802 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Photochemical Reactions of ꢀ-Bromoacetylarenes
Table 1. Product Yield in Photolysis of 1-BAN, 2-BAN, and 9-BAA upon Irradiation of 308 nm
Laser Pulse
Product yield (mol/mol precursor)
Solvent
Ar–COCH3
Ar–CH3
Ar–CH2Br
1-BAN
2-BAN
9-BAA
PhH
CH3CN
PhH
CH3CN
PhH
CH3CN
31%
19%
35%
19%
22%
27%
6%
none
<1%
none
<1%
none
7%
16%
<1%
none
46%
30%
(JASCO CT-25), was detected by a photomultiplier (Hamamatsu
Photonics, R 928), and was stored in a storage scope (Iwatsu TS-
8123) and the signals were transferred to a personal computer
(NEC PC-9801VM). Concentrations of BAP, 1-BAN, 2-BAN,
and 9-BAA were adjusted to an absorbance of 1.0 at 308 nm.
The experiments were performed under argon or oxygen atmos-
pheres.
rangement and decarbonylation reactions occurred efficiently
or not.
Experimental
Materials. ꢀ-Bromoacetophenone (BAP; ArCOCH2Br; Kanto
Chem. Co. Inc.) was used after recrystallization from methanol. 1-
(ꢀ-Bromoacetyl)naphthalene (1-BAN; ArCOCH2Br) and 2-(ꢀ-
bromoacetyl)naphthalene (2-BAN; ArCH2Br) were prepared from
1-acetylnaphthalene (2-AN; ArCOCH3) and 2-acetylnaphthalene
(2AN), respectively, by direct bromination with bromine.14 9-(ꢀ-
Bromoacetyl)anthracene (9-BAA; ArCOCH2Br) was prepared
from 9-acetylanthracene (9-AA; ArCOCH3) by bromination with
triphenylammonium tribromide, as described in detail elsewhere.7
Toluene, 1-methylnaphthalene (1-MN; ArCH3), 2-methylnaph-
thalene (2-MN; ArCH3) and 9-methylanthracene (9-MN; ArCH3),
which were used as authentic samples for product analysis, were
used as received. Benzyl bromide and 2-bromomethylnaphthalene
(2-BMN; ArCH2Br), which were used as authentic samples, were
used as received. 1-Bromomethylnaphthalene (1-BMN; ArCH2Br)
and 9-bromomethylanthracene (9-BMA; ArCH2Br), which were
used as authentic samples in product analysis, were prepared from
1-methylnaphthalene (1-MN; ArCH3) and 9-methylanthracene (9-
MA; ArCH3), respectively by bromination with N-bromosuccin-
imide, as described in detail elsewhere.7,15 1,3-Cyclohexadiene,
which was used as a radical scavenger, was used as received.
Benzene (for spectroscopy, Kanto Chem. Co. Inc.) and acetoni-
trile (for spectroscopy, Kanto Chem. Co. Inc.) were used as re-
ceived.
Results and Discussion
Products on Photoirradiation. The quantitative analyses
of the products were performed by gas chromatography (GC)
and GC–MS for 1-BAN, 2-BAN, and 9-BAA. The concentra-
tions of the precursor and final products were determined by us-
ing naphthalene or 2,5-diacetylpyrazine as external standards.
The sample solution (1:0 ꢃ 10ꢄ2 M, 3.0 mL) was irradiated us-
ing 308 nm laser pulses of 40 mJ pulseꢄ1 with a repetition of 4
Hz. The results are summarized in Table 1.
As mentioned in the introduction, it was reported that a naph-
ꢁ
thylmethyl radical (ArCH2 ) was observed in the transient ab-
sorption spectra of 2-BAN in benzene and in acetonitrile.7
However, our product analyses showed that a main product in
the photochemical reaction was 2-acetylnaphthalene (2-AN;
ArCOCH3) and the product yield of 2-methylnaphthalene
(2-MN; ArCH3) and 2-bromomethylnaphthalene (2-MN;
ArCH2Br) was less than 1% in benzene.
On the other hand, 1-BAN and 9-BAA gave products
(ArCH3 and ArCH2Br) produced from an arylmethyl radical
ꢁ
(ArCH2 ) as shown in Table 1. As a typical example, the time
Measurement. Product Analysis of the Photoreaction: An
excimer laser (XeCl 308 nm, pulse width ꢂ20 ns, Lambda physics
LEXtra 100) was used for excitation of the sample solution in a
1 cm ꢃ 1 cm quartz cell in benzene and acetonitrile. The concen-
trations of all samples were adjusted to 1:0 ꢃ 10ꢄ2 M. The experi-
ments were performed under argon atmosphere. Product analysis
of the photoreaction of bromoacetylarene was carried out by gas
chromatography (Shimadzu GC-14A) and GC–MS (Shimadzu
GCMS-QP1100EX). A typical irradiation time was 10–20 min,
corresponding to 10–20% conversion of the starting materials.
The products increased in proportion to the reaction time at the in-
itial stage. Secondary reactions, which occurred by irradiation of
the products, was neglected. The reaction yield of this photoreac-
tion was determined by gas chromatography. The GC peak areas
for these products were quantified by use of external standards
(naphthalene and 2,5-diacetylpyrazine) and authentic samples.
Transient Absorption Spectroscopy: An excimer laser (XeCl
308 nm, pulse width ꢂ20 ns, Lambda physics LPX 100) was used
for excitation of the sample in a 1 cm ꢃ 1 cm quartz cell. A pulsed
xenon arc lamp (USHIO UXL-159) was used as a monitoring light
source. The monitoring light passed through a monochromator
development of the concentrations of the initial compound and
the products was observed, the results for 1-BAN in benzene is
shown in Fig. 1. The concentration of 1-BAN decreased in ap-
proximate proportion to the irradiation time at the initial stage
with a concomitant increase in the concentration of 1-acetyl-
naphthalene (1-AN; ArCOCH3) and 1-bromomethylnaphtha-
lene (1-BMN; ArCH2Br). When the irradiation time was longer
than 10 min, 1-BAN disappeared probably due to a secondary
photocleavage reaction. Therefore, ratios within 30% conver-
sion of the initial compound were used for the determination
of the product yields. The quantum yield of the photocleavage
reaction was determined in benzene as 0.3 for 1-BAN
(Table 2).7
The difference in reactivity of an addition reaction in a naph-
thalene ring between the 1 position and the 2 position is gener-
ally explained by molecular orbital calculations of a transition
state and an electron density of a frontier molecular orbital. For
example, the nitration of a naphthalene ring occurs at the 1 po-
sition much faster than at the 2 position.16 When an electrophil-
ic substitution reaction occurs in an aromatic ring, the reaction