Practical synthesis of unsymmetrical tetraarylethylenes and their application for the preparation of [triphenylethylene-spacer-triphenylethylene] triads

Article abstract of DOI:10.1021/jo701474y

(Chemical Equation Presented) We have demonstrated that reactions of diphenylmethyllithium with a variety of substituted benzophenones produces corresponding tertiary alcohols that are easily dehydrated, without any need for purification, to produce various unsymmetrical and symmetrical tetraarylethylenes in excellent yields. The simplicity of the method allows for the preparation of a variety of ethylenic derivatives in multigram (10-50 g) quantities with great ease. The methodology was successfully employed for the preparation of various triphenylethylene (TPE)-based triads (i.e., TPE-spacer-TPE) containing polyphenylene and fluoranyl-based spacers. The ready availability of various substituted tetraarylethylenes allowed us to shed light on the effect of substituents on the oxidation potentials (E_ox) of various tetraarylethylenes. Moreover, the electronic coupling among the triphenylethylene moieties in various TPE-spacer-TPE triads was briefly probed by electrochemical and optical methods.

Full text of DOI:10.1021/jo701474y

Practical Synthesis of Unsymmetrical Tetraarylethylenes and Their
Application for the Preparation of
[Triphenylethylene-Spacer-Triphenylethylene] Triads
Moloy Banerjee, Susanna J. Emond, Sergey V. Lindeman, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881
Rajendra.Rathore@marquette.edu
ReceiVed July 7, 2007
We have demonstrated that reactions of diphenylmethyllithium with a variety of substituted benzophenones
produces corresponding tertiary alcohols that are easily dehydrated, without any need for purification, to
produce various unsymmetrical and symmetrical tetraarylethylenes in excellent yields. The simplicity of
the method allows for the preparation of a variety of ethylenic derivatives in multigram (10-50 g) quantities
with great ease. The methodology was successfully employed for the preparation of various triphenyl-
ethylene (TPE)-based triads (i.e., TPE-spacer-TPE) containing polyphenylene and fluoranyl-based
spacers. The ready availability of various substituted tetraarylethylenes allowed us to shed light on the
effect of substituents on the oxidation potentials (Eox) of various tetraarylethylenes. Moreover, the electronic
coupling among the triphenylethylene moieties in various TPE-spacer-TPE triads was briefly probed
by electrochemical and optical methods.
Introduction
of ethylenic double bondssa process of fundamental importance
both in biological and in materials chemistry.5 For example,
the mechanism of vision is believed to occur by a rotation
around a carbon-carbon double bond, following a photon
absorption, that sets off a chain of events that leads to data
storage of enormous complexity.6 The extended π-systems based
on tetraarylethylenes are also potentially important candidates
for incorporation into various organic optoelectronic and op-
tomechanical switching and storage devices,7 as well as for the
The tetraphenylethylene (TPE) motif has attracted consider-
able attention by virtue of its rich electrochemical1 and excited-
state properties2 as well as its extensive usage as an electron-
transfer catalyst in a variety of polymerization3 and coupling
reactions.4 For several decades, TPE and its derivatives have
served as models to study photoinduced cis-trans isomerization
* Corresponding author. (414) 288-7066.
(1) (b) Schreivogel, A.; Maurer, J.; Winter, R.; Baro, A.; Laschat, S.
Eur. J. Org. Chem. 2006, 15, 3395. (b) Wolf, M. O.; Fox, H. H.; Fox, M.
A. J. Org. Chem. 1996, 61, 287.
(2) (a) Shultz, D. A.; Fox, M. A. Tetrahedron Lett. 1988, 29, 4377. (b)
Sun, Y.-P.; Fox, M. A. J. Am. Chem. Soc. 1993, 115, 747. (c) Schuddeboom,
W.; Jonker, S. A.; Warman, J. M.; de Haas, M. P.; Vermeulen, M. J. W.;
Jager, W. F.; de Lange, B.; Feringa, B. L.; Fessenden, R. W. J. Am. Chem.
