Copper-catalyzed coupling of (E)-bromostilbene with phenols/azole: ESI-MS detection of intermediates by using an ionically-tagged ligand

Article abstract of DOI:10.1002/adsc.201100925

A system based on copper/1,10-phenanthroline efficiently promotes the coupling between phenols or pyrazole with (E)-bromostilbene. (E)-1-Aryloxy-1,2-diphenylethenes were obtained from the coupling with phenols in good to excellent yields (69-90%). The exc

Full text of DOI:10.1002/adsc.201100925

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DOI: 10.1002/adsc.201100925
Copper-Catalyzed Coupling of (E)-Bromostilbene with Phenols/
Azole: ESI-MS Detection of Intermediates by Using an Ionically-
Tagged Ligand
Jones Limberger,a Bꢀrbara C. Leal,a Davi F. Back,b Jairton Dupont,a,*
and Adriano L. Monteiroa,*
a
Laboratory of Molecular Catalysis, Instituto de Quꢁmica – UFRGS, Av. Bento GonÅalves 9500, 91501-970, CP 15003,
Porto Alegre, RS, Brazil
Fax : (+55)-51-3308-7304; e-mail: jairton.dupont@ufrgs.br or adriano.monteiro@ufrgs.br
Laboratory of Inorganic Materials, Departamento de Quꢁmica – UFSM. Av. Roraima 1000, 97105-900, Santa Maria, RS,
b
Brazil
Received: November 28, 2011; Revised: January 31, 2012; Published online: May 15, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100925.
Abstract: A system based on copper/1,10-phenan-
throline efficiently promotes the coupling between
phenols or pyrazole with (E)-bromostilbene. (E)-1-
Aryloxy-1,2-diphenylethenes were obtained from
the coupling with phenols in good to excellent
yields (69–90%). The exception was the reaction in-
volving a phenol containing an electron-withdraw-
ing cyano group that required a longer reaction
time and gave only 49% yield. Kinetic studies indi-
cated the participation of the vinyl halide in the
rate-determining step. Under the conditions em-
ployed, the activation of the vinyl halide via a radi-
cal pathway was discarded using a radical scavenger
test. By using an ionically-tagged 1,10-phenanthro-
line derivative as the ligand, various copper-based
ions were detected through ESI(+)-MS. These ions
suggested that formation of the active species
Most copper-catalyzed coupling reactions involve
aryl halides as the electrophile partner, leading to
CACTHNUTRGENNUG(aryl) N or CCAHTUNGTNER(NUGN aryl) O bond formation. However,
À
À
vinylic amines and ethers are compounds widely used
as intermediates in organic synthesis, and some excel-
lent examples of copper-catalyzed vinylation of phe-
nols and azoles have been reported. For instance,
amino ethers,[4] N,N-dimethylglycine[2a] and pyridinyl-
benzoimidazole[5] have been used as ligands in the
coupling of vinyl bromides and iodides with phenols.
In 2006, Taileffer and co-workers reported the cou-
pling of phenols and azoles with b-bromostyrene.[6]
The same group published a detailed study regarding
the structure/activity of bi- and tetradentate nitrogen
ligands in the arylation of phenols and azoles. These
ligands were successfully used in N- and O-vinylation
of 1-bromo-2-methylpropene.[7] A b-keto ester was
also used as a ligand to efficiently couple styryl bro-
mides with phenols, thiophenols and imidazoles.[8]
With respect to the reaction mechanism, it is cur-
rently believed that the reaction occurs via the forma-
tion of an intermediate, [LnCu(I)-Nu][9] (where L is
the ligand and Nu is the nucleophile), prior to aryl
halide activation. However, it remains unclear if this
[phenCuOArACHTUNGTRENNUNG(HOAr)2] precedes the vinyl halide ac-
tivation.
