Topochemical photodimerization of (E)-3-benzylidene-4-chromanone derivatives from β-type structures directed by halogen groups

Article abstract of DOI:10.1016/j.tet.2011.09.087

Halogen substituent plays an important role in the crystalline packing of aromatic compounds. The [2+2] photocycloaddition of (E)-3-benzylidene-4- chromanones in the crystalline state was investigated, and halogen substitution has been adopted to organize molecules with proper arrangement for photodimerization. Not halogen bonds, but the electron-withdrawing property of halogen atoms can enhance the face-to-face π-π interactions. Therefore, F, Cl or Br substitution at the para position of phenyl gave rise to almost the same β-structures with face-to-face π-stacking. Only resulted β-structures can undergo photodimerization, which gave the syn-HH (syn-head-to-head) products with high regio-/stereoselectivity.

Full text of DOI:10.1016/j.tet.2011.09.087

Tetrahedron 67 (2011) 9093e9098
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Topochemical photodimerization of (E)-3-benzylidene-4-chromanone derivatives
from
b-type structures directed by halogen groups
*
Xue-Ming Cheng, Zhi-Tang Huang, Qi-Yu Zheng
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2011
Received in revised form 14 September 2011
Accepted 20 September 2011
Available online 25 September 2011
Halogen substituent plays an important role in the crystalline packing of aromatic compounds. The [2þ2]
photocycloaddition of (E)-3-benzylidene-4-chromanones in the crystalline state was investigated, and
halogen substitution has been adopted to organize molecules with proper arrangement for photo-
dimerization. Not halogen bonds, but the electron-withdrawing property of halogen atoms can enhance
the face-to-face
rise to almost the same
p
e
p
interactions. Therefore, F, Cl or Br substitution at the para position of phenyl gave
-structures with face-to-face -stacking. Only resulted -structures can undergo
b
p
b
Keywords:
Solid reaction
Photodimerization
(E)-3-Benzylidene-4-chromanone
Halogen group
photodimerization, which gave the syn-HH (syn-head-to-head) products with high regio-/
stereoselectivity.
Ó 2011 Elsevier Ltd. All rights reserved.
Crystal engineering
1. Introduction
methoxy,8 acetoxy,1b and trifluoromethy9 have been used as
steering groups for the arrangement of aromatic rings in the crystal
In the past several decades, [2þ2] photocyclodimerization in the
crystalline state has been investigated extensively.1 The seminal
study of Schmidt and Wegner established the fundamental top-
ochemical postulate for the field of crystal engineering directed
reaction, which requires that the potentially reactive double bonds
must be aligned parallel to each other, and that the distance be-
engineering. These substituents tend to steer the crystal packed in
a photoactive form ( -structure) characterized by a short axis ca.
4 A.
b
10
ꢀ
Recently, Sakamoto et al. reported two examples of photo-
dimerization of chromane derivatives in solution and solid state.11
Solid-state photolysis of chromone-2-carboxylic esters selectively
gave anti-head-to-tail photodimers (Scheme 1). It has been known
that (E)-3-benzylidene-4-chromanones (homoiso-flavonoids) re-
lated to flavonoids has good biological activity.12 Here, we report
their solid photodimerization behaviors. The halogen has been in-
troduced in the phenyl moiety to control the arrangement of the
substrates in the crystalline state, which promoted the occurring
and regio-/stereoselectivity of the photodimerization. Moreover,
the effect of halogen substitution at different positions on the
crystal packing has been systematically investigated.
2
ꢀ
tween them should be less than 4.2 A. A variety of innovative
approaches have been adopted to organize molecules with proper
arrangement for photodimerization, which include molecular
templates, hosteguest complexes, coordination polymers, micro-
environment, and directing crystal packing.3 Among these elabo-
rate strategies, crystal engineering is a valuable tool to construct the
desired architecture in the solid state.4 For example, hydrogen
bonding has been successfully used to direct molecular arrange-
ment for [2þ2] photodimerization in the crystal.3j Weak face-to-
face
pep interaction was also considered for this topochemical
transformation. But due to the sensitivity of the crystal structure to
molecular information, more cooperative intermolecular in-
teractions are needed. Therefore, fluoro,5 chloro,6 bromo,7
* Corresponding author. Tel.: þ86 010 62652811; fax: þ86 010 62554449; e-mail
Scheme 1.
address: zhengqy@iccas.ac.cn (Q.-Y. Zheng).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.09.087

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X.-M. Cheng et al. / Tetrahedron 67 (2011) 9093e9098
2. Results and discussion
p
e
p
interaction than unsubstituted dimer.15 However, the situa-
tions for halogen substitution at other positions of phenyl are little
complicated, and only 1i, 1j gave the -structures. Unfavorable
interaction among the non- -structures may be due to the stronger
Viewing the crystal structure of (E)-3-benzylidene-4-
chromanone (1a),13 the C]C bonds are parallel, but the nearest
distance between them is 5.270 A, more than 4.2 A (Fig. 1).
