Competitive photocyclization/rearrangement of 4-aryl-1,1-dicyanobutenes controlled by intramolecular charge-transfer interaction. Effect of medium polarity, temperature, pressure, excitation wavelength, and confinement

Article abstract of DOI:10.1039/c1pp05038a

A series of 4-aryl-1,1-dicyanobutenes (1a-1f) with different substituents were synthesized to control the intramolecular donor-acceptor or charge-transfer (C-T) interactions in the ground state. Photoexcitation of these C-T substrates led to competitive cyclization and rearrangement, the ratio being critically controlled by various environmental factors, such as solvent polarity, temperature and static pressure, and also by excitation wavelength and supramolecular confinement (polyethylene voids). In non-polar solvents, the rearrangement was dominant (>10:1) for all examined substrates, while the cyclization was favoured in polar solvents, in particular at low temperatures. Selective excitation at the C-T band further enhanced the cyclization up to >50:1 ratios. More importantly, the cyclization/rearrangement ratio was revealed to be a linear function of the C-T transition energy. However, the substrates with a sterically demanding or highly electron-donating substituent failed to give the cyclization product. The Royal Society of Chemistry and Owner Societies.

Full text of DOI:10.1039/c1pp05038a

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S. Mariappan, M. T. Mwangi, M. C. Tiedman and L. R.
MacGillivray, Photochem. Photobiol. Sci., 2011, 10, 1384
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Chirogenic [3 + 2]-photocycloaddition reactions of 2-
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Hydroxy-group effect on the regioselectivity in a
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The synthesis and stereospecific solid-state
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PAPER
Competitive photocyclization/rearrangement of 4-aryl-1,1-dicyanobutenes
controlled by intramolecular charge-transfer interaction. Effect of medium
polarity, temperature, pressure, excitation wavelength, and confinement†
Tadashi Ito, Emi Nishiuchi, Gaku Fukuhara, Yoshihisa Inoue* and Tadashi Mori*
Received 27th January 2011, Accepted 15th February 2011
DOI: 10.1039/c1pp05038a
A series of 4-aryl-1,1-dicyanobutenes (1a–1f) with different substituents were synthesized to control the
intramolecular donor–acceptor or charge-transfer (C–T) interactions in the ground state.
Photoexcitation of these C–T substrates led to competitive cyclization and rearrangement, the ratio
being critically controlled by various environmental factors, such as solvent polarity, temperature and
static pressure, and also by excitation wavelength and supramolecular confinement (polyethylene
voids). In non-polar solvents, the rearrangement was dominant (>10 : 1) for all examined substrates,
while the cyclization was favoured in polar solvents, in particular at low temperatures. Selective
excitation at the C–T band further enhanced the cyclization up to >50 : 1 ratios. More importantly, the
cyclization/rearrangement ratio was revealed to be a linear function of the C–T transition energy.
However, the substrates with a sterically demanding or highly electron-donating substituent failed to
give the cyclization product.
The photochemistry of donor–acceptor systems has been stud-
Introduction
ied mostly from the electron transfer point of view. The rate of
Despite the significant developments in mechanistic organic
electron transfer is dependent on the strengths of both donor
photochemistry in recent decades, the precise control of regio- and
and acceptor molecules, and usually governed by the Marcus
equation.8 The effect of donor–acceptor interaction has thus been
discussed in terms of the rate of electron transfer and the lifetime of
charge-separated state, where the resulting charge-separated state
is eventually quenched by the back electron transfer, reverting to
the initial state. Although the selective excitation of the charge–
transfer (C–T) band by irradiating at the absorption edge is
feasible,9 less attention has been paid to the photochemical trans-
formation of donor–acceptor molecules and the studies are rather
limited. We have recently investigated the effects of excitation
wavelength on the product’s (dia)stereoselectivity in several chiral
donor–acceptor systems.10–12 Thus, the direct (or local) excitation
of one of the components in chiral fumarate-stilbene10,11 or chiral
benzoate-diphenylethene12 systems leads to the formation of a
conventional exciplex via the excited singlet state. In contrast, the
selective C–T band excitation of a ground-state complex affords
a distinctly different excited species, i.e., excited C–T complex,
accordingly providing quite different product distributions.
In this study, we investigated the photoreaction of a series
of 4-aryl-1,1-dicyanobutenes (1a–1f) with varying degrees of in-
tramolecular C–T interaction. In addition to the donor/acceptor
abilities, steric bulk is known to critically affect the donor–acceptor
interaction.13 Hence, we examined the steric effect on the photore-
actions by using more sterically congested substrates (1d and 1e).
The photolysis of 1 afforded the cyclization and rearrangement
products in almost quantitative manner (Scheme 1), the ratio
stereoselectivities is still one of the most intriguing and challenging
tasks in photochemistry.1 In contrast to the conventional thermo-
chemical counterparts, photochemical transformations occurring
in the excited states are more susceptible to environmental variants,
enabling us to manipulate the selectivities by temperature, solvent
polarity, pressure and other factors.2 Additionally, the properties
and reactivity of excited or reactive species can be modulated by
using various techniques such as singlet/triplet sensitization and
energy/electron transfer, and also by directly altering excitation
wavelength.3 Recently, weak non-covalent interactions, such as
hydrogen bonding,4 cation–p,5 donor–acceptor6 and hydrophobic
interactions, have also been used to control photoreactions in
the solid and solution phases. Thus, a wide variety of cavities,
supercages, voids or receptor sites of cyclodextrins, cucurbiturils,
zeolites, polymer matrices, metal–organic frameworks, organic
cages, dendrimers, micelles and biomolecules have also been
exploited to manipulate the photochemical outcomes, the choice
of which may depend on the nature of the targeted reactions.7
Department of Applied Chemistry, Graduate School of Engineer-
ing, Osaka University, 2-1 Yamada-oka, Suita, 565-0821, Japan.
E-mail: tmori@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7923; Tel: +81-
6-6879-7921
† This article is published as part of a themed issue in honour of Yoshihisa
Inoue’s research accomplishments on the occasion of his 60th birthday.
This journal is
The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 1405–1414 | 1405
©

priate benzyl bromides with acetylacetone. The benzyl bromides
needed for the preparation of 1c and 1e were prepared by
the literature procedures.14,15 4-Methoxyphenyl-2-butanone (for
preparation of 1f) and other benzyl bromides (for 1a, 1b, and
1d) were commercially available.
