Photo-ring-opening efficiency of 2,2-diphenyl-2H-1-benzopyran evaluated from addition reactivity of amines to its ring-opened isomers

Article abstract of DOI:10.1246/bcsj.81.641

Irradiation of 2,2-diphenyl-2H-l-benzopyran (DPBP) in the presence of a primary or a secondary amine gave an addition product. With a large excess of amine, the apparent quantum yield for product formation reached 0.5. This value corresponds to a minimal quantum yield for formation of DPBP colored ring-opened forms.

Full text of DOI:10.1246/bcsj.81.641

Short Articles
Bull. Chem. Soc. Jpn. Vol. 81, No. 5, 641–643 (2008)
641
Photo-Ring-Opening Efficiency
of 2,2-Diphenyl-2H-1-benzopyran
Evaluated from Addition
Reactivity of Amines
Before irradiation
0.2
After irradiation
0.1
to Its Ring-Opened Isomers
ꢀ1
Masako Sakuragi, Yuji Kawanishi,1
Yasuzo Suzuki,1 and Hirochika Sakuragi2
0.0
200
300
400
500
600
Wavelength/nm
1Nanotechnology Research Institute, National Institute
of Advanced Industrial Science and Technology,
1-1-1 Higashi, Tsukuba 305-8565
Figure 1. Absorption spectra observed before and after
irradiation of a cyclohexane solution of DPBP at 313 nm.
2Department of Chemistry, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-6571
O
OH
O
Ph
Ph
h
ν
H2NR
Ph
Ph
NH Ph
R
Received October 19, 2007; E-mail: m-sakuragi@aist.go.jp
DPBP
Ph
Scheme 1.
Irradiation of 2,2-diphenyl-2H-1-benzopyran (DPBP)
in the presence of a primary or a secondary amine gave an
addition product. With a large excess of amine, the apparent
quantum yield for product formation reached 0.5. This value
corresponds to a minimal quantum yield for formation of
DPBP colored ring-opened forms.
under the same conditions, the color was not changed at all
upon irradiation, however. Evaporation of the amine and sol-
vent from the reaction mixture gave a colorless oily product,
which was proved to be a 1,4-adduct of the DPBP ring-opened
form, 1,1-diphenyl-3-(2-hydroxyphenyl)-3-butylamino-1-pro-
pene by precise-mass (m=e 357.2123, C25H27NO) and proton
NMR measurements (Scheme 1). The NMR spectrum
displayed signals due to butyl-group protons, a methyne
(ꢀ 4.45, d), and an ethylenic proton (ꢀ 6.30, d) instead of the
ethylenic protons of the pyran ring (ꢀ 6.20 and 6.58, d) of
the starting DPBP. The signals due to the methyne and ethyl-
enic protons are quite similar to those of the 1,4-adduct of
DPBP and methanol reported by Padwa et al.4 Propylamine,
diethylamine, morpholine, piperidine, and 2-ethanolamine
were also found to add to DPBP upon irradiation in cyclohex-
ane and 1,4-dioxane under similar conditions. The adducts
were formed almost quantitatively. Their molecular structures
were proved to be similar to that of the butylamine adduct by
comparing their NMR spectra. The 1,4-addition of amines was
confirmed to be a thermal process by allowing an amine to re-
act with DPBP after UV-irradiation. Actually, an acetonitrile
solution of DPBP was irradiated at 313 nm for 30 s and mor-
pholine was put into the reaction pot immediately after irradi-
ation. The formation of the morpholine adduct was established
by HPLC analysis.
2H-1-Benzopyran is a core unit of well-known photochro-
mic compounds such as spiropyrans.1 On irradiation of 2,2-
diphenyl-2H-1-benzopyran (DPBP), as an example, with UV
light in solution, the colorless solution becomes pink. The
structure of this colored photoproduct was proved to be a cy-
clohexadienone with a butadiene unit produced by opening
of the pyran ring.2 We reported dynamic behavior of the pri-
mary products of 2H-1-benzopyrans in the picosecond to milli-
second time domain.3 Methyl- or phenyl-substituted 2H-1-ben-
zopyrans were reported to suffer methanol addition under illu-
mination, and the adduct formation was attributed to a thermal
process of the ring-opened forms of 2H-1-benzopyrans.4
Recently, photoaffinity labeling of cytochrome P450 3A4 by
lapachenole, a naturally occurring 2H-naphthopyran, was re-
ported.5 This is a trapping of cysteine residues (RSH) by the
primary photoproduct of the 2H-naphthopyran. We have inves-
tigated photochemical behavior of DPBP in the presence of
amines, and have found that primary and secondary amines
add effectively to the ring-opened form of DPBP. This reaction
was applied to photo-initiated surface grafting methods on
polymer film.6 In the present work, the rates and efficiencies
were measured for the addition of various amines to the
ring-opened form of DPBP, and the quantum yield was esti-
mated for the ring opening of DPBP.