Soc. 1993, 115, 3286. (d) Ma, J.; Dutt, G. B.; Waldeck, D. H.; Zimmt, M.
B. J. Am. Chem. Soc. 1994, 116, 10619. (e) Lenderink, E.; Duppen, K.;
Wiersma, D. A. J. Phys. Chem. 1995, 99, 8972.
(4) (a) Yamada, Y.; Yoshida, Z.; Oda, R. Tetrahedron 1971, 27, 4845.
(b) Schmidbaur, H.; Hager, R.; Huber, B.; Mueller, G. Chem. Ber. 1988,
121, 1341. (c) Wolf, M. O.; Fox, H. H.; Fox, M. A. J. Org. Chem. 1996,
61, 287.
(5) (a) Waldeck, D. H. Chem. ReV. 1991, 91, 415. (b) Allen, M. T.;
Whitten, D. G. Chem. ReV. 1989, 89, 1691 and references therein.
(6) (a) Schoenlein, R. W.; Peteanu, L. A.; Mathies, R. A.; Shank, C. V.
Science 1991, 254, 412. (b) Wang, Q.; Schoenlein, R. W.; Penteanu, L. A.;
Mathies, R. A.; Shank, C. V. 1994, 266, 422. (c) Wang, Q.; Kochendoerfer,
G. G.; Schoenlein, R. W.; Verdegem, P. J. E.; Lugtenburg, J.; Mathies, R.
A.; Shank, C. V. J. Phys. Chem. 1996, 100, 17388.
(3) Wang, G.; Janssens, K.; Van Oosterwijck, C.; Yakimansky, A.; Van
Beylen, M. Polymer 2005, 46, 295.
10.1021/jo701474y CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/20/2007
8054
J. Org. Chem. 2007, 72, 8054-8061

Practical Synthesis of Unsymmetrical Tetraarylethylenes
SCHEME 1. Two-Step Synthesis of Polychromophoric
Hexakis-(tetraphenylethylene) Benzene
preparation of two-dimensional molecular scaffolds for bichro-
mophoric donor-acceptor dyads for energy/charge transport.8
Thus, an efficient and scalable methodology for the prepara-
tion of these tetraarylethylenes is imperative. Although sym-
metrical tetraarylethylenes are easily prepared via McMurry
coupling of corresponding benzophenones by low-valent tita-
nium,9 the unsymmetrical tetraarylethylenes are, however, not
as readily accessible. McMurry coupling of two different
benzophenones is not of general synthetic use as it produces a
mixture of symmetrical and mixed products in a nearly statistical
mixture, unless one of the components is used in sacrificial
excess.10 Furthermore, the procedures that are generally utilized
for the preparation of unsymmetrical tetraarylethylenes either
require the multistep synthesis of precursors or suffer from low
yields. The reactions of resonance stabilized selenoketones with
diaryldiazomethane,11 benzophenone hydrazones with disele-
nium dibromide,12 the Pd-catalyzed three-component coupling
of aryl iodides/internal alkynes/arylboronic acids,13 and the Pd-
catalyzed sequential assembly using a vinyl 2-pyrimidyl sulfide
core14 give either a mixture of products or are not amenable
for large-scale syntheses.
We have recently reported15 that easily prepared diphenyl-
methyllithium from diphenylmethane and n-butyllithium in
tetrahydrofuran at 0 °C add to hexakis(4-benzoylphenyl)benzene
to afford the corresponding hexaalcohol, which can be readily
dehydrated using an acid catalyst to afford hexakis(tetraphe-
nylethylene)benzene in nearly quantitative yield15 (i.e., Scheme
1).
SCHEME 2. Representative Two-Step Reaction Sequence
for Synthesis of Various Tetraaryllethylenes
Herein, we report that the reaction sequence depicted in
Scheme 1 is general and can be applied for the preparation of
multigram quantities of unsymmetrical tetraarylethylenes as well
as the parent tetraphenylethylene with remarkable ease using
appropriately substituted benzophenones. The ready availability
of multigram quantities of mono-bromoTPE (i.e., 4-bromophe-
nyltriphenylethylene), using the protocol in Scheme 1, allowed
the preparation of a number of TPE-based triads (TPE-spacer-
TPE) by palladium-catalyzed coupling reactions. Also, various
tetraarylethylenes and polychromophoric triads, prepared herein,
were utilized for the exploration of the structure-property
relationship of their electrochemical and spectral behavior as
follows.
