First synthesis of nitro-substituted 2,2-diphenyl-2H-1-benzopyrans via the ipso-nitration reaction

Article abstract of DOI:10.1016/j.tet.2007.05.113

The first synthesis of a series of nitro-substituted 2,2-diphenyl-2H-1-benzopyrans is reported. Our synthetic approach is based on a linear synthesis in two steps from appropriate brominated 2,2-diphenyl-2H-1-benzopyrans 12-17, which requires the preliminary preparation of bromophenols 7-11. These latter were easily obtained by the reaction of phenols 1-5 with a mild and selective brominating agent tetrabutylammonium tribromide (TBA·Br₃). The key intermediates 12-17 were efficiently elaborated through an univocal classic chromenization between the commercially available 1,1-diphenyl-2-yn-1-ol and the brominated phenols 6-11. The compounds 12-17 so obtained were converted into arylboronic acids 18-23 by a metalation/boronylation sequence, followed by acid hydrolysis. From advanced building blocks 18-23, the introduction of nitro group, which constitutes the ultimate step of our strategy, was achieved by an ipso-nitration reaction using the Crivello's reagent. This highly selective method provides only the ipso-nitrated products 24-29 in moderate to high yield.

Full text of DOI:10.1016/j.tet.2007.05.113

Tetrahedron 63 (2007) 8242–8249
First synthesis of nitro-substituted 2,2-diphenyl-2H-1-
benzopyrans via the ipso-nitration reaction
a
Lahoussine Bougdid, Arnault Heynderickx, Stephanie Delbaere and Corinne Moustrou
a
b
a,
*
ꢀ
a
´
´
´
´
´
Groupe de Chimie Organique et Materiaux Moleculaires, UMR CNRS 6114, Universite de la Mediterranee,
´
Faculte des Sciences de Luminy, case 901, 163 Avenue de Luminy, 13288 Marseille cedex 9, France
Laboratoire de Physique et d’Application RMN, Faculte des Sciences Pharmaceutiques et Biologiques,
b
´
´
Universite de Lille 2, BP 83, 59006 Lille cedex, France
Received 10 April 2007; revised 22 May 2007; accepted 26 May 2007
Available online 2 June 2007
Abstract—The first synthesis of a series of nitro-substituted 2,2-diphenyl-2H-1-benzopyrans is reported. Our synthetic approach is based on
a linear synthesis in two steps from appropriate brominated 2,2-diphenyl-2H-1-benzopyrans 12–17, which requires the preliminary prepara-
tion of bromophenols 7–11. These latter were easily obtained by the reaction of phenols 1–5 with a mild and selective brominating agent
tetrabutylammonium tribromide (TBA$Br3). The key intermediates 12–17 were efficiently elaborated through an univocal classic chromeni-
zation between the commercially available 1,1-diphenyl-2-yn-1-ol and the brominated phenols 6–11. The compounds 12–17 so obtained were
converted into arylboronic acids 18–23 by a metalation/boronylation sequence, followed by acid hydrolysis. From advanced building blocks
18–23, the introduction of nitro group, which constitutes the ultimate step of our strategy, was achieved by an ipso-nitration reaction using the
Crivello’s reagent. This highly selective method provides only the ipso-nitrated products 24–29 in moderate to high yield.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
The spirocarbon atom can change reversibly its hybridization
from sp3 to sp2, allowing or stopping the electron flow be-
tween different parts of the molecular system. This con-
comitant electronic change, transforming an unconjugated
off-state to a conjugated on-state, can be used to control
and modulate the physical properties of organic molecular
material, which depend on the conjugation state. Thus, the
photochromic system working through an externallight stim-
ulus can be the basic unit for a molecular switch.5 Each indi-
vidual chromene constitutes attractive building blocks for the
design and the synthesis of molecular switches at a meso or
macroscopic scale. The incorporation of photochromic units
into such macromolecular architectures requires the prelim-
inary preparation of functionalized individual photochromic
sections. Organic chemist must have outstanding synthetic
tools at its disposal to introduce various functionalizations
into photochromic skeleton. In this context, we have previously
In the last decades, a renewal of interest in the design and the
synthesis of 2H-1-benzopyrans (chromenes) has been ob-
served due to their industrial applications for variable optical
transmission materials (ophthalmic glasses and lenses)1 and
their novel applications in emerging optoelectronic and pho-
tonic technologies.2 The photochromic properties of the 2H-
chromene structure arise from an equilibrium between a
closed form and a set of open forms (photomerocyanines).3
The initial closed form (CF) absorbs in the UV region and,
through excited intermediates, gives cleavage of the Csp3–
O bond and isomerization. It leads to different stereoisomers
of quasi-planar opened form (OF) absorbing in the visible
range. Both opened isomers are thermally instable and the
visible light stimulates photobleaching, leading to the initial
closed form4 as depicted in Scheme 1.
NO2
C6H5
C6H5
NO2
C6H5
C6H5
NO2
1 (UV)
C6H5
C6H5
O
2 (Vis) and / or Δ
O
O
Colorless closed form (CF)
Colored open forms (OF)
Scheme 1. Photochromic equilibrium for 2H-1-benzopyrans.
Keywords: Heterocycles; Synthesis design; ipso-Nitration; Photochromism.
* Corresponding author. Fax: +33 491829301; e-mail: moustrou@luminy.univ-mrs.fr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.113

L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
8243
demonstrated that the palladium-catalyzed cross-coupling
reactions could be successfully applied to [2H]-chromenes.6
Surprisingly, to the best of our knowledge, the introduction of
a nitro group into the benzopyran or naphthopyran part has
never been investigated. This unsolved synthetic problem is
undoubtedly attributable to the lack of regioselectivity,
harshness of nitration reaction conditions. The classical ni-
tration methods are not suitable.7 They led very often to the
formation of various tarry by-products.
character is totally inhibited. In view of these disappointing
results, it became apparent that introduction of the nitro
group must occur after formation of the chromenic structure.
Thus, we designed the retrosynthetic analysis depicted in
Scheme 3.
The adopted synthetic strategy began with the synthesis of
the brominated chromenes 12–17, which were submitted
to a metalation/boronylation sequence, followed by acidic
hydrolysis to furnish the advanced boronic acids 18–23. Fi-
nally, the introduction of the nitro group should be realized
by an ipso-nitration reaction, which completes the retrosyn-
thetic approach. The overall synthetic procedure from 12–17
to 24–29 exhibits three different reaction types. Synthesis of
the brominated chromenes 12–17 requires the preliminary
preparation of the suitable brominated phenols 7–11. Con-
version of phenols 1–5 into their brominated homologues
7–11 was achieved using a mild and selective brominating
agent: tetrabutylammonium tribromide (TBA$Br3)12
(Scheme 4).
