which is frequently
performed with acetone, acetophenone and benzophenone (BP).
The triplet–triplet energy transfer takes place from the lowest
excited triplet state of sensitizer to substrate. Their aromatic ketone
long-lived, making it possible for the reactants to diffuse and collide
with each other. In addition, excited aromatic ketones can photo-
sensitize hydrogen abstraction from suitable donors though with
low efficiency, energy transfer to many substrates [14]. Hydrogen
bond is proposed to play crucial roles in promoting the photochem-
ical reactions [15]. Thus, aromatic ketones can be efficient as triplet
photosensitizers.
Ionic liquids have attracted growing attention due to their
advantages as reagents and catalysts with tunable features for
various targeted chemical tasks [16]. In particular, imidazolium
salts present more rapidly increasing interest in photochem-
istry [17], which possess of photochemical inertness as excellent
media. These observations led to the idea that an ionic liq-
uid containing a photosensitizing chromophore might be used
as photosensitizer and solvent. Imidazolium moiety would make
a
homogenously supported ionic liquid. Thus, the approach
of functional ionic liquids (imidazole-tagged aryl ketones) as
photosensitizers was shown in Scheme 1, which contained a photo-
sensitizing chromophore by a covalent linking of aromatic ketones
and imidazolium units.
In view of these precedents, it would be of much interest to
explore the potential photosensitization of phenacylimidazoliums
(PIms) in [2 + 2] photocycloadditions. Herein, we present the exper-
imental results of photocycloaddition of 1,4-dihydropyridines
(DHPs) catalyzed by PIms 1a–e and 2. Laser flash photolysis and
theoretical calculations have been performed to rationalize the
stepwise mechanism of triplet PIm photosensitized cycloaddition.
∗
Corresponding author. Tel.: +86 10 67396642; fax: +86 10 67396642.
E-mail address: hongyan@bjut.edu.cn (H. Yan).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.05.013

14
X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
2.4. Fluorescence quenching
O
O
N
N
N
N
Fluorescence measurements were performed on an Edinburgh
FL900 spectrofluorometer equipped with a hydrogen flash lamp.
All samples in acetonitrile were placed into quartz cells of 1 cm
path length. The excitation wavelengths were selected as 312 and
305 nm for photosensitizers 1e and 2. Relative fluorescence life-
times were measured for MeCN solutions containing the sensitizers
(0.3 mM) and various concentrations of 1,4-dihydropyridine 3a.
The fluorescence-quenching rate constants were determined from
a Stern–Volmer analysis.
Br
N
Br
R
1e
1a R= H
1b R= CH3
1c R= Cl
1d R= Ph
O
N
Br
2
Scheme 1. Structures of photosensitizers.
2.5. Laser flash photolysis
2. Experimental
Transient absorption spectra were performed on a LP-920
pump-probe spectroscopic setup (Edinburgh). The excited source
was the unfocused third harmonic (355 nm) output of a Nd:YAG
laser (Continuum surelite II). The pulse duration was 7 ns and the
energy of the laser beam was 13 mJ/pulse. The monitor light from
a pulsed 450-W Xe arc lamp was collected by a focusing lens and
directed to a grating monochromator. The monitor light was set
perpendicular to the laser beam. The output of the monochroma-
tor was monitored using a photomultiplier tube (PMT). The signal
from the PMT was recorded on a transient digitizer (Tektronix TDS
3012B oscilloscope). The output signal was transferred to a personal
computer for data analysis. The samples were placed in a sealed
quartz cuvette and purged with nitrogen for 15 min prior to the
LFP experiments. All measurements were carried out at 22 2 ◦C.
The transient spectra were obtained by a point-to-point technique,
monitoring the change of absorbance (ꢁA) after the laser flash at
intervals of 10 nm over the spectral range 350–800 nm, averaging
at least 10 decays at each wavelength. The error estimated on the
rate constants was 10%.