0 °C produced an orange-red solution of diphenylmethyllithium
within 30 min. To this highly colored solution was added solid
4-methylbenzophenone (100 mmol) at 0 °C, and the resulting
mixture was allowed to warm to room temperature and was
stirred for 6 h. The reaction was quenched with saturated
aqueous ammonium chloride solution followed by a standard
aqueous workup, affording the corresponding alcohol in nearly
quantitative yield (see Scheme 2). The crude alcohol was
dissolved in toluene and refluxed in the presence of a catalytic
amount of p-toluenesulfonic acid with an azeotropic removal
of water using a Dean-Stark trap to furnish tolyltriphenyleth-
ylene (2) in 96% isolated yield (33.5 g, ∼97 mmol) by simple
crystallization from a mixture of dichloromethane and methanol
(i.e., Scheme 2).
Results and Discussion
Synthesis of Tetraarylethylenes. Thus, an addition of
n-butyllithium (2.5 M in hexanes, 100 mmol) to a solution of
diphenylmethane (105 mmol) in tetrahydrofuran (100 mL) at
Using a procedure akin to that in Scheme 2, a number of
benzophenone derivatives were reacted with freshly prepared
diphenylmethyllithium to afford the corresponding alcohols,
which were dehydrated, without any purification, to produce
the corresponding tetraarylethylenes in excellent yield as listed
in Table 1. The efficacy of the synthetic protocol in Scheme 2
is clearly evident by excellent yields of various substituted
tetraarylethylenes using methoxy-, methyl-, or phenyl-substituted
benzophenones (see Table 1). The structures of various tet-
raarylethylenes were established by 1H/13C NMR spectroscopy,
comparison with the authentic samples where available, and
X-ray crystallography of a representative example (6). Various
characterization data are compiled in the Supporting Information.
It is to be further noted that the reaction of 4-bromoben-
zophenone (entry 4) with diphenylmethyllithium was carried
out at a lower temperature (-78 °C) for 3 h followed by slow
(7) (a) Ito, A.; Nakano, Y.; Kato, T.; Tanaka, K. Chem. Commun. 2005,
403. (b) Mori, T.; Inoue, Y. J. Phys. Chem. A 2005, 109, 2728.
(8) Sengupta, S. Synlett 2004, 1191.
(9) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405 and references therein.
(10) (a) Duan, X.-F.; Zeng, J.; Lu, J.-W.; Zhang, Z.-B. J. Org. Chem.
2006, 71, 9873. (b) McMurry, J. E.; Krepski, L. R. J. Org. Chem. 1976,
41, 3929.
(11) Okuma, K.; Kojima, K.; Ohta, H. Phosphorus, Sulfur Silicon Relat.
Elem. 1993, 80, 259.
(12) Okuma, K.; Kojima, K.; Oyama, K.; Kubo, K.; Shioji, K. Eur. J.
Org. Chem. 2004, 820.
(13) (a) Zhou, C.; Larock, R. C. J. Org. Chem. 2005, 70, 3765. (b) Zhou,
C.; Emrich, D. E.; Larock, R. C. Org. Lett. 2003, 5, 1579.
(14) Itami, K.; Mineno, M.; Muraoka, N.; Yoshida, J. J. Am. Chem. Soc.
2004, 126, 11778.
(15) (a) Mladenova, G.; Chen, L.; Rodriquez, C. F.; Siu, K. W. M.;
Johnston, L. J.; Hopkinson, A. C.; Lee-Ruff, E. J. Org. Chem. 2001, 66,
1109. (b) Rathore, R.; Burns, C. L.; Abdelwahed, S. A. Org. Lett. 2004, 6,
1689.