Herein, we report the first synthesis of a series of nitro-
substituted 2,2-diphenyl-2H-1-benzopyrans. The introduction
of a nitro group into different positions of the benzopyranic
part has been achieved. The resolution of this challenging
synthetic problem opens new perspectives towards improve-
ments in photochromic properties of benzopyrans.
2. Results and discussion
Commonly, two synthetic methods are used for the building
of the 2,2-diphenyl-2H-1-benzopyrans. The usual approach
is based on a ‘one-pot reaction’ between an appropriate phe-
nol and the commercially available 1,1-diphenyl-2-yn-1-ol.8
This condensation takes place in an apolar solvent (CH2Cl2,
toluene) under acid catalysis (PTSA,9 PPTS6c,10). The
reaction proceeds via a Claisen-like [3,3]-sigmatropic
rearrangement of alkynyl aryl ether, resulting from phenol
‘O-alkylation’, which is followed by enolization and a
[1,5]-sigmatropic H-shift. An electrocyclization completes
the process. The second method involves the reaction of
the a,b-unsaturated aldehyde with titanium(IV) salts of phe-
nols11 leading to the C-alkylation in a ortho-position; subse-
quent electrocyclization yields the 2H-1-benzopyran moiety.
In the beginning of our investigations relating to the prepa-
ration of the 2,2-diphenyl-6-nitro-2H-1-benzopyran, these
two synthetic methods have been tested: the 4-nitrophenol
was reacted with the commercially available 1,1-diphenyl-
2-yn-1-ol and the b-phenylcinnamaldehyde, respectively
(Scheme 2).
OH
OH
X1
X2
X1
X2
TBA.Br3 (1eq)
CH2Cl2, MeOH
r.t
X4
X4
X3
X3
7-11
1-5
Scheme 4.
The reaction of commercial phenols 1–5 with a molar
quantity of TBA$Br3, in dichloromethane–methanol solu-
tion, for 1 h, at room temperature, gave selectively the
expected monobromophenols 7–11.13 This very simple pro-
cedure for bromination of phenols proceeded rapidly and
smoothly. The active species, which generates Br+ is pre-
sumably methyl hypobromite produced from the reaction
of TBA$Br3 with methanol.13b,14 The isolated yields are
good to excellent ranging from 88 to 98%, after recrystalli-
zation from methanol–water. The results are summarized in
Table 1.
In both cases, no positive result was obtained even with ex-
tended reaction time. Only unchanged starting materials
were recovered. The strong electron-withdrawing property
of the nitro group deactivates the phenol. Its nucleophilic
The commercially available 4-bromophenol 6 and the previ-
ous monobrominated phenols 7–11 were subsequently
involved in the standard chromenization procedure as
outlined in Scheme 5.
OH
OH
OH
C
(ii)
C6H5
C6H5
(i)
C6H5
C6H5
C6H5
C6H5
CH
CH CHO
O
NO2
NO2
NO2
(i): PTSA, CH2Cl2, Δ; (ii): Ti(OEt)4, Toluene, Δ
Scheme 2. Two classic procedures for the synthesis of benzopyranic structures.
OH
OH
C
C6H5
C6H5
NO2
C6H5
C6H5
B(OH)2
C6H5
C6H5
Br
C6H5
C6H5
CH
O
O
O
Br
24 - 29
18 - 23
12 - 17
6 - 11
Scheme 3. Retrosynthetic analysis.

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L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
Table 3. Yields in the preparation of boronic acids 18–23
Table 1. Preparation of the monobrominated phenols
Entry
X1
X2
X3
X4
Bromination (yield %)a
Entry Starting
Boronation X1
(yield %)a
X2
X3
X4
brominated reaction
phenol
1
2
3
4
5
CH3
CH3
CH3
Br
H
CH3
H
H
CH3
Br
Br
Br
CH3
Br
H
H
CH3
CH3
CH3
7 (93)
8 (93)
9 (98)
10 (97)
11 (88)
1
2
3
4
5
6
12
13
14
15
16
17
18 (79)
19 (89)
20 (77)
21 (67)
22 (65)
23 (35)
H
H
H
B(OH)2
B(OH)2
H
H
H
CH3
CH3
CH3
B(OH)2
H
H
CH3 B(OH)2
H
H
B(OH)2 CH3
CH3 CH3
a
All the yields are for pure, isolated products.
CH3 B(OH)2 CH3
a
All the yields are for pure, isolated products.
OH
X4
OH
C
X1
X2
C6H5
C6H5
PTSA, CH2Cl2
r.t.
C6H5
C6H5
CH
X3
O
X4
retrosynthetic approach (Scheme 3). The replacement of
boronic acid group with nitro group was accomplished by
ipso-nitration reaction.17 In this context, two methods were
recently reported. The first one was based on the use of
Crivello’s reagent18 (NH4NO3/(CF3CO)2O).19 The second
involved the system AgNO3/(CH3)3SiCl.20 We applied the
first methodology, for which trifluoroacetylnitrate seems to
be the reactive nitrating agent. The typical procedure of
nitration began with the preparation of nitrating agent: tri-
fluoroacetic anhydride was slowly and carefully added to
a mixture of ammonium nitrate (1.1 equiv) in acetonitrile,
with vigorous stirring until all solids had dissolved. The
boronic acid (1 equiv) 18–23, previously dissolved in aceto-
nitrile in the presence of trifluoroacetic anhydride, was re-
acted with the prepared nitrating agent at ꢀ35 ꢁC, to give
only one compound: the desired ipso-nitrated product 24–
29 (Scheme 7). Considering the high oxophilicity of boron,
the intermediate active nitrating agent interacted electroni-
cally with the boronic acid group, favouring the nitration
at the ipso-position.
X3
X2
X1
12 - 17
6 - 11
Scheme 5. Preparation of the chromenic key intermediates 12–17.
The photochromic compounds 12–17 were obtained in satis-
factory yield varying from 54 to 85% (Table 2), from con-
densation of suitable monobrominated phenols 6–11 with
the commercially available 1,1-diphenyl-2-yn-1-ol. The re-
action was carried out in dichloromethane, at room temper-
ature in the presence of a catalytic amount of PTSA. The
reaction was monitored by TLC and was stopped when the
starting brominated phenol was consumed.
Halide-metal exchange of the brominated benzopyrans 12–
17 with 2 equiv of n-BuLi at ꢀ40 ꢁC in dry THF,15 afforded
the corresponding lithium species, which were transmeta-
lated at ꢀ90 ꢁC with an excess of tributylborate (Scheme 6).