According to the Schmidt’s postulate, it could not undergo photo-
dimerization, which was confirmed by our experiment. Therefore,
we considered to introduce a halogen group onto the phenyl for
b
pep
b
ꢀ
ꢀ
electron density and lower symmetry of phenyl group.
All molecules of 1def, 1i, and 1j in the crystal structures are
arranged in a syn-head-to-head
b manner with a slight offset
(Table 6). These parameters basically conform to the topochemical
postulate.Althoughwedidnotgetthecrystalstructureof1h, thelater
photoreaction indicated that it should have similar packing as 1i.
realizing the
b-structure packing by using the additional halogen-
/halogen interaction.10
Fig. 1. The chemical structure of substituted (E)-3-benzylidene-4-chromanones 1ael and arrangement of closest neighbors in their crystal structures except 1h.
The reactants 1ael were synthesized,14 and their crystals were
grown from a mixture of light petroleum, ethyl acetate, and meth-
anol. Except 1h, that we could not get a single crystal suitable for X-
rayanalysis, all structures weredeterminedand the arrangements of
closest neighbors in these crystals are shown in Fig. 1.
Table 1
Weak intermolecular contacts in 1d
ꢀ
ꢀ
Actually, in contrast with compounds 1aec, halogen (F, Cl or Br)
substitution at the para position of phenyl (1def) gave rise to al-
H/O or H/C/A
CeH/O or CeH/C/ꢀ
C/O or C/C/A
C12eH12A/O2
C9eH9B/O2
C9eH9A/O1
C16eH16A/C4
2.689
2.395
2.578
2.761
162.98
139.18
135.30
144.42
CeO/C or CeC/C/ꢀ
153.41
100.92
100.07
CeH/F/ꢀ
141.25
3.608
3.209
3.355
3.577
most the same
b-structures with face-to-face p-stacking. But in
most -structures crystals, halogen/halogen interaction can be
b
ignored (Tables 1e5) because the shortest halogen/halogen dis-
tances are more than the sum of the van der Waals radii. The fa-
vorable p-stacking could be attributed to the electron-withdrawing
property of halogen atoms. According to the HuntereSanders
model, electron-withdrawing substituents diminish the electron
density in the p-cloud of the substituted ring, and result in stronger
ꢀ
ꢀ
O/C or C/C/A
C/C or C/C/A
C7eO2/C9
C3eC2/C3
C5eC6/C5
3.209
3.386
3.399
4.339
3.898
3.898
ꢀ
ꢀ
H/F/A
C/F/A
C3eH3A/F1
2.669
3.461

X.-M. Cheng et al. / Tetrahedron 67 (2011) 9093e9098
9095
Table 2
O
O
Weak intermolecular contacts in 1e
X
O
hv
ꢀ
ꢀ
H/O or H/C/A
CeH/O or CeH/C/ꢀ
C/O or C/C/A
O
X
C13eH13A/C2
C16eH16A/C5
C12eH12A/O2
C7eH7A/O2
2.888
2.833
2.589
2.465
2.551
151.75
140.61
155.07
145.90
138.21
3.752
3.618
3.474
3.331
3.355
crystal state
O
O
X
Scheme 2.
C7eH7B/O1
Table 3
Weak intermolecular contacts in 1f
Table 7
The irradiation of (E)-3-benzylidene-4-chromanones 1ael in the solid state
CeH/O or CeH/C/ꢀ
C/O or C/C/A
ꢀ
ꢀ
H/O or H/C/A
Entry
Compd
Timea (h)
Convb (%)
Yield (%)
C12eH12A/C2
C15eH15A/C5
C9eH9B/O2
C9eH9A/O1
C16eH16A/O1
2.840
2.864
2.571
2.487
2.578
140.05
153.68
138.15
146.77
153.69
CeH/Br/ꢀ
147.24
3.619
3.739
3.374
3.359
3.456
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
40
40
40
10
30
30
40
6
30
40
40
40
d
d
d
82
97
98
d
89
93
95
d
d
d
d
d
72
38
37
d
60
47
54
d
d
ꢀ
ꢀ
H/Br/A
3.032
C/Br/A
3.865
C3eH3A/Br1
9
Table 4
Weak intermolecular contacts in 1i
10
11
12
1j
1k
1l
CeH/O or CeH/C/ꢀ
C/O or C/C/A
ꢀ
ꢀ
H/O or H/C/A
a
C12eH12A/C2
C15eH15A/C5
C16eH16A/O1
C9eH9A/O1
C9eH9B/O1
C9eH9B/O2
2.859
2.869
2.667
2.616
2.700
2.669
C/C/A
3.392
3.341
161.53
123.88
165.12
140.55
135.61
134.38
CeC/C/ꢀ
104.35
83.22
3.623
3.489
3.594
3.439
3.477
3.435
C/C/A
3.969
3.466
The reaction was reached at the photostationary state in the cited irradiation.