2.1. General procedures
All chemicals were bought from commercial suppliers and
used without further purification. 1,4-Dihydropyridines 3a–g were
given by cyclocondensation of aromatic aldehyde, ethyl propiolate,
and amine in acetic acid [7]. Melting points were determined on a
XT-4A melting point apparatus and were uncorrected. Infrared (IR)
spectra were recorded as thin films on KBr plates with a Bruker ver-
tex 70 spectrophotometer. 1H and 13C NMR spectra were obtained
using a Bruker ARX-400 MHz spectrophotometer and chemical shift
values are expressed in ı values (ppm) relative to tetramethylsilane
(TMS) as internal standard. Analytical thin-layer chromatography
(TLC) was carried out on aluminum-baked silica gel plates (60 HF-
254, Kangbinuo). Electrospray ionization mass spectra (ESI-MS)
were measured on a ZAB-HS and ESQUIRE6000 mass spectrome-
ter. High-resolution mass spectrometry (HRMS) was conducted at
Agilent 6210.
2.2. General procedure for the synthesis of phenacylimidazolium
bromides
2.6. Computational details
All calculations were performed with the use of the Gaussian
03 series of programs [18]. The geometrical structures were opti-
mized using the two-layered ONIOM [19] method. The reactive site
of molecules included 1,4-dihydropyridine rings and the carbonyl
group of PIm, while the carbethoxyl groups, the phenyl and imi-
dazole groups were included in the second layer. The two levels
of theory used for calculations were the density functional theory
(DFT) at Becke’s three parameter exchange functional and correla-
tion functional of Lee–Yang–Parr (B3LYP) [20] with the standard
6-31G* basis set [21] for the high level and the semiempirical
PM3 method [22] for the low level. This methodology has been
successfully applied to study structures and reaction mechanisms
for the photocycloaddition between 2-cyclohexenones and 1,1-
diethoxyethylene [23]. Open-shell systems have been described
within a spin-unrestricted formalism. All the stationary points were
characterized by harmonic vibrational frequency analysis in order
to verify that minima and transition structures have zero and one
imaginary frequency, respectively. Moreover, the electronic struc-
tures of stationary points were analyzed by the natural bond orbital
(NBO) approach [24].
Bromine (20 mmol) was added to a solution of appropriate ace-
tophenone (18 mmol) in chloroform (50 mL) containing HBr (4
drops) at 0 ◦C and stirred at room temperature for 4 h. The reaction
mixture was diluted with saturated aqueous NaHCO3. The sepa-
rated organic layer was washed with brine, then dried over NaSO4,
and evaporated to give a crude compound that was phenacyl bro-
mide. 1-Methylimidazole (18 mmol) was syringed into a stirred
solution of phenacyl bromide in 50 mL of ethanol, and the mixture
was kept at room temperature for 16 h or at reflux for 8 h. After
completion of the reaction as indicated by TLC, the solid, which
separated, was collected and then triturated extensively and dried
in vacuo to give the desired salt. Pure samples were obtained after
recrystallization from appropriate solvent (acetone or methanol).
Analytical and spectroscopic data for the compounds 1a–e and 2
are described in the Supporting Information.
2.3. General procedure for the photocycloaddition in the presence
of PIm photosensitizers
1,4-Dihydropyridines 3 or syn-dimer 5 (1 mmol) containing
0.05 equiv. of PIms such as 1a–e and 2 were dissolved in 50 mL of the
appropriate solvent in an ACE glass of photochemical reactor fitted
with a centrally positioned quartz cooling jacket. The solution was
purged with dry nitrogen for 20 min, and irradiated with a 250 W
medium-pressure mercury lamp (320 nm < ꢀ < 400 nm) centered at
ꢀ = 365 nm inside a quartz immersion well, under continuous mag-
netic stirring. After completion of the reaction as indicated by TLC,
the solvent was evaporated and the residue was chromatographed
in a silica gel column (ethyl acetate/petroleum ether, 1:5).
3. Results and discussion
3.1. Phenacylimidazoliums (PIms) as photosensitizers
The [2 + 2] photocycloadditions of 1,4-dihydropyridines 3a–g
were carried out by irradiation of medium-pressure mercury lamp,
in the absence and in the presence of photosensitizers PIms 1a–e
and 2 (Scheme 2). The influence of solvents and photosensitizers
was investigated, and the results are summarized in Table 1.

X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
15
EtOOC
0.05 equiv. of benzophenone (entry 3). By comparison, irradiation
of 3a, in the presence of PIm 1a, led to the products in 78% yield
(entry 4). The initially formed adduct 5a was submitted to forma-
tion of 6a in 85% yield resulted from a [2 + 2] cycloaddition reaction
of the intramolecular double bonds (entry 5). In these cases the
irradiation times were obviously shortened by the addition of PIm
1a, even under a low-power light source (250 W).