(a) 4-Substituted 2-butanones. These compounds were pre-
pared by procedures similar to those described in the literature.16
A mixture of benzylbromide (8.0 mmol), acetylacetone (0.80 g,
8.0 mmol), and anhydrous potassium carbonate (1.10 g, 8.0 mmol)
in methanol (20 mL) was refluxed for 16 h under nitrogen atmo-
sphere. The reaction mixture was cooled to room temperature and
the solvent was removed under a reduced pressure. The resulting
residue was treated with ethyl acetate (20 mL) and water (20 mL),
and the organic layer was collected. The aqueous layer was further
extracted with ethyl acetate (3 ¥ 15 mL). The combined organic
phase was washed with brine (3 ¥ 15 mL), dried over anhydrous
sodium sulfate, and the solvent was removed under a reduced
pressure. The resulting oily material was chromatographed on
silica gel using an 8 : 2 mixture of hexane and ethyl acetate as
eluent to give the corresponding butanones as oil in 29–64% yield.
Scheme 1 Photoreactions of 4-aryl-1,1-dicyanobutenes (1a–1f).
of which was shown to be a critical function of environmental
factors such as medium polarity, temperature, pressure, as well as
excitation wavelength. We will demonstrate how and to what extent
the strength of C–T interaction affects the product distribution,
and reveal that the combined use of multiple factors (e.g., solvent,
temperature and pressure) significantly enhances the product
selectivity.
(b) 4-Aryl-1,1-dicyanobutenes. These were prepared by pro-
cedures similar to those in the literature.17 4-Substituted 2-
butanone (5.1 mmol), malononitrile (0.34 g, 5.1 mmol), acetic
acid (0.30 mL, 5.1 mmol) were dissolved in toluene (25 mL).
Ammonium acetate (0.11 g, 1.4 mmol) was added and the
mixture was refluxed with a Dean–Stark apparatus for 16 h under
nitrogen. The solution was cooled to room temperature and the
resulting mixture was washed with saturated aqueous sodium
hydrogen carbonate (3 ¥ 20 mL) and dried over anhydrous sodium
sulfate. The solution containing the crude product was filtrated,
concentrated in vacuo and chromatogrammed on silica gel with
hexane–ethyl acetate (8 : 2) eluent to give the corresponding 4-
aryl-1,1-dicyanobutenes in 28–95% yield.
Experimental
Instrumentation
UV-vis absorption spectra were recorded on a JASCO V-550 or V-
560 spectrometer fitted with an ETC-505T temperature controller.
1H (400 MHz) and 13C NMR (100 MHz) spectra were recorded in
CDCl3 on JEOL GX-400 or JNM-ECP400, and chemical shifts
are reported in ppm relative to Me4Si. Gas chromatographic (GC)
analyses were performed on Shimadzu GC-2014, and the reactant
conversion and product yield were determined on a capillary
column ZB-5 (30 m ¥ 0.25 mm i.d. ¥ 0.25 mm d.f.). GC peaks
were integrated on a Shimadzu GC Solution (version 2.3). GC-
MS was obtained on a Hewlett Packard HP 5890 Series II fitted
with an HP 5971A mass-selective detector at an applied voltage
of 70 eV. Elemental analyses were performed at the Center for
Chemical Analyses, Osaka University. Column chromatography
was performed on silica gel 60 (230–400 mesh ASTM) with a
Yamazen PREP UV-10V detector and pump Model 540 or by
GPC (JAIGEL-H column) on JAI LC-908.
1,1-Dicyano-2-methyl-4-phenyl-1-butene (1a).
1
4-Phenyl-2-butanone. (0.77 g, 64%). H NMR (lit.18): d 2.16
(3H, s), 2.78 (2H, t, J = 7.6 Hz), 2.92 (2H, t, J = 7.6 Hz), 7.19–7.23
(3H, m) and 7.27–7.32 (2H, m).
1,1-Dicyano-2-methyl-4-phenyl-1-butene (1a). (0.86 g, 85%).
1H NMR (lit.17): d 2.26 (3H, s), 2.90 (4H, s), and 7.18–7.35 (5H,
m).
1,1-Dicyano-2-methyl-4-(4-methylphenyl)-1-butene (1b).
4-(4-Methylphenyl)-2-butanone. (0.80 g, 62%). 1H NMR
(lit.18): d 2.14 (3H, s), 2.31 (3H, s), 2.74 (2H, t, J = 7.6 Hz), 2.86
(2H, t, J = 7.6 Hz) and 7.06–7.10 (4H, m).
Materials
Solvents (Aldrich or Wako, spectrophotometric grade) and
reagents – benzylbromide (Wako, 98% purity), acetylacetone
(Kanto, 99.5%), potassium carbonate (Wako, 99.5%), malonon-
itrile (Wako, 98%), acetic acid (Wako, 99.7%), ammonium ac-
etate (Kanto, >97%), p-methylbenzyl bromide (Wako, 98%), 3,5-
dimethylbenzylbromide (Alfa Aesar, 98%), m-xylene (Wako, 98%),
paraformaldehyde (Kanto, 92%), hydrobromic acid (Wako, 47–
49%), 1-phenylhexane (Wako), 4-(4-methoxyphenyl)-2-butanone
(Aldrich) – were used as received.
1,1-Dicyano-2-methyl-4-(4-methylphenyl)-1-butene
(1b).
(0.99 g 95%). H NMR: d 2.25 (3H, s), 2.34 (3H, s), 2.86–2.89
(4H, m), 7.06 (2H, d, J = 8.1 Hz) and 7.13 (2H, d, J = 8.1 Hz); 13
1
C
NMR: d 21.1, 22.8, 33.3, 39.7, 86.5, 111.6, 111.9, 128.1, 129.5,
135.3, 136.6 and 181.4; MS (EI): m/z = 210 (M+, 7%), 106 (10),
105 (100); EA: Found: C, 79.88; H, 6.58; N, 13.10%. Calcd for
C14H14N2: C, 79.97; H, 6.71; N, 13.32%.
1,1-Dicyano-2-methyl-4-(4-hexyllphenyl)-1-butene (1c).