Keywords: copper; cross-coupling; ESI-MS; ion-
tagged ligand; vinylation
À
C X activation occurs through an oxidative addition
Cu(I)-Cu
A
The copper-catalyzed coupling of phenols and azoles studies,[9e,10] reactions with well-defined Cu
ACHTUNGTRENNUNG
with aryl or vinyl halides providing aryl or vinyl pounds[11] and radical-presence tests[9d,12]] or via a radi-
ethers and amines represents a synthetic method that cal SET/IAT Cu(I)-Cu(II) pathway (as shown in theo-
has clearly grown in importance in the last decade.[1] retical studies[13] and ESI-MS experiments in combi-
The judicious choice of appropriate ligands (e.g., di- nation with radical scavenger tests[14]).
A
In this way, we report here the employment of an
use of milder conditions and lower amounts of ESI(+)-MS technique in order to detect reaction in-
copper. Therefore, the scope of the reaction has been termediates in the copper-catalyzed vinylation of phe-
extended and various synthetic intermediates[2] and nols. For this, we synthesized and used ionically-
targets[3] have been obtained.
tagged 1,10-phenanthroline derivative ligand L1 that
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Jones Limberger et al.
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[a]
À
À
Table 1. C O and C N coupling of (E)-bromostilbene and 1,1,2-bromotriphenylethene.
Entry
1
R1/R2
Product
Conversion [%]
97
Yield [%][b]
96 (83)
H/4-OMe
2
H/2,5-OMe
80
76 (69)
3
H/4-Br
100
56
97 (82)
51 (49)
96 (90)
4[c]
H/4-CN
Ph/4-OMe
5[d]
99
6[d]
H/pyrazole
85
85 (78)
[a]
Reaction conditions (for GC analysis): vinyl bromide (0.5 mmol), phenol (0.75 mmol), K3PO4·H2O (1 mmol), CuI
(10 mol%), 1,10-phenanthroline (10 mol%), dioxane (2 mL).
Yields refer to GC yields (with undecane as an internal standard). Values in brackets refer to yields of isolated products
using a 2-mmol scale.
CuI (20 mol%), 1,10-phenanthroline (20 mol%), 1208C, 24 h.
Reaction time: 24 h.
[b]
[c]
[d]
was used as a probe. Moreover, (E)-bromostilbene (10 mol%), 1,10-phenanthroline (10 mol%) and
was used as substrate, allowing the synthesis of tri- K3PO4·H2O (2 equiv) in dioxane at 1008C for 6 h.
À
and tetrasubstituted vinyl ethers and N-vinylpyrazole
Table 1 describes the results obtained for the C N
À
in good to excellent yields, using 1,10-phenanthroline and C O coupling between phenols/azoles with (E)-
as ligand. At the end, a viable mechanistic pathway of bromostilbene (1a) and bromotriphenylethene (1b).
copper-catalyzed vinylations of phenols was proposed
Good to excellent yields were obtained for the cou-
based on the results obtained from ESI(+)-MS, pre- pling of both an electron-rich phenol (Table 1,
liminary kinetic studies and a radical scavenger ex- entry 1) and slightly electron-deficient phenols
periment.
(Table 1, entries 2 and 3). However, when we used
À
We chose to study the C O coupling of (E)-bro- a phenol substituted with the electron-withdrawing
mostilbene using CuI as the catalyst precursor and CN group, the reaction gave only moderate substrate
commercially available 1,10-phenanthroline as the conversion and product yield even with longer reac-
ligand. Initially, we performed a screening of bases tion times, (Table 1, entry 4, 24 h) probably due to its
(Cs2CO3, K2CO3, CsF, KF and K3PO4·H2O), solvents poor nucleophilicity compared with the other phenols
(MeCN, DMF, PhMe and dioxane) and temperatures tested. It is worth mentioning that 4-bromophenol re-
(50–1008C). The optimized best results were obtained acted not only as a nucleophile partner but also as an
À
using a simple catalytic system composed of CuI electrophile in the Caryl O coupling reaction. Under
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Copper-Catalyzed Coupling of (E)-Bromostilbene with Phenols/Azole
Scheme 1. Synthesis of the ligand L1.