The conv and the yield were determined by 1H NMR.
b
The progress of the reaction was monitored by 1H NMR spec-
troscopy and thin layer chromatography. The dimers were purified
by column chromatography and the crystals were grown from
a mixture of light petroleum and ethyl acetate. X-ray structure and
1H NMR spectroscopy analysis indicated the stereochemistry of
products is syn-HH (syn-head-to-head) (Fig. 2). The low yield was
due to deformation of the substrates in the reaction and the lack of
molecular flexibility. It is obvious that high regio-/stereoselectivity
in forming photodimers is attributed to the constricted arrange-
ment in the crystalline state.
ꢀ
ꢀ
C4eC1/C4
C1eC2/C5
H/Br/A
2.945
CeH/Br/ꢀ
124.88
C/Br/A
3.575
ꢀ
ꢀ
C4eH4A/Br1
Table 5
Weak intermolecular contacts in 1j
CeH/O or CeH/C/ꢀ
C/O or C/C/A
ꢀ
ꢀ
H/O or H/C/A
C9eH9A/O2
C13eH13A/O1
C14eH14A/O1
2.572
2.719
2.667
142.35
126.05
120.46
CeH/F or CeH/H/ꢀ
126.50
3.224
3.303
3.594
ꢀ
ꢀ
H/F or H/H/A
C/F or C/H/A
C15eH15A/F1
C3eH3A/F1
C3eH5A/H5A
2.540
2.613
2.396
3.198
3.492
3.122
154.06
132.95
Table 6
The geometric parameters of photoactive compoundsa
ꢀ
No
Compd
d (A)
q1 (deg)
q2 (deg)
q3 (deg)
1
2
3
4
5
1d
1e
1f
1i
3.898
3.906
3.943
3.969
3.882
0
0
0
0
0
92.36
90.97
90.89
92.03
90.90
75.52
75.35
75.47
73.37
79.83
1j
a
The structure of 1h was not determined. q3 is the angle between the >C]C<and
C]C/C]C plane.
Fig. 2. Crystal structures of the photodimerization products.
Powder crystals were placed between two quartz slides and ir-
radiated with a 500 W high-pressure mercury lamp under argon
It is easy to understand why 1c, 1g, and 1l could not undergo
dimerization from Fig. 1. The nearest double bonds of 1c and 1l are
and Pyrex filter at room temperature. For those
b
-form arranged
compounds, 1def, 1hej,
a clean photodimerization occurred
ꢀ
parallel and the distance between them is 5.485, 5.328 A, re-
(Scheme 2, Table 7).
ꢀ
spectively, more than 4.2 A. The double bonds of 1g are not parallel

9096
X.-M. Cheng et al. / Tetrahedron 67 (2011) 9093e9098
(
q1¼53.67ꢀ), although the nearest distance between them is
mixture was refluxed for 24 h and then diluted with water. Filtra-
tion afforded the crude product, which was crystallized from
methanol to give the pure product.
ꢀ
3.935 A.
A few examples were reported that double bonds are within
16
ꢀ
4.2 A, but photochemically inert. Both of 1b, 1k are arranged in
form (anti-head-to-tail) with potentially reactive double bonds
a
4.2.1. (E)-3-Benzylidene-4-chromanone 1a20. Colorless crystal,
(Fig. 1). In contrast to the above photoactive compounds, the
neighboring moieties of the C]C bonds in 1b, 1k are not parallel.
Probably due to the steric compression, these moieties hamper
close movement of double bonds toward each other. Thus unpar-
allel groups act as stationary impediments to the
photodimerization.
