The synthesis and photochromism of a 2,2-diaryl-6-styryl-2H-[1]benzopyran: Unexpected palladium-mediated ring-contraction of a 6-bromo-2,2-diaryl-2H-[1] benzopyran

Article abstract of DOI:10.1016/j.dyepig.2011.05.005

The ligand-free Pd-mediated reaction between styrene and a 6-bromo-2,2-diaryl-2H-[1]benzopyran proceeded anomalously to give a 2-(diarylmethyl)-5-styrylbenzofuran via a tandem Heck coupling - ring-contraction process; none of the styryl substituted 2,2-diaryl-2H-[1] benzopyran was observed. A 2,2-diaryl-6-styryl-2H-[1]benzopyran resulted from the condensation between 4-hydroxystilbene and a 1,1-diarylpropynol and which exhibited photochromism through the reversible electrocyclisation of the pyran unit; no isomerisation - electrocyclisation of the stilbene moiety was detected.

Full text of DOI:10.1016/j.dyepig.2011.05.005

Dyes and Pigments 92 (2012) 825e830
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Short communication
The synthesis and photochromism of a 2,2-diaryl-6-styryl-2H-[1]benzopyran:
Unexpected palladium-mediated ring-contraction of a 6-bromo-2,
2-diaryl-2H-[1]benzopyran
*
Christopher D. Gabbutt, B. Mark Heron , Colin Kilner, Suresh B. Kolla
Department of Colour Science, School of Chemistry, The University of Leeds, Leeds LS2 9JT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The ligand-free Pd-mediated reaction between styrene and a 6-bromo-2,2-diaryl-2H-[1]benzopyran
proceeded anomalously to give a 2-(diarylmethyl)-5-styrylbenzofuran via a tandem Heck coupling e
ring-contraction process; none of the styryl substituted 2,2-diaryl-2H-[1]benzopyran was observed.
A 2,2-diaryl-6-styryl-2H-[1]benzopyran resulted from the condensation between 4-hydroxystilbene and
a 1,1-diarylpropynol and which exhibited photochromism through the reversible electrocyclisation of the
pyran unit; no isomerisation e electrocyclisation of the stilbene moiety was detected.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 14 April 2011
Received in revised form
4 May 2011
Accepted 5 May 2011
Available online 13 May 2011
Keywords:
Heck reaction
Benzopyran
Benzofuran
Hydroxystilbene
Photochromism
Ring-contraction
1. Introduction
mediated by Pd(OAc)2 an appreciable yield of the ring-contracted
naphthofuran 2 resulted (Scheme 1) [5]. Whilst ring-contractions
The phenomenon of photochromism has attracted considerable
interest over the last three decades [1]. A considerable proportion
of this attention has been directed towards the highly commercially
successful naphthopyrans [2] and related pyran-derived systems
[3] where the excellent photochromic response is due to
the photochemical electrocyclic ring-opening of the pyran ring
to afford intensely coloured photo-merocyanines; a subsequent
thermal cyclisation with concomitant fading of the colour
completes the process. With the ever increasing structural
complexity of the naphthopyran unit resulting from the need for
superior photochromic performance it comes as no surprise to note
that transition metal-mediated chemistry has been applied in this
area [4]. In a recent study the Pd-mediated cyanation, carboxylation
and trimethylsilylethynylation of the 2-bromo-3,3-diaryl-3H-
naphtho[2,1-b]pyran 1 was described leading to new substituted
naphthopyrans which displayed modified photochromic properties
[5]. Of particular interest and of relevance to our studies on naph-
thopyrans [6,7] was the observation that upon attempted cyanation
of naphthopyrans e.g. 3 [7], 4 [8] and dihydronaphthopyrans 5 [9]
to naphthofurans are known (Scheme 1) we were interested by
the Pd-mediated ring-contraction and now report our observations
on attempts to access the photochromic styrylbenzopyran 6 by
a Pd-mediated Heck reaction.
2. Experimental
2.1. Equipment
Unless otherwise stated, reagents were used as supplied by the
major chemical catalogue companies. NMR spectra were recorded
on
a
Bruker Avance 400 MHz spectrophotometer (1H NMR
400 MHz, 13C NMR 100 MHz) for sample solutions in CDCl3 with
tetramethylsilane as an internal reference. The crystal structure
determination was carried out at 150 K on a Bruker-Nonius Apex X8
diffractometer equipped with an Apex II CCD detector and using
graphite monochromated Mo-Ka radiation from an FR591 rotating
anode generator. The structure was solved by direct methods and
refined using SHELXL-97. FT-IR spectra were recorded on a Perkin
Elmer Spectrum One spectrophotometer system equipped with
a golden gate ATR attachment (neat sample). UVevisible spectra
* Corresponding author. Tel.: þ44 113 3432925; fax: þ44 113 3436565.
