Synthesis of 1-vinylidene-naphthofurans: A Thermally reversible photochromic system that colors only when adsorbed on silica gel

Article abstract of DOI:10.1021/jo400710r

A set of new 1-vinylidene-1,2-dihydro-naphtho[2,1-b]furans were unexpectedly obtained in the reaction of 2-naphthol with readily available 1,1,4,4-tetraarylbut-2-yne-1,4-diols. Surprisingly, when adsorbed in silica gel, these compounds exhibit photochromism at room temperature, whereas not in solution and in the solid state. UV or sunlight irradiation leads, in a few seconds, to the formation of intense pink/violet to green colors that bleach completely in a few minutes in the dark. These new compounds also exhibit reversible acidochromic properties in solution: addition of TFA leads to the opening of the furan ring and addition of a proton to the allene function, leading to a conjugated and violet tertiary carbocation that returned immediately to the uncolored allenic structure upon addition of a weak base.

Full text of DOI:10.1021/jo400710r

Article
pubs.acs.org/joc
Synthesis of 1‑Vinylidene-naphthofurans: A Thermally Reversible
Photochromic System That Colors Only When Adsorbed on Silica Gel
Ceu M. Sousa,† Jerome Berthet,‡ Stephanie Delbaere,‡ and Paulo J. Coelho*,†
́
†Centro de Química - Vila Real, Universidade de Tras
́
-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
‡Universite
France
́
Lille Nord de France, CNRS UMR 8516, UDSL, Faculte des Sciences Pharmaceutiques et Biologiques, F-59006, Lille,
́
S
* Supporting Information
ABSTRACT: A set of new 1-vinylidene-1,2-dihydro-naphtho[2,1-
b]furans were unexpectedly obtained in the reaction of 2-naphthol
with readily available 1,1,4,4-tetraarylbut-2-yne-1,4-diols. Surprisingly,
when adsorbed in silica gel, these compounds exhibit photochromism
at room temperature, whereas not in solution and in the solid state.
UV or sunlight irradiation leads, in a few seconds, to the formation of
intense pink/violet to green colors that bleach completely in a few
minutes in the dark. These new compounds also exhibit reversible acidochromic properties in solution: addition of TFA leads to
the opening of the furan ring and addition of a proton to the allene function, leading to a conjugated and violet tertiary
carbocation that returned immediately to the uncolored allenic structure upon addition of a weak base.
activation, and the thermal relaxation kinetic can be controlled by
introducing proper substituents in the structure.13
INTRODUCTION
■
Over the last decades, interest in photochromic compounds has
been growing.1 These molecules are able to switch between two
states, usually exhibiting different colors, under the action of light
and have been applied with success in ophthalmic photochromic
lenses that reversibly darken under sunlight.2 Because other
properties, such as the refractive index, dielectric constant, and
geometry, can be switched upon irradiation with light, these
molecules have an enormous potential to be used in other
applications that do not require necessarily a color change, such
as optical data storage devices,3 electronic display systems,4 logic
gates,5 and molecular imaging.6 The most important classes of
thermally reversible photochromic compounds include naph-
thopyrans, spiropyrans, spiroxazines, fulgides, and azo com-
pounds, but new structures are now and then discovered.7 The
UV or visible light activation of these compounds promotes a
chemical reaction leading to the formation of one or more species
with a different absorption spectrum, which, in the absence of
light, returns thermally, with variable speed, to the initial form.
The phenomenon occurs typically in solution or when the
molecules are dispersed in polymeric matrixes, although the
kinetics of the process is highly affected by the nature of the host
matrix.8 A wide range of chemically different matrixes, where the
organic photochrome is dispersed or covalently linked, have been
investigated: purely organic medium, for example, poly(methyl
methacrylate) (PMMA),9 hybrid inorganic−organic composites
(ormosils),10 or purely inorganic silicate matrixes.11 In some
cases, the photochromism was also detected in the solid state.12
Because of the versatility of the naphthopyran entity, there has
been an intense research on this class of molecules:
naphthopyrans are easy to prepare, they show a high resistance
to photodegradation, different colors can be produced after UV
Naphthopyrans are usually prepared through the reaction of
naphthols with 1,1-diarylpropynols in acid media at reflux.14 The
mechanism involves the acid-catalyzed formation of an ether,
followed by Claisen rearrangement, enolization, 1,5-hydrogen
shift, and electrocyclization (Scheme 1).15 The reaction is
compatible with various substituents and has been used for the
synthesis of hundreds of different naphthopyrans.