1
4-Hexylbenzylbromide. (3.47 g, 71%). H NMR (lit.14): d 0.88
General procedures for preparing 4-aryl-1,1-dicyanobutenes
(3H, t, J = 6.9 Hz), 1.30 (6H, bs), 1.55–1.66 (2H, m), 2.59 (2H, t,
J = 7.8 Hz), 4.49 (2H, s), 7.17 (2H, d, J = 7.2 Hz) and 7.27 (2H, d,
J = 7.2 Hz).
4-Aryl-1,1-dicyanobutenes were prepared from the corresponding
butenones, which were obtained by the condensation of appro-
1406 | Photochem. Photobiol. Sci., 2011, 10, 1405–1414 This journal is
The Royal Society of Chemistry and Owner Societies 2011
©

4-(4-Hexylphenyl)-2-butanone. (0.91 g, 29%). 1H NMR: d 0.88
(3H, t, J = 6.9 Hz), 1.30 (6H, bs), 1.54–1.61 (2H, m), 2.14 (3H, s),
2.56 (2H, t, J = 7.6 Hz), 2.75 (2H, t, J = 7.6 Hz), 2.86 (2H, t, J =
7.6 Hz) and 7.07–7.11 (4H, m). 13C NMR: d 14.2, 22.8, 29.2, 29.5,
30.2, 31.7, 31.9, 35.7, 45.5, 128.3, 128.7, 138.2, 140.9 and 208.3;
MS (EI): m/z = 232 (M+, 76%), 175 (48), 161 (100), 147 (31),
104 (33); EA: Found: C, 82.59; H, 10.48%. Calcd for C16H24O: C,
82.70; H, 10.41%.
band-pass filter. After a certain period of time, the photolyte was
directly analyzed by GC.
Irradiation under high pressure was conducted in a pressure
vessel fitted with sapphire windows and connected to a plunger
pump. A 0.15 mL quartz cuvette (light path, 0.2 cm) filled with a
sample solution under air was placed in the high-pressure vessel,
which was thermostatted by circulating water through the reactor
body. The pressure was applied with the pump, and its magnitude
was measured by a pressure gauge. The sample solution was
photoirradiated through one of the sapphire windows. After the
photolysis, the pressure was released and the resulting photolyte
was directly analyzed by GC.
Typical procedures for photoreaction in a cavity of polyethy-
lene film were described elsewhere.20 Briefly, cleaned and doped
polyethylene film was placed in a 1 cm quartz cell capped with a
rubber septum, and was purged with nitrogen for 20 min. After the
photoirradiation, the film was successively immersed in a series
of dichloromethane aliquots (50 mL) until no absorption was
detected in the UV spectrum of the last aliquot. The combine
extract was concentrated in vacuo and the residue was analyzed by
GC.
1,1-Dicyano-2-methyl-4-(4-hexylphenyl)-1-butene
(1c).
1
(0.95 g, 86%). H NMR: d 0.88 (3H, t, J = 7.0 Hz), 1.25–1.36
(6H, m), 1.55–1.63 (2H, m), 2.25 (3H, s), 2.58 (2H, t, J = 7.9 Hz),
2.83–2.91 (4H, m), 7.08 (2H, d, J = 8.1 Hz) and 7.14 (2H, d, J =
8.1 Hz); 13C NMR: d 22.1, 22.9, 39.3, 46.3, 87.0, 111.8, 126.6,
127.3, 129.0, 143.5 and 180.5; MS (EI): m/z = 280 (M+, 4%),
176 (15), 175 (100), 104 (11); EA: Found: C, 81.44; H, 8.69; N,
10.09%. Calcd for C19H24N2: C, 81.38; H, 8.63; N, 9.99%.
1,1-Dicyano-2-methyl-4-(3,5-dimethylphenyl)-1-butene(1d).
4-(3,5-Dimethylphenyl)-2-butanone. (0.82 g, 55%). 1H NMR
(lit.19): d 2.14 (3H, s), 2.28 (6H, s), 2.73 (2H, t, J = 7.7 Hz), 2.82
(2H, t, J = 7.7 Hz), 6.80 (2H, s) and 6.83 (1H, s).
1,1-Dicyano-2-methyl-4-(3,5-dimethylphenyl)-1-butene (1d).
In preparative-scale photolyses, a 1–5 mM acetonitrile solution
of 4-aryl-1,1-dicyanobutene (250 mL) in a donut-shaped flask was
irradiated for 6–35 h under nitrogen atmosphere with a 30 W
low-pressure mercury lump (lex = 254 nm). After irradiation,
the photolyte was concentrated and separated and purified by
repeated GPC with CHCl3 as eluent.
◦
1
(0.52 g, 50%). M.p. 126 C. H NMR: d 2.25 (3H, s), 2.34 (3H,
s), 2.78–2.90 (4H, m), 7.06 (2H, d, J = 8.1 Hz) and 7.13 (2H,
d, J = 8.1 Hz); 13C NMR: d 21.3, 22.9, 33.7, 39.8, 87.0, 111.7,
112.0, 126.1, 128.8, 138.4, 138.6 and 181.4; MS (EI): m/z = 224
(M+, 12%), 120 (9), 119 (100); EA: (Found: C, 80.32; H, 7.23; N,
12.39%. Calcd for C15H16N2: C, 80.32; H, 7.19; N, 12.49%).
1-Dicyanomethyl-1,5-dimethylindane (2b). Yield: 31%; 1H
NMR: d 1.64 (3H, s), 2.18 (1H, td, J = 8.5, 13.8 Hz), 2.37 (3H,
s),2.40 (1H, td, J = 8.5, 13.8 Hz), 2.91–3.06 (2H, m),3.72 (1H, s)
and 7.10–7.26 (3H, m); 13C NMR: d 21.4, 24.1, 29.2, 33.7, 38.0,
50.5, 112.1, 112.2, 123.7, 125.1, 130.0, 137.1, 139.8 and 143.3;
C14H14N2(210.1157) HRMS: 210.1155.
1,1-Dicyano-2-methyl-4-(2,4-dimethylphenyl)-1-butene (1e).
2,4-Dimethylbenzylbromide. (2.64 g, 70%). 1H NMR (lit.15):
d2.30 (3H, s), 2.38 (3H, s), 4.51 (2H, s), 6.99–7.00 (2H, m) and
7.19 (1H, d, J = 7.5 Hz).