E-mail address: b.m.heron@leeds.ac.uk (B.M. Heron).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.005

826
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 825e830
R1
R1
2.2. Preparation of (E)-4-hydroxystilbene (11)
Pd(OAc)2, dppf, Na2CO3
K4[Fe(CN)6]
NMP, 120 oC, 44 h
Br
Ar
Ar
Ar
n-Butyllithium (1.6 M in hexanes, 56.25, mL, 90 mmol) was
added portionwise by syringe over 1 h to a cold (ꢁ10 ꢂC) stirred
suspension of benzyltriphenylphosphonium chloride (35.0 g,
90 mmol) in anhydrous THF (150 mL). Upon completion of the
addition the mixture was stirred for 30 min at ꢁ10 ꢂC and a solution
of 4-hydroxybenzaldehyde (5.0 g, 41 mmol) in THF (80 mL), was
added drop wise over 20 min and the resulting solution was stirred
for 3 h at room temperature. The mixture was poured into water
(200 mL), neutralised with aqueous saturated ammonium chloride
solution (150 mL) and extracted with ethyl acetate (3 ꢀ 70 mL). The
combined organic layers were washed with water (100 mL) and
dried (anhyd. Na2SO4). Removal of the solvent gave a pale brown
semi-solid that was eluted from silica (30% EtOAc/hexane) to afford
the crude product as a mixture of the cis- and trans- isomers.
Heating a solution the foregoing crude isomer mixture in hexane
(100 mL) containing a catalytic amount of iodine (20 mg) gave the
title product 11 as an off-white solid 5.78 g, 72%, mp 184e186 ꢂC (lit.
mp ¼ 186 ꢂC [10]), dH 4.76 (1H, s, eOH), 6.83 (2H, m, AreH), 6.94
(1H, d, J ¼ 16.3 Hz, styryl), 7.03 (1H, d, J ¼ 16.3 Hz, styryl),7.34 (2H,
m, AreH), 7.42 (2H, m, AreH), 7.49 (2H, m, AreH).
O
O
Ar
2
1
R
1 = CO2Me, Ar = 4-FC6H4
Ar
365 nm irradiation
PhMe
Ar
Ar
O
O
Ar
3
Ar = 4-MeS(O2)C6H4
OH
OH
CO2Me
Br
CO2Me
5 mol% CuI, NaOMe
MeOH, heat 56 h
O
O
4
R
R
S8, 200 o
C
O
O
O
O
2.3. Preparation of 6-bromo-2,2-bis-(4-methoxyphenyl)-2H-[1]
benzopyran (8)
5
R = H, Me, Ph
Aluminium oxide (activated, acidic, Brockmann 1, w150 #)
(3.0 g) was added in a single portion to a warm (ca. 50 ꢂC) stirred
solution of the 4-bromophenol (2.0 g, 11.6 mmol) and 1,1-bis-(4-
methoxyphenyl)prop-2-yn-1-ol 7 (3.1 g, 11.6 mmol) in toluene
(60 mL). The resulting suspension was heated under reflux until no
propynol remained by TLC (ca. 2 h). The solution was cooled
(w30 ꢂC) and filtered and the spent alumina catalyst was washed
with warm toluene (2 ꢀ 30 mL). Evaporation of the toluene from
the combined washings and filtrate afforded the crude product
which was eluted from silica using 10% EtOAc in PhMe, to afford the
Scheme 1. Ring-contractions of naphthopyrans to naphthofurans.
were recorded for spectroscopic grade CH2Cl2 solutions of the
samples (10 mm path length quartz cuvette, PTFE capped, ca.