RESULTS AND DISCUSSION
■
Synthesis. In the course of our studies on the photo-
chromism of naphthopyrans substituted in the pyran-double
bond,16 we explored the reaction of 2-naphthol 1 with the readily
Scheme 1. Mechanism for the Acid-Catalyzed Synthesis of
Naphthopyrans from Naphthols and 1,1-Diarylpropynols
Received: April 4, 2013
© XXXX American Chemical Society
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available 1,1,4,4-tetraarylbut-2-yne-1,4-diols 2a−2c17 (Scheme
2), which could afford directly a naphtho[2,1-b]pyran possessing
Scheme 3. Mechanism for the Formation of 1-Vinylidene-
naphtho[2,1-b]furan 3a and the Allenylnaphthol 4
Scheme 2. Synthesis of 1-Vinylidene-naphtho[2,1-b]furans
3a−3c (a: R = H; b: R = F; c: R = OCH3)
a reactive alcohol function as substituent in position 1. In fact,
treating 2-naphthol 1 with diol 2a (Ar = Ph) in CHCl3 in the
presence of an acid catalyst (p-TSA) led to the complete
disappearance of the starting materials after 60 min, at room
temperature, with the formation of a new photochromic
compound. However, the absence of any ethylenic-pyran
hydrogen in the 1H NMR spectrum, the presence of a low-field
signal at 202.9 ppm in the 13C NMR spectrum, and a shift of the
typical pyran sp3 carbon atom, usually around 80 ppm, but here at
95.1 ppm, excluded a naphthopyran structure and pointed to the
formation of a completely different structure: the naphthofuran
3a (47% yield) presenting a substituted allene function (Scheme
2).
The allenylnaphthol 4 is stable in the solid state, but not in
solution, being completely converted to the naphthopyran 5 after
a few days in a CDCl3 solution (Scheme 4). The formation of the
Scheme 4. Thermal Evolution of Allenylnaphthol 4 into
Naphthopyran 5 in CDCl3 Solution
The structure of this new compound was unambiguously
established using 2D NMR experiments. In particular, H−13C
1
naphthopyran structure 5 was confirmed by the appearance in
the 1H NMR spectrum of a singlet at 5.82 ppm, characteristic of
the ethylenic-pyran proton H-2, and the typical sp3 pyran carbon
C-3, observed at 82.9 ppm in the 13C NMR spectrum. 1H−13C
scalar correlation between proton H-2 and carbon C-3 was
measured in HMBC experiments, which confirmed structure 5.
Photochromic Properties. The 1-vinylidene-naphtho[2,1-
b]furans 3a−3c are not photochromic in the solid state or in
solution using polar (CH2Cl2, Et2O, CHCl3, CH3CN, DMSO,
EtOH, CH3CO2H) or apolar (toluene) solvents, even at low
temperature (toluene or acetone at −80 °C). Nevertheless, they
show reversible photochromic properties when adsorbed in silica
gel at room temperature:18 when silica gel was added to a
solution of 1-vinylidene-naphthofurans 3a in CH2Cl2 and the
solvent was removed by evaporation at atmospheric pressure, we
obtained a white powder of silica gel doped with compound 3a
that turned pink/violet when exposed to UV light/sunlight for 15
s and faded back completely to white, in the dark, in less than 5
min (Figure 1).
long-range scalar correlations (HMBC) were measured between
protons H-2′/H-6′ at 7.16 ppm and C-1 at 117.7 ppm (3J) and C-
2 at 202.9 ppm (4J) that proved the allene structure. Carbon C-3
at 114.9 ppm showed no correlation through 2, 3, and 4 bonds.
The sp3 carbon C-4 at 95.1 ppm is correlated with protons H-2″/
H-6″ at 7.47 ppm (3J), and finally, the furan carbon C-4a at 157.9
ppm is correlated with H-5 at 7.29 ppm (2J) and H-6 at 7.82 ppm
(3J).