1
4-(2,4-Dimethylphenyl)-2-butanone. (1.23 g, 54%). H NMR:
d2.14 (3H, s), 2.28 (6H, s), 2.73 (2H, t, J = 8.0 Hz), 2.82 (2H, t, J =
8.0 Hz), 6.93–7.01 (3H, m); 13C NMR (100 MHz, CDCl3): d19.2,
20.9, 26.7, 30.0, 44.1, 126.8, 128.5, 131.1, 135.7, 135.8, 136.0 and
208.2; C12H16O (176.1201) HRMS: 176.1207.
1,1-Dicyano-2-methy◦l-4-(2,4-dimethylphenyl)-1-butene (1e).
(0.44 g, 28%). M.p. 63 C. 1H NMR: d 2.30 (6H, s), 2.32 (3H, s),
2.88–2.87 (4H, m), 6.97 (2H, d, J = 1.1 Hz) and 7.00 (1H, br s);
13C NMR: d 19.2, 21.1, 23.1, 31.2, 38.9, 86.6, 111.7, 111.9, 127.2,
128.9, 131.7, 133.8, 135.8, 137.0 and 181.4; MS (EI): m/z = 224
(M+, 6%), 120 (10), 119 (100); EA: Found: C, 80.34; H, 7.03; N,
12.44%. Calcd for C15H16N2: C, 80.32; H, 7.19; N, 12.49%.
3,3-Dicyano-2-methyl-4-(4-methylphenyl)-1-butene
(3b).
Yield: 8%; 1H NMR:d 2.05 (3H, q, J = 0.7 Hz), 2.37 (3H,
s), 3.26 (2H, s), 5.28 (1H, q, J = 0.7 Hz), 5.44 (1H, bs), 7.19 (2H,
d, J = 8.1 Hz) and 7.25 (2H, d, J = 8.1 Hz); 13C NMR: d 19.0,
21.3, 43.2, 45.7, 114.3, 118.6, 128.9, 129.7, 130.2, 135.1 and 138.9;
C14H14N2(210.1157) HRMS: 210.1149.
1-Dicyanomethyl-5-hexyl-1-methylindane (2c). Yield: 24%; 1H
NMR: d 0.88 (3H, t, J = 7.1 Hz), 1.25–1.36 (6H, m), 1.56–1.62(2H,
m), 2.13–2.21 (1H, m), 2.36–2.42 (1H, m), 2.61 (2H, t, J = 7.8 Hz),
2.91–3.06 (2H, m), 3.71 (1H, s),7.12 (1H, d, J = 8.7 Hz), 7.16 (1H,
s) and 7.17 (1H, d, J = 8.7 Hz); 13C NMR: d 14.2, 22.7, 24.1,
29.1, 29.4, 31.8, 31.8, 33.9, 36.0, 38.2, 50.7, 112.3, 112.3, 123.2,
125.2, 129.5, 140.1, 142.5 and 143.4; C19H24N2(280.1939) HRMS:
280.1937.
1,1-Dicyano-2-methyl-4-(4-methoxyphenyl)-1-butene
(1f).
◦
1
(3.77 g, 83%). M.p. 89 C. H NMR: d 2.25 (3H, s), 2.84–2.87
(2H, m), 3.80 (3H, s), 6.86 (2H, d, J = 8.6 Hz) and 7.09 (5H, d,
J = 8.6 Hz); 13C NMR: d23.0, 33.1, 40.0, 55.4, 111.7, 111.9, 114.4,
129.4, 130.5, 158.8 and 181.3; MS (EI): m/z = 226 (M+, 5%), 122
(9), 121 (100); EA: Found: C, 74.31; H, 6.24; N, 12.38%. Calcd
for C14H14N2O: C, 74.39; H, 6.23; N, 12.40%.
3,3-Dicyano-2-methyl-4-(4-hexylphenyl)-1-butene (3c). Yield:
1
9%; H NMR: d 0.88 (3H, t, J = 7.1 Hz), 1.26–1.35 (6H, m),
1.56–1.64(2H, m), 2.04 (3H, q, J = 0.7 Hz), 2.61 (2H, t, J = 7.9
Hz), 3.26 (2H, s), 5.28 (1H, q, J = 0.7 Hz), 5.43 (1H, s), 7.19 (2H,
d, J = 8.1 Hz) and 7.26 (2H, d, J = 8.1 Hz); 13C NMR: d 14.2, 19.0,
22.7, 29.1, 31.4, 31.8, 35.8, 43.3, 45.7, 114.3, 118.6, 129.0, 129.1,
130.2, 135.1 and 143.9; C19H24N2(280.1939) HRMS: 280.1937.
Irradiation procedures
A 0.1 mM solution of 4-aryl-1,1-dicyanobutene in acetonitrile or
other appropriate solvent was placed in a 1 cm quartz cell and
purged with nitrogen for 10 min. This was irradiated typically
at 20 ◦C with a 300 W xenon lamp (Asahi Spectra, MAX-301)
fitted with a UV mirror module and HQBP254/280/290/300-UV
3,3-Dicyano-2-methyl-4-(3,5-dimethylphenyl)-1-butene
(3d).
1
Yield: 8%; H NMR: d 2.04 (3H, q, J = 0.7 Hz), 2.33 (6H, s),
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Table 1 Charge-transfer band maxima and oxidation potentials of 1a–1f
3.20 (2H, s), 5.28 (1H, q, J = 0.7 Hz), 5.46 (1H, bs), 6.96 (2H, s)
and 7.26 (1H, s); 13C NMR: d19.0, 21.4, 43.4, 45.6, 114.3, 118.4,
128.1, 130.6, 131.9, 135.3 and 138.5; C15H16N2(224.1313) HRMS:
224.1308.