J. Org. Chem, Vol. 72, No. 21, 2007 8055

Banerjee et al.
TABLE 1. Tetraarylethylenes Synthesized Using the Two-Step
Procedure in Scheme 2a,b
TABLE 2. Synthesis of TPE-Spacer-TPE Triads Using the
Two-Step Procedure in Scheme 2a,b
a All reactions were performed using the genral procedure (see text).
b Progress of the reactions was monitored by TLC. c Isolated yield.
FIGURE 1. (Left) Ball-and-stick model of the molecular structure of
11 obtained by X-ray crystallography. (Right) Partial space-filling
representation of [11‚2CH2Cl2] showing the sandwiching of CH2Cl2
molecules between a pair of phenyls via C-H‚‚‚π interactions.
is interesting to note that a pair of dichloromethane molecules
is sandwiched between two pairs of cofacially juxtaposed phenyl
moieties in 11 in a way that they allow effective C-H‚‚‚π
interactions, as shown by a partial space-filling representation
in Figure 1 (right). Moreover, Figure 1 (left) also shows that a
cofacial juxtaposition of the phenyls and central phenylene in
11 should allow an effective electronic coupling between the
two triphenylethylene moieties via a contribution from through-
space electronic coupling16 (vide infra). The X-ray structure of
13 was also established by X-ray crystallography, and the
structural details are available in the Supporting Information.
The ready availability of triphenylethylene-based triads
separated by mono- and di-phenylene spacers (i.e., 11 and 12)
prompted us to obtain the higher homologues containing longer
polyphenylene spacers using mono-bromotetraphenylene (4)
(vide infra). Indeed, a standard Suzuki coupling of 2 equiv of
4 with the benzene-1,4-diboronate ester, biphenyl-4,4′-dibor-
onate ester, or 9,9′-dihexylfluorene-2,7-diboronic acid afforded
the corresponding bichromophoric TPE derivatives (14-16) in
excellent yields (i.e., Scheme 3). With a series of TPE-based
triads (Table 2 and Scheme 3) and various tetraarylethylenes
(Table 1) at our disposal, we next examined their electrochemi-
cal and optical properties as follows.
a All reactions were performed using the genral procedure (see text).
b Progress of the reactions was monitored by TLC. c Isolated yield. Pure
products were obtained in most cases by simple crystallization from a
mixture of dichloromethane and methanol acetonitrile.
warming to 22 °C to avoid halogen-metal exchange. Dehydra-
tion of the resulting alcohol afforded the mono-bromotetraphe-
nylethylene 4 in good yield. This particular ethylene derivative
holds the potential to be utilized for the preparation of a variety
of macromolecular derivatives by palladium-catalyzed coupling
reactions (vide infra).
Preparation of TPE-Based Triads (TPE-Spacer-TPE).
The reaction of diphenylmethyllithium with bis-benzoyl sub-
strates according to the procedure in Scheme 2, with a simple
change in the stoichiometries, provided ready access to TPE-
spacer-TPE triads with phenylene, bis-phenylene, and 9,9-
dihexylfluorene as spacers. Various triads in Table 2 were
obtained in excellent yields with the reactions being clean and
complete and having no traces of the products arising from
incomplete reactions of the bis-benzoyl substrates (Table 2).
The molecular structure of TPE-Ph-TPE (11) was estab-
lished by X-ray crystallography and is displayed in Figure 1. It
(16) (a) Rathore, R.; Kochi, J. K. Can. J. Chem. 1999, 77, 913. (b)
Chebny, V. J.; Shukla, R.; Rathore, R. J. Phys. Chem. A 2006, 110, 13003.
