Photochemical reactions of hydroarenes with N-bromosuccinimide

Article abstract of DOI:10.1246/bcsj.75.769

The photochemical reactions of 1,2,3,4-tetrahydronaphthalene (THN), 9,10-dihydrophenanthrene (DHP), 9,10-dihydroanthracene (DHA), and acenaphthene (AN) with N-bromosuccinimide (NBS) were investigated under N₂ atmosphere at room temperature. The results show that the relative reactivities of the hydroarenes toward a photochemical reaction with NBS are THN < DHP < AN << DHA, which is consistent with the stabilities of the radicals produced by benzylic hydrogen abstraction from the hydroarenes. Photochemical reactions of THN and DHP mainly afforded dehydrogenated products, while the photobrominations of the dehydrogenated products from AN and DHA with NBS proceeded readily.

Full text of DOI:10.1246/bcsj.75.769

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 769–771 (2002)
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Photochemical Reactions of Hydroarenes with N-Bromosuccinimide
Photochemical Reactions of Hydroarenes
Z.-M. Zong et al.
Zhi-Min Zong, Wei-Hong Zhang,# Qun Jiang,## Jin Lu,## and Xian-Yong Wei*
School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
(Received July 16, 2001)
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The photochemical reactions of 1,2,3,4-tetrahydronaphthalene (THN), 9,10-dihydrophenanthrene (DHP), 9,10-di-
hydroanthracene (DHA), and acenaphthene (AN) with N-bromosuccinimide (NBS) were investigated under N2 atmo-
sphere at room temperature. The results show that the relative reactivities of the hydroarenes toward a photochemical re-
action with NBS are THN < DHP < AN << DHA, which is consistent with the stabilities of the radicals produced by
benzylic hydrogen abstraction from the hydroarenes. Photochemical reactions of THN and DHP mainly afforded dehy-
drogenated products, while the photobrominations of the dehydrogenated products from AN and DHA with NBS pro-
ceeded readily.
01
01
tom flask. After replacing air in the flask with nitrogen, the flask
was covered up with a gum cap and then irradiated at room tem-
perature with a 200-W high-pressure mercury arc while the solu-
tion in the flask was being stirred magnetically. After being irradi-
ated for a prescribed period of time, the solution was sampled with
a syringe through the gum cap. The substrate and products in the
solution sampled were identified by GC-MS (Shimadzu QP-5000)
if necessary and quantified by GC (Shimadzu GC-17A).
As an important reagent for organic synthesis, N-bromosuc-
cinimide (NBS) has been widely used in bromination,1,2 dehy-
drogenation,3 and other reactions.4,5 When using methylarenes
as substrates, a bromine atom of NBS can substitute for either
an aromatic or a benzylic hydrogen atom, depending on the
substrate types and the reaction conditions. It is necessary for
the efficient use of NBS to understand the reactivities and reac-
tion mechanisms of different substrates with NBS under mild
conditions.
Results and Discussion
Futamura and Zong6 investigated the photobromination of
some mono- and dimethylarenes with NBS at room tempera-
ture, finding that the benzylic hydrogen atom of mono- and
dimethylarenes can be selectively replaced with Br from NBS
to afford bromomethyl-, (bromomethyl)methyl- and bis(bro-
momethyl)arenes. Can the benzylic hydrogen of other com-
pounds be photochemically replaced with Br from NBS? To
answer this, we investigated the photochemical reactions of
1,2,3,4-tetrahydronaphthalene (THN), 9,10-dihydrophenan-
threne (DHP), 9,10-dihydroanthracene (DHA), and acenaph-
thene (AN) with NBS.
Time profiles of the substrate conversions and product
yields are provided as Figs. 1–4. It can be seen that in the ini-
tial step, the reactivities of the substrates in the photochemical
reaction decreased in the order DHA >> AN >> DHP >>
THN.
THN, DHP, and DHA are routinely used as hydrogen do-
nors, since their benzylic hydrogen atoms are readily abstract-
ed. Studies show that their rates of benzylic hydrogen abstrac-
tion decrease in the order DHA >> AN > DHP > THN.7–10
It can be deduced from the data in Table 1 that once a benzylic
hydrogen atom is abstracted, an adjacent β-hydrogen atom (as
for THN) or a hydrogen atom on another benzylic carbon (as
for DHP and DHA) is more readily abstracted. Therefore, the
abstraction of the first benzylic hydrogen atom should be the
rate-determining step. The reactivities of the compounds to-
ward a photochemical reaction with NBS thus decrease in the
order DHA >> DHP > THN.
The relative reactivities of alkylarenes toward benzylic hy-
drogen abstraction can be evaluated in terms of the stabilities
of the resulting radicals. The resonance energy (RE), based on
structure-resonance theory, is considered to be an important in
dex for comparing the radical stabilities. Hence, the relative
reactivities of alkylarenes toward benzylic hydrogen abstrac-
tion can be compared by the RE values of the corresponding
radicals, i.e., the larger is the RE value of a radical, the more
easily does benzylic hydrogen abstraction from its precursor
alkylarenes proceed. When a benzylic hydrogen is abstracted,
DHP, AN, and DHA are converted to 9,10-dihydrophenan-
thren-9-yl, acenaphthen-1-yl, and 9,10-dihydroanthracen-9-yl
Experimental
Materials. All of the reagents used were commercially pur-
chased. NBS was used as received without additional purification.
The impurities in THN were removed by washing in turn with 5%
aqueous H2SO4, 5% Na2CO3, and water followed by distillation
under nitrogen. DHP was purified by silica-gel column chroma-
tography using hexane as an eluate. The purifications of DHA,
AN, and biphenyl were performed by recrystallization from etha-
nol.
General Procedure. A substrate (1 mmol), biphenyl (0.5
mmol, used as internal standard), benzene (20 mL, used as sol-
vent), and NBS (2.2 mmol) were put into a 100 mL, spherical-bot-
# Present address: Department of Applied Chemistry, Jiangsu
Institute of Petrochemical Technology, Changzhou 213016,
Jiangsu, China.
## Present address: Department of Chemical Engineering and
Biotechnology, College of Material and Chemical Engineering,
Zhejiang University, Yuquan, Hangzhou 310027, Zhejiang, China.

