Temperature dependence of photosensitized nucleophilic addition to alkenes

Article abstract of DOI:10.1246/bcsj.20110149

Photosensitized nucleophilic additions of 2,5-dimethyl- 2,4-hexadiene and 2,3-dimethyl-2-butene were performed in a microchannel reactor using phenanthrene and p-dicyanobenzene as a redox pair. The quantum yields for the formation of photo-NOCAS products were remarkably enhanced at lower temperatures. The exciplex formation disturbed the reaction at higher temperatures.

Full text of DOI:10.1246/bcsj.20110149

1130 Bull. Chem. Soc. Jpn. Vol. 84, No. 10, 1130-1132 (2011)
Short Articles
Nu
Nu
h
ν
HNu
−H+
PH
Temperature Dependence of
Photosensitized Nucleophilic
Addition to Alkenes
1PH*
NC
Nu
DCB
− CN
DCB
DCB
PH
CN
CN
2
1
Scheme 1. Photosensitized nucleophilic addition reaction.
Jin Matsumoto,* Yusuke Yoshinaga,
Akiko Hamasaki, Tomohiro Kawasaki,
Me Me
Me
Nu
Me
CN
+
HNu
i
³
Me
Toshiaki Yamashita, Tsutomu Shiragami,
Me
+
Me
Me
(E1/2ox 0.86 V)
NC
NC
and Masahide Yasuda*
DCB
2a; Nu= NHMe
2b; Nu= OMe
1a
Department of Applied Chemistry, Faculty of Engineering,
University of Miyazaki, 1-1 Gakuen-Kibanadai-Nishi,
Miyazaki 889-2192
Me Me
Nu
Me
i
Me
+
DCB
+
HNu
Me
Me Me
NC
Me
(E1/2ox 1.22 V)
2c; Nu= OMe
2d; Nu= OEt
2e; Nu= OPrn
Received May 23, 2011
E-mail: jmatsu@cc.miyazaki-u.ac.jp
1b
Scheme 2. The PH/DCB redox-photosensitized nucleophilic
addition (PNA) of 1 in MCR: (i) h¯, PH.
Photosensitized nucleophilic additions of 2,5-dimethyl-
2,4-hexadiene and 2,3-dimethyl-2-butene were performed in
a microchannel reactor using phenanthrene and p-dicyano-
benzene as a redox pair. The quantum yields for the
formation of photo-NOCAS products were remarkably
enhanced at lower temperatures. The exciplex formation
disturbed the reaction at higher temperatures.
0.4
0.3
0.2
0.1
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(A)
(B)
Φ
Φ
A pair of aromatic hydrocarbon and aromatic nitrile is the
most familiar donor and acceptor pair to enable the sensitizing
of a variety of photoinduced electron-transfer reactions.1
However, this pair tends to undergo a strong exciplex interac-
tion, competing with the formation of free ion radicals even in
polar solvents.2 Therefore the generation of free ion radicals has
been enhanced by applying polar solvent3 and salt effects.4
Also, temperature effects are a useful method to control the
interaction between the donor and acceptor. In general the
encounter rate becomes higher at high temperature.5 Here, we
report on a temperature dependence on redox-sensitization of
the pair of phenanthrene (PH) and p-dicyanobenzene (DCB)
toward nucleophilic addition reaction of simple alkenes 1,
which proceeds through the generation of cation radicals of 1 by
the redox-photosensitization by the PH/DCB pair (Scheme 1).1
The PH/DCB-photosensitized nucleophilic addition (PNA)
with nucleophiles (HNu) such as MeNH2 and alcohols were
applied to 2,5-dimethyl-2,4-hexadiene (1a) and 2,3-dimethyl-2-
butene (1b) which had no absorption at >300 nm.6 In order to
prevent the further reaction of the products and control the reac-
tion temperature precisely, the PNA was examined by the flow
system using a microchannel reactor (MCR) which was kept at a
given reaction temperature (T, 5-50 °C). The reactant solution
flowed from an inlet into the microchannel using a microsyringe
pump and was irradiated >290 nm with a high pressure Hg
0
10 20 30 40 50
0
10 20 30 40 50
T/°
C
T/°C
Figure 1. The temperature dependence of Φ for (A) the
formation of 2a ( ) and 2b ( ) from the PNA reaction of
1a and (B) the formation of 2c ( ), 2d ( ), and 2e ( )
from the PNA reaction of 1b.