Substrate
l
CT/nm, ECT/eVa
Eox/V vs. SCEb
DGET/eVc
1a
1b
1c
1d
1e
1f
266, 4.66 (264, 4.70)
270, 4.59 (269, 4.61)
270, 4.59 (270, 4.59)
271, 4.58 (270, 4.59)
273, 4.54 (273, 4.54)
277, 4.48 (278, 4.46)
2.37
2.11
2.14
2.09
1.98
1.63
-0.29
-0.39
-0.34
-0.41
-0.40
-0.61
1-Dicyanomethyl-1,5,7-trimethylindane (2e). Yield: 33%; 1H
NMR:d 1.63 (3H s), 2.17 (1H, ddd, J = 4,7, 7.3, 13.9 Hz), 2.23 (3H,
s), 2.33 (3H, s), 2.39 (1H, ddd, J = 4,7, 7.3, 13.9 Hz), 2.87–2.91 (2H,
m), 3.71 (1H, s), 6.95 (1H, s) and 6.98 (1H, s); 13C NMR: d19.1,
21.4, 24.5, 28.1, 34.1, 37.8, 50.9 112.3, 112.3, 121.1, 131.0, 134.6,
137.5, 138.8 and 143.3; C15H16N2(224.1313) HRMS: 224.1312.
a C–T band maxima (in acetonitrile) determined by deconvolution of
the UV spectra of 1a–1f. The values in parentheses were obtained
in methylcyclohexane. b Oxidation potentials obtained in acetonitrile
using n-Bu4N+PF6 as a supporting electrode. c Free energy change for
-
3,3-Dicyano-2-methyl-4-(2,4-dimethylphenyl)-1-butene
(3e).
1
(intramolecular) electron transfer in the excited singlet state of 1, calculated
by using the Rehm–Weller equation.23 The following values were used in the
estimation of DGET. Ered = -1.69 V (1,1-dicyano-2-methylpropene),24 A =
-0.06 V (acetonitrile). The 0-0 transition energies upon singlet excitation
Yield: 19%; H NMR: d 2.08 (3H, q, J = 0.7 Hz), 2.32 (3H, s),
2.40 (3H, s), 3.32 (2H, s), 5.32 (1H, q, J = 0.7 Hz), 5.51 (1H, bs),
7.03–7.06 (2H, m) and 7.25–7.27 (1H, m); 13C NMR: d19.1, 20.0,
21.2, 39.7, 45.2, 114.5, 118.3, 127.2, 127.6, 130.9, 132.1, 135.7,
137.4 and 138.8; C15H16N2(224.1313) HRMS: 224.1311.
were estimated from those of the C–T band (at the wavelength of eCT
100).
=
3,3-Dicyano-2-methyl-4-(4-methoxyphenyl)-1-butene
(3f).
1
Yield: 7%; H NMR: d 2.04 (3H, q, J = 0.7 Hz), 3.24 (2H, s),
3.82 (2H, s), 5.28 (1H, q, J = 0.7 Hz), 5.43 (1H, bs), 6.90 (2H,
d, J = 8.6 Hz) and 7.28 (2H, d, J = 8.6 Hz); 13C NMR: d 19.0,
42.9, 45.8, 55.4, 114.4, 118.6, 123.9, 129.5, 131.6, 135.0 and 160.1;
C14H14N2O (226.1106) HRMS: 226.1102.
1a–1f. The excitation energy (ECT) of the C–T band was found to
be a linear function of the oxidation potential (Fig. 2a; slope =
0.24, intercept = 4.08, r = 0.99). The fact that the degree of
C–T interaction evaluated from its excitation energy is linearly
proportional to the donor strength of the molecule indicates
insignificant contribution of the steric effects upon ground-state
C–T interaction at least in1a–1f.
Results and discussion
Ground-state donor–acceptor interactions of
4-aryl-1,1-dicyanobutenes
Intramolecular donor–acceptor interactions of 1a–1f in the
ground state were investigated by absorption spectroscopy. The
UV spectra of 1a–1f showed a long-wavelength band, ascribable
to the charge–transfer (C–T) band. This band was not observed
in the spectrum of donor (p-xylene) or acceptor (1,1-dicyano-2-
methylpropene) component of 1b (Fig. 1a). The degree of donor–
acceptor interaction was dependent on the nature of substituent,
and the absorption edge of the C–T band of 1a–1f was gradually
shifted to the red, as the donor strength of the aryl group
increased (Fig. 1b). Table 1 summarizes the excitation energies
of the deconvoluted C–T bands and the oxidation potentials of
Fig. 2 (a) Relationship between the excitation energy of the deconvoluted
C–T band (ECT) and the oxidation potential of 1. (b) Relationship between
the logarithm of 2/3 ratio and ECTupon excitation at 254 nm (blue) and
280 nm (green) in acetonitrile at 20 ◦C. The values for 1e (open circles) are
doubled for statistical corrections. See text.
The solvent effect on the intramolecular C–T interaction was
rather small, as the lCT value was slightly shifted by 0–2 nm, by
changing from polar acetonitrile to non-polar methylcyclohexane.
The opposite shift observed for 1f may indicate possible mixing of
some n–p* character.
The ground-state donor–acceptor interaction of 4-aryl-1,1-
dicyanobutenes arises from the through-space interaction in
folded conformation(s) as well as the through-bond interaction
in extended conformation(s).21 Fig. 3 illustrates the ground-
state geometries of 1b optimized by dispersion-corrected density
functional theory calculations.22 Three conformations were found
stable upon rotation around the C3–C4 bond; i.e., gauche+, gauche-
and trans conformers. The relative energies of these conformers
were 0, 2.5 and 5.4 kJ mol-1, respectively, at the DFT-D-B97-
D/TZV2P level in the gas phase. The cyclization in the excited
state appears to be possible only from the gauche (or related
Fig. 1 (a) UV spectral examinations of donor–acceptor interaction
of 1b; black: spectrum of 1b, blue: spectral sum of p-xylene and
1,1-dicyano-2-methylpropene, red: difference spectrum (black - blue). All
experimental spectra were obtained for 0.1 mM solution in acetonitrile at
20 ◦C. (b) UV spectra of 0.1 mM of 1a–1f in acetonitrile at 20 ◦C. black:
1a, blue: 1b, green: 1d, yellow: 1c, pink: 1e, red: 1f. Inset: expansion of the
C–T band region.
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indicating the formation of a highly polarized excited species
stabilized in polar acetonitrile. The fluorescence quantum yield
of 1b in acetonitrile was determined as 0.007, by using p-xylene as
a reference.