8056 J. Org. Chem., Vol. 72, No. 21, 2007

Practical Synthesis of Unsymmetrical Tetraarylethylenes
SCHEME 3. Suzuki Coupling of Mono-bromotetraphenylethylene 4 with Various Diboronic Acid Spacers
TABLE 3. Electrochemical Oxidation Potentials of Representative
Tetraarylethylenesa
CHART 1. Anisyl versus Tolyl Stabilization of Cationic
Charge
solutions in anhydrous dichloromethane containing 0.2 M tetra-
n-butylammonium hexafluorophosphate (TBAH) as the sup-
porting electrolyte. The common unifying feature in all the
cyclic voltammograms of tetraarylethylenes (in Table 1) is the
presence of two closely coupled one-electron oxidation waves
with the first wave completely reversible and the degree of
reversibility of the second wave dependent upon the nature and
substitution pattern and the possibility of electrochemically
induced chemical reactions at the electrode (vide infra).18 The
potentials corresponding to the first and second oxidations were
referenced to added ferrocene, as an internal standard, and are
compiled in Table S1 in the Supporting Information along with
the separation between the two oxidation potentials (∆E1/2). To
briefly comment on the effect of substitution patterns on the
first and second oxidation potentials of various methyl- and
methoxy-substituted tetraarylethylenes, their Eox values together
with tetratolylethylene (17)19 and tetraanisylethylene (18)19 are
shown in Table 3.
Predictably, both the first and the second oxidation potentials
of various tetraarylethylenes decrease with an increasing number
of electron-donating methyl or methoxy substituents in com-
parison to the parent tetraphenylethylene. Also the reversibility
of the two redox processes (or the stability of the cation radical
and dication) reaches a maximum in tetra-substituted derivatives
17 and 18.19
Also noteworthy are the ∆Eox values (i.e., the separation
between the first and the second oxidation potentials), which
are relatively larger for unsymmetrical methoxy-substituted
derivatives 5 and 6 (i.e., 232 and 268 mV, respectively) as
compared to the symmetrical tetraanisylethylene (120 mV) (see
Figure 2). In contrast, however, both symmetrical and unsym-
metrical methyl-substituted tetraphenylethylene 18, 2, and 3
showed rather similar ∆Eox values (∼300 ( 25 mV). Such a
disparate behavior among the methyl- and methoxy-substituted
a In anhydrous dichloromethane as a 0.2 mM solution containing 0.2 M
TBAP at a scan rate of 200 mV/S and at 25 °C. All potentials were calibrated
with ferrocene as an added internal standard [Eox (ferrocene) ) 0.45 V vs
SCE].
Electrochemical Studies of the Tetraarylethylenes. A
number of studies17 on the tetraarylethylene systems has shown
that they are generally oxidized and reduced in two successive
one-electron transfer steps and that the one-electron oxidation/
reduction products have a tendency to disproportionate with
the position of equilibrium dependent upon ion-pairing and
solvation. Herein, we examine the effect of substitutions on the
oxidation potentials (Eox) of various tetraarylethylenes as
follows.
The Eox values for various tetraarylethylenes were determined
electrochemically at a platinum electrode as 2 × 10-3
M
(17) (a) Farnia, G.; Maran, F.; Sandone, G. J. Chem. Soc., Faraday
Trans.1 1986, 82, 1885. (b) Grzeszczuk, M.; Smith, D. E. J. Electroanal.
Chem. 1984, 162, 189. (c) DeGroof, B.; Levin, G.; Szwarc, M. J. Am. Chem.
Soc. 1977, 99, 474. (d) Lundgren, B.; Levin, G.; Claesson, S.; Szwarc, M.
J. Am. Chem. Soc. 1975, 97, 262. (e) Troll, T.; Baizer, M. M. Electrochim.
Acta 1974, 19, 951.
J. Org. Chem, Vol. 72, No. 21, 2007 8057

Banerjee et al.
TABLE 4. Electrochemical Oxidation Potentials of Various
TPE-Spacer-TPE Triadsa
E 0ox (I)
E 0ox (II)
(V vs SCE)
∆ (mV) E 0ox (II) -
E 0ox (I)
triad
(V vs SCE)
11
12
14
15
13
16
1.218
1.258
1.286
1.29
1.142
1.246
1.38
1.383
1.331
162
125
45
0
220
67
1.362
1.313
a In anhydrous dichloromethane as a 0.2 mM solution containing 0.2 M
TBAHP at a scan rate of 200 mV/s and at 25 °C.