steps. In order to introduce a good leaving group, al-
cohol 4 was treated with MeSO2Cl and Et3N. The me-
sylated intermediate was then reacted with 1,2-dime-
thylimidazole. After that, the counter-ion was ex-
changed by the addition of KPF6, giving the desired
ionophilic ligand L1 in 12% overall yield. Before
Figure 1. ORTEP diagram for the product 2b.
the studied conditions, the reaction was chemoselec- a final purification, ESI-MS analysis (Figure 2) indi-
À
tive and only the product obtained from Cvinyl Br
bond activation was observed (Table 1, entry 3). The
tetrasubstituted vinyl ether 2e was obtained in 90%
yield (Table 1, entry 5). The same method was then
extended to the N-nucleophile pyrazole and the N-vi-
nylpyrazole 2f was obtained in 78% yield.
The vinyl ethers (2a–2d) have only been obtained
in the literature as the Z isomer by a gold-catalyzed
hydrophenoxylation of diphenylacetylene.[15] We were
able to obtain an X-ray crystal structure of compound
2b that unambiguously established the E configura-
tion (Figure 1). As we have observed for Pd-catalyzed
coupling reactions,[16] the configuration of the double
bond remained unchanged during the vinyl bromide
activation reaction and the product showed the same
stereochemistry as the substrate.
After the synthesis of vinylic compounds, we tried
to determine the reaction mechanism through
ESI(+)-MS analyses and radical scavenger tests
(Scheme 4). Recently, two studies concerning the use
of ESI(+)-MS to determine the intermediates of the
copper-catalyzed arylation of amines were pub-
lished.[14,17] Our group has published some studies con-
cerning the utilization of ionophilic ligands in organo-
metallic biphasic catalysis.[18] This type of ionically-
tagged ligand is also very useful for detecting inter-
mediates in ESI(+)-MS experiments because neutral
species are transformed into ionic species by the iono-
philic ligand. For this reason, we synthesized the
ligand L1 as demonstrated in Scheme 1.
In the first step, neocuproine 3 was oxidized to the
corresponding monoaldehyde using SeO2. After filtra-
tion, NaBH4 and methanol were added to the solution
and alcohol 4 was obtained in 24% yield over two CuI, L1 and L2.
Figure 2. ESI(+)-mass spectrum of the reaction between
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Table 2. Results of the ESI(+)-MS experiments.[a]
Exp.
1
Sequence of
addition/time
Copper-based ion
CG
results
1. CuI+(L1+L2)/1 h
2. phenol+base/1 h
3. (E)-bromostilbene/1 h
1. CuI+(L1+L2)/1 h
2. (E)-bromostilbene/1 h
3. phenol+base/1 h
all reagents/1 h
1. I1, I2
2. none
3. none
1. I1, I2
2. I1
3. I1–I10
I1–I10
No coupling products
34% yield
2
3
38% yield
[a]
Reaction conditions: see Table 1.
cated the presence of the by-product L2 that was shown in Figure 3). In Exp. 3, where all reagents were
formed from the reaction of the mesylated intermedi- added simultaneously, the same ions as observed in
ate with Et3N as the nucleophile. We decided to use Exp. 2–3 were detected. The proposed ions are shown
the mixture of L1 and L2 for the ESI-MS experi- in Figure 3. Concerning the structure of the inter-
ments since both could be a potential probe in the mediates, at first, it is important to note that the theo-
search for intermediates.
retical and experimental isotopic distributions of the
A mixture of L1 and L2 was used as a probe to proposed copper ions are in agreement (see the Sup-
detect intermediates in the coupling between (E)-bro- porting Information). It is also noteworthy that some
mostilbene and 4-methoxyphenol catalyzed by CuI. copper-based intermediates formed from the attack of
Three different experiments were performed, varying the phenol to the benzylic position of the ligands L1
the sequence of the addition of reagents as shown in and L2 were observed (I5, I7 and I9) in conjunction
Table 2.