The photochemistry of (E)-3-benzylidene-4-chromanones in
solution was also examined. Same as reported before,17 photo-
dimerization was not observed. However, we isolated product 3a18
when irradiating the solution of 1a in acetonitrile (Scheme 3),
which is similar to Chawla and Marathe’s reports.19 The isolated
yield of this reaction is 45% in the presence of oxygen and iodine.
The mechanism may include the 1O2 initiated ene reaction, then
elimination of H2O.
yield 79%, 1H NMR (400 MHz, CDCl3):
d
8.03 (dd, J¼7.6, 1.6 Hz, 1H,
ArH), 7.89 (s, 1H, CH), 7.51e7.41 (m, 4H, ArH), 7.33e7.30 (m,
J¼1.2 Hz, 2H, ArH), 7.10e7.06 (m, 1H, ArH), 6.97 (d, J¼8.0 Hz, 1H,
ArH), 5.36 (d, J¼1.6 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d
182.37, 161.34,137.62,136.02, 134.60, 131.14, 130.16, 129.63, 128.91,
128.14, 122.24, 122.09, 118.10, 67.80. HRMS (EI): calcd for C16H12O2
(Mþ): 236.0837; found: 236.0835.
4.2.2. (E)-3-(4-Methylbenzylidene)-4-chromanone1b20. Colorless
crystal, yield 51%, 1H NMR (400 MHz, CDCl3):
d
8.03 (dd, J¼8, 2 Hz,
1H, ArH), 7.86 (s, 1H, CH), 7.51e7.46 (m, 1H, ArH), 7.27e7.21 (m, 5H,
ArH), 7.09e7.05 (m, 1H, ArH), 6.97 (d, J¼8.4 Hz, 1H, ArH), 5.37 (d,
J¼2 Hz, 2H, CH2), 2.41 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3):
d
182.47, 161.31, 140.14, 137.81, 135.97, 131.82, 130.35, 129.70, 128.15,
122.31, 122.07, 118.08, 67.95, 21.68. HRMS (EI): calcd for C17H14O2
(Mþ): 250.0994; found: 250.0990.
4.2.3. (E)-3-(4-Methoxybenzylidene)-4-chromanone 1c20. Colorless
crystal, yield 69%, 1H NMR (400 MHz, CDCl3):
d
8.02 (dd, J¼8, 1.6 Hz,
1H, ArH), 7.84 (s, 1H, CH), 7.50e7.46 (m, 1H, ArH), 7.28 (d, J¼8.8 Hz,
2H, ArH), 7.09e7.05 (m, 1H, ArH), 6.98e6.95 (m, 3H, ArH), 5.38 (dd,
J¼2 Hz, 2H, CH2), 3.86 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3):
d
182.38, 161.19, 160.95, 137.52, 135.86, 132.26, 129.13, 128.10,
Scheme 3.
127.25, 122.35,122.04, 118.03, 114.49, 68.00, 55.62. HRMS (EI): calcd
for C17H14O3 (Mþ): 266.0943; found: 266.0947.
4.2.4. (E)-3-(4-Fluorobenzylidene)-4-chromanone
1d21. Colorless
8.03 (dd, J¼7.6,
3. Conclusion
crystal, yield 61%, 1H NMR (400 MHz, CDCl3):
d
1.6 Hz, 1H, ArH), 7.83 (s, 1H, CH), 7.52e7.48 (m, 1H, ArH), 7.32e7.29
(m, 2H, ArH), 7.17e7.06 (m, 2H, ArH), 6.97 (d, J¼8.4 Hz, 1H, ArH),
In summary, we present here the [2þ2] photocycloaddition of
(E)-3-benzylidene-4-chromanones in the crystalline state. Halogen
substituent in phenyl group plays an important role in crystal
packing. Not halogen bonds, but the electron-withdrawing prop-
5.33 (d, J¼1.6 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d 182.23,
164.65, 162.15, 161.27, 136.40, 136.12, 132.20, 132.12, 130.97, 130.73,
128.14, 122.18, 118.11, 116.26, 116.05, 67.67. HRMS (EI): calcd for
C16H11O2F (Mþ): 254.0743; found: 254.0746.
erty of halogen atoms can enhance the face-to-face
teractions. Therefore, F, Cl or Br substitution at the para position of
phenyl gave rise to almost the same -structures with face-to-face
-stacking. Only resulted -structures can undergo photo-
pep in-
b
4.2.5. (E)-3-(4-Chlorobenzylidene)-4-chromanone
crystal, yield 70%, 1H NMR (400 MHz, CDCl3):
1e22. Colorless
8.02 (dd, J¼8, 1.6 Hz,
p
b
d
dimerization, which gave the syn-HH (syn-head-to-head) products
with high regio-/stereoselectivity. This work will inspire more ideas
about photodimerization in organic synthesis.