During acidic workup, the resulting crude boronic esters, not
isolated, were hydrolysed to give the corresponding boronic
acids16 18–23 in 35–89% yield. The results are reported in
Table 3.
X4
X4
C6H5
C6H5
C6H5
C6H5
NH4NO2, (CF3CO)2O
CH3CN, -35 °C
X3
X3
O
O
X1
X2
X1
X2
After the successful synthesis of advanced intermediates
18–23, our attention turned towards the introduction of the
nitro group, which constitutes the ultimate step of our
18 - 23
24 - 29
Scheme 7.
After purification by silica gel chromatography, the yields in
isolated compounds were moderated to good (32–80%). The
results are reported in Table 4. It is noteworthy that in all
cases, we observed the complete absence of dinitroproducts.
Table 2. Yields in the synthesis of brominated 2,2-diphenyl-2H-1-benzopyr-
ans 12–17
Entry Starting
brominated (yield %)a
phenol
Chromenisation X1
X2
X3
X4
1
2
3
4
5
6
6
7
8
9
10
11
12 (74)
13 (69)
14 (71)
15 (85)
16 (54)
17 (76)
H
CH3
H
H
Br
Br
H
H
H
CH3
3. Conclusion
CH3 CH3 Br
CH3
Br
H
H
Br
In conclusion, we have described the first efficient and
highly selective synthesis of a series of nitro-substituted
2,2-diphenyl-2H-1-benzopyrans 24–29 in two steps, starting
from their brominated homologues 12–17, which were
CH3 CH3
CH3
H
CH3 Br
a
All the yields are for pure, isolated products.
X4
X3
X2
X4
C6H5
C6H5
1°) n-BuLi, THF, -40 °C to -90 °C
C6H5
O
X3
C6H5
O
2°) B(OBu)3, -90 °C to r.t.
3°) H2O/H+
X1
12 - 17
X1
X2
18 - 23
Scheme 6.

L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
Table 4. Yields of ipso-nitration: target molecules 24–29
8245
phenols were obtained from commercial sources, and used
without further purification. 2-Methylphenol (1) was pur-
chased from Avocado. 2,3-Dimethylphenol (2) and 3,5-di-
methylphenol (5) were purchased from Lancaster and
Fluka, respectively. 2,5-Dimethylphenol (3), 3,4-dimethyl-
phenol (4) and 4-bromophenol (6) were purchased from
Aldrich Chemical Co. Brominated phenols (7–11) have
been described before.13
Entry Starting
ipso-Nitrated X1
X2
X3
X4
boronic acid product
(yield %)a
1
2
3
4
5
6
18
19
20
21
22
23
24 (49)
25 (71)
26 (80)
27 (61)
28 (52)
29 (32)
H
H
H
NO2
NO2
H
H
H
CH3
CH3
CH3
NO2
H
CH3 NO2
H
H
NO2 CH3
CH3 CH3
CH3 NO2 CH3
4.3. General procedure (1) of ‘chromenization’ for the
synthesis of compounds 12–17
a
All the yields are for pure, isolated products.
A 50 mL round-bottomed flask was charged with 1,1-diphe-
nylprop-2-yn-1-ol (2.46 g, 11 mmol), the appropriate bro-
mophenol (10 mmol), a catalytic amount of p-toluene
sulfonic acid (PTSA) and dry dichloromethane (20 mL),
purged with argon and stirred at room temperature for 6–
10 h. The progress of the reaction was monitored by TLC
(pentane/Et2O, 1:1). After complete disappearance of the
bromophenol, the reaction mixture was washed with brine
(3ꢂ20 mL). The organic layer was dried with MgSO4, fil-
tered and concentrated to dryness under reduced pressure.
initially obtained by a classical chromenization between the
commercially available 1,1-diphenyl-2-yn-1-ol and various
brominated phenols 6–11. These key intermediates were
subsequently converted into boronic acids 18–23 by a metal-
ation/boronylation sequence, followed by acidic hydrolysis.
We have successfully carried out regioselective electrophilic
nitration reaction of these key buildings blocks, using the
Crivello’s reagent, which constitutes the key step of our
strategy. The operative conditions were fully compatible
with the benzopyranic structure. The introduction of the
nitro group takes place exclusively at the ipso-position of
the arylboronic acids.
The crude material was purified by column chromatography
(SiO2, cyclohexane/dichloromethane gradient 100:0 to
50:50). Data of the individual compounds are given below.
4. Experimental section
4.1. General remarks
4.3.1. 6-Bromo-2,2-diphenyl-2H-1-benzopyran (12). This
compound was obtained by general procedure (1), from the
commercially available 4-bromophenol 6 (1.73 g, 10 mmol)
and 1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol). The
product 12 was isolated as a light yellow solid (2.69 g,
Melting points were determined in capillary tubes on a Buchi
510 apparatus and are uncorrected. Glassware used in the re-
actions described below was dried for a minimum of 12 h in
an oven at 120 ꢁC. All the reactions were performed in stan-
dard glassware under an inert atmosphere of Ar. Fourier
transform IR spectra were recorded on a Matson Polaris
spectrophotometer from samples as KBr pellets or as solu-
tions in CCl4. The frequencies of band positions are given
in cmꢀ1. Nuclear magnetic resonance (1H and 13C NMR)
spectra were recorded on either a Bruker AC250 (250 and
62.5 MHz, respectively) or a Bruker Avance-DPX300 (300
and 75 MHz, respectively) spectrometer. Chemicals shifts
are reported in parts per million (d) relative to the nondeuter-
ated solvent peak. Coupling constants (J values) are ex-
pressed in hertz (Hz), and spin multiplicities are indicated
by the following symbols: s (singlet), d (doublet), t (triplet),
q (quadruplet), m (multiplet), br s (broad singlet). Elemental
analyses were performed on an LECO-932-CNS analyzer.
Column chromatography was carried out using silica gel
60 230–400 mesh (Merck & Co.). Silica TLC was conducted
on precoated aluminium sheets (60 F254) with a 0.2 mm
thickness (Aldrich Chemical Co.).