In the first experiment (Exp. 1), CuI, L1 and L2 (m/z=96) and triethylamine (m/z=101). Several
were allowed to react for 1 h in dioxane at 1008C and [Cu
(phen)2]+ species were detected (I6, I7 and I9).
with the respective leaving group dimethylimidazole
AHCTUNGTRENNUNG
the mixture was analyzed by ESI(+)-MS. Thereby, These species are similar to those described by Taille-
the ions I1 and I2 and the free ligands L1 and L2 fer and co-workers as very active catalysts in the ary-
were detected (Figure 2). The fact that there was free lation of phenols, even at very low concentrations of
ligand even in the presence of a chelating diamine copper (10 ppm).[19] Another important intermediate
could indicate that a portion of the copper was not in detected was I10. In this species, the ionophilic probe
À
solution, but in a copper reservoir as previously sug- was not cleaved and it contains a Cu OAr bond. The
gested.[19] Next, we added 4-methoxyphenol and formation of species Cu Nu has been described in
À
K3PO4·H2O; however, after 1 h of reaction, no the arylation of phenols[20] and other nucleophiles.[9]
copper-based ions were observed. The same result (no These species have been reported as capable to react
ions detected) was obtained after the addition of (E)- with aryl halides under mild conditions.[9,20] Moreover,
bromostilbene. Finally, the reaction was analyzed by some species of copper-containing phenol molecules
GC and the coupling product 2a was also not ob- as the ligand were observed (I8 and I10), indicating
served.
that the phenol can act as a nucleophile and as
Next, we decided to change the order of addition a ligand.
(Exp. 2), starting with CuI+(L1+L2) (Exp. 2-1) fol-
In order to observe the participation of radical in-
lowed by bromostilbene (Exp. 2-2) and phenol+base termediates in the catalytic cycle, we performed the
(Exp. 2-3). Initially, the same ions I1 and I2 were ob- coupling of (E)-bromostilbene with 4-methoxyphenol
served. After the addition of (E)-bromostilbene, no in the absence and presence of 2,6-di-tert-butyl-4-
changes were observed on the mass spectrum, indicat- methylphenol (BHT), a well-known radical scavenger.
ing that the halides did not react directly with inter- No significant differences were observed in the ab-
mediates I1 and I2 and, consequently, the formation sence (conversion: 37%), with 10 mol% (conversion:
of a phenoxide species precedes halide activation. Fi- 41%) or 100 mol% (conversion: 41%) of the radical
nally, after the addition of the phenol and the base, inhibitor (Scheme 2). The presence of a radical mech-
a complete change in the ESI(+)-MS was observed anism was proposed for the coupling reaction of iodo-
and various ions could be identified (the ions are toluene with diphenylamine using TEMPO as a radical
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Copper-Catalyzed Coupling of (E)-Bromostilbene with Phenols/Azole
Figure 3. ESI(+)-mass spectrum of the reaction between (E)-bromostilbene and 4-methoxyphenol, using the ligands L1 and
L2, and K3PO4·H2O as base.
scavenger.[14] When we used BHT for the same reac-
Additionally, we performed some preliminary ki-
tion and under similar conditions as described in the netic studies in order to determine the involvement of
literature, we also observed a significant decrease in the phenol and the halide in the rate-determining
the yield (Scheme 3). Therefore, at least for the reac- step. We carried out a series of reactions by varying
tion conditions used in our work for the coupling of the concentration of 4-methoxyphenol or bromostil-
(E)-bromostilbene with phenol, the radical pathway is bene. These reactions were stopped with low conver-
not operational. Further evidence for the non-involve- sions and the initial rates were determined; therefore,
ment of radicals is the maintenance of the E stereo- we were able to determine the reaction order as
chemistry in the coupling product, because in a radical a function of phenol and vinyl halide. As expected,
pathway an isomeric E/Z mixture would be expected. a first-order reaction was obtained for the halide, in-
dicating its participation in the rate-determining step
(see the Supporting Information). On the other hand,
À
À
Scheme 2. Radical scavenger test using BHT in C O vinyla-
tion.