In order to ascertain this mechanism and to clarify the reac-
tion process, decay behavior of the ring-opened form was in-
vestigated in the absence and presence of various amines
and methanol. After irradiation of DPBP in solution for 60 s
at 313 nm, disappearance of the ring-opened form was fol-
lowed by measuring its absorbance at 410 nm at 20 ꢁC. From
the absorbance change, the first-order decay rate constants
of the DPBP ring-opened form were determined, as listed in
Table 1. The decay rate constants are smaller in hetero-
atom-containing solvents rather than in hydrocarbon solvents.
On irradiation of a cyclohexane solution of DPBP at
313 nm, the color of the solution varied from initially colorless
to pink; main absorption bands appeared around 410 and
455 nm (Figure 1). In the presence of butylamine, otherwise
Published on the web May 14, 2008; doi:10.1246/bcsj.81.641

642 Bull. Chem. Soc. Jpn. Vol. 81, No. 5 (2008)
ꢀ 2008 The Chemical Society of Japan
Table 1. Rate Constants for Decay of the DPBP Ring-
Opened Form at 20 ꢁC
Table 3. Apparent Quantum Yields for Addition of Amines
(A) to DPBP
Solvent
Rate constant/sꢂ1
Lifetime/h
Apparent quantum yield
Amine
Cyclohexane
Hexane
Ethyl acetate
1,4-Dioxane
Acetonitrile
Tetrahydrofuran
8:9 ꢃ 10ꢂ4
3:5 ꢃ 10ꢂ4
1:2 ꢃ 10ꢂ4
3:8 ꢃ 10ꢂ5
2:8 ꢃ 10ꢂ5
2:6 ꢃ 10ꢂ5
0.31
0.79
2.3
7.3
9.9
In 1,4-dioxane In cyclohexane In cyclohexane
[A] = 0.2 M
[A] = 2 mM
[A] = 2 mM
Piperidine
0.45
0.30
0.13
0.08
0.30
0.18
0.10
0.04
0.50
0.51
0.47
—
Morpholine
Diethylamine
Butylamine
11
Table 2. Rate Constants for Addition of Amines and Meth-
anol to the DPBP Ring-Opened Form at 20 ꢁC
0.5
0.4
0.3
0.2
0.1
Rate constant/Mꢂ1 sꢂ1
Addend
pKb
In 1,4-dioxane
In cyclohexane
Piperidine
2200
960
480
360
3
1300
480
550
90
2.88
5.30
3.07
3.40
—
Morpholine
Diethylamine
Butylamine
Methanol
16
In 1,4-dioxane
In cyclohexane
0.0
0
O
O
Ph
500
1000
1500
2000
2500
O
O
Ph
Rate Constant/M-1s-1
Ph
Ph
Ph
Ph
Ph
Figure 2. Relationship between the apparent quantum
yield and the addition rate constant.
ZZ
ZE
EZ
EE
Ph
Scheme 2.
perimental), the addition of amines to the DPBP ring-opened
form is more exothermic and lower in activation energy than
the methanol addition.
In the presence of amines the decay rate of the DPBP ring-
opened form was very much accelerated, depending upon the
species and concentrations. After short-period irradiation of
DPBP in solution at 313 nm, an amine solution was added,
and the spectral change of the irradiated solution was followed
at 410 nm at 20 ꢁC. The second-order rate constants were cal-
culated by analyzing the initial 10% decrease in absorbance as
a reaction of the ring-opened form with amines or methanol.
Concentrations of the DPBP ring-opened form were calculated
from its molar absorption coefficient (") (see Experimental).