Starting from substituted diols 2b−2c (R = F, OMe), we
obtained the corresponding 1-vinylidene-1,2-dihydro-naphtho-
[2,1-b]furans 3b (19%) and 3c (46% yield). When the reaction
between 1 and diol 2a was quenched after 10 min, the formation
of the expected 1-vinylidene-naphtho[2,1-b]furan 3a was
detected by TLC along with a small amount of another
compound 4, with lower Rf, that showed similar photochromic
properties and was also isolated and characterized (Scheme 3).
The structure elucidation of this compound showed that it has
two OH groups and an allene function, characterized by the
carbon resonances of 109.2 ppm (C-1), 112.2 ppm (C-3), and
208.3 ppm (C-2), being similar to the allene intermediate in the
naphthopyran synthesis (Scheme 1). The allenylnaphthol 4 is
clearly an intermediate in this reaction since, in the presence of p-
TSA, it is completely converted into the 1-vinylidene-naphtho-
[2,1-b]furan 3a, and the photochromic behavior, observed in the
TLC plates, is probably due to its conversion into compound 3a
promoted by the silica. The isolation of a small amount of this
allenylnaphthol from the reaction mixture indicates that the
reaction between 2-naphthol 1 and diol 2a follows, very likely,
the same mechanism leading to naphthopyrans, shown in
Scheme 1, except that the intermediate 4 after rotation and
dehydration is converted into the 1-vinylidene-naphtho[2,1-
b]furan 3a (Scheme 3).
Figure 1. Photographs of silica gel doped with compound 3a, in an
aluminum sheet, before and after UV irradiation (365 nm) for 15 s.
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Acidochromic Properties. These compounds show also
acidochromic properties. Addition of TFA to a CHCl3 uncolored
solution of compound 3a leads to the appearance of a violet
color, characterized by two bands in the visible spectrum (λmax
=
451 and 580 nm) that bleached when NEt3 was added to the
solution (Figure 2). The process is completely reversible, and
upon subsequent addition of acid, the violet coloration appears
again. The same reversible behavior was observed with
compounds 3b (λmax = 448 and 588 nm) and 3c (λmax = 503
and 658 nm) with a notable bathochromic shift of both bands in
the latter due to the presence of the dimethoxy substituents in the
aromatic rings. This reaction was also followed by NMR:
addition of TFA to a CDCl3 solution of 3a led to a complete
change in the aromatic part of the 1H NMR spectrum, and upon
addition of NEt3, the signals of the naphthofuran 3a appeared
again (Figure 3).
Figure 2. Absorption spectra of (black) naphthofuran 3a, (red) after the
addition of CF3COOH, (pink) after the addition of CF3COOH + NEt3,
and (blue) after subsequent addition of CF3COOH.
The 1H NMR spectrum of the colored solution suggests that
an addition of H+ to the allene function occurred with the
opening of the furan ring and formation of a tertiary carbocation.
This assumption was confirmed by 13C NMR with the
disappearance of the low-field allenic signal at 202.9 ppm (C-
2) and the appearance of two signals at 185.9 and 186.7 ppm
assigned, respectively, to a carbonyl (C-4a) and a tertiary
carbocation (C-4). The chemical shifts of H-2′, H-3′, and H-4′ at
6.59, 6.92, and 7.11 ppm, respectively, are more shielded than
those in compound 3a. This strong upfield effect can be
explained by π stacking between the phenyl and the naphthalene
part in the opened form. Moreover, the 1D NOESY spectrum
showed dipolar correlations between the ethylenic proton H-2 at
7.31 ppm and the aromatic protons H-2b/H-2c at 7.45−7.49
ppm, which confirmed the structure of this opened structure
(Scheme 5).
The photochromic behavior of compounds 3a−3c is
reproducible and can be repeated several times without any
sign of degradation: TLC analysis of compounds 3a−3c after 10
UV/dark cycles showed that they remained unchanged. The
color change is dependent on the amount of water in the silica gel
and does not occur in hydrated silica, but the photochromic
behavior can be recovered after heating a sample of hydrated
silica doped with compound 3a (oven, 60 °C, 1 day). On the
other hand, after UV activation of the doped silica powder, the
addition of any solvent leads to the immediate disappearance of
the color, indicating that the compound is no longer adsorbed in
the silica. The behavior observed with silica gel could not be
extended to other oxides (Al2O3, SnO2, MgO, TiO2) or pure
SiO2 (quartz), which means that the terminal OH groups present
in the silica gel are probably necessary for the observation of the
phenomenon.