Scheme 3. Radical scavenger test using BHT in C N aryla-
tion.
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Scheme 4. Catalytic cycle proposed for the copper-catalyzed vinylation of phenols.
we found that the reaction rate was also dependent philes. Through ESI(+)-MS, we described the forma-
on the phenol concentration in a variable order de- tion of the species [LCuI], [L2Cu]+ [LCu(I) OAr]
À
pendence on the phenol concentration. These results that is preceding halide activation. Furthermore, with
are in agreement with the ESI-MS experiments which kinetic studies and radical inhibition studies, we could
indicate that the phenol can act as a nucleophile and determine the participation of the halide in the rate-
as a ligand.
determining step and the non-involvement of radicals
Based on the results obtained from ESI(+)-MS, in the reaction pathway, respectively. Further studies
preliminary kinetic studies and the radical scavenger concerning the application of this reaction and mech-
experiment we propose viable mechanistic pathways anistic studies are in progress in our group.
for the copper/1,10-phenanthroline-catalyzed vinyla-
tion of phenols. At first the coordination of the ligand
to CuI takes place, providing a species (phen)CuI,
which was detected as I1 and I2 in the ESI(+)-MS.
Thereafter, an intermediate [(phen)2Cu]+ is formed.
The formation of this intermediate was supported by
Experimental Section
the detection of I6, I7 and I9. Next, the formed inter-
Typical Procedure for the Coupling of (E)-Bromo-
stilbene or Bromotriphenylethene with Phenols or
Azoles
À
mediate Cu OAr was detected as I10 which is report-
ed to be capable to react with the organic halides.[20]
In terms of vinyl halide activation, since the radical
pathway is not operational, we can argue in favour of
An oven-dried resealable Schlenk flask was charged with
an oxidative addition to form a Cu
Although these species are not common, arylcop-
per
(III) complexes have been identified recently,[11a,21]
and arylcopper(III) intermediates containing nitrogen
ligands are calculated to be kinetically accessible
under the reaction conditions.[9d] Then, the [(phen)-
CuOAr] intermediate would be reformed by a puta-
tive reductive elimination followed by attack of the
phenoxide to the copper species.
ACHTUNGTNER(NUNG III) intermediate. (E)-bromostilbene or bromotriphenylethene (2.0 mmol),
phenol or azole (3.0 mmol), 1,10-phenanthroline (36 mg,
0.2 mmol), CuI (38 mg, 0.2 mmol) and K3PO4·H2O (4 mmol,
920 mg), evacuated and back-filled with argon. Then, diox-
ane (8 mL) was added, and the reaction mixture was stirred
at 1008C for the desired time (6 h for phenols and 24 h for
azoles). Next, the reaction mixture was cooled, filtered and
analyzed by GC and GC-MS. The product was further puri-
fied by column chromatography (silica gel in hexane/ethyl
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1
acetate) and analyzed by H NMR and 13C NMR. Crystal-
In summary, we have described herein the efficient
line samples were obtained through crystallization in hot
coupling between (E)-bromostilbene and nucleo- ethanol.
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Copper-Catalyzed Coupling of (E)-Bromostilbene with Phenols/Azole
8.9 Hz, 2H), 7.22–7.07 (m, 5H), 7.32–7.24 (m, 3H), 7.38 (d,
Synthesis of the Ionophilic Ligand L1
J=8.9 Hz, 2H), 7.49–7.42 (m, 2H); 13C NMR: d=115.0,
115.2, 120.3, 126.6, 128.2, 128.4, 128.8, 129.0, 129.4, 132.5,
133.9, 135.1, 152.7, 155.7; elem. anal. calcd. for C20H15BrO:
C 68.39, H 4.30; found: C 68.18, H 4.32.