2.5 ꢀ 10ꢁ4 mol dmꢁ3) using a Cary 50 Probe spectrophotometer
equipped with a single cell Peltier temperature controlled (20 ꢂC)
stirred cell attachment with activating irradiation provided by an
Oriel 150 Watt xenon arc lamp source (Newport 66906), xenon
ozone free arc lamp (Newport 6255), distilled water liquid filter
(Newport 6177), multiple filter holder (Newport 62020), UG11 filter
(Newport FSO-UG11), fibre optic coupler (Newport 77799) and
liquid light guide (Newport 77557). All compounds were homo-
title benzopyran
8 as colourless microcrystals 1.32 g, 27%,
mp ¼ 100e101 ꢂC, nmax 1608.2, 1506.1, 1470.7, 1301.8, 1252.5,
1240.6, 1175.3, 1029.1, 983.5, 946.3, 817.0, 702.1, 586.6, 556.9 cmꢁ1
,
dH 3.79 (6H, s, [OMe]2), 6.13 (1H, d, J ¼ 9.8 Hz, 3eH), 6.50 (1H, d,
J ¼ 9.8 Hz, 4eH), 6.73 (1H, d, J ¼ 8.3 Hz, 8eH), 6.82 (4H, m, AreH),
7.11 (1H, d, J ¼ 2.3 Hz, 5eH), 7.18 (1H, dd, J ¼ 8.3, 2.3 Hz, 7eH), 7.28
(4H, m, AreH); dC 55.26, 82.58, 113.04, 113.50, 118.29, 122.00,
123.04, 128.31, 128.95, 130.57, 131.96, 136.77, 151.66, 159.02. Found
C, 65.20; H, 4.55; [M þ Hþ] 423.0591 (Br79). C23H19BrO3 requires C,
65.26; H, 4.52; [M þ Hþ] 423.0590 (Br79).
geneous by TLC (Merck TLC aluminium sheets, silica gel 60 F254
)
using a range of eluent systems of differing polarity and flash
column chromatography was performed on chromatography silica
gel (Fluorochem, 40e63 micron particle size distribution). Mass
spectra were recorded at the National EPSRC Mass Spectrometry
Service Centre, Swansea.
An
An
An
An
An
O
An
6
O
O
An = 4-MeOC6H4
colour 1
colourless
An
An
H
H
An
An
An
An
O
O
O
colourless
colour 2
Scheme 2. Proposed modes of photochromism of styrylbenzopyran 6.

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 825e830
827
Ph
Br
Br
H
(i)
O
(ii)
+
An
An
An
An
O
An
A
n
H
O
O
7
8
9
=
n
A
-
e
H4
OC6
4 M
Reagents and conditions:
(i) acidic Al2O3, PhMe, heat; (ii) styre.ne, Pd(OAc)2 (0.05 mol%),
o
,
K PO , , -dimethylacetamid
e
, N2 rt 1h then 140 C for 20 h
N N
3
4
Scheme 3. Tandem Heck reaction d ring-contraction.
2.4. Preparation of 2,2-bis-(4-methoxyphenyl)-6-styryl-2H-[1]
benzopyran (6)
expensive phosphine ligand [13]. This latter approach appeared
attractive for the synthesis of the styryl substituted benzopyran 6
from the bromo- precursor 8 which is available from bromophenol
and a prop-2-yn-1-ol using established methodology [14]. Styr-
ylbenzopyran 6 was expected to display photochromism through
cycling of the pyran ring e photomerocyanine and may also
undergo the alternative photochemically induced process involving
E- to Z- photoisomerisation of the stilbene unit followed by an
electrocyclisation ultimately leading to a phenanthrene system
(Scheme 2). The photocyclisation of a series of 6-phenylvinyl and
6-(2-naphthyl)vinyl substituted benzopyran-2-ones (coumarins)
has been reported and the resulting helical phenanthropyranone
derivatives were treated with phenylmagnesium bromide to afford
diaryl substituted photochromic phenanthropyrans [15].
The 6-bromobenzopyran 8 was accessed in surprisingly low
yield (27%), albeit from relatively inexpensive starting materials, by
the acid-catalysed condensation between 4-bromophenol and 1,1-
bis(4-methoxyphenyl)prop-2-yn-1-ol 7 to afford an intermediate
propargylic ether which readily rearranges in situ to the benzo-
pyran (Scheme 3). The formation of 8 was confirmed by 1H NMR
spectroscopy which displayed a key AB system with a doublet at
The title naphthopyran 6 was obtained in a similar manner to
that described for the foregoing bromobenzopyran 8, from
4-hydroxystilbene 11 (2.0 g, 10 mmol) and 1,1-bis-(4-methox-
yphenyl)prop-2-yn-1-ol 7 (2.7 g, 10 mmol) after elution from silica
with 20% EtOAc in PhMe, as colourless microcrystals, 1.1 g, 24%,
mp ¼ 142e143 ꢂC, nmax 1609.7, 1507.8, 1488.7, 1301.0, 1247.7, 1173.7,
1030.5, 961.4, 948.8, 828.6, 816.6, 707.1, 566.8 cmꢁ1
, dH 3.79 (6H, s,
[OMe]2), 6.14 (1H, d, J ¼ 9.8 Hz, 3eH), 6.61 (1H, d, J ¼ 9.8 Hz, 4eH),
6.86 (5H, m, AreH), 6.91 (1H, d, J ¼ 16.3 Hz, styryl-H), 7.01 (1H, d,
J ¼ 16.3 Hz, styryl-H), 7.16 (1H, d, J ¼ 2.1 Hz, 5eH), 7.22 (1H, m,
AreH),7.28 (1H, dd, J ¼ 8.3, 2.1 Hz, AreH), 7.33 (6H, m, AreH), 7.46
(2H, m, AreH); dC 55.26, 82.57, 113.47, 121.21, 122.90, 124.50, 126.26,
126.75, 127.22, 127.82, 128.20, 128.33, 128.64, 129.69, 130.58, 137.19,
137.64, 152.48, 158.94. Found C, 83.35; H, 5.80; [M þ Hþ] 447.1955.