1
7.40 mmol, 74%). Mp 125–126 ꢁC; H NMR (250 MHz,
CDCl3): d¼6.13 (d, J¼10.0 Hz, 1H), 6.47 (d, J¼10.0 Hz,
1H), 6.72 (d, J¼7.5 Hz, 1H), 7.04 (d, J¼2.5 Hz, 1H), 7.12
(dd, J¼2.5, 7.5 Hz, 1H), 7.15–7.35 (m, 10H); 13C NMR
(62.5 MHz, CDCl3): d¼82.9 (OC), 113.2 (C), 118.3
(CH]), 122.4 (CH]), 123.0 (C), 127.0 (4ꢂCH]), 127.7
(2ꢂCH]), 128.2 (4ꢂCH]), 129.0 (CH]), 130.2 (CH]),
132.0 (CH]), 144.4 (2ꢂC), 151.6 (C). Anal. Calcd for
C21H15BrO: C, 69.43; H, 4.16; Br, 21.99. Found: C, 69.52;
H, 4.23; Br, 22.01. FTIR (KBr): n¼3055, 3026, 2968,
2924, 1629, 1597, 1472, 1446, 1416, 1265, 1242, 1212,
1163, 1128, 1053, 993, 945, 915, 876, 816, 767, 752, 701,
558 cmꢀ1
.
4.3.2. 6-Bromo-2,2-diphenyl-8-methyl-2H-1-benzopyran
(13). This compound was obtained by general procedure
(1), from 4-bromo-2-methylphenol 7 (1.87 g, 10 mmol) and
1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol). The product
13 was isolated as a white solid (2.60 g, 6.89 mmol, 69%).
Mp 73–74 ꢁC; 1H NMR (250 MHz, CDCl3): d¼2.17 (s,
3H), 6.09 (d, J¼10.0 Hz, 1H), 6.46 (d, J¼10.0 Hz, 1H),
6.89 (d, J¼2.5 Hz, 1H), 7.00 (d, J¼2.5 Hz, 1H), 7.12–7.41
(m, 10H); 13C NMR (62.5 MHz, CDCl3): d¼15.6 (CH3),
82.7 (OC), 112.6 (C), 122.5 (C), 122.9 (CH]), 126.7
(CH]), 126.8 (4ꢂCH]), 127.6 (2ꢂCH]), 127.8 (C),
128.2 (4ꢂCH]), 129.7 (CH]), 133.2 (CH]), 144.8
(2ꢂC), 149.5 (C). Anal. Calcd for C22H17BrO: C, 70.04; H,
4.54; Br, 21.18. Found: C, 69.97; H, 4.48; Br, 21.17. FTIR
(KBr): n¼3085, 3054, 3023, 2990, 2963, 2919, 2848, 1636,
1597, 1570, 1491, 1462, 1446, 1381, 1232, 1212, 1170,
4.2. Chemicals
THF was distilled prior to use from sodium benzophenone
ketyl under argon, while dichloromethane (CH2Cl2) was dis-
˚
tilled from calcium hydride, and stored over 3 A molecular
sieves. Acetonitrile and methanol were purchased from
S.D.S. Chemicals Co. and are used as supplied. Tetrabutyl-
ammonium tribromide (TBA$Br3) was commercially avail-
able from Aldrich, and was used as received. All starting
1096, 1055, 994, 908, 862, 767, 757, 715, 697, 564 cmꢀ1
.

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L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
4.3.3. 6-Bromo-7,8-dimethyl-2,2-diphenyl-2H-1-benzo-
pyran (14). This compound was obtained by general proce-
dure (1), from 4-bromo-2,3-dimethylphenol 8 (2.01 g,
10 mmol) and 1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol).
The product 14 was isolated as a white solid (2.78 g,
7.1 mmol, 71%). Mp 116–117 ꢁC; 1H NMR (250 MHz,
CDCl3): d¼2.20 (s, 3H), 2.22 (s, 3H), 6.05 (d, J¼10.0 Hz,
1H), 6.45 (d, J¼10.0 Hz, 1H), 6.99 (s, 1H), 7.12–7.37 (m,
10H). 13C NMR (62.5 MHz, CDCl3): d¼12.9 (CH3), 20.0
(CH3), 82.7 (OC), 116.3 (C), 120.3 (C), 123.0 (CH]), 126.1
(C), 126.8 (4ꢂCH]), 127.0 (CH]), 127.5 (2ꢂCH]),
128.1 (4ꢂCH]), 129.0 (CH]), 137.3 (C), 145.0 (2ꢂC),
149.3 (C). Anal. Calcd for C23H19BrO: C, 70.59; H, 4.89;
Br, 20.42. Found: C, 70.65; H, 4.93; Br, 20.52. FTIR
(KBr): n¼3059, 3026, 2922, 2848, 1639, 1598, 1488, 1446,
1412, 1376, 1231, 1171, 1101, 1050, 971, 941, 906, 881,
(CH3), 81.8 (OC), 116.8 (CH]), 118.9 (C), 121.0 (CH]),
126.6 (C), 127.0 (4ꢂCH]), 127.5 (2ꢂCH]), 128.1
(4ꢂCH]), 128.8 (CH]), 133.8 (C), 139.1 (C), 144.7
(2ꢂC), 151.3 (C). Anal. Calcd for C23H19BrO: C, 70.59;
H, 4.89; Br, 20.42. Found: C, 70.52; H, 4.79; Br, 20.45.
FTIR (KBr): n¼3058, 3024, 2919, 2850, 1626, 1600,
1489, 1446, 1391, 1308, 1227, 1190, 1176, 1153, 1073,
1029, 960, 911, 851, 766, 752, 699, 555 cmꢀ1
.
4.4. General procedure (2) for the synthesis of boronic
acids 18–23
A stirred solution of the appropriate brominated benzopyran
(2.75 mmol) in anhydrous THF (20 mL) under an Ar atmo-
sphere was cooled to ꢀ40 ꢁC. A 2.5 M solution of n-BuLi in
hexane (2.2 mL, 5.5 mmol) was added dropwise over 10 min
and stirring was continued for 2 h at ꢀ40 ꢁC. The resulting
solution was cooled to ꢀ90 ꢁC and tributylborate (2.3 mL,
8.26 mmol) was added at once via a syringe. The resulting
mixture was maintained at ꢀ90 ꢁC for 1 h, then allowed to
warm to room temperature. The reaction was quenched by
the addition of 1 N aqueous HCl solution (20 mL) and ex-
tracted with Et2O (3ꢂ40 mL). The combined organic layers
were dried with MgSO4 and filtered. The solvent was re-
moved under reduced pressure to give a crude residue, which
was subjected to column chromatography (SiO2; eluent: cy-
clohexane/ethyl acetate gradient 100:0 to 70:30) to afford
the pure desired product. The data for the individual com-
pounds are given below.
766, 755, 714, 696, 506 cmꢀ1
.