When the photochemical reactions of 3b in the presence of PIms
1a–e and 2 were performed in tetrahydrofuran/methanol (1:1), the
substituted naphthyl derivative 1e was found to be more efficient,
and used for all the other preparative experiments. Cycloadducts
6b–g were isolated in better yields ranging from 35 to 63% (entries
6–16), by contrast with the uncatalyzed reactions [7b]. Specifically,
the combination of high yield of the cage dimer 6f along with the
short irradiation times were quite remarkable and indicated a sur-
prisingly efficient process which contrasted sharply the lack of 1e
in 20% yield for 6 h [7b] (entry 15).
R
H
COOEt
N
N
H
EtOOC
EtOOC
R
R
COOEt
4
EtOOC
COOEt
hv
sens.
EtOOC
R
N
H
3
H
N
R
H
EtOOC
EtOOC
COOEt
COOEt
COOEt
N
hv
sens.
N
H
6
N
3a: R = H
H
COOEt
5
R
R
3b: R = C6H5
3c: R = 4-OCH3C6H4
3d: R = 2,4-(OCH3)2C6H3
3e: R = 3,4,5-(OCH3)3C6H2
3f: R = 2,3,4-(OCH3)3C6H2
3g: R = 2,4,5-(OCH3)3C6H2
Scheme 2. Photocycloadditions of 1,4-dihydropyridines catalyzed by triplet pho-
tosensitizers.
As mentioned above, the presence of PIms 1a–e and 2 led to
the more efficient photocycloadditions of DHPs 3a–g with a low
power lamp (250 W) for shorter times. As triplet photosensitizers,
PIms had an important influence on the improvement of the photo-
chemical reactions, and was assumed as sensitizers promoting the
substrate to an excited state, presumably the T1 state. Therefore,
in order to gain a better understanding of the reaction mechanism
and the role of the photosensitizers, the intermediacy of such tran-
sients was investigated by laser flash photolysis (LFP) experiments
allowing the quenching rate constants for the employed photosen-
sitizers to be determined, whereas theoretical calculations offered
an interpretation of the whole process.
Table 1
Photocycloaddition of 1,4-dihydropyridines 3a–g sensitized by PIms 1a–e and 2.
Entry
Sens.
Reactant
Conditionsb
Yields (%)
4
6
1a
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
3a
3a
3a
3a
5a
3b
3b
3b
3b
3b
3b
3c
3d
3e
3f
24 h, 400 W, THF
12 h, 250 W, THF
12 h, 250 W, THF
12 h, 250 W, THF
39
9
36
7
Benzophenone
11
37
12
41
85
45
47
48
40
53
45
49
60
63
35
47
1a
1a
1a
1b
1c
1d
1e
2
1e
1e
1e
1e
1e
1 h, 250 W, THF
3.5 h, 250 W, THF/MeOH
3.5 h, 250 W, THF/MeOH
3.5 h, 250 W, THF/MeOH
3.5 h, 250 W, THF/MeOH
3 h, 250 W, THF/MeOH
3.5 h, 250 W, THF/MeOH
3 h, 250 W, THF/MeOH
3 h, 250 W, THF/MeOH
3 h, 250 W, THF/MeOH
4 h, 250 W, THF/MeOH
4 h, 250 W, THF/MeOH
3.2. Time-resolved studies
Laser flash photolysis of deaerated acetonitrile solutions were
performed at 355 nm (Nd:YAG, 7 ns laser pulse, unfocused ca.
13 mJ/pulse). The transient absorption spectra of 1e shows two
peaks centered at 370 and 430 nm, respectively, a weaker broad
band between 610 and 690 nm (Fig. 1A). Decay kinetics for the two
peaks were the same, indicating the presence of a triplet transient
with a lifetime of approximately 2 ␮s, and they were not dependent
on the concentration of 1e (the self-quenching process was negligi-
ble). This species was attributed to the characteristic triplet–triplet
absorption of naphthoyl chromophore in accordance with that of
2-acetonaphthone triplet state independently generated [25]. The
3 g
a
Described in the literature [7a].