lamp. Irradiation time was equal to the retention time (Z) in the
MCR which was adjusted to be 9.4, 32, 63, 94, and 118 s by
changing the flow rate (F) in 10, 3.0, 1.5, 1.0, and 0.8 ¯L min
Z = 60 © V/F where the channel volume (V) was 1.57 ¯L.
¹1
:
The PNA of 1a with MeNH2 and MeOH in MCR gave
2a and 2b, respectively. Similarly the PNA of 1b with ROH
(R = Me, Et, and n-Pr) gave 2c-2e (Scheme 2).7 The quantum
yields (Φ) for the formation of 2a-2e were determined at
various T. Figure 1 shows the T-dependence of Φ. As T became
lower, the Φ became higher. The chemical and quantum yields
of 2a-2e in the PNA reaction under the optimized T and Z
are summarized in Table 1.
As shown in Scheme 1, the PNAwas initiated by the electron
transfer from the excited singlet state of PH to DCB to give the
cation radical of PH (PH•+) and the anion radical of DCB
(DCB•¹). This step was examined by the quenching of the
fluorescence PH (-max = 365 nm) by DCB in MeCN-H2O
³ Present address: Department of Chemical Science and
Engineering, Miyakonojo National College of Technology,
Miyakonojo, Miyazaki 885-8567
Published on the web October 15, 2011; doi:10.1246/bcsj.20110149

J. Matsumoto et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 10 (2011) 1131
0.80
0.75
0.70
0.65
0.60
0.60
Table 1. Yields of 2a-2e in PNA Reaction of 1 in MCR
under the Optimized Conditionsa)
(A)
(B)
0.55
0.50
0.45
0.40
0.35
0.30
PH
/mM
2c)
(yield/%) /%d)
Conv.
1
HNu
Z/sb)
Φ
1a
1a
1be)
1be)
1be)
0.05
0.05
0.10
0.10
0.10
MeNH2
MeOH
MeOH
EtOH
63
63
94
118
118
2a (93)
2b (96)
2c (67)
2d (62)
2e (67)
94
96
100
100
100
0.29
0.28
0.54
0.18
0.12
n-PrOH
a) Irradiation was performed for an MeCN-H2O solution
(19:1) containing 1 (0.2 M), DCB (0.1 M), PH (0.05 M), and
MeNH2 (0.2 M) and an MeCN-ROH solution (2:1) containing
1 (0.2 M), DCB (0.1 M), and PH (0.05 M) at 5 °C. b) Reaction
time (Z) in MCR. c) Chemical yields based on DCB used.
d) The conversion of DCB. e) In MeCN-ROH (3:1).
0
10 20 30 40
0
10 20 30 40
T/°C
T/°C
Figure 3. Temperature-dependence of (A) solvent parame-
ter (ETN) and (B) viscosity (©) of MeCN-H2O (19:1) ( )
and MeCN-MeOH (3:1) ( ).
1
kRI
β
PH* + DCB
PH* + DCB
PH DCB
Radical ions
2
T/°C
0.8
0.6
0.4
0.2
0
50
40
30
20
10
5
kEX
1[PH DCB]*
Exciplex
1/
τ
PH*
PH
kQ = kRI + kEX
(1)
Φ
=
β
kRI[DCB]/(1/
τ
+ kRI[DCB] + kEX[DCB]) (2)
420
440
460
480
500
Scheme 3.
Wavelength/nm
generate the cation radical of 1, since the oxidation potential
of PH (1.29 V vs. Ag/AgNO3)8 was higher than those of 1a
(0.86 V)6 and 1b (1.22 V).6 The hole-transfer process proceeded
at near diffusional controlled limits (1010 M¹1 s¹1). On the
Figure 2. Temperature dependence of the exciplex emis-
sion between PH and DCB.
