Chemistry Letters Vol.35, No.4 (2006)
349
Table 1. Products and yields of photochemical transformation
of chlorinated dibenzo-p-dioxins in methanol
oxygen-saturated solution. These results can be attributed to
ꢀ
secondary photochemical reaction from T1 of 6-CBP and
5,6-DCBP produced from 1-CDD and 1,2-DCDD, respectively.
Yield/%c
Conditiona Timeb
ꢀ
CDDs
Reductive dechlorination from T1 is a general process of chlori-
CDDs DD CBPs BP
1
2
nated aromatics. Another possibility for the pathway leading to
BP is secondary photochemical rearrangement of DD produced
by dechlorination of CDDs. This route can be ruled out, because
DD was absent under the condition where BP was produced
1
1
1
2
2
2
1
1
1
2
2
2
2
2
2
a
-CDD
-CDD
-CDD
-CDD
-CDD
-CDD
,2-DCDD
,2-DCDD
,2-DCDD
,3-DCDD
,3-DCDD
,3-DCDD
,7-DCDD
,7-DCDD
,7-DCDD
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
3 h
3 h
3 h
5 min
5 min
10 min
1 h
1 h
1 h
10 min 24.3h
10 min
3 h
10 min
10 min
60 min 40.2h 49.1
—
—
—
—
—
—
0
0
0
87.4
11.7d 18.8
21.4d trace
0
0
0
0
11.6 16.6e
(
Table 1).
In conclusion, it has been revealed that the singlet excited
0
95.7
0
28.8e
0
0
13.0
0
13.0
0
state of CDDs in methanol undergoes regioselective homolysis
of carbon–oxygen bond leading to CBPs, while reductive
dechlorination of the dioxin skeleton is dominant process for
the triplet excited state. Investigation on photolysis of other
CDDs is in progress. Details of this study will be published
elsewhere in the near future.
f
0
0
78.9g trace
0
13.0
12.0
0
0
0
i
0
0
i
0
55.9h 19.9
0
9.5j
15.2j
0
0
0
0
0
0
0
0
0
References and Notes
1
2
G. C. Choudhry, O. Hutzinger, Residue Rev. 1991, 84, 113.
a) Y. Ogata, K. Takagi, I. Ishino, Tetrahedron 1970, 26, 2703.
b) N. Haga, H. Takayanagi, J. Org. Chem. 1996, 61, 735.
A: >300 nm, Ar; B: >300 nm, O2; C: >360 nm, Ar, acetophenone
3
4
a) B. Guan, P. Wan, J. Chem. Soc., Chem. Commun. 1993, 409.
b) B. Guan, P. Wan, J. Photochem. Photobiol., A 1994, 80, 199.
a) R. S. Nohr, J. G. Mcdonald, U. Kogelschatz, G. Mark, H.-P.
Schuchmann, C. von Sonntag, J. Photochem. Photobiol., A 1994,
ꢂ3
b
(
5:0 ꢁ 10 M). Irradiation time was controlled so that conversion
c
of CDDs may be in the range of 15–50%. Yields based on CDDs
consumed. 6-CBP. 5-CBP. 5,6-DCBP. 1-CDD (58.5%) and 2-
d
e
f
g
h
i
j
0
CDD (20.5%). 2-CDD. 4,5-DCBP. 5,5 -DCBP.
79, 141. b) S. Kieatiwong, L. Nguyen, V. Hevert, M. Hackett, G.
Miller, M. Miille, R. Mitzel, Environ. Sci. Technol. 1990, 24, 1575.
Bond dissociation energy of carbon–chlorine (chlorobenzene) and
5
carbon–oxygen bond (diphenyl ether) is 39813 and 368 kJ mol
,
ꢂ1 13
O
h
ν
(>300 nm)
respectively, which is expected to be competitively homolyzed upon
excitation.