As such, the magnitude of the observed splitting of the
oxidation waves (∆Eox) in various triads qualitatively reflects
the extent of electronic coupling between the two TPE moieties,
and the electronic interaction is highly dependent on the
planarity of the (poly)phenylene bridge (vide infra).21 With an
increasing degree of torsional freedom between the phenylene
rings going from one phenylene spacer (in 11) to four phenylene
spacers (in 15) completely prevents electronic coupling between
the TPE moieties.
Qualitatively, the electronic coupling between the TPE
moieties can be enhanced by deliberate planarization of the
molecular conformation of the bridging spacer. For example,
the electronic coupling through the bis-p-phenylene spacer can
be dramatically improved by bridging the two p-phenylene units
together in a rigid coplanar conformation by a chemical -CR2-
link between their ortho positions. The resulting fluorene-bridged
triad 13 showed a greater splitting of the oxidation waves (∆Eox
) 220 mV) than the bis-p-phenylene spacer (∆Eox ) 125 mV)
in 12. In fact, the more planar 2,7-diphenylfluorene spacer in
16 allows a weak electronic interaction between the TPE
moieties when compared to the structurally similar tetrakis-p-
phenylene spacer in 15, with multiple degrees of torsional
freedom, which shows only a single two-electron oxidation wave
(compare Figure 3A,B). We hope that spectroscopic and X-ray
crystallographic studies of the one- and two-electron oxidized
triads (Table 4) presently underway will provide further insight
into the mechanism of electronic coupling in the various triads
presented previously.
FIGURE 2. Cyclic- and square-wave voltammogram of the methoxy-
substituted tetraphenylethylene derivatives 5, 6, and 18 as a 0.2 mM
solution in dichloromethane containing 0.2 M TBAH as the supporting
electrolyte at 22 °C and scan rate of 200 mV s-1
.
tetraphenylethylene derivatives arise, at least in part, due to the
fact that a single p-anisyl group stabilizes a cationic charge much
more effectively than the p-tolyl or phenyl groups (see Chart
1).19
Owing to the extensive interest in poly(phenylenevinylene)
derivatives for the preparation of electroactive materials,20
herein, we briefly apply electrochemical techniques to probe
electronic coupling between the two triphenylethylene moieties
separated by either a poly-p-phenylene spacer of increasing
length or the fluorene and 2,7-diphenylfluorene spacers as well-
defined triads (TPE-spacer-TPE). The square-wave voltam-
mograms of various triads recoded under identical conditions
are displayed in Figure 3, and the Eox values are compiled in
Table 4.
When the two TPE moieties were separated through a single
p-phenylene bridge as in 11, two well-resolved reversible
oxidation waves centered at 1.22 and 1.38 V with a separation
of ∆Eox ) 162 mV were observed (Figure 3). Varying the spacer
from a simple p-phenylene to bis-p-phenylene to tris-p-phe-
nylene to tetrakis-p-phenylene showed a steady decrease in ∆Eox
values of 162 to 125 to 45 mV to an eventual merger of the
two oxidation waves in triad 15 (with tetrakis-p-phenylene as a
spacer) into a single two-electron oxidation wave at 1.29 V
(Figure 3).21
Absorption and Emission Studies of TPE-Spacer-TPE
Triads. Tetraphenylethylene and its derivatives are known to
have exceedingly short singlet excited-state lifetimes, negligible
fluorescence quantum yields in solution, and slow rates of
intersystem crossing.22 Herein, we explore the emission/optical
behavior of tetraphenylethylene moieties when incorporated into
various triads (vide infra).
For example, the absorption spectra as a 2.2 × 10-5
M
solution in dichloromethane showed the same general spectral
band shape as in tetraphenylethylene with a poorly resolved
vibrational structure for the various triads with poly-p-phenylene
(Figure 4A) and fluoranyl-based (Figure 4B) spacers. The
absorption maxima of various TPE-(Ph)n-TPE triads show
slight bathochromic shifts up to the bis-phenylene spacer while
the molar absorptivity increases with an increasing number of
phenylene rings in the spacer up to the tris-phenylene spacer.
