that light-induced tautomerization of â-dicarbonyl com-
pounds can take place.10 Introduction of the fluorine atom
at the C2 position in the case of 1,3-diarylpropan-1,3-diones
completely changed the ground state situation, and practically
Scheme 5. Product Distribution in Photolysis of 1d in
CH3CNa
1
in all cases only the diketone form is present. By using H
NMR spectra and UV spectra, we can determine that 2-fluoro
compounds cannot photochemically isomerize to the enol
form. On the other hand, the photolysis of 2-bromo-1,3-
diphenylpropan-1,3-dione11 (9) resulted in a complex reaction
mixture containing the products derived from R-cleavage
(Scheme 4). No photocyclization to flavone occurred.
Scheme 4. Product Distribution in Photolysis of 9 in CH3CN
a Irradiation time: 2 h; λ ) 352 nm; c(substrate) ) 0.002 M.
shows that in the case of 1c the conversion to flavone is a
little higher. In the case of 1d, the fluorine atom bonded at
the C2 position completely changes the course of the
reaction. The formation of the 3-fluoroflavone derivative
becomes the dominant pathway, and R-cleavage occurs only
to a minor extent.
In conclusion, products in the photolysis of 2-halo-sub-
stituted 1,3-diphenylpropan-1,3-diones strongly depend on
the type of halogen atom bonded at C2 and also on the
position of electron-donor groups in the phenyl rings.
2-Fluoro derivatives are photostable, and 2-bromo derivatives
resulted in R-cleavage processes. On the other hand, 2-chloro
derivatives resulted in photocyclization to flavone or R-cleav-
age.
These results led us to prepare and study the photolysis
of 2-chloro-2-fluoro derivatives. We were able to prepare
2-chloro-2-fluoro-1,3-diphenylpropan-1,3-dione (1c) and
2-chloro-2-fluoro-1,3-di(4-methoxyphenyl)propan-1,3-di-
one (1d), but we were unable to prepare 2-chloro-2-fluoro-
1(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione.
Over 2 h irradiation of an acetonitrile solution of 1c, 55%
conversion and formation of 3-fluoroflavone (2c)12 as the
sole product were observed (φ ) 0.036 ( 0.002). Complete
conversion was obtained after 4 h of irradiation. After
isolation, the structure of product 2c was determined by its
spectroscopic data and by comparison to the GC retention
time of an authentic sample.12
Photolysis of 1d under identical reaction conditions
resulted in 59% conversion, and besides 3-fluoro-2-(4-
methoxyphenyl)-7-methoxylflavone13 (2d), p-methoxyben-
zaldehyde, and p-methoxybenzoic acid, products of R-cleav-
age were observed (Scheme 5).
In both cases (1c and 1d), we did not detect any
3-chloroflavone derivatives indicating that in 2-chloro-2-
fluoro-substituted compounds also no cyclization products
derived from C-F bond cleavage were formed. The com-
parison of the results obtained from 1a and 1c or 1e and 1d
The effect of halogen atoms can be explained by an
internal heavy-atom effect. It is known that heavy-atom
substituents greatly enhance spin-orbital coupling and there-
fore also enhance the rates of intersystem crossing.1 The spin
orbital constant is proportional to Z4, where Z is the atomic
number and increases from fluorine to bromine. R-Cleavage
for several ketones is essentially a triplet (n,π*) state process.
Only R-cleavage in the case of 2-bromo derivatives is
connected with the most rapid intersystem crossing from the
singlet to the triplet state.
Quenching experiments using naphthalene and piperylene
have shown that photolysis of 2-chloro-1,3-diphenylpropan-
1,3-dione to flavone is not quenched by triplet quenchers,
so it must proceed through a singlet or a short-lived triplet
state. The fact that the photocyclization to flavone is superior
in the case of 2-chloro-1-(3,5-dimethoxyphenyl)-3-phenyl-
propan-1,3-dione (φ ) 0.059 ( 0.002) to that in 2-chloro-
1,3-diphenylpropan-1,3-dione (φ ) 0.022 ( 0.002) and that
the cyclization exclusively takes place on an activated phenyl
ring can be explained by the addition step of an electrophilic
oxygen atom in the excited carbonyl group to the benzene
ring which is more favored by meta than by ortho/para donor
groups.
(10) Markov, P. Chem. Soc. ReV. 1984, 13, 69-96.
(11) Kosˇmrlj, J.; Kocˇevar, M.; Polanc, S. Synth. Commun. 1996, 20,
3583-3592.
(12) Hodson, H. F.; Madge, D. J.; Wissowson, D. A. Synlett 1998, 9,
973-974.
(13) White crystalline compound: mp ) 163-165 °C. Spectroscopic
data: 1H NMR (CDCl3) δ 3.90 (s, 3H), 3.93 (s, 3H), 6.94(d, J ) 2.3 Hz),
7.00 (dd, J ) 9; 2.2 Hz), 7.05 (d, J ) 9Hz), 8.00,(d, J ) 9 Hz), 8.17 (d,
J ) 9Hz); 19F NMR (CDCl3) δ -163.27 (s); Mass spectrum m/e 300(100),
272(10), 257(50); HRMS calcd for C17H13FO4 300.07979, found 300.08007.
Acknowledgment. We thank Prof. Dr. Mahesh K. Lak-
shman (CUNY City College) for fruitful discussions and Dr.
Org. Lett., Vol. 9, No. 20, 2007
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