THF/MeOH = 1:1.
b
Irradiation of a tetrahydrofuran solution of 3a for 24 h resulted
in a mixture of the anti-4a and cage dimer 6a, in 75% total yield
with a ratio of 39/36 (Table 1, entry 1). With a low-power Hg lamp
(250 W) for 12 h, the mixture was obtained in 16% yield (entry
2). Under these conditions, 3a was irradiated in the presence of
Fig. 1. Transient absorption spectra of the acetonitrile solutions of 1e (0.3 mM) excited with a pulsed laser at 355 nm: (A) 0.4 ␮s (ꢀ), 2 ␮s (᭹), 4 ␮s (ꢁ) and 12 ␮s (ꢂ) after
the laser pulse; (B) in the presence of 3a (0.3 mM), 0.8 ␮s (ꢀ), 2 ␮s (᭹), 5 ␮s (ꢁ) and 12 ␮s (ꢂ) after the laser pulse. Inset: single-exponential fitting of the absorption decay
recorded at 430 and 500 nm, respectively.

16
X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
Fig. 2. Transient absorption spectra of the acetonitrile solutions of 2 (0.3 mM) excited with a pulsed laser at 355 nm: (A) 30 ns (ꢀ), 140 ns (᭹), 360 ns (ꢁ) and 810 ns (ꢂ) after
the laser pulse; (B) in the presence of 3a (1.0 mM), 0.04 ␮s (ꢀ), 3 ␮s (᭹), 10 ␮s (ꢁ) and 20 ␮s (ꢂ) after the laser pulse. Inset: single-exponential fitting of the absorption decay
recorded at 530 and 480 nm, respectively.
latter absorption in the 610–690 nm region was assigned to the
lowest triplet state of the imidazole chromophore [26].
Quenching of the PIm and BPIm triplets by 3a were analyzed by
means of the Stern–Volmer linear relationships, where the recip-
rocal lifetimes (1/ꢂ) of the excited state in the absence and in the
presence of a quencher (Q) are related to the concentration of
quencher ([Q]). The triplet quenching rate constants, kq(T1) were
obtained from the slopes of the straight lines according to the fol-
lowing equation:
This experiment was performed upon addition of 3a to identify
the possible transient reaction intermediates and to determine the
triplet quenching rate constant (Fig. 1B). In the presence of 3a, dis-
appearance of the PIm 1e triplet was followed by formation of a
new transient with absorption maxima at 500 nm (strong), and a
shoulder (low intensity) in the 400–450 nm region.
1
ꢂ
It has been reported that the quenching of the naphthalene (NP)
triplet by the ground state of aromatic ketones (AK) led to H-atom
transfer via triplet exciplexes 3(NPı+· · ·AKı−) [27]. Pérez-Prieto
et al. reported the similar formation of a polarized hydrogen-
bonded InHı+· · ·BTı− exciplex in the intermolecular photoreaction
between 2-benzoylthiophene (BT) and indole (InH) [28]. As elec-
tron donors with electronic features similar to that of indole [29],
DHP have the potential to form 3(DHP· · ·ketone)* triplet complexes.
Thus, the transient absorption at 500 nm in Fig. 1B might well be
ascribed to the 3(DHP· · ·PIm)* exciplex. This type of species has
been obtained by hydrogen abstraction from 1,4-dihydropyridine
derivatives. In the presence of increasing amounts of 3a, the LFP of
1e has provided the evidence of the 3(DHP· · ·PIm)* triplet complex
by a significant reduction of the 500 nm signal. More importantly,
the exciplex intermediate lifetime of 0.5 ␮s is lower than that of
PIm triplet in the absence of 3a, suggesting the occurrence of
triplet–triplet energy transfer to produce a polarized hydrogen
bond [30].
As expected, LFP of imidazole-tagged benzophenone 2 (BPIm)
alone gives its well-characterized triplet state with strong signals
identical in lifetime, absorption maxima, and changes in optical
density (Fig. 2A). The absorption spectrum with the maximum
at 530 nm was safely assigned to the BP triplet state [31]. The
latter transient signal at 650 nm was ascribed to the absorption
of Im triplet state as described above. Quenching of the triplet
by 3a was also investigated (Fig. 2B). After longer delay times, a
residual absorption peaking at 530 nm was followed triplet disap-
pearance in the presence of 3a. Although the BPIm triplet retained
the capability to form hydrogen-bonded exciplex with 3a, the capa-
bility of n␲*aromatic ketones made it easier to achieve direct
H-abstractionby comparisonwithPIm. Accordingly, thisquenching
process resulted in formation of BPIm ketyl radical with its absorp-
tion at 560 nm, which was clearly different with PIm to form the
triplet complex. It was remarkable that formation of the Ph2C•OH
ketyl radical from BP triplets was associated with a bathochromic
shift of ca. 20 nm (from 520 to 540 nm) [32], in the case of BPIm the
analogous process led to the similar shift (from 530 to 560 nm).