The increase in molar absorptivity is, however, modest and in
fact dies down at the tris-phenylene spacer (Figure 4A). The
(18) (a) Stuart, J. D.; Ohnesorge, W. E. J. Am. Chem. Soc. 1971, 93,
4531. (b) Svanholm, U.; Ronlan, A.; Parker, V. D. J. Am. Chem. Soc. 1974,
96, 5108.
(19) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am.
Chem. Soc. 1998, 120, 6931.
(20) (a) Hoerhold, H. H.; Helbig, M.; Weiss, D.; Raabe, D. Materials
Science Forum; Material Science High Technolology, MASHTEC ’90: New
York, 1990; Pt. 21990, Vol. 62-64, p 411. (b) Helbig, M.; Hoerhold, H.
H. Makromol. Chem. 1993, 194, 1607. (c) Moerner, W. E. Science 1997,
277, 1059. (d) Savvateev, V.; Yakimov, A.; Davidov, D. AdV. Mater. 1999,
11, 519 and references therein.
(21) Sun, S.; Lindeman, S. V.; Rathore, R.; Kochi, J. K. J. Chem. Soc.,
Perkin Trans. 2 2001, 1585.
(22) (a) Shultz, D. A.; Fox, M. A. J. Am. Chem. Soc. 1989, 111, 6311.
(b) Ma, J.; Zimmt, M. B. J. Am. Chem. Soc. 1992, 114, 9723.
8058 J. Org. Chem., Vol. 72, No. 21, 2007

Practical Synthesis of Unsymmetrical Tetraarylethylenes
FIGURE 3. (A) Coalescence of two one-electron waves with increasing length of poly-p-phenylene spacer to a single two-electron wave in the
case of the quaterphenyl spacer. (B) Square-wave voltammograms showing the effect of planarization of the spacers in the form of incorporation
of fluorene moieties in 13 and 16 in comparison to 12 and 15, respectively.
FIGURE 5. Emission spectral profile of the various triads and 4,4-
di-t-butylquaterphenyl (QP) as a 3 × 10-4 M solution in dichlo-
romethane at 22 °C.
FIGURE 4. Comparison of the UV-vis spectral profile of the TPE-
spacer-TPE triads as a 2.2 × 10-5 M solution in dichloromethane.
As such, the observation of an intense emission from the triad
15, containing a quaterphenyl spacer, is surprising and would
suggest that the emission must be arising from a singlet energy
transfer from TPE moieties to the quaterphenyl spacer. Indeed,
a comparison of the emission spectrum of 4,4-di-t-butylquater-
phenyl (QP),23 obtained under similar conditions with that of
15, showed a reasonable spectral similarity (see Figure 5).
Further studies, using time-resolved spectroscopy, will be
required to pinpoint the origin of observed emission in various
triads.
spacer has a more pronounced effect on the absorption intensity
in the case of the fluoranyl containing triads (see Figure 4B).
The emission spectra were recorded as a 3 × 10-4 M solution
in dichloromethane, but no emission was detected at that
concentration for the parent tetraphenylethylene and the triads
containing the p-phenylene, bis-p-phenylene, and tris-p-phe-
nylene spacers. The triad containing the quaterphenyl spacer
(i.e., 15), however, showed an intense emission, while the triads
with fluoranyl spacers (13 and 16) yielded only weak fluores-
cence emissions (Figure 5).
(23) Banerjee, M.; Lindeman, S. V.; Rathore, R. J. Am. Chem. Soc. 2007,
129, 8070.
J. Org. Chem, Vol. 72, No. 21, 2007 8059

Banerjee et al.
1-(4-Methoxyphenyl)-1,2,2-triphenylethylene (5). Light yellow
solid, mp 130-132 °C (lit.13b mp 132-134 °C); 1H NMR (CDCl3,
400 MHz) δ 3.76 (s, 3H), 6.68 (d, J ) 8.88 Hz, 2H), 6.99 (d, J )
8.88 Hz, 2H), 7.05-7.17 (m, 10H);13C NMR (CDCl3, 100 MHz)
δ 55.2, 113.2, 126.42, 126.43, 126.5, 127.8, 127.9, 131.52, 131.55,
131.57, 132.7, 136.3, 140.2, 140.7, 144.16, 144.21, 158.2.