(b) Effects of C–T interaction on product selectivity. The
local excitation of non-substituted 1a at 254 nm in acetonitrile
at 20 ◦C afforded rearranged 3a as the major product, together
with a smaller amount of cyclized 2a, the 2/3 ratio being 0.5
(Table 2).17 In contrast, the substrates of stronger C–T character
predominantly photocyclized under the same conditions. For
instance, the photoirradiation of methyl- and hexylphenyl deriva-
tives 1b and 1c afforded the corresponding cyclization products
with 2/3 ratios of 3.0 and 3.1, respectively. Despite the increased
C–T character, 2,4-dimethylphenyl derivative 1e afforded a lower
2/3 ratio of 2.6, which however should be doubled, as one of
the ortho positions to be attacked upon cyclization is blocked
by the methyl substituent. Indeed, the logarithm of 2/3 ratio
plotted against the C–T excitation energy (ECT) gave a linear
relationship, as shown in Fig. 2b (where the values for 1e were
statistically corrected (doubled) for the number of available ortho
position). The nice linear plot indicates that the rearrangement and
cyclization are competing (trade-off) processes and the cyclization
is accelerated as the intramolecular C–T interaction becomes
stronger. Unfortunately, 3,5-dimethylphenyl derivative 1d did
not afford any cyclization product, probably due to the steric
hindrance of the methyl groups at adjacent meta positions. The
failure of 1f to give the cyclization product would be ascribed to
the efficient back electron transfer in the geminate radical ion pair
generated upon photoexcitation, formation of which is supported
by the highly negative free energy change (Table 1).
Fig. 3 Optimized geometries of three most stableconformations of 1b
optimized at the DFT-D-B97-D/TZV2P level.
syn-periplanar) conformations, as the bond-forming carbon–
carbon distance is not suitable in the trans (anti-periplanar)
conformation.
Photochemistry of 4-aryl-1,1-dicyanobutenes
(a) Effects of irradiation period on product selectivity. Pho-
toreaction of 4-aryl-1,1-dicyanobutenes (1) affords cyclization and
rearrangement products, as shown in Scheme 1. The photochem-
istry of 4-phenyl-1,1-dicyanobutene 1a was extensively investi-
gated by Cookson et al. in early 1980s.17 Although the pronounced
wavelength dependence was observed for the product ratio upon
high-energy excitation in the photoreaction of 1a, the irradiation
at short wavelengths may evoke secondary photoreaction(s) of the
primary products.
Hence, we first examined the time course of the product
distribution in the photoreaction of 1b at two different excitation
wavelengths (Fig. 4). The formation of both 2b and 3b gradually
became slower as the reaction proceeded (simply due to the
internal filter effect of the photoproducts), but no apparent
formation of byproducts was observed on GC even after 2 h of
irradiation at 254 and 280 nm. The 2/3 ratio was virtually kept
constant at 2.8–3.1 upon irradiation at 254 nm and at 14–17 at
280 nm at least for up to 30 min. Thereafter, the photoreaction
was gradually slowed down and the ratio appreciably deviated
from the initial value upon prolonged irradiations. We therefore
employ the data at conversions lower than 15% (typically 6–10 min
irradiation) in the following discussion to avoid any complexities,
except for the photoreaction in polyethylene films, where the
cyclization was observed only after prolonged irradiations.
(c) Effects of solvent polarity on product selectivity. In nonpo-
lar methylcyclohexane, the photorearrangement was the dominant
pathway, irrespective of the substituent introduced. The reaction
of 1b was examined also in dichloromethane and diethyl ether,
but only a small amount of 2b (cyclization) was obtained.
However, the formation of cyclization product became substantial
in polar acetonitrile (Table 2). The possible reaction mechanism for
photoreaction of 4-aryl-1,1-dicyanobutenes is shown in Scheme 2.
The electron transfer (ET) from the aromatic donor of 1 to the
dicyanoethylene acceptor is initiated by the photoexcitation of
1. The intramolecular electron transfer is energetically feasible
in acetonitrile, as judged from the negative free energy change
(DGET) for this process (Table 1). The ET process is prohibited in
non-polar solvent due to the lack of effective solvation. Instead,
another possible reaction channel, i.e., the rearrangement process
starts to occur, as this sigmatropic benzyl shift is a concerted
process and does not involve any polar intermediate.25 Although
the formation of 3 is claimed to occur through the extended (trans)
conformer, we do not rule out the possibility of its formation from
the other conformers.
Fig. 4 Yields of 2b (open circles) and 3b (closed circles) upon irradiation
of 1b at (a) 254 or (b) 280 nm in acetonitrile at 20 ◦C.
(d) Effects of excitation wavelength on product selectivity. The
cyclization/rearrangement (2/3) ratio was a critical function of the
irradiation wavelength and significantly increased upon excitation
at longer wavelengths. For example, the 2/3 ratio was enhanced
from 0.5 to 6.3 in the case of 1a by changing the irradiation
The product distribution was not affected very much upon
irradiation in the presence of air (2/3 = 2.9 under air or 3.0 under
nitrogen, with 254 nm in acetonitrile at 20 ◦C for 1b), supporting
that the reaction proceeds through the singlet manifold. A weak
broad fluorescence was observed for 1b in acetonitrile at long
wavelengths (lmax = 434 nm) with a large Stokes shift of 14 000 cm-1,
◦
wavelength from 254 to 290 nm (in acetonitrile at 20 C). Thus,
an exclusive formation of cyclization product 2b was achieved in
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©

Table 2 Product distributions in the photoreaction of 1a–1fa
Substrate
Solvent
CH3CN
Temperature/◦C
20
lex/nmb
t/minc
Conversion (%)
Yield of 2 (%)
Yield of 3 (%)
2/3 ratio
0.5 (0.5)f,g
1a
254
280
290
300
254
280
254
280
16
50
180
240
16
50
6
7
6
1.1
1.7
1.9
2.1
0.5
0.3
3.4
6
6.3 (8.1)f
e
-40
6
5
9
7
1.6
1.7
< 0.01
< 0.01
1.2
0.2
5.6
2.1
1.3
8.5
< 0.01
MCHd
20
40
< 0.01
1b
CH3CN
20
254
280
300
254
280
254
280
254
280
254
280
254
280
254
280
8
8
60
8
8
8
8
8
8
10
10
6
20
6
6
7
8
6
6
6
6
6
6
4
3
4
3
9
4
4.5
3.0
5.0
3.3
1.6
3.5
1.7
3.7
1.8
< 0.01
< 0.01
< 0.01
0.3
0.6
0.1
1.5
0.2
< 0.01
0.7
0.07
0.6
0.05
0.5
0.04
2.3
0.5
1.4
0.9
3.0
15
> 50
4.7
23
5.8
34
7.4
45
< 0.01
< 0.02
< 0.01
0.3
0.1
0.1
0
-20
-40
20
CH2Cl2
Et2O
20
MCHd
20
6.2
1.4
6
1c
CH3CN
20
254
280
300
254
280
254
280
8
8
60
8
8
6
5
6
8
5
5
9
4
2.8
2.4
5.1
2.9
1.0
0.6
0.1
0.9
0.1
< 0.01
0.5
0.03
6.5
1.2
3.1
24
> 50
5.8
33
0.1
-40
MCHd
CH3CN
MCHd
CH3CN
MCHd
CH3CN
MCHd
20
6
0.1
1d
1e
1f
20
20
20
20
20
20
254
280
300
254
280
60
120
240
6
11
< 0.01
< 0.01
9.5
1.0
< 0.01
< 0.01
6
e
2
2
< 0.01
< 0.01
0.4
0.2
< 0.01
< 0.01
20
254
280
300
254
280
8
8
60
6
4
7
9
10
6
2.1
2.5
6.9
< 0.01
< 0.01
0.8
0.2
< 0.01
3.5
2.6
13
> 50
< 0.01
< 0.01
6
1.9
254
280
300
254
280
60
120
240
10
14
< 0.01
1.9
0.9
< 0.01
e
e
2
< 0.01
< 0.01
e
120
a Solutions of 1a–1f (0.1 mM) were irradiated at 254, 280, 290 or 300 nm under nitrogen. b Excitation wavelength. c Irradiation period. d MCH =
methylcyclohexane. e Not detected. f Ref. 17. g Irradiated at 250 nm.