= k0 + kq(T1) × [Q]
Noteworthy, the quenching rate constants for 3a were found to
be 2.4 × 109 M−1 s−1 (1e) and 1.5 × 109 M−1 s−1 (2) (Fig. 3A and
B). From these data, it was established that the energy transfer
occurred at almost diffusion controlled rate, in good accordance
with the high efficiency of photosensitization.
3.3. Theoretical calculations
First, the theoretical study of the photocycloaddition of DHP
3a as a model of 1,4-dihydropyridines has been investigated by
means of DFT methods at the ONIOM(UB3LYP/6-31G*:PM3) level,
aiming to elucidate the details of each elementary step, the struc-
tures of stationary points, the rate- and stereodetermining step of
this reaction. Then, the PIm-promoted photocycloaddition of DHP
is discussed theoretically to explain the role of the triplet photo-
sensitizer on the formation of the final outcome of the cycloadduct
6a (CA).
3.3.1. Uncatalyzed photocycloaddition of DHP
Due to the existence of two symmetrical double bonds of
DHP, the photocycloaddition follows a generally accepted stepwise
mechanism with formation of triplet biradical intermediate [33].
Thus, the different stationary points of this cycloaddition have been
depicted in Scheme 3 together with the atom numbering.
The first step involves the attack of the triplet excited double
bond to the other ꢃ bond forming a triplet biradical intermediate
IN1(T1), which can evolve to the singlet biradical IN1(S0) through
an intersystem crossing (isc). The activation energy associated to
the formation of a C2 C5 bond via TS1(T1) is 4.3 kcal/mol above
the triplet reactants. Formation of intermediate IN1(T1) is exother-
mic by 10.7 kcal/mol. The biradicals IN1(T1) and IN1(S0) differ by
only 2.9 kcal/mol and present very close geometrical parameters
(see Fig. S1 in Supporting Information), thereby confirming that
these two stationary points sit near the intersystem crossing region
between the ground-state singlet and triplet energy surfaces of

X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
17
TS2(S0)
IN1(S0)
syn(S0)
isc
hv
H
N
H
N
COOEt
COOEt
H
N
EtOOC
COOEt
COOEt
COOEt
TS1(T1)
TS2(T1)
3
4
2
1
5
6
8
7
+
N
H
EtOOC
N
EtOOC
N
EtOOC
COOEt
COOEt
COOEt
H
H
syn(T1)
DHP(S0)
DHP(T1)
IN1(T1)
TS3(T1)
H
N
H
N
COOEt
COOEt
COOEt
COOEt
EtOOC
EtOOC
EtOOC
IN2(S0)
N
H
EtOOC
N
H
CA(S0)
IN2(T1)
Scheme 3. Proposed mechanism for [2 + 2] photocycloaddition of DHP.
the system. However, formation of the second C C bond from this
intermediate on the triplet energy surface via TS2(T1) is uphill by
36.2 kcal/mol. This would be the rate-limiting step for the triplet
pathway, and indicate that triplet biradical IN1(T1) is kinetically
stable. On the other hand, the cycloaddition might take place along
the ground state through a transition state TS2(S0) with a very low
activation energy of 2.5 kcal/mol. As a consequence, formation of
the cycloadduct syn(S0) is faster than the intramolecular [2 + 2]
cycloaddition, in clear agreement with the experimental findings.
In the intramolecular [2 + 2] cycloaddition, which the residual
C
C bonds of syn adduct affords the final photocycloaddition prod-
uct, the reaction profile shown in Fig. S2 is similar to that previously
found for the first reaction. Thus, its first step consists on the forma-
tion of a new C C bond to yield the intermediate IN2(T1) through
the TS3(T1) transition state. This step has a large energy barrier
13.1 kcal/mol, which fulfills the need for the rotation of the two
DHP planes in the excited cycloadduct syn(T1) to reach IN2(T1). At
the last ring-closure step the intermediate IN2(T1) allows its con-
version into IN2(S0) through intersystem crossing, which can give
the final cycloaddition product CA(S0) in a barrierless process.