4.3.4. 6-Bromo-5,8-dimethyl-2,2-diphenyl-2H-1-benzo-
pyran (15). This compound was obtained by general proce-
dure (1), from 4-bromo-2,5-dimethylphenol 9 (2.01 g,
10 mmol) and 1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol).
The product 15 was isolated as a white solid (3.33 g,
1
8.51 mmol, 85%). Mp 153–154 ꢁC; H NMR (250 MHz,
CDCl3): d¼2.25 (s, 3H), 2.35 (s, 3H), 6.21 (d, J¼10.0 Hz,
1H), 6.84 (d, J¼10.0 Hz, 1H), 7.18 (br s, 1H), 7.21–7.46
(m, 10H); 13C NMR (62.5 MHz, CDCl3): d¼15.5 (CH3),
18.1 (CH3), 81.6 (OC), 116.2 (C), 120.9 (C), 121.4 (CH]),
125.2 (C), 126.7 (4ꢂCH]), 127.5 (2ꢂCH]), 128.1
(4ꢂCH]), 129.3 (CH]), 130.8 (C), 133.7 (CH]), 144.9
(2ꢂC), 149.8 (C). FTIR (KBr): n¼3060, 3024, 2952, 2921,
2853, 1625, 1593, 1493, 1450, 1380, 1364, 1236, 1230,
1203, 1168, 1095, 1062, 1031, 970, 907, 864, 770, 754,
700, 573 cmꢀ1. Anal. Calcd for C23H19BrO: C, 70.59; H,
4.89; Br, 20.42. Found: C, 70.56; H, 4.79; Br, 20.48.
Boronic acids in general present a host of difficulties with re-
gard to analysis. The principal difficulty is their spontaneous
condensation to boroxines to varying degrees.21
The strong O–B signal at 1310–1380 cmꢀ1 is usually visible,
but not as clearly as for monoboronic acids.22
4.3.5. 8-Bromo-5,6-dimethyl-2,2-diphenyl-2H-1-benzo-
pyran (16). This compound was obtained by general proce-
dure (1), from 2-bromo-4,5-dimethylphenol 10 (2.01 g,
10 mmol) and 1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol).
Product 16 was isolated as a light yellow solid (2.11 g,
5.4 mmol, 54%). Mp 97–98 ꢁC; 1H NMR (250 MHz,
CDCl3): d¼2.10 (s, 3H), 2.23 (s, 3H), 6.23 (d, J¼10.0 Hz,
1H), 6.74 (d, J¼10.0 Hz, 1H), 7.06 (br s, 1H), 7.09–7.28
(m, 6H), 7.38–7.48 (m, 4H); 13C NMR (62.5 MHz, CDCl3):
d¼14.5 (CH3), 19.6 (CH3), 82.1 (OC), 107.6 (C), 121.2
(CH]), 121.6 (C), 126.7 (4ꢂCH]), 127.4 (2ꢂCH]),
128.1 (4ꢂCH]), 129.8 (CH]), 130.7 (C), 131.8 (C),
133.2 (CH]), 144.7 (2ꢂC), 147.4 (C). Anal. Calcd for
C23H19BrO: C, 70.59; H, 4.89; Br, 20.42. Found: C, 70.43;
H, 4.75; Br, 20.45. FTIR (KBr): n¼3055, 3024, 2969,
2920, 2857, 1620, 1489, 1448, 1368, 1271, 1231, 1190,
As a consequence of incomplete decoupling of the B–C
spin–spin coupling by 11B (or 10B) quadrupolar relaxation,23
the signal of carbon bonded to the boron atom, which should
be very broad, was not observed.
4.4.1. 6-Dihydroxyborane-2,2-diphenyl-2H-1-benzo-
pyran (18). This compound was obtained by general proce-
dure (2), starting from 12 (1 g, 2.75 mmol). The product 18
was isolated as a white solid (713 mg, 2.17 mmol, 79%).
Mp>260 ꢁC; 1H NMR (300 MHz, CDCl3): d¼6.19 (d,
J¼9.9 Hz, 1H), 6.73 (d, J¼9.9 Hz, 1H), 7.00 (d, J¼8.1 Hz,
1H), 7.26 (tt, J¼1.5, 8.3 Hz, 2H), 7.33 (dd, J¼1.5, 7.0 Hz,
4H), 7.44 (dd, J¼7.0, 8.3 Hz, 4H), 7.81 (d, J¼1.5 Hz, 1H),
7.96 (dd, J¼1.5, 8.1 Hz, 1H); 13C NMR (75 MHz, CDCl3):
d¼83.2 (OC), 116.2 (CH]), 120.5 (C), 123.3 (CH]),
127.0 (4ꢂCH]), 127.6 (2ꢂCH]), 128.2 (4ꢂCH]),
128.6 (CH]), 134.3 (CH]), 137.6 (CH]), 144.7 (2ꢂC),
156.5 (C). FTIR (KBr): n¼3054, 1646, 1612, 1577, 1510,
1475, 1445, 1338, 1253, 1124, 1089, 981, 932, 906, 832,
1056, 978, 951, 907, 750, 699, 572 cmꢀ1
.
4.3.6. 6-Bromo-5,7-dimethyl-2,2-diphenyl-2H-1-benzo-
pyran (17). This compound was obtained by general proce-
dure (1), from 4-bromo-3,5-dimethylphenol 11 (2.01 g,
10 mmol) and 1,1-diphenylprop-2-yn-1-ol (2.46 g, 11 mmol).
Product 17 was isolated as a white solid (2.97 g,
749, 693 cmꢀ1
.
4.4.2. 6-Dihydroxyborane-2,2-diphenyl-8-methyl-2H-1-
benzopyran (19). This compound was obtained by general
procedure (2), starting from 13 (1.04 g, 2.75 mmol). The
product 19 was isolated as a white solid (838 mg,
2.45 mmol, 89%). Mp>260 ꢁC; 1H NMR (300 MHz,
1
7.59 mmol, 76%). Mp 109–110 ꢁC; H NMR (250 MHz,
CDCl3): d¼2.25 (s, 3H), 2.31 (s, 3H), 6.10 (d, J¼10.0 Hz,
1H), 6.67 (s, 1H), 6.75 (d, J¼10.0 Hz, 1H), 7.13–7.38 (m,
10H); 13C NMR (62.5 MHz, CDCl3): d¼18.9 (CH3), 24.4

L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
8247
CDCl3): d¼2.37 (s, 3H), 6.17 (d, J¼9.9 Hz, 1H), 6.77 (d,
J¼9.9 Hz, 1H), 7.15–7.55 (m, 10H), 7.79 (d, J¼2.3 Hz,
1H), 7.92 (d, J¼2.3 Hz, 1H). 13C NMR (75 MHz, CDCl3):
d¼15.8 (CH3), 83.0 (OC), 120.1 (C), 123.8 (CH]), 125.0
(C), 126.8 (4ꢂCH]), 127.5 (2ꢂCH]), 128.1 (5ꢂCH]),
132.2 (CH]), 138.6 (CH]), 145.2 (2ꢂC), 154.5 (C).