1-CDD, 2-CDD, and 2,3-DCDD were synthesized according to
S1
O
O
Cln
C−O
scission
6
7
Clm
the literature.14 Preparation of 1,2-DCDD and 2,7-DCDD was
15
16
CDDs (m + n = 1 or 2)
successfully achieved by a modified method of the literature.
ꢀ
The energy of S1 of CDDs estimated from the edge (ꢁ with "
h
ν
ꢂ1
ꢂ1
ꢂ1
of 2 cm
(
kJ mol (2,3-DCDD); 369 kJ mol (2,7-DCDD). These values
M
1-CDD); 371 kJ mol (2-CDD); 361 kJ mol (1,2-DCDD); 347
) of UV–vis spectrum is as follows: 377 kJ mol
ꢂ1 ꢂ1
(
>360 nm)
senstizer
ꢂ1
ꢂ1
Clm
Cln
are lower than the energy of 302.5 nm light from the Hg lamp that
ꢂ1
O
CBQs
corresponds to 395 kJ mol
.
T1
8
A methanol solution of CDDs was irradiated in a cylindrical irradi-
ation flask (Pyrex, 200 mL or 500 mL) using a 400 W high-pressure
mercury lamp (RIKO UVL-400-HA, internal irradiation type) with a
Pyrex jacket. Argon or oxygen gas was bubbled before and through-
out the irradiation. Evaporation of solvent and isolation by repeated
chromatography (silica gel, hexane–acetone as a developing sol-
vent) of the residual photolysate afforded products to allow instru-
mental analysis (EI-MS, H and C NMR) as listed in Table 1.
Formation of CBPs was dramatically enhanced when irradiation was
performed in the presence of NaBH4 as a hydride donor, by which
CBQs is facilely reduced to CBPs.3
C−Cl
scission
hydrogen
donor
OH
O
1
13
Clm
Cln
9
1
O
OH
DD
CBPs
0 Triplet energy of CDDs estimated by phosphorescence spectra is
ꢂ1
2
and 2,7-DCDD, respectively,
acetophenone (311 kJ mol ).
78, 284, 275, and 257 kJ mol for 1-CDD, 2-CDD, 2,3-DCDD,
which is lower than that of
Scheme 2.
14;17
ꢂ1
framework (Scheme 2). Occurrence of reductive dechlorination
ꢀ
1
1
1 J. R. Siegman, J. J. Houser, J. Org. Chem. 1982, 47, 2773.
2 R. S. Davidson, J. W. Goodin, G. Kemp, in Advances in Physical
Organic Chemistry, ed. by V. Gold, D. Bethell, Academic Press,
London, 1984, Vol. 20, p. 191.
from T1 of CDDs, despite higher bond dissociation energy of
carbon–chlorine bond than the triplet energy of CDDs, may be
ascribed to the contribution from the mechanism that involves
electron transfer from solvent molecule to CDDs to give radical
anion of CDDs.11 Production of DD on irradiation of 2-CDD and
13 S. L. Murov, I. Carmichael, G. L. Hug, in Handbook of Photochem-
istry, 3rd ed., Marcel Dekker, New York, N.Y., 1993, p. 280.
14 A. E. Pohland, G. C. Yang, J. Agric. Food Chem. 1972, 20, 1093.
15 J. E. Oliver, J. Heterocycl. Chem. 1984, 21, 1073.
16 O. Aniline, in Advances in Chemistry Series, ed. by E. H. Blair,
American Chemical Society, Wasington, 1973, Vol. 120, Chap. 14,
p. 126.
2
,3-DCDD with >300 nm light in the absence of oxygen can be
ꢀ
ꢀ
rationalized in terms of intersystem crossing from S1 to T1 of
these CDDs.
Formation of BP was observed on irradiation of 1-CDD and
1,2-DCDD with >300 nm light. In each case, BP was absent in
17 V. G. Klimenko, R. N. Nurmukhametov, J. Fluoresc. 1998, 8, 128.