The visible spectra of these colored species present two
absorption maxima. The band around 450 nm resembles the
absorption band of a merocyanine dye (open form of the
naphthopyran), while the second maxima, between 580 and 658
Figure 3. 1H NMR spectra of (a) 3a in CDCl3, (b) after addition of CF3COOH, and (c) after subsequent addition of NEt3.
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Scheme 5. Acidochromism of Naphthofuran 3a and Dipolar Correlations between H-2 and H-2b/H-2c
nm, may be justified by the extension of the conjugation between
the merocyanine and the carbocation.19
monoexponential equation, suggesting that, most likely, a single
colored species is formed after light activation.
The formation of this conjugated opened species in an acidic
medium suggests that the UV irradiation of compound 3a,
dispersed in silica gel, may also lead to a heterolytic cleavage of
the C−O bond with formation of a conjugated zwitterionic
species stabilized by the polar OH terminal groups of the silica
gel (Scheme 6). This type of stabilization of a zwitterionic
Scheme 6. Photochromic Equilibrium for 1-Vinylidene-
naphtho[2,1-b]furan 3a
Figure 4. Color space coordinates evolution (a* and b*) during the
thermal fading of compound 3a.
colored form through hydrogen bonding by the residual acidic
silanol groups has also been observed in spiropyrans and
spirooxazines.20 The structure of this zwitterionic species is
different from the one observed upon addition of acid but allows
also the extended conjugation between the naphthalene and the
phenyl rings.
Kinetic Studies. For the measurement of the color of the
doped silica solid after UV irradiation and kinetic analysis of the
thermal decoloration process, we used a colorimeter that
characterizes the color developed in terms of their tonality,
luminosity, and saturation using the L*a*b* color space
coordinates (L* is the luminosity, +a* indicates toward the
red, −a* toward the green, +b* direction yellow, −b* direction
blue). The silica gel doped with naphtho[2,1-b]furans 3a−3c is
white before irradiation, and it has nearly the same color
coordinates as the pure silica gel. After irradiation with UV light
(6 W, 365 nm) for 15 s, all the color coordinates change and the
doped powder acquired pink/violet (3a, 3b) or green (3c)
colorations that bleached in the dark and returned to the initial
white coloration with variable kinetics depending on the nature
of the aryl substituents (Table 1).
Figure 5. Color space coordinate L* (luminosity) evolution during the
thermal fading of compound 3a.
For naphtho[2,1-b]furan 3a, a lifetime of 76 s of the violet/
pink opened form was measured at 20 °C. The introduction of
electron acceptor fluoro atoms in the para position of the aryl
substituents (compound 3b) led to an increase of the bleaching
The time evolution of the three color coordinates L*,a*,b*
during the thermal decoloration of compound 3a is shown in
Figures 4 and 5. These curves can be very closely fitted to a
Table 1. Color Space Coordinates (L*a*b*) before and after UV Irradiation, Bleaching Rate Constants, and Half-Life Time for
Compounds 3a−3c Adsorbed in Silica Gel
compound
before UV light
after UV light
k (s−1
)
t1/2 (s)
3a (R = H)
3b (R = F)
3c (R = OMe)
silica gel
90.64; 3.13; 1.32 (white)
92.65; 0.76; 3.59 (white)
84.88; −7.12; 8.55 (white)
92.31; 0.70; 2.37 (white)
53.35; 41.61; −0.72 (violet/pink)
75.54; 20.15; −2.40 (violet/pink)
46.04; −23.35; 8.17 (green)
0.0091
0.020
76
35
0.0012
577
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dry THF (50 mL) was stirred at room temperature for 3 days. The
mixture was then filtered with Celite, and the solvent was removed
under reduced pressure. The residue was diluted with water (50 mL)
and extracted with AcOEt (2 × 40 mL). The organic phase was dried
over Na2SO4, and a part of the solvent was removed under reduced
pressure. During the solvent evaporation, the ketone, a white solid, is
removed by fractionation precipitation. The remaining oil is composed
mainly of the diol and can now be easily purified
1,1,4,4-Tetra(4-fluorophenyl)but-2-yne-1,4-diol (2b). The crude
product was precipitated with CH2Cl2, and the crystals were filtered and
washed with petroleum ether to afford 2b as white crystals (0.437 g from
8.80 g of 4,4′-difluorobenzophenone, 5% yield). mp 158.4−165.4 °C. IR
(KBr, cm−1): 3553, 3448, 1606, 1504, 1413, 1345, 1231, 1156, 1095,
997, 845, 822, 735, 573. 1H NMR (400 MHz, CDCl3): 7.50 (m, 8H),
7.48 (t, J = 8.9 Hz, 8H), 2.82 (s, 2H, OH). EI-MS (TOF) m/z (%): 444
(2), 428 (10), 416 (5), 321 (6), 243 (4), 218 (70), 214 (11), 123 (100),
95 (61), 75 (22). HRMS calcd for C28H16OF4 (M − H2O): 444.1137.