FTIR (KBr): n¼3058, 2969, 2920, 1637, 1598, 1489, 1445,
1381, 1338, 1207, 1167, 1102, 1055, 987, 905, 735,
J¼10.0 Hz, 1H), 6.63 (s, 1H), 6.87 (d, J¼10.0 Hz, 1H),
7.24 (tt, J¼1.7, 8.4 Hz, 2H), 7.30 (dd, J¼6.9, 8.4 Hz, 4H),
7.40 (dd, J¼1.7, 6.9 Hz, 4H); 13C NMR (75 MHz, CDCl3):
d¼18.2 (CH3), 21.4 (CH3), 81.9 (OC), 115.0 (CH]),
117.0 (C), 120.4 (CH]), 127.0 (4ꢂCH]), 127.4 (CH]),
127.5 (2ꢂCH]), 128.1 (4ꢂCH]), 135.5 (C), 141.0 (C),
145.2 (2ꢂC), 152.5 (C). FTIR (KBr): n¼3480–3104, 3085,
3058, 3025, 2956, 2919, 2850, 1632, 1613, 1596, 1562,
1491, 1462, 1446, 1372, 1313, 1224, 1160, 1141, 1092,
695 cmꢀ1
.
1073, 1031, 979, 941, 907, 844, 756, 723, 696 cmꢀ1
.
4.4.3. 6-Dihydroxyborane-7,8-dimethyl-2,2-diphenyl-
2H-1-benzopyran (20). This compound was obtained by
general procedure (2), starting from 14 (1.07 g,
2.75 mmol). The product 20 was isolated as a white solid
(754 mg, 2.12 mmol, 77%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.29 (s, 3H), 2.63 (s, 3H), 6.11 (d,
J¼9.6 Hz, 1H), 6.67 (d, J¼9.6 Hz, 1H), 7.25 (tt, J¼1.5,
8.5 Hz, 2H), 7.31 (dd, J¼1.5, 7.0 Hz, 4H), 7.45 (dd,
J¼7.0, 8.5 Hz, 4H), 7.68 (s, 1H); 13C NMR (75 MHz,
CDCl3): d¼11.7 (CH3), 19.0 (CH3), 83.1 (OC), 117.7 (C),
123.9 (CH]), 124.4 (C), 126.8 (4ꢂCH]), 127.2 (CH]),
127.4 (2ꢂCH]), 128.1 (4ꢂCH]), 133.4 (CH]), 145.4
(2ꢂC), 147.2 (C), 153.5 (C). FTIR (KBr): n¼3059, 3027,
2923, 1632, 1598, 1569, 1445, 1379, 1326, 1217, 1167,
4.5. General procedure (3) for the synthesis of nitrated
compounds 24–29
A 50 mL two-necked flask equipped with a magnetic stirrer
was charged with boronic acid (1 mmol) and dry acetonitrile
(20 mL), purged with argon, and cooled to ꢀ35 ꢁC. With
rapid stirring, trifluoroacetic anhydride (1 mL) was added
dropwise over 10 min with a syringe. Stirring was continued
at ꢀ35 ꢁC. Under argon atmosphere, in a separate flask, am-
monium nitrate (88 mg, 1.1 mmol) and dry acetonitrile
(10 mL) were mixed and the resulting mixture was cooled
to 0 ꢁC. To this vigorously stirred suspension, trifluoroacetic
anhydride (6–8 mL) was carefully and slowly added until all
solids had dissolved. The nitrating agent, thus prepared, was
transferred by syringe into the reaction flask within 10 min.
Stirring was continued for 2 h at ꢀ35 ꢁC and the reaction
mixture was allowed to warm to room temperature. The sol-
vent was removed under reduced pressure and the residue
was diluted with Et2O (30 mL). The resulting mixture was
washed with water (2ꢂ20 mL) and brine (3ꢂ20 mL), dried
with MgSO4, filtered and concentrated to dryness under re-
duced pressure. The crude material was purified by column
chromatography (SiO2; eluent: cyclohexane/Et2O gradient
100:0 to 70:30) to yield the ipso-nitrated product.
1098, 965, 907, 739, 696, 638 cmꢀ1
.
4.4.4. 6-Dihydroxyborane-5,8-dimethyl-2,2-diphenyl-
2H-1-benzopyran (21). This compound was obtained by
general procedure (2), starting from 15 (1.07 g,
2.75 mmol). The product 21 was isolated as a white solid
(656 mg, 1.84 mmol, 67%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.31 (s, 3H), 2.69 (s, 3H), 6.19 (d,
J¼10.1 Hz, 1H), 6.95 (d, J¼10.1 Hz, 1H), 7.23 (tt, J¼1.5,
8.5 Hz, 2H), 7.31 (dd, J¼1.5, 7.0 Hz, 4H), 7.45 (dd, J¼7.0,
8.5 Hz, 4H), 7.79 (s, 1H). 13C NMR (75 MHz, CDCl3):
d¼15.9 (CH3), 17.1 (CH3), 81.9 (OC), 119.6 (C), 121.1
(CH]), 122.3 (C), 126.8 (4ꢂCH]), 127.4 (2ꢂCH]),
127.9 (CH]), 128.1 (4ꢂCH]), 139.9 (CH]), 140.5 (C),
145.3 (2ꢂC), 154.0 (C). FTIR (KBr): n¼3059, 3026, 2921,
2854, 1632, 1573, 1447, 1390, 1372, 1322, 1219, 1167,
4.5.1. 2,2-Diphenyl-6-nitro-2H-1-benzopyran (24). This
compound was obtained by general procedure (3), starting
from 18 (328 mg, 1 mmol). The product 24 was isolated as
a white powder (162 mg, 0.49 mmol, 49%). Mp 134 ꢁC;
1H NMR (300 MHz, CDCl3): d¼6.30 (d, J¼9.9 Hz, 1H),
6.67 (d, J¼9.9 Hz, 1H), 6.97 (d, J¼8.8 Hz, 1H), 7.24–7.41
(m, 10H), 7.94 (d, J¼2.7 Hz, 1H), 8.04 (dd, J¼2.7, 8.8 Hz,
1H); 13C NMR (75 MHz, CDCl3): d¼84.5 (OC), 116.8
(CH]), 120.9 (C), 121.8 (CH]), 122.4 (CH]), 125.6
(CH]), 126.9 (4ꢂCH]), 128.1 (2ꢂCH]), 128.4
(4ꢂCH]), 130.7 (CH]), 142.0 (C), 143.9 (2ꢂC), 158.0
(C). Anal. Calcd for C21H15NO3: C, 76.53; H, 4.59; N,
4.25. Found: C, 76.49; H, 4.71; N, 4.19. FTIR (KBr):
n¼3054, 3027, 1647, 1614, 1576, 1509, 1475, 1446, 1335,
1067, 1022, 969, 906, 739, 695, 638 cmꢀ1
.