In a typical synthesis, 2,9-dimethyl-1,10-phenanthroline 3
(2.00 g, 9.60 mmol) was stirred with selenium dioxide
(1.60 g, 14.40 mmol) in 60 mL of dioxane. The reaction mix-
ture was heated at 1008C for 2 h. After that, the mixture
was filtered and the dioxane was evaporated. Next, sodium
borohydride (2.0 g, 0.05 mol) and 100 mL of methanol were
added and the mixture was stirred at room temperature for
16 h. The volume was reduced to 25 mL, water was added
and the crude product was extracted with chloroform. At
the end the product was purified by column chromatography
(alumina) resulting in (2-methyl-1,10-phenanthrolin-9-yl)me-
thanol 4; yield: 24%. The characterization data of product 4
were in agreement with those in the literature.[22]
Product 4 was then stirred with methanesulphonyl chlo-
ride (0.34 g, 3.00 mmol) and triethylamine (0.35 g,
3.45 mmol) in 20 mL of dichloromethane for 1 h at 258C.
Then, the mixture was filtered through a neutral alumina
plug and the solution was concentrated to a volume of
5 mL. Next, a solution of 1,2-dimethyl-1H-imidazole (0.38 g,
3.91 mmol) in 20 mL of acetonitrile were added and the
mixture was stirred for 16 h at 258C. The solvent was then
evaporated and the resulting solid was washed with ethyl
4-[(E)-1,2-Diphenylvinyloxy]benzonitrile (2d): White
solid, yield: 49%, mp: 98–998C. IR (KBr): n=3096, 3057,
2224, 1649, 1601, 1502, 1446, 1242, 1169 cmÀ1; MS (EI,
70 eV): m/z (rel. intensity)=297 (32), 179 (87), 178 (100),
165 (6), 152 (21), 106 (6), 105 (76), 77 (13); 1H NMR
(300 MHz, CDCl3): d=6.48 (s, 1H), 7.09 (d, J=9.0 Hz, 2H),
7.30–7.13 (m, 8H), 7.41–7.34 (m, 2H), 7.52 (d, J=9.0 Hz,
2H); 13C NMR (75 MHz, CDCl3): d=105.5, 117.9, 118.0,
118.8, 127.2, 128.3, 128.5, 128.8, 129.2, 129.2, 133.2, 134.0,
134.4, 150.6, 160.5; elem. anal. calcd. for C21H15NO: C 84.82,
H 5.08, N 4.71; found: C 84.47, H 5.12, N, 4.69.
1-[(E)-1,2-diphenylvinyl]-1H-pyrazole (2e): Colourless oil,
yield: 78%. IR (neat): n=3057, 3024, 1641, 1598, 1514, 1498,
1444, 1392, 1321, 1193 cmÀ1; MS (EI, 70 eV): m/z (rel. inten-
sity)=245 (61, M+), 178 (100), 152 (12); 1H NMR
(300 MHz, CDCl3): d=6.25 (dd, J=2.4, 1.84 Hz, 1H), 6.99–
6.92 (m, 2H), 7.11–7.01 (m, 3H), 7.24–7.18 (m, 2H), 7.41–
7.24 (m, 5H), 7.63 (d, J=1.3 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d=106.4, 118.7, 126.7, 127.9, 128.9, 129.1, 129.2,
130.3, 134.2, 135.0, 138.0, 140.7; elem. anal. calcd. for
C17H14N2: C 82.90, H 5.73, N 11.37; found: C 82.90, H 5.93,
N 11.27.
ether (3ꢃ10 mL). Finally,
a
solution of potassium
hexafluorophosphate (0.85 g, 4.60 mmol) in 20 mL of water
ACHTUNGTRENNUNG
was added, giving a pale yellow solid. The solid was filtered
and dried under vacuum, furnishing L1; global yield: 515 mg
(12%).