(19:1) at various T. The rate constant (kQ) was determined from
the Stern-Volmer constants and the fluorescence lifetime (¸) of
PH (57.5 ns).8 As the kQ depended on T as has been reported in
other hand, the rate constant for the nucleophilic reaction of
¹1 10
MeNH2 to PH•+ was found to be 2 © 108 M¹1 s
.
This is a
a variety of donor-acceptor systems,5 the kQ values gradually
reason why hole transfer from PH•+ to 1 proceeded efficiently
without the occurrence of the nucleophilic attack of MeNH2 to
PH•+. The cation radical of 1 allowed the nucleophilic addition
of HNu to give adduct radicals which underwent to the ipso-
substitution on DCB•¹ to give the photo-NOCAS (photo-
chemical nucleophile-olefin combination, aromatic substitu-
tion) products 2 combined between HNu, 1, and DCB.
¹1
increased from 6.75 © 109 M¹1 s
M
at 5 °C to 1.11 © 1010
¹1 s¹1 at 40 °C. Thus, the kQ values were smaller at the lower
T, although the Φ values were larger at the lower T. Arrhenius
plots of ln kQ vs. 1/(T + 273) led to the activation enthalpy
(Ea = 10.0 kJ mol¹1) and the frequency factor (log A = 11.8).5
1
Moreover, a weak exciplex emission of [PH¢DCB]* was
remarkably observed at 442 nm with an isoemissive point at
420 nm at higher T, as shown in Figure 2 which was the
spectral subtraction between the fluorescences in the absence
and the presence of DCB (5 mM). These suggested that the
formation of the radical ion (PH•+ DCB•¹) took place
competitively with the exciplex formation at higher T.
Scheme 3 can be postulated as the quenching process of PH
with DCB when kEX and kRI are defined to be the rate constants
for the formation of the exciplex and the radical ions, respec-
tively. The kQ was equal to summation of kEX and kRI (eq 1). As
mentioned above, log A was small. Therefore, the Ea can be as-
signed to be the activation energy for the formation of the ex-
ciplex from PH* and DCB. Probably the kRI was constant to T.
The Φ can be represented by eq 2 where ¢ denotes the
efficiency of the process from 1•+ to 2. The Φ values were
almost the same in the PNA of 1a by MeNH2 and MeOH with
different nucleophilicities. Moreover, the nucleophilic addition
to the localized positive charge of 1•+ is expected to be faster
than the case of PH•+ where the positive charge delocalized
over the ³-conjugation. Therefore, the nucleophilic addition to
1•+ was thought to be near the diffusion-controlled limit
(1010 M¹1 s¹1). It was deduced that the quantum yield (¢) for
the process from 1•+ to 2 was high and independent of T.
In general, the exciplex formation occurred in less polar
solvents whereas the radical ion pair formed predominantly in
polar solvents.9 The solvent parameter, ET values, of MeCN-
N
H2O (19:1) and MeCN-MeOH (3:1) were measured in various
N
T and were plotted against T in Figure 3A. The ET values of
these solvents were higher at lower T. Moreover, the plots of
viscosity (©) against T are shown in Figure 3B. The © of the
mixed solvents were higher at lower T. Therefore, it is expected
that the exciplex formation is favorable at higher T, due to low
polarity and low viscosity.
A hole transfer from PH•+ to 1 could occur exothermically to

1132 Bull. Chem. Soc. Jpn. Vol. 84, No. 10 (2011)
© 2011 The Chemical Society of Japan
1H NMR: ¤ 0.99 (s, 6H), 1.10 (t, J = 6.9 Hz, 3H), 1.38
(s, 6H), 3.27 (quint, J = 6.9 Hz, 4H), 7.53 (d, J = 8.8 Hz, 2H),
7.59 (d, J = 8.8 Hz, 2H). 13C NMR: ¤ 15.96, 20.46, 24.24,
45.91, 56.52, 77.89, 109.13, 119.42, 129.52, 130.51, 153.44.