Found: 444.1127.
rate (t1/2 = 35 s), while electron-donating methoxy groups in the
same position (compound 3c) promoted the formation of more
stable opened forms with a green coloration and a much higher
lifetime (t1/2 = 577 s). These results are in accordance with the
zwitterionic character proposed for the structure of the colored
forms (Scheme 6).
The easiness of the synthesis of these new 1-vinylidene-1,2-
dihydronaphthofurans and their photochromic performance,
under UV or sunlight irradiation, at room temperature when
adsorbed in silica gel, make them good candidates for new
photochromic materials with thermally reversible behavior.
CONCLUSION
■
A new photochromic system with a 1-vinylidene-naphthofuran
structure was discovered. These compounds were easily obtained
in the reaction of 1-naphthol with 1,1,4,4-tetraarylbut-2-yne-1,4-
diols at room temperature and show photochromic properties
only when they are adsorbed in silica gel. Sunlight irradiation of
silica gel doped with these compounds generates in a few seconds
a deep pink/green color that fades thermally within a few
minutes in the dark. The 1-vinylidene-naphthofurans also show
acidochromic properties: addition of TFA to an uncolored
solution of compound 3a leads to the immediate development of
an intense violet coloration that bleaches immediately when a
weak base (NEt3) is added. NMR studies on these solutions show
that, in an acid medium, the heterolytic opening of the furan ring
and addition of a proton to the allene occur, leading to a highly
conjugated cationic species that reverts completely to the initial
allenic structure in a weak basic medium. The UV irradiation of
compound 3a in silica gel may lead also to a heterolytic cleavage
of the C−O bond with formation of a conjugated zwitterionic
species stabilized by the polar OH terminal groups of the silica
gel.
1,1,4,4-Tetra(4-methoxyphenyl)but-2-yne-1,4-diol (2c). The crude
product was purified by flash chromatography (0−50% AcOEt/
petroleum ether) to afford pure 2c as yellow crystals (1.42 g from
9.76 g of 4,4′-dimethoxybenzophenone, 56% yield). mp 136.5−138.0
°C. IR (KBr, cm−1): 3398, 2959, 2934, 2833, 1585, 1613, 1511, 1460,
1
1414, 1362, 1249, 1175, 1106, 1032, 987, 832, 588. H NMR (400
MHz,CDCl3): 7.48 (d, J = 8.7 Hz, 8H), 6.83 (d, J = 8.7 Hz, 8H), 3.77 (s,
12H, OCH3), 2.80 (s, 2H, OH). EI-MS (TOF) m/z (%): 492 (11), 490
(10), 476 (23), 464 (14), 243 (10), 242 (81), 211 (33), 135 (100), 107
(15), 92 (19), 77 (19). HRMS calcd for C32H28O5 (M − H2O):
492.1937. Found: 492.1936.
Synthesis of 1-Vinylidene-1,2-dihydro-naphtho[2,1-b]furans
(3a−3c). 4-Toluenesulfonic acid monohydrate (5 mg) was added to a
stirred solution of 2-naphthol (280 mg) and diol 2a−2c (1 equiv) in
CHCl3 (10 mL), and the violet solution thus formed was maintained at
room temperature until TLC examination indicated that no diol
remained (about 1 h). After addition of water (60 mL), the organic
phase was separated and the aqueous phase was extracted with CHCl3 (2
× 40 mL). The combined organic extracts were dried (Na2SO4), and the
solvent was removed under reduced pressure to give an oil that was
purified by column chromatography and recrystallization.