C31H26O3 requires C, 83.38; H, 5.87; [M þ Hþ] 447.1955.
2.5. Heck coupling reaction between styrene and 6-bromo-2,2-bis-
(4-methoxyphenyl)-2H-[1]benzopyran (9)
d
6.13 (3-H) and at
d
6.50 (4-H) with J ¼ 9.8 Hz and a signal at
d 82.5
in the 13C NMR spectrum assigned to C-2; these chemical shifts and
coupling constants are typical for 2H-[1]benzopyrans [16].
Heating a solution of 8 with styrene in N,N-dimethylacetamide
containing K3PO4 and palladium acetate according to the procedure
Pd(OAc)2 (0.05 mol %) was added to a stirred suspension of
6-bromo-2,2-bis-(4-methoxyphenyl)-2H-[1]benzopyran (1.0 g, 2.4
mmol), styrene (0.3 g, 2.9 mmol) and K3PO4 (0.7 g, 3.3 mmol) in
N,N-dimethylacetamide (15 mL) under a nitrogen atmosphere and
the mixture was stirred for 20 h at 140 ꢂC. The cooled mixture was
poured in to water (70 mL) and extracted with ethyl acetate
(3 ꢀ 40 mL). The combined organic layers were washed well with
water (5 ꢀ 50 mL), dried (Na2SO4 anhyd.) and evaporated to afford
a dark brown gum which was eluted from silica using 20% EtOAc in
hexane to afford 2-[1,1-bis-(4-methoxyphenyl)methyl]-5-styr-
ylbenzo[b]furan 9 as off-white microcrystals 0.31 g, 30%, mp ¼
122e124 ꢂC, nmax 1608.3, 1583.8, 1507.0, 1467.1, 1450.9, 1242.8,
1173.8, 1028.7, 960.7, 830.0, 806.2, 742.5, 689.6, 609.8, 582.0,
499.3 cmꢁ1
, dH 3.80 (6H, s, [OMe]2), 5.48 (1H, s, methine-H), 6.25
(1H, s, 3eH), 6.85 (4H, m, AreH), 7.05 (1H, d, J ¼ 16.3 Hz, styryl-H),
7.14 (4H, m, AreH),7.18 (1H, d, J ¼ 16.3 Hz, styryl-H), 7.22 (1H, m,
AreH), 7.36 (3H, m, AreH), 7.42 (1H, dd, J 8.5, 1.7 Hz, 6eH), 7.51 (2H,
m, AreH), 7.57 (1H, d, J 1.7 Hz, AreH); dC 49.68, 55.27, 105.39,
110.83, 111.30, 113.92, 120.85, 125.00, 126.95, 128.22, 128.67, 129.30,
129.58, 130.53, 131.86, 133.46, 137.47, 154.64, 158.54, 160.99. Found
C, 83.25; H, 5.80 [Mþ] 446.1876. C31H26O3 requires C, 83.38; H, 5.87;
[Mþ] 446.1876.
3. Discussion
The Heck reaction [11] offers an efficient route to vinyl
substituted aromatic and heteroaromatic compounds from either
the bromo- or iodo- substituted precursors using a range of Pd-
complexes and associated ligands [12]. One modification of the
Heck reaction is the use of Pd(OAc)2 without the need of a typically
Fig. 1. X-Ray crystal structure of styrylbenzofuran 9.