1H, ArH), 7.81 (s, 1H, CH), 7.52(m, 1H, ArH), 7.44e7.42 (m, 2H, ArH),
7.24 (s, 1H, ArH), 7.10e7.06 (m, 1H, ArH), 6.98 (d, J¼8.4 Hz, 1H, ArH),
5.31 (d, J¼1.6 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d 182.15,
161.31, 136.20, 135.80, 133.01, 131.63, 131.39, 129.26, 128.18, 122.24,
122.13, 118.15, 67.66. HRMS (EI): calcd for C16H11O2Cl (Mþ):
270.0448; found: 270.0451.
4. Experimental section
4.1. General
4.2.6. (E)-3-(4-Bromobenzylidene)-4-chromanone
crystal, yield 75%, 1H NMR (400 MHz, CDCl3):
1f22. Colorless
8.02 (dd, J¼8,1.6 Hz,
Melting points were determined on a Laboratory Devices X-4
apparatus and are uncorrected. IR spectra were taken on a Nicolet
380 FT-IR spectrometer in KBr film and reported in cmꢁ1. 1H NMR
and 13C NMR spectra were measured on a Bruker DMX 400 MHz
d
1H, ArH), 7.79 (s, 1H, CH), 7.59 (d, J¼8.4 Hz, 2H, ArH), 7.52e7.48 (m,
1H, ArH), 7.18 (d, J¼8.4 Hz, 2H, ArH), 7.10e7.06 (m, 1H, ArH), 6.98 (d,
J¼8.4 Hz, 1H, ArH), 5.30 (d, J¼2 Hz, 2H, CH2); 13C NMR (100 MHz,
spectrometer in CDCl3 with chemical shift (d) given in parts per
CDCl3):
d 182.16, 161.33, 136.25, 136.23, 133.46, 132.23, 131.73,
million relative to TMS as internal standard. HRMS (EI) was de-
termined by using Micromass GCT MS instrument (Waters). X-ray
crystallographic analysis was performed with a Rigaku R-AXIS
RAPID IP.
131.58, 128.19, 124.10, 122.25, 122.13, 118.16, 67.66. HRMS (EI): calcd
for C16H11O729Br (Mþ): 313.9942; found: 313.9940; calcd for
C16H11O821Br (Mþ): 315.9922; found: 315.9926.
4.2.7. (E)-3-(3-Fluorobenzylidene)-4-chromanone
crystal, yield 70%, 1H NMR (400 MHz, CDCl3):
1g23. Colorless
8.02 (dd, J¼8, 1.6 Hz,
4.2. General procedure for the synthesis of 3-benzylidene-4-
chromanones derivatives
d
1H, ArH), 7.81 (s, 1H, CH), 7.52e7.48 (m, 1H, ArH), 7.45e7.40 (m, 1H,
ArH), 7.14e7.06(m, 3H, ArH), 7.02e6.97 (m, 2H, ArH), 5.32 (d,
A round-bottom flask was charged with the 4-chromanone
(3.4 mmol) and aromatic-aldehyde (3.6 mmol). This was dissolved
in methanol (20 mL), and concentrated hydrochloride (10 mL). The
J¼2 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d 182.15, 164.15,
161.69, 161.38, 136.70, 136.62, 136.26, 136.11, 136.09, 132.18, 130.60,
130.52, 128.19, 125.88, 125.85, 122.25, 122.10, 118.18, 116.85, 116.69,

X.-M. Cheng et al. / Tetrahedron 67 (2011) 9093e9098
9097
116.63, 116.48, 67.63. HRMS (EI): calcd for C16H11O2F (Mþ):
254.0743; found: 254.0746.
4.3.1. 3-Benzoyl-chromone 3a18. White solid, yield 45%, 1H NMR
(400 MHz, CDCl3):
8.30 (s, 1H, CH), 8.26 (dd, J¼8, 1.6 Hz, 1H, ArH),
7.85 (t, J¼7.6 Hz, 2H, ArH), 7.77e7.72(m, 1H, ArH), 7.61e7.53 (m, 2H,
ArH), 7.50e7.45(m, 3H, ArH); 13C NMR (100 MHz, CDCl3):
192.09,
174.92, 158.84, 156.30, 137.40, 134.60, 133.72, 129.80, 128.63, 126.72,
126.34, 125.43, 125.23, 118.53; APCI-MS: m/z 251.28 [MþH]þ. HRMS
(EI): calcd for C16H10O3 (Mþ): 250.0630; found: 250.0626.