1,2-Dihydro-1-(2,2-diphenyl)vinylidene)-2,2-diphenylnaphtho-
[2,1-b]furan (3a). The product was purified by column chromatography
(5−10% ethyl acetate/petroleum ether) and then recrystallized from
petroleum ether to afford white crystals of 3a (0.455 g from 0.758 g of
diol 2a, 47% yield). mp 193.4−195.9 °C. IR (KBr, cm−1): 3060, 3027,
1626, 1592, 1516, 1457, 1440, 1269, 1247, 1180, 1017, 962, 806, 751,
696. 1H NMR (300 MHz, CDCl3): 8.26 (d, J = 8.2 Hz, 1H), 7.85 (d, J =
8.2 Hz, 1H), 7.82 (d, J = 8.8 Hz, 1H), 7.43−7.51 (m, 5H), 7.36 (t, J = 7.5
Hz, 1H), 7.20−7.31 (m, 13H), 7.16 (d, J = 7.9 Hz, 4H). 13C NMR (75
MHz, CDCl3): 95.1, 112.6, 113.7, 114.9, 117.5, 122.4, 123.9, 127.5,
127.9, 128.0, 128.1, 128.3, 128.6, 128.9, 129.3, 130.0, 130.3, 132.0, 136.6,
143.2, 157.9, 202.9. EI-MS (TOF) m/z (%): 499 (11), 498 (43), 497
(11), 496 (29), 469 (16), 468 (76), 467 (11), 454 (11), 421 (32), 420
(10), 392 (20), 391 (100), 389 (32), 387 (16), 376 (11), 331 (13), 315
(10), 313 (20). HRMS calcd for C38H26O: 498.1984. Found: 498.1989.
1,2-Dihydro-1-(2,2-di(4-fluorophenyl)vinylidene)-2,2-di(4-fluoro-
phenyl)naphtho[2,1-b]furan (3b). The product was purified by
column chromatography (3% ethyl acetate/petroleum ether) and then
recrystallized from petroleum ether to afford white crystals of 3b (0.216
g from 0.445 g of diol 2b, 19% yield). mp 124.4−127.7 °C. IR (KBr,
cm−1): 3061, 1634, 1604, 1505, 1460, 1271, 1236, 1156, 1012, 962, 832,
797, 748, 573. 1H NMR (300 MHz, CDCl3): 8.18 (d, J = 8.3 Hz, 1H),
7.88 (d, J = 8.4 Hz, 1H), 7.85 (d, J = 8.8 Hz, 1H), 7.51 (t, J = 7.9 Hz, 1H),
7.34−7.46 (m, 5H), 7.28 (d, J = 8.9 Hz, 1H), 7.12 (dd, J = 8.6 Hz, J = 5.4
Hz, 4H), 6.89−7.06 (m, 8H). 13C NMR (75 MHz, CDCl3): 93.8, 112.4,
112.9, 115.1, 115.3, 115.6, 115.9, 121.9, 124.0, 128.1, 129.1, 129.3, 129.6,
130.1, 130.2, 132.0, 132.3, 138.6, 157.6, 160.9, 164.2, 202.1. 19F NMR
(282 MHz, CDCL3): −113.51 (tt, J = 8.4 Hz, J = 5.4 Hz, 1F), −113.92
(tt, J = 8.5 Hz, J = 5.4 Hz, 1F). EI-MS (TOF) m/z (%):570 (100), 476
(10), 475 (51), 474 (11), 445 (14), 367 (18), 351 (16), 349 (14), 201
(14). HRMS calcd for C38H22OF4: 570.1607. Found: 570.1605.