4.4.5. 8-Dihydroxyborane-5,6-dimethyl-2,2-diphenyl-
2H-1-benzopyran (22). This compound was obtained by
general procedure (2), from 16 (1.07 g, 2.75 mmol). The
product 22 was isolated as a white solid (637 mg,
1.79 mmol, 65%). Mp>260 ꢁC; 1H NMR (300 MHz,
CDCl3): d¼2.18 (s, 3H), 2.22 (s, 3H), 6.12 (d, J¼10.1 Hz,
1H), 6.88 (d, J¼10.1 Hz, 1H), 7.20–7.40 (m, 10H), 7.45 (s,
1H); 13C NMR (75 MHz, CDCl3): d¼14.9 (CH3), 19.7
(CH3), 83.0 (OC), 118.9 (C), 120.9 (CH]), 127.0 (4ꢂ
CH]), 127.8 (2ꢂCH]), 128.1 (CH]), 128.4 (4ꢂCH]),
129.6 (C), 136.7 (C), 137.1 (CH]), 144.1 (2ꢂC), 156.0
(C). FTIR (KBr): n¼3500–3100, 3058, 2957, 2922, 2854,
1647, 1595, 1490, 1443, 1391, 1326, 1256, 1219, 1053,
1248, 1224, 1124, 1086, 983, 902, 832, 747, 695 cmꢀ1
.
4.5.2. 2,2-Diphenyl-8-methyl-6-nitro-2H-1-benzopyran
(25). This compound was obtained by general procedure
(3), starting from 19 (343 mg, 1 mmol). The product 25
was obtained as a dark brown solid (243 mg, 0.71 mmol,
934, 904, 758, 734, 699 cmꢀ1
.
1
71%). Mp 98 ꢁC; H NMR (300 MHz, CDCl3): d¼2.33 (s,
4.4.6. 6-Dihydroxyborane-5,7-dimethyl-2,2-diphenyl-
2H-1-benzopyran (23). This compound was obtained
by general procedure (2), starting from 17 (1.07 g,
2.75 mmol). The product 23 was isolated as a white powder
(343 mg, 0.962 mmol, 35%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.26 (s, 3H), 2.29 (s, 3H), 6.12 (d,
3H), 6.26 (d, J¼10.1 Hz, 1H), 6.65 (d, J¼10.1 Hz, 1H),
7.20–7.50 (m, 10H), 7.79 (d, J¼2.3 Hz, 1H), 7.91 (d,
J¼2.3 Hz, 1H); 13C NMR (75 MHz, CDCl3): d¼15.9
(CH3), 84.3 (OC), 120.1 (CH]), 120.3 (C), 122.3 (CH]),
126.3 (CH]), 126.7 (4ꢂCH] and C), 127.9 (2ꢂCH]),
128.4 (4ꢂCH]), 130.1 (CH]), 141.2 (C), 144.1 (2ꢂC),

8248
L. Bougdid et al. / Tetrahedron 63 (2007) 8242–8249
155.9 (C). Anal. Calcd for C22H17NO3: C, 76.95; H, 4.99; N,
4.08. Found: C, 76.86; H, 5.11; N, 4.10. FTIR (KBr):
n¼3064, 3000, 2900, 2857, 1635, 1600, 1588, 1514, 1447,
1333, 1242, 1165, 1089, 994, 973, 940, 915, 893, 749,
(CH]), 118.3 (C), 119.32 (CH]), 126.3 (C), 126.9
(4ꢂCH]), 127.8 (2ꢂCH]), 128.2 (4ꢂCH]), 129.9
(CH]), 131.2 (C), 144.2 (2ꢂC), 146.8 (C), 153.2 (C).
Anal. Calcd for C23H19NO3: C, 77.29; H, 5.36; N, 3.92.
Found: C, 77.34; H, 5.28; N, 3.88. FTIR (KBr): n¼3059,
3031, 2956, 2922, 2852, 1650, 1608, 1583, 1563, 1514,
1462, 1444, 1363, 1315, 1305, 1240, 1212, 1189, 1102,
1090, 1001, 985, 949, 931, 851, 836, 780, 766, 758,
699 cmꢀ1
.
4.5.3. 7,8-Dimethyl-2,2-diphenyl-6-nitro-2H-1-benzo-
pyran (26). This compound was obtained by general proce-
dure (3), starting from 20 (356 mg, 1 mmol). The product 26
was isolated as a dark yellow solid (286 mg, 0.80 mmol,
697 cmꢀ1
.
1
80%). Mp 143 ꢁC; H NMR (300 MHz, CDCl3): d¼2.28
(s, 3H), 2.40 (s, 3H), 6.23 (d, J¼9.9 Hz, 1H), 6.61 (d,
J¼9.9 Hz, 1H), 7.20–7.45 (m, 10H), 7.50 (s, 1H); 13C
NMR (75 MHz, CDCl3): d¼12.2 (CH3), 16.3 (CH3), 83.8
(OC), 118.3 (C), 120.6 (CH]), 122.3 (CH]), 126.2 (C),
126.8 (4ꢂCH]), 127.8 (2ꢂCH]), 128.3 (4ꢂCH]),
129.8 (CH]), 133.9 (C), 144.3 (3ꢂC), 153.1 (C). Anal.
Calcd for C23H19NO3: C, 77.29; H, 5.36; N, 3.92. Found:
C, 77.18; H, 5.27; N, 4.03. FTIR (KBr): n¼3059, 3028,
2922, 2852, 1640, 1608, 1557, 1512, 1446, 1379, 1347,
1267, 1243, 1175, 1098, 1026, 966, 932, 912, 880, 763,
Acknowledgements
The 300 MHz NMR facilities were funded by the Region
ꢁ
Nord-Pas-de-Calais, The Ministere de La Jeunesse, de
l’Education Nationale et de la Recherche and FEDER.
This collaboration work was realized within the framework
CNRS GDR 2466.
696 cmꢀ1
.