L1: Pale yellow solid, yield: 12%, mp: 2958C (decomp.).
1H NMR (300 MHz, DMSO-d6): d=2.80 (s, 3H), 2.90 (s,
3H), 3.90 (s, 3H), 5.90 (s, 2H), 7.60–7.80 (m, 4H), 7.91–7.98
(m, 2H), 8.38 (d, J=8.2 Hz, 1H), 8.54 (d, J=8.2 Hz, 1H);
13C NMR (75 MHz, DMSO-d6): d=10.2, 25.1, 34.8, 52.5,
121.0, 121.6, 122.4, 123.8, 125.1, 126.7, 126.8, 127.7, 136.5,
137.6, 144.2, 144.4, 146.4, 153.4, 158.6; HR-MS: m/z=
303.1616, calcd. for C19H19N4: 303.1604.
Characterization of New Products
(E)-1-(4-Methoxyphenoxy)-1,2-diphenylethene (2a): White
solid, yield: 83%, mp 79–808C. IR (KBr): n=3053, 2960,
2839, 1644, 1505, 1226, 1205 cmÀ1; MS (EI, 70 eV): m/z (rel.
intensity)=302 (26, M+), 197 (32), 178 (80), 152 (19), 124
(100), 109 (17); 1H NMR (300 MHz, CDCl3): d=3.77 (s,
3H), 6.07 (s, 1H), 6.84 (d, J=9.1 Hz, 2H), 7.17–6.99 (m,
7H), 7.33–7.23 (m, 3H), 7.50–7.43 (m, 2H); 13C NMR: d=
55.5, 111.4, 114.6, 120.6, 126.0, 128.0, 128.2, 128.7, 128.8,
129.5, 134.7, 135.8, 149.6, 155.0, 155.5; elem. anal. calcd. for
C21H18O2: C 83.42, H 6.00; found: C 83.47, H 6.09.
Acknowledgements
We thank CNPq, CAPES and INCT-Catꢀlise for partial fi-
nancial support. We also thank CNPq (J.L.) and CAPES
(B.C.L.) for providing fellowships.
(E)-1-(3,5-Dimethoxyphenoxy)-1,2-diphenylethene (2b):
White solid, yield: 69%, mp 95–968C. IR (KBr): n=3020,
2967, 2837, 1595, 1475, 1355, 1205, 1137, 1064 cmÀ1; MS (EI,
70 eV): m/z (rel. intensity)=332 (45, M+), 227 (18), 178
(100), 154 (63), 125 (24), 105 (31); 1H NMR (300 MHz, References
CDCl3): d=3.72 (s, 6H), 6.13 (t, J=2.2 Hz, 1H), 6.27 (d, J=
2.2 Hz, 2H), 6.33 (s, 1H), 7.17–7.07 (m, 5H), 7.27–7.23 (m,
3H), 7.46–7.40 (m, 2H); 13C NMR (75 MHz, CDCl3): d=
55.3, 94.9, 97.0, 115.2, 126.5, 128.1, 128.3, 128.8, 129.3, 134.3,
135.4, 152.5, 158.5, 161.3; elem. anal. calcd. for C22H20O3: C
79.50, H 6.06; found: C 79.58, H 5.96. Crystals suitable for
X-ray diffraction analyses were obtained by recrystallization
from ethanol. CCDC 850009 contains the supplementary
crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(E)-1-(4-Bromophenoxy)-1,2-diphenylethene (2c): White
solid, yield: 82%, mp 88–908C. IR (KBr): n=3058, 3023,
1644, 1581, 1480, 1230, 1054 cmÀ1; MS (EI, 70 eV): m/z (rel.
intensity)=352 (10), 350 (10, M+), 178 (100), 152 (17);
1H NMR (300 MHz, CDCl3): d=6.32 (s, 1H), 6.98 (d, J=
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