2-(4-Cyanophenyl)-2,3-dimethyl-3-propoxybutane (2e):
1H NMR: ¤ 0.87 (t, J = 7.4 Hz, 3H), 0.99 (s, 6H), 1.38
(s, 6H), 1.45 (qt, J = 7.4, 6.3 Hz, 2H), 3.18 (t, J = 6.3 Hz, 2H),
7.52 (d, J = 8.8 Hz, 2H), 7.58 (d, J = 8.8 Hz, 2H). 13C NMR:
¤ 11.04, 20.34, 23.72, 24.24, 46.02, 62.71, 77.67, 109.11,
119.42, 129.57, 130.49, 153.44.
Irradiation
(A)
Inlet
(B)
Pyrex glass
1000 μm
41 μm
100 μm
Pyrex glass
Channel
Outlet
Scheme 4. (A) Top view and (B) cross section of MCR:
width = 100 ¯m; depth = 41 ¯m, length = 500 mm, vol-
ume (V) = 1.57 ¯L.
Determination of Quantum Yield. The conversion curve
was obtained from the PNA reaction under varying irradiation
times controlled by the flow rate at a given T. From the slope
of the straight line of the conversion curve, the production
amounts of 2 per second were obtained. Light intensity per
second adsorbed by sensitizer (PH) was determined to be
Therefore, the T-dependence on Φ can be attributed to the
T-dependence on the interaction of PH* to DCB.
1
Thus, kQ became higher but Φ for the PNA reaction were
lower at higher T where the exciplex formation occurred
considerably. Judging from these results, it was concluded that
the exciplex formation retarded the PNA reaction of 1. Thus,
we found the PNA which is a convenient synthetic method to
introduce functional groups to alkenes can be enhanced by
lowering the reaction temperature.
¹1
7.28 © 10¹6 einstein s using the photoamination of PH (0.1
M) with MeNH2 (0.5 M) in the presence of DCB (0.1 M) in the
MCR as chemical actinometer where the quantum yield for
the formation of 9-methylamino-9,10-dihydrophenanthrene has
been found to be 0.141.10 Here the concentration of PH of
actinometer was set to be the same as that for the PNA reaction.
Thus the Φ for the formation of 2 was calculated by the
division of the production amounts of 2 per one second by light
intensity per one second.
Experimental
Instruments.
1H NMR (400 MHz) and 13C NMR (100
MHz) spectra were taken on a Bruker AV 400M spectrometer
for CDCl3 solutions with tetramethylsilane used as an internal
standard. The viscosity (©) of the mixed solvents was measured
on a viscometer SV-10 (A&D Co., Limited). A Pyrex glass
MCR (ICC-SY500) was purchased from Institute of Micro-
chemical Technology (IMT, Kanagawa, Japan). The channel
shape was as follows: channel width: 100 ¯m, channel depth:
41 ¯m, cross-section area: 3.14 © 10¹5 cm2, length: 50 cm, V:
1.57 ¯L (Scheme 4). The MCR was set in a water bath which
was kept at a given T by the temperature-controlling circulator.
The PNA Reaction in MCR. An MeCN-H2O solution
(19:1) containing 1a (0.2 M), DCB (0.1 M), and PH (0.05 M),
and MeNH2 (0.2 M) was introduced to the MCR under
irradiation. The reaction solution (1 mL) was collected from
the outlet of the MCR and subjected to separation by column
chromatography to give 2a. In the cases of PNA reaction of
alcohols (ROH), MeCN-ROH (R = Me, Et, and n-Pr) solution
(2:1 and 3:1) containing 1 (0.2 M), DCB (0.1 M), and PH (0.05
or 0.1 M) was irradiated in MCR to give 2b-2e.
Measurements of Solvent Polarity (ETN). The solvent
polarity of MeCN-H2O (19:1) and MeCN-MeOH (3:1) at
various T was evaluated by the solvent parameter, ET(30), which
¹1
was the wavelength in kcal mol at the absorption maxima
of
2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate
(Reichardt’s dye)11 in the sample solvent. The ET(30) was trans-
formed to ET by the equation: ETN = (ET(30) ¹ 30.7)/32.4.
N
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(2d):