d
4.2.8. (E)-3-(3-Chlorobenzylidene)-4-chromanone
solid, yield 65%, 1H NMR (400 MHz, CDCl3):
1h24. White
8.02 (dd, J¼8, 1.2 Hz,
d
d
1H, ArH), 7.79 (s, 1H, CH), 7.52e7.48(m, 1H, ArH), 7.40 (s, 1H, ArH),
7.38 (s, 1H, ArH),7.29 (s, 1H, ArH), 7.19 (t, J¼4.4 Hz, 1H, ArH), 7.09 (t,
7.2 Hz, 1H, ArH), 6.98(d, J¼8.4 Hz, 1H, ArH), 5.31 (d, J¼1.2 Hz, 2H,
CH2); 13C NMR (100 MHz, CDCl3):
d
182.13, 161.39, 136.34, 136.29,
4.4. General procedure for the photodimerization of 3-
benzylidene-4-chromanone derivatives
135.92, 134.98, 132.33, 130.22, 129.84, 129.63, 128.21, 128.15, 122.28,
122.10, 118.20, 67.61. HRMS (EI): calcd for C16H11O2Cl (Mþ):
270.0448; found: 270.0451.
Powdered 3-benzylidene-4-chromanone derivatives 1ael
sandwiched two quartz plates charged in Pyrex tube were irradi-
ated with a 500-W high-pressure mercury lamp in argon. Crude
photolysate was subjected to silica gel column chromatography
using petroleum ether: dichloromethane (1:2). Solid dimers were
recrystallized from a mixture of petroleum ether and EtOAc.
4.2.9. (E)-3-(3-Bromobenzylidene)-4-chromanone
crystal, yield 75%, 1H NMR (400 MHz, CDCl3):
1i22. Colorless
8.02 (dd, J¼8, 1.6 Hz,
d
1H, ArH), 7.78 (s, 1H, CH), 7.55e7.48 (m, 2H, ArH), 7.45 (s, 1H, ArH),
7.33 (t, J¼8 Hz, 1H, ArH), 7.23(d, J¼7.6 Hz, 1H, ArH), 7.11e7.07 (m,
1H, ArH), 6.98 (d, J¼8.4 Hz, 1H, ArH), 5.31 (d, J¼2 Hz, 2H, CH2); 13
C
NMR (100 MHz, CDCl3):
d
182.10, 161.40, 136.64, 136.29, 135.80,
4.4.1. syn-HH dimer 2d. Colorless crystal, mp: 255e256 ꢀC; IR (KBr
132.72, 132.53, 132.38, 130.45, 128.57, 128.21, 123.06, 122.28, 122.11,
118.20, 67.59. HRMS (EI): calcd for C16H11O729Br (Mþ): 313.9942;
found: 313.9939; calcd for C16H11O821Br (Mþ): 315.9922; found:
315.9927.
film):
n ;
1689,1608,1510,1480,1462,1308,1220, 756, 731, 409 cmꢁ1
1H NMR (400 MHz, CDCl3):
d
7.52 (dd, J¼8, 1.6 Hz, 2H, ArH),
7.22e7.18 (m, 2H, ArH), 6.97e6.92 (m, 8H, ArH), 6.73 (t, J¼7.6 Hz,
2H, ArH), 6.56 (d, J¼8.4 Hz, 2H, ArH), 5.20 (s, 2H, CH), 4.76 (d,
J¼13.2 Hz, 2H, CH2), 4.46 (d, J¼13.2 Hz, 2H, CH2); 13C NMR
4.2.10. (E)-3-(2-Fluorobenzylidene)-4-chromanone 1j20. Colorless
(100 MHz, CDCl3): d 190.38, 162.94, 161.15, 160.49, 136.17, 131.38,
crystal, yield 55%, 1H NMR (400 MHz, CDCl3):
d
8.04 (dd, J¼8, 1.6 Hz,
131.31, 131.04, 127.64, 121.18, 119.74, 117.01, 115.50, 115.29, 68.78,
54.09, 39.21. HRMS (EI): calcd for C32H22O4F2 (Mþ): 508.1486;
found: 508.1494.