3.3.2. PIm-photocatalyzed DHP cycloaddition
The stepwise mechanism of PIm-promoted photocycloaddition
of DHP has also been studied using DFT methods (as shown in
Scheme 4). Since flexibility of 3-methylimidazolium in 1a signif-
icantly increases computational time, the moiety is replaced by
imidazole group, and the counterion of the catalyst, Br−, is believed
to act as a spectator and thus is not considered in the calcula-
tions. This simplified reaction system is still a valid model, since all
important structural and electronic features of studied system are
remained. Therefore, four TSs, six biradical intermediates, and the
cycloadduct have been located and characterized. The optimized
geometries of the TSs are given in Fig. 4, while the corresponding
potential energy profile is represented in Fig. 5.
Energy profile shows that the formation of PIm-complexed DHP
occurs through a weakly hydrogen bond between a DHP molecule
and the carbonyl oxygen atom of PIm at the early stage in this
reaction. Initially, the C2 position of excited DHP–PIm complex can
attack the C5 position of DHP in the ground state via transition state
TS4(T1). The calculated energy barrier is only 2.6 kcal/mol and thus
very low in comparison with the barrier for the uncatalyzed DHP
photocycloaddition (4.3 kcal/mol). These energies point to a very
fast formation of the catalyst after irradiation of the reaction mix-
ture and evaluate the PIm catalyst effectively accelerate the first
step. For all the subsequent steps the reaction proceeds downhill.
This process leads to the open-chain intermediate IN3(T1), which is
Fig. 3. Stern–Volmer plots for triplet quenching of 0.3 mM acetonitrile solutions of
(A) 1e or (B) 2 in the presence of increasing amounts of 3a.

18
X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
Scheme 4. Proposed mechanism for PIm-catalyzed photocycloaddition of DHP.
located 10.7 kcal/mol below the reactants. After a hydrogen trans-
fer from secondary amine of the DHP ring to the PIm, the formation
of intermediate complex IN4(T1) is exothermic by 6.6 kcal/mol.
All attempts to detect the corresponding transition state for the
approach were unsuccessful. This means that these results support
the feasibility of the proposed hydrogen transfer without energy
barrier. The subsequent ring-closing process to generate interme-
diate IN5(T1) through the transition state TS5(T1) is also the rate
determining step and presents an energy barrier of 19.3 kcal/mol.
The energy of activation for this stepwise process is lower than
that for the uncatalyzed reactions by more 16 kcal/mol. Finally, the
dissociation of PIm from IN5(T1) leads to the formation of the inter-
molecular cycloadduct syn-PIm(T1) which is exothermic by about
12 kcal/mol in terms of free energy.
At the subsequent photoreaction, the formation of cyclobu-
tane ring (C3 C4 C7 C8) occurs through the intramolecular [2 + 2]
cycloaddition. The following mechanism is similar to the bimolecu-
lar cycloaddition in Scheme 4, and the PIm may also be considered
to form a coordination bond with DHP ring of the syn adduct. In
principle, the attack of C3 atom of the double bond to C8 atom
in complex syn-PIm(T1) allows the formation of the intermedi-
ate IN6(T1). However, the energy barrier associated with TS6(T1)
is 30.7 kcal/mol, which is higher than those of the mechanism with
no the catalyst. This unfavorable activation energy is mainly due
to the strain associated with the formation of the four-membered
ring at the TS6(T1). Therefore, the biradical IN6(T1) is kinetically
and thermodynamically very unstable, undergoing fast hydrogen
abstraction from the PIm triplet state to yield the intermediate
IN7(T1). Furthermore, the activation energy associated to the inter-
mediate IN8(T1) is only of 2.9 kcal/mol, and the formation of [2 + 2]
cycloadduct is strongly exothermic by 51.0 kcal/mol.