The rate constants for quenching of the DPBP ring-opened
form by typical amines and methanol were determined in
1,4-dioxane and cyclohexane, as listed in Table 2, where
pKb values of the amines are also listed. The ring-opened
forms of DPBP consist of four isomers (Scheme 2).3 The spe-
cies with a lifetime in the minute-to-hour range can be as-
signed to the EE isomer in analogy with 3,3-diphenyl-3H-
naphtho[2,1-b]pyran.7 The rate constants can be ascribed to
those for addition of the amines to the EE isomer of the DPBP
ring-opened forms. Table 2 shows that the addition of amines
is faster in 1,4-dioxane than in cyclohexane. Plot of logarithms
of the rate constants against pKb gave a straight line except for
morpholine, showing that the more basic, the more reactive the
amines. These facts indicate that the quenching of the ring-
opened form is a nucleophilic addition of amines to the enone
skeleton. Table 2 also shows that the amines are much more
reactive with the ring-opened form than methanol. Taking into
account that the methanol adduct thermally reverts more easily
to the original components than do the amine adducts (see Ex-
The photochemical addition of amines to DPBP has been
shown not to be a simple photochemical reaction but to be a
photochemical and thermal two-step reaction. However, it is
important to determine photochemical efficiency (apparent
quantum yield) of the reaction, since this reaction is initiated
with absorption of photons by DPBP. As an example, a solu-
tion of DPBP and morpholine in 1,4-dioxane or cyclohexane
was irradiated at 313 nm, and the formed adduct and the re-
maining DPBP were analyzed quantitatively with HPLC at
low conversion (within 10%) of DPBP. The amount of the
remaining DPBP was also determined by spectral change at
333 nm since the absorbance at this wavelength is exclusively
due to DPBP. The apparent quantum yields (ꢀa) for adduct
formation in the presence of 2 mM of amines are dependent
upon the amines and higher in 1,4-dioxane than in cyclohex-
ane, as listed in Table 3. It is noticeable that the amine and sol-
vent dependence of ꢀa is quite similar to that observed in the
addition rate measurements. In Figure 2 is depicted the rela-
tionship between ꢀa and the rate constant. Figure 2 shows that
ꢀa is well correlated with the addition rate constant; the effi-
ciency of adduct formation is controlled by the addition rate.
In the presence of 0.2 M of amines, ꢀa’s were higher than
those in the presence of 2 mM of amines, and reached approx-
imately 0.5 independently of the amines employed. Compari-
son of the decay and quenching data in Tables 1 and 2 indi-
cates that the observed ring-opened form is almost completely
quenched by the 0.2 M amines. Consequently, ꢀa of 0.5 corre-

M. Sakuragi et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 5 (2008)
643
Quenching Efficiency of the DPBP Ring-Opened Form by
Amines. Immediately after irradiation of a solution (2 mL) of
DPBP (1 ꢃ 10ꢂ4 M) in 1,4-dioxane or cyclohexane for 60 s at
20 ꢁC by a 500-W high-pressure mercury lamp through U-340
glass filter, 10 mL of an amine (1 ꢃ 10ꢂ2 M) solution was added
to this solution, and the decrease in absorbance at 410 nm was
followed at 20 ꢁC.
sponds to the quantum yield for formation of the ring-opened
EE form, the most long-lived isomer. Also, this value is as-
sumed to be a minimal quantum yield for photocleavage of
DPBP, which generates the short-lived EZ and other forms
as well as the EE form. To the best of our knowledge, this
is the first report on the quantum yield for ring opening of
2H-1-benzopyran analogs.
Apparent Quantum Yields for Reaction of DPBP and
Amines. A solution (2 mL) of DPBP (2 ꢃ 10ꢂ3 M) and morpho-
line (0.2 M) in cyclohexane was placed in a quartz cell, deaerated
by bubbling with N2 for 15 min, and irradiated for 30 s by a 500-W
high-pressure mercury lamp through Kenko UV-30 and U-340
glass filter. The number of photons absorbed by DPBP was deter-
mined by using fulgide in toluene as chemical actinometer.8 The
concentrations of the adduct and the remaining DPBP were deter-
mined by HPLC through a Merck LiChrosorb Si60 column eluting
with ethanol–hexane (2:8 by volume).
A solution (2 mL) of DPBP (2 ꢃ 10ꢂ3 M) and an amine
(2 ꢃ 10ꢂ3 M) was irradiated for 35 s under similar conditions.
The absorbance of the solution at 333 nm, where "’s of DPBP
are 500 cm2 mmolꢂ1 in cyclohexane and 350 cm2 mmolꢂ1 in
1,4-dioxane, was measured every 5 s.
Molar Absorption Coefficient of the DPBP Ring-Opened
Form. A solution of DPBP (1 ꢃ 10ꢂ4 M) in acetonitrile was ir-
radiated for 30 s by a 500-W high-pressure mercury lamp through
U-340 glass filter. Immediately after measurement of the UV–vis
absorption spectrum of the solution, 5 mL of morpholine was add-
ed to this solution. The concentrations of DPBP before and after
these operations were determined by HPLC through a Merck
LiChrosorb RP-18 column eluting with acetonitrile–water (95:5
by volume). The molar absorption coefficient of the DPBP ring-
opened form was estimated to be 6600 cm2 mmolꢂ1 at 410 nm
in acetonitrile by assuming that all the ring-opened form observed
with the absorption spectrum reacted with morpholine added.
Thermal Stability of the DPBP-Amine Adducts. Proton
NMR spectra of some adducts were followed at 50 ꢁC in CDCl3.
The signals due to the amine adducts gradually decreased in inten-
sity and those due to DPBP increased. A half amount of the dieth-
ylamine adduct reverted to DPBP in 40 h, a 25% amount of butyl-
amine adduct in 200 h, and a less than 10% amount of morpholine
adduct in 600 h.