1H, ArH), 7.85 (d, J¼1.2 Hz, 1H, CH), 7.52e7.48 (m, 1H, ArH),
7.44e7.38 (m, 1H, ArH), 7.23e7.14 (m, 3H, ArH), 7.10e7.06 (m, 1H,
ArH), 7.12e7.04 (m, 1H, ArH), 6.97 (d, J¼8.4 Hz, 1H, ArH), 5.19 (t,
J¼1.6 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d
182.09, 161.98,
4.4.2. syn-HH dimer 2e. Colorless crystal, mp: 288e289 ꢀC; IR (KBr
161.60, 159.49, 136.18, 133.09, 131.65, 131.57,131.13, 131.10, 130.51,
130.49, 128.23, 124.43, 124.39, 122.67, 122.52, 122.18, 118.18, 116.42,
116.20, 68.14, 68.09. HRMS (EI): calcd for C16H11O2F (Mþ):
254.0743; found: 254.0746.
film):
(400 MHz, CDCl3):
n ;
1688, 1607, 1491, 1479, 1460, 1307, 754, 733 cmꢁ1 1H NMR
d
7.51 (dd, J¼7.6, 1.2 Hz, 2H, ArH), 7.24e7.18 (m,
6H, ArH), 6.93 (d, J¼8.4 Hz, 4H, ArH), 6.73 (t, J¼7.2 Hz, 2H, ArH),
6.56 (d, J¼8.4 Hz, 2H, ArH), 5.19 (s, 2H, CH), 4.75 (d, J¼12.8 Hz, 2H,
CH2), 4.43 (d, J¼12.8 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
4.2.11. (E)-3-(2-Chlorobenzylidene)-4-chromanone 1k22. Colorless
d 190.23, 161.14, 136.23, 133.84, 132.95, 131.08, 128.64, 127.63,
crystal, yield 61%, 1H NMR (400 MHz, CDCl3):
d
8.05 (dd, J¼8, 1.6 Hz,
121.20, 119.66, 117.03, 68.66, 54.10, 39.28. HRMS (EI): calcd for
1H, ArH), 7.96 (s, 1H, CH), 7.52e7.48 (m, 2H, ArH), 7.36e7.32 (m, 2H,
ArH), 7.24 (s, 1H, ArH), 7.14e7.07 (m, 2H, ArH), 6.97 (d, J¼8.4 Hz, 1H,
ArH), 5.31(d, J¼1.6 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
C32H22O4Cl2 (Mþ): 540.0895; found: 540.0901.
4.4.3. syn-HH dimer 2f. Colorless crystal, mp: >300 ꢀC; IR (KBr
d
182.29,161.53,136.24,135.17,134.71,133.14,132.69,130.71,130.53,
film):
(400 MHz, CDCl3):
n
1688, 1608, 1479, 1460, 1307, 1010, 756, 424 cmꢁ1; 1H NMR
130.30, 128.24, 126.84, 122.22, 118.19, 67.75. HRMS (EI): calcd for
C16H11O2Cl (Mþ): 270.0448; found: 270.0451.
d
7.51 (dd, J¼7.6, 1.2 Hz, 2H, ArH), 7.38 (d,
J¼8.4 Hz, 4H, ArH), 7.23e7.18 (m, 2H, ArH), 6.87 (d, J¼8.4 Hz, 4H,
ArH), 6.73 (t, J¼8 Hz, 2H, ArH), 6.56 (d, J¼8.4 Hz, 2H, ArH), 5.17 (s,
2H, CH), 4.74 (d, J¼13.2 Hz, 2H, CH2), 4.45 (d, J¼13.2 Hz, 2H, CH2);
4.2.12. (E)-3-(2-Bromobenzylidene)-4-chromanone 1l22. Colorless
crystal, yield 50%, 1H NMR (400 MHz, CDCl3):
d
8.05 (dd, J¼8, 1.6 Hz,
13C NMR (100 MHz, CDCl3):
d 190.20, 161.14, 136.24, 134.36, 131.60,
1H, ArH), 7.90 (s, 1H, CH), 7.68 (dd, J¼8.0, 0.8 Hz, 1H, ArH),
7.52e7.48 (m, 1H, ArH), 7.40e7.36 (m, 1H, ArH), 7.29e7.25 (m, 1H,
ArH), 7.13e7.07 (m, 2H, ArH), 6.97 (d, J¼8 Hz, 1H, ArH), 5.17 (d,
131.41, 127.63, 121.21, 121.04, 119.65, 117.04, 77.55, 68.65, 54.07,
39.32. HRMS (EI): calcd for C32H22O749Br81Br (Mþ): 629.9864;
found: 629.9858.