lengths at the transition state in the catalytic reaction are rela-
tively loose compared with the case of noncatalytic reaction, which
may be responsible for the low barrier. Noteworthy, the most sig-
nificant data in the catalytic reaction are the distances between
acidic hydrogen of DHP and the carbonyl oxygen atom of PIm
(H O), as well as this hydrogen and the N1 nitrogen atom of
the DHP (N1 H). DHP coordinates to the PIm in this transition
˚
state as evidenced by the distances of H O bond of 1.904 A. At
the intermediates IN3(T1) and IN4(T1), the distances of N1 H and
˚ ˚
O are 1.015 and 2.028 A for IN3(T1) and 1.982 and 0.989 A for
H
IN4(T1), respectively. The shorter H O distances found in these
species confirm that the hydrogen atom has been transferred from
the donor to the PIm carbonyl group, while the process seems
to stabilize the resulting complexes IN4(T1). At TS5(T1) associ-
ated with the attack of the radical carbon atom to the imine, the
˚
lengths of the C1 C6 forming bond is 2.836 A, while the four-
membered ring become nearly coplanar, with the dihedral angle
C1 C2 C5 C6 of −39.9◦. In IN5(T1), the C1 C6 is already formed,
with the dihedral angle C3 C4 C7 C8 of −37.1◦, where the lengths
˚
of N1 H and H O are 2.004 and 0.990 A. The dissociation of the
H
O bond generates the syn-PIm(T1) complex as indicated by
˚
the shortening N1 H (1.022 A) and lengthening H O distance
(2.183 A). The length of the C3 C8 bond being formed during the
˚
attack of DHP moiety to the DHP–PIm moiety in syn-PIm(T1) is
˚
3.535 A at TS6(T1). When hydrogen transfer occurs, the distances
˚
of N1 H and H O change from 1.014 and 2.066 A in IN6(T1) to
˚
1.963 and 0.990 A in IN7(T1), respectively. Finally, in the isolated
PIm molecule the cage dimer formation proceeds, while the H
O
˚
bond-forming interaction is weak (H O = 2.217 A) in the ground
state.
To understand the electronic features of the catalytic reaction,
the charge transfer (CT) and the bond order (BO) [34] were ana-
lyzed. At the TS4(T1) associated with the attack of DHP to the
DHP–PIm complex, the BO value of the C2 C5 forming bond is 0.23.
In IN3(T1) intermediate the BO value between the key atoms (H O)
Fig. 4 shows the optimized structures for all intermediates and
transition states involved in this pathway. At the TS4(T1), the
˚
lengths of the C2 C5 forming bond is 3.240 A, whereas the dis-
˚
tance between the C1 and C6 atoms remains at 3.766 A. These

X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
19
Fig. 4. Geometries of the different stationary points involved in the PIm-photocatalyzed DHP cycloaddition optimized by the ONIOM(UB3LYP/6-31G*:PM3) method (upper
layer: ball and stick. Lower layer: wireframe). The bond lengths between the atoms directly involved in the reaction are given in angstroms.
is 0.03, which indicates that a significant interaction between DHP
and the PIm stabilizes this intermediate. The natural atomic charges
have been shared between the DHP and the PIm fragments. The CT
located at the PIm(H) fragment is 0.05 e (almost negligible) from
IN3(T1) to IN4(T1), suggesting that one electron and one proton are
transferred in a concerted fashion. The N1 C1 BO value changes
from 1.23 in IN3(T1) to 1.81 in IN4(T1) along the process. At the
TS5(T1) associated with the ring closure, the BO value of C1 C6
is 0.12. For back hydrogen transfer with the separation of PIm
from IN5(T1), the CT also remains at 0.05 e. Likewise, the electronic
nature of the further cyclization points out that the participation of
PIm plays a critical role for the stabilization of those intermediates,
governing thus the formation of the final product CA as previously
observed.
In summary, a comparison of PIm-photocatalyzed DHP cycload-
dition with the uncatalyzed reaction reveals some interesting
behaviors. PIm assists the formation of the biradical, which is ini-
tialized by the attack of a molecule DHP to the DHP–PIm complex
with a low barrier. Subsequent hydrogen transfer from N H to
the PIm carbonyl group followed by ring closure gives rise to a
cycloadduct intermediate. The energy results obtained for these
further steps are kinetically and thermodynamically favorable as a
consequence of formation of the cycloadduct, allowed to explain
the efficient PIm-catalyzed effect.

20
X. Zhu et al. / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 13–20
Fig. 5. Schematic potential energy profile of photocycloaddition of DHP sensitized by PIm at the ONIOM(UB3LYP/6-31G*:PM3) level of calculation.
4. Conclusion
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Acknowledgments
This work was financially supported by the National Natural Sci-
ences Foundation (no. 20872009) and Natural Sciences Foundation
of Beijing (no. 200710005002).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
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