828
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 825e830
Pd(OAc)2
OAc
An
DMA heat
Pd
An
An
Pd
X
Pd0
X
X
An
OAc
n
A
O
n
A
O
O
8
X = Br
An = 4-MeOC6H4
An
[Base-H]
•
H
Base
X
An
Pd
An
An
X
X
An
O
9
O
An
O
10
X=CH=CHPh
Scheme 4. Proposed mechanism for the ring-contraction process.
reported by Yao et al. [13], resulted in a complex multi-component
mixture from which the major product, a non-photochromic
compound 9 was isolated in 30% yield after elution from silica
(Scheme 3). No photochromic compound was detected by routine
TLC examination of the reaction mixture. The 1H NMR spectrum of
9 displayed signals for a trans-styryl function (with J ¼ 16.4 Hz),
confirming the success of the Heck process, but did not show any
expected doublets that could be attributed to the benzopyran ring
chloride and 4-hydroxybenzaldehyde with subsequent isomer-
isation achieved upon heating the crude isomer mixture in hexane
containing a catalytic amount of iodine [22]. Heating 11 with pro-
pynol 7 (An ¼ 4-MeOC6H4) resulted in a mediocre yield of 6 (24%)
after column chromatography (Scheme 5). 1H NMR spectroscopy
was used to confirm the structure of 6 which displayed two AB
systems; the former with doublets at
attributed to the pyran ring protons and the latter system at lower
field ( 6.91 and 7.01) with a larger vicinal coupling constant of
d
6.14 (J ¼ 9.8 Hz) and
d 6.61
and instead displayed a singlet at
d
5.95 and at
d
7.25. The structure
d
of this compound was elucidated as the styrylbenzofuran 9 by
X-ray crystallography (Fig. 1) [17].
16.3 Hz confirming the E-geometry of the styryl unit.
The photochromic response of the new benzopyrans 6 and 8 in
CH2Cl2 solution (ca. 2.5 ꢀ 10ꢁ4 mol dmꢁ3) was next investigated.
The bromobenzopyran 8 displayed photochromism when sub-
jected to irradiation with light of wavelength 270e380 nm (150
Watt, 5 min) and generated a maroon shade (lmax 522 nm) asso-
ciated with the photomerocyanine tautomer. However, the photo-
generated colour was transient and faded almost instantaneously
upon cessation of irradiation indicating the strong preference of 8
to remain in the ring-closed state. The weak photochromism of
other simple diaryl substituted 2H-[1]benzopyrans has been noted
[23] and more recently the photochromism of 6- and 7- aryl
substituted 2H-[1]benzopyrans has been explored [24]. Similar
treatment of the styryl substituted isomer 6 resulted in the
generation of a far longer lived greeneblue colour, the absorption at
longer wavelength and persistence being in accord with the
extended chromophore. The absorption spectrum of 6 is presented
in Fig. 2 and shows a typical photochromic response with maxima
The precise mechanism for the ring-contraction leading to 9 is at
present unknown, however in the presence of dimethylacetamide
at high temperature it is probable that nanoparticulate Pd0 is
generated in situ [18e20]. Addition of nucleophilic Pd0 to C-4 of the
benzopyran 8, a process comparable with the many examples of
addition of Pd0 to allylic acetates with displacement of acetate ion
leading to a p-allyl Pd species [21], proceeds with ring-opening and
subsequent capture of the palladium to generate the palladacycle
10. Deprotonation with either added base (K3PO4) or acetate ion
regenerates Pd0 and an allenylphenol, which presumably
undergoes a 5-exo-dig cyclisation to the benzofuran 9 (Scheme 4).
The ring-contraction sequence operates in tandem with the
expected Heck reaction resulting in the overall conversion of 8 / 9.
Further exploration of the nature of the Pd species and the gener-
ality of the tandem ring-contraction d Heck sequence is under
investigation.
In an effort to synthesise the target benzopyran 6 we returned to
more traditional chemistry and prepared (E)-4-hydroxystilbene 11
in 72% yield by a Wittig reaction between an excess of the ylide
derived from deprotonation of benzyltriphenylphosphonium
at 580 nm (broad band) and at 446 nm. The half-life (t , s) for the
½
persistence of the photogenerated species was determined by
following the rate of decrease of the photogenerated colour at each
of these wavelength maxima over 1 min period after excitation to
Scheme 5. Preparation of styrylbenzopyran 6.

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 825e830
829
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Fig. 2. Absorption spectrum of styrylbenzopyran 6.
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resulted in t values of 1.8 s; it should be noted that this value is
½
approaching the limit of detection using the present apparatus.