J¼2 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d 182.28, 161.54,
136.84,136.25,134.98,133.49,132.40,130.79,130.50,128.23,127.44,
125.09, 122.22, 118.20, 67.67. HRMS (EI): calcd for C16H11O729Br
(Mþ): 313.9942; found: 313.9946; calcd for C16H11O821Br (Mþ):
315.9922; found: 315.9926.
4.4.4. syn-HH dimer 2h. Colorless crystal, mp: 219e220 ꢀC; IR (KBr
film):
CDCl3):
n
1689, 1608, 1478, 1306, 1219, 756 cmꢁ1; 1H NMR (400 MHz,
d
7.52 (dd, J¼8,1.6 Hz, 2H, ArH), 7.24e7.16 (m, 6H, ArH), 6.99
(s, 2H, ArH), 6.87 (d, J¼7.6 Hz, 2H, ArH), 6.73 (t, J¼7.6 Hz, 2H, ArH),
6.56 (d, J¼8.4 Hz, 2H, ArH), 5.21 (s, 2H, CH), 4.77 (d, J¼13.2 Hz, 2H,
CH2), 4.49 (d, J¼13.2 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
4.3. Procedure for photoreaction of (E)-3-benzylidene-4-
d 190.04, 161.14, 137.42, 136.25, 134.48, 129.76, 129.61, 127.86,
chromanone
127.66, 127.28, 121.22, 119.64, 117.02, 68.63, 54.18, 39.54. HRMS (EI):
calcd for C32H22O4Cl2 (Mþ): 540.0895; found: 540.0902.
A quartz tube was charged with a solution of (E)-3-benzylidene-
4-chromanone (50 mg, 0.2 mmol) and catalytic amount of I2 in
acetonitrile saturated with oxygen (10 mL). The mixture was irra-
diated with a high-pressure mercury lamp (500 W) for 24 h. The
reaction mixture was then concentrated under vacuum and the
residue was purified by silica gel column chromatography using
petroleum ether:EtOAc (2:1) as eluent.
4.4.5. syn-HH dimer 2i. Colorless crystal, mp: 219e221 ꢀC; IR (KBr
film):
CDCl3):
n ;
1688, 1608, 1479, 1307, 913, 743 cmꢁ1 1H NMR (400 MHz,
d
7.52 (dd, J¼8, 1.6 Hz, 2H, ArH), 7.38 (d, J¼8.4 Hz, 2H, ArH),
7.23e7.19 (m, 2H, ArH), 7.15e7.10 (m, 4H, ArH), 6.92 (d, J¼7.6 Hz, 2H,
ArH), 6.73 (t, J¼7.6 Hz, 2H, ArH), 6.56 (d, J¼8.4 Hz, 2H, ArH), 5.20 (s,
2H, CH), 4.77 (d, J¼13.2 Hz, 2H, CH2), 4.48 (d, J¼13.1 Hz, 2H, CH2);

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d
190.02, 161.15, 137.68, 136.26, 132.68,
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130.23, 129.88, 128.31, 127.67, 122.66, 121.24, 119.65, 117.03, 68.63,
54.19, 39.51. HRMS (EI): calcd for C32H22O749Br81Br (Mþ): 629.9864;
found: 629.9856.
4.4.6. syn-HH dimer 2j. Colorless crystal, mp; 256e257 ꢀC; IR (KBr
film): 1690, 1608, 1481, 1461, 1307, 756, 417 cmꢁ1
;
1H NMR
(400 MHz, CDCl3):
d
7.54 (dd, J¼8, 1.6 Hz, 2H, ArH), 7.25e7.17 (m,
6H, ArH), 7.05e6.98 (m, 6H, ArH), 6.73 (t, J¼7.2 Hz, 2H, ArH), 6.53
(d, J¼8.4 Hz, 2H, ArH), 5.39 (s, 2H, CH), 4.83 (d, J¼12.8 Hz, 2H, CH2),
4.58 (dd, J¼12.8, 1.2 Hz, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d
189.38, 162.16, 161.21, 159.72, 135.91, 130.79, 130.76, 128.88,
128.80, 127.78, 123.81, 123.78, 123.56, 123.42, 121.06, 119.68, 116.86,
116.18, 115.96, 68.49, 68.451, 54.61, 34.32. HRMS (EI): calcd for
C32H22O4F2 (Mþ): 508.1486; found: 508.1493.
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We are grateful for financial support from the National Natural
Science Foundation of China (20932004), the Major State Basic
Research Development Program of China (2011CB932501), and the
Chinese Academy of Sciences.
Supplementary data
Crystallographic reports (CCDC numbers: 815957e815970).
Supplementary data associated with this article can be found, in the
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