acetonitrile upon excitation of 1b at the C–T band edge (300 nm).
Considering the fact that both folded (gauche) and extended
(trans) conformations are possible for 4-aryl-1,1-dicyanobutenes
(vide supra), we may conclude that the folded conformer is
selectively excited when irradiated at longer wavelengths to give
the cyclization product (Scheme 2). The theoretical calculations for
the electronic transition of these three conformers of 1b at the TD-
DFT-BH-LYP/TZV2P level26 revealed that the gauche conformers
possess the C–T band at longer wavelengths than the trans
conformer, i.e., 250 and 263 nm for g conformations and 249 nm
for t conformation. The upward-shifted linear ln (2/3) versus ECT
plot for the 280 nm excitation, compared to the 254 nm excitation
(Fig. 2b), supports this conclusion, indicating the higher popula-
tion of folded C–T conformer absorbing at the longer wavelength.
(e) Effects of temperature on product selectivity. The tem-
perature effects on chemo-, regio- and stereoselectivity have been
investigated in a wide variety of photochemical reactions.2 We also
examined the temperature effects on the photoreaction of 1a–1c.
In all the cases examined, the 2/3 ratios increased with decreasing
temperature (Table 2). Somewhat unexpectedly, the temperature
effect on 2/3 ratio was appreciably smaller for hexyl derivative 1c
than for methyl derivative 1b, possibly due to the greater freedoms
of 1c that make the folded structure less favorable.
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©

kinetics of a (photo)chemical process is less common, applying
static pressure is known to alter the chemo- and stereoselectivity,
provided that an appreciable difference exists in activation and/or
reaction volume between the relevant competing processes.27
Indeed, the 2/3 ratio was considerably increased by applying
pressure of up to 400 MPa to the photoreaction of 1a–1c (Table 3).
Thus, the plots of ln (2b/3b) against pressure gave good straight
lines for the photoreaction of 1b at 254 and 280 nm, as shown in
Fig. 5b. From the slopes of the plots, the differential activation
volumes were determined as DDV‡2/3 = -8.7 and -5.1 cm3 mol-1for
254 and 280 nm excitation, respectively. These values are at
least one order of magnitude larger than those found in the
intermolecular donor–acceptor system.11 The negative differential
activation volumes indicate that the transition state for 2b is more
compact than that for 3b, which may be consistent with the smaller
volume of 2b than that of 3b (the van der Waals volumes were
calculated for 1b, 2b, and 3b as 128, 125, and 128 cm3 mol-1,
respectively).
Scheme 2 Mechanism for photoreaction of 4-aryl-1,1-dicyanobutenes.
(g) Photoreactions in polyethylene films. The confinement
effect on photoreaction has been extensively explored. Among
various supramolecular systems used for that purpose, polymer
matrices are rather unique, putting a passive pressure on the
substrate accommodated in polymer void, and hence frequently
utilized for controlling photochemical behavior.20 The nature of
polymer, such as crystallinity and branching, affects the passive
pressure and therefore the photochemical outcomes. Polyethylene
films are particularly intriguing in their templating effect, as the
cavity shape is preserved for a certain period of time during the
course of reaction. Hence, we investigated the photoreaction of 1a–
1c in selected polyethylene films of different crystallinities (PE0–
PE74) (Table 4). The photolysis of 1a or 1c in both PE0 and PE46
matrices afforded only the rearrangement product, probably due
to the nonpolar nature of the matrices. Quite interestingly, the
photoreaction of 1b in PE0 afforded a much larger amount of
cyclization product, the 2/3 ratio being 0.1 and 0.7 upon local
(254 nm) and C–T (280 nm) excitations, respectively. This behavior
is quite a contrast to the fact that the photochemical behavior
of 1b and 1c (i.e., methyl versus hexyl) resembles to each other
in solution phase. This may be explained by assuming that the
folded (gauche) conformations are slightly more favored in this
polymer matrix for 1b than for 1c (with a lengthy hexyl chain),
leading to the increased formation of cyclization product. Thus, it
is shown that the cavity of polyethylene film is sensitive to the shape
of substrate, right matching of which is essential for controlling
the product selectivity in polyethylene matrices. However, further
detailed studies of the photoreaction of 1 in polymer matrices was
not feasible, as the material balance was rather low in these media,
accompanying formation of considerable amounts of unidentified
products.