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The typically high yielding acidic alumina catalysed propargyl
alkynol route to naphthopyrans affords benzopyrans in only
moderate yield when applied to phenols. The ligand-free Heck
functionalisation of
a bromobenzopyran was complicated by
a competing ring-contraction when Pd(OAc)2 was employed as the
Pd source. The Pd species involved in the contraction process is
most probably nanoparticulate Pd0 as a consequence of the severe
applied reaction conditions. The styrylbenzopyran displays rela-
tively weak photochromism generating a green-blue shade which
[7] Gabbutt CD, Heron BM, Kolla SB, Kilner C, Coles SJ, Horton PN, et al. Ring
contraction during the 6
p-electrocyclisation of naphthopyran valence tauto-
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fades relatively rapidly (t ¼ 1.8 s). Under the applied irradiation
½
conditions neither isomerisation nor electrocyclisation of the styryl
unit of 6 to afford a phenanthropyran was detected.
Acknowledgement
[10] Güsten H, Salzwedel M. Die kernmagnetischen resonazspektren substituierter
trans-stilbene. Tetrahedron 1967;23:173e85.
The authors thank Yorkshire Forward and James Robinson Ltd.,
[Vivimed Laboratories (Europe)] for a studentship (SBK). The
Worshipful Company of Clothworkers’ of the City of London are
thanked for a millennium grant for the purchase of a Bruker Avance
NMR instrument and for an annual grant for the purchase of a Cary
50 Probe spectrophotometer and associated irradiation sources.
The EPSRC are thanked for access to the National Mass Spectrom-
etry Service (Swansea).
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[17] X-Ray crystal structure details for styrylbenzofuran 9: Colourless plate;
Crystal size ¼ 0.19 ꢀ 0.07 ꢀ 0.01 mm; T ¼ 150(2) K;
l
¼
¼ 0.71073 Å [Mo-
Ka]; Crystal system
dimensions ¼ a ¼ 8.9630(4) Å
22.5140(12)
(calculated)
¼
monoclinic; Space group
P21; Unit cell
¼ 100.7150(10)ꢂ,
a
¼ 90ꢂ, b ¼ 5.8390(2) Å
b
c
¼
Å
g
¼
90ꢂ; Volume
¼
1157.73(9) Å3;
Z
¼
2; Density
¼
1.281 Mg/m3; Absorption coefficient
¼
0.081 mmꢁ1
27.47ꢂ; Index
29; Reflections
0.0686];
;
F(000)
ranges
collected
¼
472; Data collection range
ꢁ11 11, ꢁ7
9218; Independent reflections
¼
2.64
ꢃ
q
l
ꢃ
¼
ꢃ
h
ꢃ
ꢃ
k
ꢃ
7, ꢁ27
ꢃ
ꢃ
¼
¼
2897 [R(int)
¼
[23] (a) Becker RS, Michl J. Photochromism of synthetic and naturally occurring
2H-chromenes and 2H-pyrans. J Am Chem Soc 1966;88:5931e3;
Observed reflections ¼ 1919 [I > 2
s
(I)]; Absorption correction ¼ multi-
0.9992 and 0.7708; Refinement
2897/1/309; Goodness of
(I)] ¼ R1 ¼ 0.0495, wR2 ¼ 0.0908; R
0.0958, wR2 0.1049; Largest diff. peak and
scan; Max. and min. transmission
¼
(b) Sakuragi M, Kawanishi Y, Suzuki Y, Sakuragi H. Photo-ring-opening effi-
ciency of 2,2-diphenyl-2H-[1]benzopyran evaluated from addition reactivity
of amines to its ring-opened isomers. Bull Chem Soc Jpn 2008;81:641e3.
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arylchromenes: remarkable modification of absorption properties and life-
times of o-quinoid intermediates. Org Lett 2007;9:919e22;
method
fit ¼ 0.999; Final R indices [I > 2
indices (all data) R1
¼
Full; Data/restraints/parameters
¼
s
¼
¼
¼
hole ¼ 0.172 and ꢁ0.201e.Åꢁ3. Crystallographic data (excluding structure
factors) for this structure has been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication numbers
CCDC786366. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: þ44(0)-
1223e336033 or e-mail: deposit@ccdc.cam.ac.uk).
(b) Moorthy JN, Koner AL, Samanta S, Roy A, Nau WM. Modulation of spec-
trokinetic properties of o-quionoid reactive intermediates by electronic
factors: time resolved laser flash and steady-state photolysis investigations of
photochromic 6- and 7- arylchromenes. Chem Eur J 2009;15:4289e300.