Experimental
Melting points are uncorrected. UV–vis absorption spectra
were measured with a JASCO U-560 spectrophotometer. Proton
NMR spectra were recorded with a JEOL JNM-GSX270N
(270 MHz) spectrometer. Precise-mass and ESI-mass spectra were
measured with a Hitachi M80B and a Thermo Quest Finnigan
AQA mass spectrometer, respectively.
Photoaddition of Amines. A solution of 23 mg of DPBP and
1 mL of morpholine in 30 mL of cyclohexane in a quartz cylindri-
cal cell was irradiated for 20 min under nitrogen atmosphere using
a 500-W high-pressure mercury lamp with Kenko U-340 glass fil-
ter. These irradiation operations were performed two times. The
reaction mixtures were combined, and the solvent and morpholine
were removed under reduced pressure. The resulting oil was chro-
matographed on 5 g of silica gel using hexane–ethyl acetate (4:1
by volume) as eluent. Removal of the solvent left 62 mg of white
residue, which was crystallized from hexane to give 36 mg of
a white solid, whose structure was assigned as 1,1-diphenyl-3-
(2-hydroxyphenyl)-3-morpholino-1-propene on the basis of its
characteristic spectra; mp 55–56 ꢁC; 1H NMR (CDCl3): ꢀ 2.44–
2.57 (m, 2H), 2.6–2.8 (broad s, 2H), 3.6–3.8 (m, 4H), 4.22 (d,
1H, J ¼ 10:5 Hz), 6.37 (d, 1H, J ¼ 10:5 Hz), 6.74–6.92 (m, 3H),
7.1–7.3 (m, 8H), 7.4–7.5 (m, 3H); precise-MS: m=e found,
371.1892; calcd for C25H25NO2, 371.1885.
Photoaddition of other amines to DPBP were conducted under
similar conditions. Piperidine adduct: mp 129–131 ꢁC; 1H NMR
(CDCl3): ꢀ 1.40–1.70 (m, 6H), 2.30–2.60 (m, 4H), 4.28 (d, 1H,
J ¼ 10:0 Hz), 6.38 (d, 1H, J ¼ 10:0 Hz), 6.70–6.85 (m, 2H),
6.93 (d, 1H, J ¼ 8:7 Hz), 7.10–7.20 (m, 3H), 7.20–7.30 (m, 5H),
7.30–7.45 (m, 3H); precise-MS: m=e found, 369.1992; calcd
1
for C26H27NO, 369.2093. Butylamine adduct: H NMR (CDCl3):
ꢀ 0.87 (t, 3H, J ¼ 7:3 Hz), 1.20–1.45 (m, 4H), 2.45–2.65 (m,
2H), 4.45 (d, 1H, J ¼ 10:0 Hz), 6.30 (d, 1H, J ¼ 10:0 Hz),
6.70–6.92 (m, 3H), 7.10–7.25 (m, 8H), 7.32–7.45 (m, 3H); pre-
cise-MS: m=e found, 357.2123; calcd for C25H27NO, 357.2093.
Propylamine adduct: 1H NMR (CDCl3): ꢀ 0.87 (t, 3H, J ¼
7:6 Hz), 1.40–1.50 (m, 2H), 2.40–2.60 (m, 2H), 4.45 (d, 1H, J ¼
10:0 Hz), 6.29 (d, 1H, J ¼ 10:0 Hz), 6.70–6.90 (m, 3H), 7.10–7.25
(m, 8H), 7.35–7.45 (m, 3H); ESI-MS: m=e found, 344.2; calcd
for [C24H25NO + H]þ, 344.2. Diethylamine adduct: 1H NMR
(CDCl3): ꢀ 0.92 (t, 6H, J ¼ 6:6 Hz), 2.45–2.60 (m, 2H), 2.70–
2.85 (m, 2H), 4.79 (d, 1H, J ¼ 10:5 Hz), 6.29 (d, 1H, J ¼
10:5 Hz), 6.75–6.85 (m, 2H), 7.10–7.35 (m, 12H); ESI-MS: m=e
found, 358.2; calcd for [C25H27NO + H]þ, 358.2. 2-Ethanol-
amine adduct: 1H NMR (CDCl3): ꢀ 2.70–2.90 (m, 2H), 3.50–
3.70 (m, 2H), 4.47 (d, 1H, J ¼ 10:0 Hz), 6.32 (d, 1H, J ¼ 10:0
Hz), 6.70–6.95 (m, 3H), 7.10–7.30 (m, 8H), 7.35–7.45 (m, 3H);
ESI-MS: m=e found, 368.2; calcd for [C23H23NO2 + Na]þ, 368.2.
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