EXPERIMENTAL SECTION
■
General Methods. The reactions were monitored by thin-layer
chromatography on aluminum plates precoated with silica gel 60 F254
(0.25 mm). Column chromatography (CC) was performed on silica gel
60 (70−230 mesh). The new compounds were determined to be >95%
pure by 1H NMR spectroscopy. NMR spectra in CDCl3 were recorded
at 298 K using spectrometers (v0 (1H) = 300, 400, and 500 MHz).
Chemical shifts (δ) are reported in parts per million. IR spectra were
obtained using KBr disks (wavenumbers in cm−1).
1,1,4,4-Tetraphenylbut-2-yne-1,4-diol (2a). n-Butyllithium (15
mL, 1.6 M in hexanes) was slowly added (ca. 15 min) to a cold (0 °C)
stirred solution of 1,1-diarylprop-2-yn-1-ol (2.50 g, 12 mmol) in
anhydrous tetrahydrofuran (25 mL). On completion of the addition, the
cold solution was stirred for 30 min. The benzophenone (2.41 g, 30
mmol) was added in a single portion to the solution, and the resulting
mixture was stirred at room temperature for 22 h and controlled by TLC
(the reaction was not complete). Three-fourths of the solvent was
removed under reduced pressure, and aqueous HCl (5%, 50 mL) was
added to afford a suspension of diol 2a that was filtered. The crystals
were then recrystallized from ethyl acetate/petroleum ether to give pure
2a as white crystals (3.80 g, 81% yield). mp 201.4−203.0 °C. IR (KBr,
cm−1): 3532, 3456, 3061, 3019, 2918, 2843, 2364, 2339, 1600, 1499,
1448, 1331, 1213, 1138, 1070, 1028, 995, 911, 886, 776, 743, 693, 642,
609. 1H NMR (400 MHz, CDCl3): 7.59 (d, J = 7.9 Hz, 8H), 7.39−7.23
(m, 12H), 2.86 (s, 2H, OH). EI-MS (TOF) m/z (%): 372 (32), 371
(11), 370 (14), 356 (28), 344 (22), 267 (22), 265 (27), 252 (11), 208
(10), 207 (14), 182 (64), 181 (10), 179 (13), 178 (17), 165 (19), 05
(100), 77 (50). HRMS: calcd for C28H20O (M − H2O): 372.1514.
Found: 372.1503
Synthesis of Diols (2b and 2c). The suspension of the lithium
acetylide−ethylene diamine complex (90%, 2.475 g, 26.9 mmol) and 1.5
equiv of 4,4′-difluorobenzophenone or 4,4′-dimethoxybenzophenone in
E
dx.doi.org/10.1021/jo400710r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
1,2-Dihydro-1-(2,2-di(4-methoxyphenyl)vinylidene)-2,2-di(4-
methoxyphenyl)naphtho[2,1-b]furan (3c). The product was purified
by column chromatography (15−20% ethyl acetate/petroleum ether).
The compound was then dissolved in CH2Cl2, and charcoal was added.
The suspension was heated for 30 min at 60 °C and filtered, and the
solvent was removed under reduced pressure. Recrystallization from
petroleum ether afforded slightly yellow crystals of 3c (0.550 g from
0.970 g of diol 2c, 46% yield). mp 71.9−74.9 °C. IR (KBr, cm−1): 2954,
2931, 2833, 2359, 2337, 1607, 1511, 1458, 1248, 1170, 1030, 833, 811,
584. 1H NMR (300 MHz,CDCl3): 8.25 (d, J = 8.3 Hz, 1H), 7.83 (d, J =
8.1 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H), 7.45 (t, J = 7.7 Hz, 1H), 7.39 (d, J =
8.8 Hz, 4H), 7.34 (t, J = 8.0 Hz, 1H), 7.25 (d, J = 8.8 Hz, 1H), 7.11 (d, J =
8.8 Hz, 4H), 6.78 (d, J = 8.7 Hz, 4H), 6.75 (d, J = 8.8 Hz, 4H), 3.83 (s,
6H, OCH3), 3.80 (s, 6H, OCH3). 13C NMR (75 MHz, CDCl3): 55.3,
94.5, 112.5, 113.4, 113.7, 113.8, 114.8, 116.3, 122.2, 123.5, 127.7, 128.7,
129.0, 129.1, 128.8, 129.9, 130.0, 131.4, 135.7, 157.7, 159.2, 159.3, 202.0.