The product ratios obtained in the variable-temperature pho-
toreactions of 1b at both 254 and 280 nm (Fig. 5a) were subjected
to the Eyring analysis. Thus, the ln (2b/3b) values were plotted
against the reciprocal temperature to afford a good straight line
in each case, indicating that the product selectivity is determined
in a single step (at least in the temperature range examined). The
differential activation enthalpy (DDH‡) and entropy (TDDS‡; T =
293 K) were calculated from the slope and intercept of the plot as
-8.0 and -5.2 kJ mol-1 for the local excitation at 254 nm and -9.5
and -2.5 kJ mol-1 for the C–T excitation at 280 nm, respectively,
indicating that the enthalpic factors are more favorable for
cyclization in both excitation modes. We presume that the ground-
state equilibrium (K), rather than that in the excited state, plays the
critical roles in determining the product ratio and it temperature
dependence, although the other possibilities cannot be rigorously
ruled out and remain to be elucidated (Scheme 2). In any case, the
combined use of excitation wavelength and temperature enabled
us to further enhance the product selectivity. Indeed, the 2/3 ratio
obtained in the photoreaction of 1b dramatically leaped from
3.0 upon local excitation (254 nm) at 20 ◦C up to 45 upon C–
T excitation (280 nm) at -40 ◦C.
Conclusion
Fig. 5 (a) Temperature and (b) pressure dependence of the 2b/3b ratio
obtained upon irradiation of 1b in acetonitrile at 254 (blue) and 280 nm
(green).
In this study, we investigated the photochemistry of charge-
transfer 4-aryl-1,1-dicyanobutenes under a variety of con-
ditions to demonstrate that the product selectivity, cycliza-
tion/rearrangement ratio, can be critically controlled from practi-
cally zero to over 50 by manipulating solvent polarity, temperature,
pressure, and excitation wavelength.
(f) Effects of pressure on the product selectivity. Although
using pressure as a tool for modifying thermodynamics and
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©

Table 3 Effect of pressure on photoreaction of 1a–1ca
Substrate
lex/nmb
254
t/minc
Pressure/MPa
Conversion (%)
Yield of 2 (%)
Yield of 3 (%)
2/3 ratio
1a
16
16
50
50
0.1
400
0.1
7
7
5
6
1.3
2.3
0.5
1.6
2.3
1.0
0.2
0.2
0.6
2.3
2.5
8.0
280
400
1b
1c
254
280
8
8
8
0.1
200
300
400
0.1
200
300
400
9
9
9
7
7
7
12
11
2.6
3.1
4.4
4.7
3.7
6.3
8.9
8.9
0.9
0.4
0.5
0.4
0.3
0.3
0.3
0.3
2.9
7.8
8.8
12
12
21
8
30
30
30
30
30
30
254
280
8
8
30
30
0.1
400
0.1
8
8
7
2.4
3.9
3.9
3.6
0.9
0.5
0.2
0.1
2.7
7.9
20
400
10
36
a Irradiations were performed at 254 or 280 nm under air in a high-pressure vessel. b Excitation wavelength. c Irradiation time.
Table 4 Photoreaction of 1a–1c in polyethylene filmsa
Substrate
Media
PE0
lex/nmb
Conversion (%)
Yield of 2 (%)
Yield of 3 (%)
2/3 ratio
1a
254
280
254
280
61
52
28
64
< 0.1
< 0.1
< 0.1
< 0.1
9.8
35.7
19.2
13.0
< 0.01
< 0.01
< 0.01
< 0.01
PE46
1b
PE0
254
280
254
280
254
280
254
280
254
280
61
74
70
62
20
18
63
80
52
71
1.6
7.3
11.6
10.3
4.4
12.1
18.0
18.0
2.0
11.2
14.7
21.2
0.1
0.7
PE46
PE50
PE68
PE74
< 0.1
< 0.1
< 0.1
< 0.1
< 0.1
< 0.1
< 0.1
< 0.1
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
1c
PE0
254
280
254
280
23
56
77
86
< 0.1
< 0.1
< 0.1
< 0.1
13.1
26.2
9.3
< 0.01
< 0.01
< 0.01
< 0.01
PE46
11.7
a Irradiation of 1a–1c was performed in polyethylene films at 254 (2 h) or 280 nm (4 h) and at ambient temperature (23–25 ◦C). Numbers in the acronyms
are the % crystallinities from X-ray diffraction measurement. For details of polyethylene films, see ref. 20. b Excitation wavelength.
The cyclization was favored in polar solvents at lower temper-
atures and upon selective C–T excitation at longer wavelengths.
Mechanistically, the cyclization involves the initial electron trans-
fer from the donor to acceptor moiety and the subsequent
combination of the resulting radical ion pair in the folded (gauche)
conformation to give a zwitterion, which eventually leads to
the product. Such a process is accelerated in polar solvents as
the electron transfer becomes energetically more feasible. The
folded conformers with stronger C–T character absorb at longer
wavelengths and can be selectively excited, and the subsequent
cyclization reaction is much faster than the relaxation in the excited
state. Changing the temperature should shift the equilibrium in
both ground and excited states. However, the linear relationships
found upon the Eyring treatment of the data obtained at 254
and 280 nm revealed that the ground-state equilibrium plays a
major role in determining the product selectivities. The static and
passive pressures can also be used as independent environmental
variants for altering the product selectivity. It is more important
that the cyclization/rearrangement ratio strongly depends on
the magnitude of C–T interaction and its logarithmic value is
proportional to the C–T excitation energy. Thus, the quantitative
adjustment of C–T interaction can be used as an additional
tool for precisely controlling the photochemical outcomes in
donor–acceptor systems. The combined use of multiple controlling
factors, such as solvent, temperature, pressure, wavelength, and
C–T strength, is a convenient, effective method to improve the
product selectivity. Accordingly, the exclusive cyclization (cf. the
2/3 ratio being >50 with 300 nm at 20 ◦C in acetonitrile or 45 with
280 nm at -40 ◦C) or rearrangement (cf. 2/3 ratio being <0.01 in
polymer matrices and in most of non-polar solvents) was achieved
1412 | Photochem. Photobiol. Sci., 2011, 10, 1405–1414 This journal is
The Royal Society of Chemistry and Owner Societies 2011
©

just by tuning the reaction conditions. This strategy is currently
being applied to a wide variety of asymmetric photoreactions both
in solution and in supramolecular systems, and the results will be
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Acknowledgements
Financial supports by Grant-in-Aid for Scientific Research, Japan
Society for the Promotion of Science, Mitsubishi Chemical
Cooperation Fund, and Sumitomo Foundation are gratefully
acknowledged. We thank Prof. Richard G. Weiss for providing
us the polyethylene films.
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©