770 Bull. Chem. Soc. Jpn., 75, No. 4 (2002)
Photochemical Reactions of Hydroarenes
Table 1. Dissociation Energies (kJ mol−1) of the C–H Bond in Some Hydrocarbons and Radicals.11
Structure
BDE
431.0
395.4
342.9
306.9
198.7
167.4
100.4
Fig. 1. Time profile of THN conversion and product yields.
Fig. 3. Time profile of DHA conversion and anthracene
yield.
Fig. 2. Time profile of DHP conversion and phenanthrene
yield.
Fig. 4. Time profile of AN conversion and acenaphthylene
yield.
radicals, respectively. The radicals can be structurally classi-
fied into benzyl, 1-naphthylmethyl, and diphenylmethyl radi-
cals, respectively, the RE values (kJ mol−1)12 of which increase
in the order benzyl (30.6) < 1-naphthylmethyl (44.4) < diphe-
nylmethyl (54.6). The above theoretical analysis further inter-
prets the difference among the substrates in reactivity toward a
photochemical reaction with NBS.
DHA is very reactive toward a photochemical reaction with
NBS, the conversion being more than 80% on irradiation for
only 2 min. In the initial step, DHA is mainly dehydrogenated
to anthracence by bromic and N-succinimidyl radicals derived
from NBS decomposition (Scheme 1).
The reaction of the resulting HBr with NBS produces Br2
(Scheme 2).
The reaction of Br2 with 9,10-dihydroanthracen-9-yl radical
affords 9-bromo-9,10-dihydroanthracene (BDHA) and Br
(Scheme 3).
Scheme 1.
Scheme 2.