EI-MS (TOF) m/z (%): 618 (47, M+), 617 (29), 616 (100), 589 (12),
588 (43), 587 (12), 557 (13), 511 (12), 510 (11), 482 (14), 481 (55),
350 (3), 287 (3), 227 (3), 135 (4). HRMS calcd for C42H34O5:
618.2406. Found: 618.2402.
1-(1-Hydroxy-1,1,4,4-tetraphenylbuta-2,3-dien-2-yl)-
naphthalen-2-ol (4). 1H NMR (500 MHz, CDCl3): 3.22 (s, 1H), 7.00
(broad, 4H), 7.14 (d, J = 8.6 Hz, 1H), 7.14 (ddd, J = 8.4 Hz, J = 6.9 Hz, J
= 1.4 Hz, 1H), 7.20−7.28 (m, 9H), 7.29−7.33 (m, 4H), 7.53 (s, 1H),
7.57 (broad, 4H), 7.61 (d, J = 8.2 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H), 7.70
(d, J = 8.1 Hz, 1H). 13C NMR (125 MHz, CDCl3): 83.0, 109.2, 112.1,
114.3, 118.6, 123.1, 125.2, 126.3, 127.0, 127.8, 128.0, 128.1, 128.4, 128.6,
129.0, 130.1, 133.9, 135.5, 144.4, 152.1, 208.3. EI-MS (TOF) m/z (%):
516 (5), 514 (16), 500 (53), 499 (99), 498 (100), 437 (64), 422 (50),
421 (96), 420 (73), 393 (25), 333 (80), 332 (83), 331 (90), 315 (73),
302 (80), 289 (44), 255 (57), 226 (44), 167 (75), 165 (82). HRMS
calcd for C38H28O2: 516.2089. Found: 516.5084.
1-(Hydroxydiphenylmethyl)-3,3-diphenyl-3H-naphtho[2,1-
b]pyran (5). 1H NMR (500 MHz, CDCl3): 3.37 (s, 1H), 5.82 (s, 1H),
6.99 (ddd, J = 8.5 Hz, J = 7.1 Hz, J = 1.4 Hz, 1H), 7.17 (ddd, J = 7.9 Hz, J
= 7.0 Hz, J = 0.9 Hz, 1H), 7.19−7.27 (m, 8H), 7.32 (d, J = 7.7 Hz, 4H),
7.37 (t, J = 7.8 Hz, 4H), 7.39 (d, J = 8.9 Hz, 1H), 7.53 (d, J = 7.4 Hz, 4H),
7.57 (d, J = 8.7 Hz, 1H), 7.67 (d, J = 7.9 Hz, 1H), 7.72 (d, J = 8.9 Hz,
1H). 13C NMR (125 MHz, CDCl3): 82.9, 83.2, 116.7, 119.4, 123.4,
125.8, 126.2, 127.3, 127.5, 127.7, 127.89, 127.92, 128.1, 128.9, 130.0,
130.3, 130.9, 135.1, 141.3, 143.8, 147.9, 153.1. EI-MS (TOF) m/z (%):
516 (9), 515 (13), 514 (30), 500 (43), 499 (39), 498 (79), 437 (100),
421 (56), 409 (54), 334 (87), 333 (75), 315 (32), 302 (44), 289 (30),
257 (64), 255 (34), 226 (31), 191 (32), 182 (39). HRMS calcd for
C38H28O2: 516.2089. Found: 516.5082.
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ASSOCIATED CONTENT
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NMR spectra of all new compounds and the colored species and
kinetic studies for compounds 3a−3c. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pcoelho@utad.pt.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors acknowledge FCT (Portugal’s Foundation for
Science and Technology) and FEDER for financial support
through the research unit Centro de Quimica-Vila Real (POCTI-
SFA-3-616). The 300 and 500 MHz NMR facilities were funded
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by the Reg
Jeunesse de l′Education Nationale et de la Recherche (MJENR),
and the Fonds Europeens de Developpement Regional
(FEDER).
́ ̀
ion Nord-Pas de Calais (France), the Ministere de la
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dx.doi.org/10.1021/jo400710r | J. Org. Chem. XXXX, XXX, XXX−XXX