References and notes
4.5.4. 5,8-Dimethyl-2,2-diphenyl-6-nitro-2H-1-benzo-
pyran (27). This compound was obtained by general proce-
dure (3), starting from 21 (356 mg, 1 mmol). The product 27
was isolated as a light brown solid (218 mg, 0.61 mmol,
1. (a) Crano, J. C.; Kwak, W. S.; Welch, C. N. Applied
Photochromic Polymer Systems; Ardle, C. B., Ed.; Chapman
and Hall: New York, NY, 1994; (b) Crano, J. C.; Flood, T.;
Knowles, D.; Kumar, A.; Van Germert, B. Pure Appl. Chem.
1996, 68, 1395–1398; (c) Le Naour-Sene, L. U.S. Patent
4,286,957, 1981; (d) Maltman, W. R., Threlfall, I. M. U.S.
Patent 4,851,471, 1989.
1
61%). Mp 181 ꢁC; H NMR (300 MHz, CDCl3): d¼2.29
(s, 3H), 2.47 (s, 3H), 6.30 (d, J¼10.3 Hz, 1H), 6.89 (d,
J¼10.3 Hz, 1H), 7.20–7.50 (m, 10H), 7.64 (s, 1H); 13C
NMR (75 MHz, CDCl3): d¼14.7 (CH3), 15.7 (CH3), 82.8
(OC), 120.0 (CH]), 120.2 (C), 124.3 (C), 126.7
(4ꢂCH]), 127.1 (CH]), 127.8 (2ꢂCH]), 127.9 (C),
128.3 (4ꢂCH]), 130.0 (CH]), 143.6 (C), 144.2 (2ꢂC),
154.3 (C). Anal. Calcd for C23H19NO3: C, 77.29; H, 5.36;
N, 3.92. Found: C, 77.25; H, 5.29; N, 4.12. FTIR (KBr):
n¼2922, 2853, 1626, 1558, 1511, 1451, 1376, 1320, 1274,
2. Molecular Electronics; Ashwell, G. J., Ed.; John Wiley and
Sons: New York, NY, 1992.
3. (a) Venec, D.; Delbaere, S.; Micheau, J. C.; Frigoli, M.;
Moustrou, C.; Samat, A.; Vermeersch, G. J. Photochem.
Photobiol. A: Chem. 2006, 183, 70–78; (b) Delbaere, S.;
Venec, D.; Micheau, J. C.; Frigoli, M.; Moustrou, C.; Samat,
A.; Vermeersch, G. J. Photochem. Photobiol. A: Chem. 2006,
181, 174–179.
1225, 1068, 1023, 1068, 1023, 965, 758, 699 cmꢀ1
.
4. (a) Bertelson, R. C. Techniques of Chemistry; Brown, G. H.,
Ed.; Wiley-Interscience: New York, NY, 1971; Vol. III,
pp 45–431; (b) Guglielmetti, R. Photochromism: Molecules
4.5.5. 5,6-Dimethyl-2,2-diphenyl-8-nitro-2H-1-benzo-
pyran (28). This compound was obtained by general proce-
dure (3), starting from 22 (356 mg, 1 mmol). The product 28
was isolated as a light yellow solid (186 mg, 0.52 mmol,
52%). Mp 154–155 ꢁC; 1H NMR (300 MHz, CDCl3):
d¼2.20 (s, 3H), 2.24 (s, 3H), 6.43 (d, J¼10.1 Hz, 1H),
6.89 (d, J¼10.1 Hz, 1H), 7.20–7.55 (m, 10H), 7.60 (s,
1H); 13C NMR (75 MHz, CDCl3): d¼15.3 (CH3), 19.7
(CH3), 83.0 (OC), 120.6 (CH]), 122.2 (C), 125.6 (CH]),
126.6 (4ꢂCH]), 127.7 (2ꢂCH]), 128.3 (4ꢂCH]),
129.4 (C), 130.5 (CH]), 136.1 (C), 139.2 (C), 143.9
(2ꢂC), 144.0 (C). Anal. Calcd for C23H19NO3: C, 77.29;
H, 5.36; N, 3.92. Found: C, 76.98; H, 5.45; N, 3.89. FTIR
(KBr): n¼3058, 2900, 2838, 1634, 1576, 1506, 1447,
€
and Systems; Durr, H., Ed.; Elsevier: Amsterdam, 1990;
Vol. 8, p 314; (c) Van Gemert, B. Organic Photochromic and
Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J.,
Eds.; Plenum Publishing Corporation: New York, NY, 1999;
Vol. 1, Chapter 3, p 111.
5. (a) Molecular Switches; Feringa, Ben L., Ed.; Wiley-VCH
GmbH: Weinheim, 2001; (b) Callan, J. F.; Prasanna de Silva,
A.; Magri, D. C. Tetrahedron 2005, 61, 8551–8588; (c)
ꢁ
Yassar, A.; Jaafari, H.; Rebiere-Galy, N.; Frigoli, M.;
Moustrou, C.; Samat, A.; Gugielmetti, R. Eur. Phys. J. Appl.
Phys. 2002, 18, 3–8; (d) Yassar, A.; Garnier, F.; Jaafari, H.;
ꢁ
Rebiere-Galy, N.; Frigoli, M.; Moustrou, C.; Samat, A.;
1333, 1245, 1100, 1058, 997, 966, 744, 699 cmꢀ1
.
Guglielmetti, R. Appl. Phys. Lett. 2002, 80, 4297–4299; (e)
Golovkova, T. A.; Kozlov, D. V.; Neckers, D. C. J. Org.
Chem. 2005, 70, 5545–5549; (f) de Jong, J. J. D.;
Tiemersma-Wegma, T. D.; van Esch, J. H.; Feringa, B. L.
J. Am. Chem. Soc. 2005, 127, 13804–13805; (g) Higashiguchi,
4.5.6. 5,7-Dimethyl-2,2-diphenyl-6-nitro-2H-1-benzo-
pyran (29). This compound was obtained by general proce-
dure (3), starting from 23 (356 mg, 1 mmol). The product 29
was isolated as a light yellow solid (115 mg, 0.32 mmol,
ꢀ
K.; Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc.
2005, 127, 8922–8923.
1
32%). Mp 128 ꢁC; H NMR (300 MHz, CDCl3): d¼2.22
(s, 6H), 6.26 (d, J¼10.2 Hz, 1H), 6.69 (s, 1H), 6.76 (d,
J¼10.2 Hz, 1H), 7.25–7.43 (m, 10H); 13C NMR (75 MHz,
CDCl3): d¼13.5 (CH3), 17.9 (CH3), 82.6 (OC), 116.7
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