Z.-M. Zong et al.
Bull. Chem. Soc. Jpn., 75, No. 4 (2002) 771
Table 2. Yields of the Products from the Photochemical Reactions of Some Hydroarenes with
NBS for 2 h
Substrate
Product yield/mol%
Main product
Naphthalene
Phenanthrene
9-Bromoanthracene
1-Bromoacenaphthylene
By product
97.1 1-Bromonaphthalene
94.5 9-Bromophenanthrene
88.9 9,10-Dibromoanthracene
68.2 1,2-Dibromoacenaphthylene
THN
DHP
DHA
AN
2.9
5.5
11.1
31.8
1.314) >> phenanthrene (9-position: 1.026) > naphthalene
(1-position: 0.994).13 Therefore, the photochemical reactions
of THN and DHP afforded only small amounts of the corre-
sponding bromoarenes.
Scheme 3.
Dehydrogenation is an important synthetic reaction. Con-
ventional dehydrogenation is usually conducted at high tem-
perature and in the presence of a catalyst, and entails disadvan-
tages, such as low product yield, difficult product separations
and catalyst deactivation by carbonaceous deposit. The photo-
chemical reaction with NBS provides an effective method for
the dehydrogenation of some hydroarenes under mild condi-
tions.
The resulting BDHA decomposes to anthracene and HBr
due to its lability (Scheme 4).
Scheme 4.
We thank the Foundation for Excellent Young University
Teachers of the State Education Commission of China, IET
Foundation for Young University Teachers and the Foundation
for Scientific Research from Open Research Laboratory of
Comprehensive Utilization of Carbon Resources, Dalian Uni-
versity of Technology.
Therefore, unlike mono- and dimethylarenes, BDHA, as the
bromo derivative of DHA, was produced only as an intermedi-
ate. A similar reaction mechanism can be presumed for the
other substrates.
After 2 h-irradiation, all of the substrates were completely
converted. However, as shown in Table 2, THN and DHP were
primarily converted to dehydrogenation products, whereas the
main products from the photochemical reactions of DHA and
AN with NBS were monobromo derivatives of polycyclic are-
nes with small amounts of dibromo derivatives, indicating that
the photobrominations of the dehydrogenation products from
DHA and AN proceeded significantly.
Taking DHA as an example, photobromination of its dehy-
drogenated product, anthracence, may proceed either by the
addition of Br· and subsequent hydrogen abstraction or by the
addition of Br2 and subsequent removal of HBr to afford 9-bro-
moanthracene (9-BA). Some of the resulting 9-BA may be
converted to 9,10-dibromoanthracene via further photobromi-
nation followed by hydrogen abstraction or by the removal of
HBr.
A similar process could be inferred for acenaphthylene, the
dehydrogenation product of acenaphthene, but the photobro-
minations of naphthalene and phenanthrene are difficult. As
quantum-chemical measures of the reactivities of unsaturated
hydrocarbons toward radical, electrophilic or nucleophilic re-
actions, superdelocalizability (Sr) values13 have been used to
interpret the differences in the reactivity among various diaryl-
methanes toward hydrogenolysis14 and hydrocracking,15 i.e., a
carbon atom in an aromatic ring with a larger Sr value accepts a
hydrogen atom more readily. During bromination, an arene ac-
cepts radical Br· or electrophilic Br2, and the abilities of arenes
to accept radical Br· or electrophilic species Br2 can be analo-
gously predicted by their Sr values: anthracene (9-position:
References
1
A. W. van der Made and R. H. van der Made, J. Org.
Chem., 58, 1262 (1993).
2
G. J. M. Gruter, O. S. Akkerman, and F. Bickelhaupt, J.
Org. Chem., 59, 4473 (1994).
3
Y. K. Shim, I. J. Youn, J. S. Chun, T. H. Park, M. H. Kim,
and W. J. Kim, Synthesis, 1990, 753.
4
5
6
N. Konigsberg, J. Am. Chem. Soc., 235,1341 (1960).
R.-L. Wang, Chem. J. Chin. Univ., 13, 845 (1992).
S. Futamura and Z.-M. Zong, Bull. Chem. Soc. Jpn., 65,
345 (1992).
7
M. L. Poutsma, Energy & Fuels, 4, 123 (1990).
8
9
X.-Y. Wei and Z.-M. Zong, Energy & Fuels, 6, 236 (1992).
Z.-M. Zong and X.-Y. Wei, Fuel Process. Technol. 41, 79
(1994).
10 X.-Y. Wei, E. Ogata, Z.-M. Zong, and E. Niki, Coal Con-
version, 18, 67 (1995).
11 S. W. Benson, J. Chem. Educ., 42, 502 (1965).
12 W. C. Herndon, J. Org. Chem., 46, 2119 (1981).
13 T. Yonezawa, C. Nagata, H. Kato, A. Imamura, and K.
Morokuma, “Guide to Quantum Chemistry,” Kagaku-Dojin Press,
Kyoto (1990).
14 S. Futamura, S. Koyanagi, and Y. Kamiya, Fuel, 67, 1436
(1988).
15 X.-Y. Wei, E. Ogata, and E. Niki, Bull. Chem. Soc. Jpn.,
65, 1114 (1992).