1,1-Diphenyl-2,2-di-(p-methoxyphenyl)ethylene (6). Light yel-
low solid, mp 156-158 °C (lit.24 mp 154-158 °C); 1H NMR
(CDCl3, 400 MHz) δ 3.76 (s, 6H), 6.68 (d, J ) 8.84 Hz, 4H), 6.99
(d, J ) 8.84 Hz, 4H), 7.06-7.16 (m, 10H); 13C NMR (CDCl3, 100
MHz) δ 55.2, 113.1, 126.2, 127.8, 131.5, 132.8, 136.5, 139.4, 140.2,
144.5, 158.2.
Conclusion
We have described herein a scalable and simple methodology
for the preparation of various unsymmetrical tetraarylethylenes
in excellent yields. The synthetic strategy was also employed
for the preparation of various TPE-based triads (TPE-spacer-
TPE) using bis-benzoyl substrates as well as palladium-catalyzed
coupling reactions using mono-bromotriphenylethylene. Redox
and photophysical properties were also evaluated to shed light
onto the structure-property relationships of various tetraaryl-
ethylene derivatives and triads. Studies are underway, especially
for the TPE-spacer-TPE triads, to establish the extent of the
electronic coupling among the triphenylethylenic donor moieties
when connected to various poly-p-phenylene spacers. These
studies will be reported in due course.
1,1-Diphenyl-2,2-di-(3,4-dimethoxyphenyl)ethylene (7). Yel-
1
low solid, mp 167-168 °C; H NMR (CDCl3, 300 MHz) δ 3.48
(s, 6H), 3.82 (s, 6H), 6.56-6.65 (m, 6H), 7.02-7.14 (m, 10H);
13C NMR (CDCl3, 75 MHz) δ 55.7, 55.8, 110.3, 115.3, 124.2, 126.3,
127.9, 131.3, 136.2, 139.5, 140.6, 144.5, 147.7, 147.9.
Experimental Section
1-(4-Biphenyl)-l,2,2-triphenylethylene (8). White solid, mp
187-189 °C; 1H NMR (CDCl3, 300 MHz) δ 7.00-7.46 (m, 24H);
13C NMR (CDCl3, 75 MHz) δ 126.4, 126.63, 126.69, 127.0, 127.3,
127.85, 127.90, 127.96, 128.9, 131.55, 131.63, 132.0, 139.0, 140.7,
140.8, 141.3, 143.0, 143.93, 143.97.
Diphenylmethylidenefluorene (9). Pale yellow solid, mp 232-
234 °C (lit.26 mp 235 °C); 1H NMR (CDCl3, 400 MHz) δ 6.66 (d,
J ) 7.96 Hz, 2H), 6.96 (t, J ) 6.96 Hz, 2H), 6.27 (t, J ) 7.1 Hz,
2H), 7.41-7.45 (m, 10H), 7.73 (d, J ) 7.36 Hz, 2H); 13C NMR
(CDCl3, 100 MHz) δ 119.5, 125.1, 126.6, 127.8, 128.4, 129.0,
129.9, 134.4, 138.9, 140.7, 143.1, 145.7.
(4-Triphenylethenylphenyl)pentaphenylbenzene (10). White
solid, mp 342-344 °C (lit.15b mp >350 °C);1H NMR (CDCl3, 400
MHz) δ 6.54-6.61 (m, 4H), 6.75-7.08 (m, 40H); 13C NMR
(CDCl3, 100 MHz) δ 125.4, 126.33, 126.40, 126.46, 126.73, 126.78,
127.5, 127.7, 127.8, 130.1, 131.0, 131.49, 131.54, 131.61, 131.66,
139.1, 140.2, 140.3, 140.40, 140.42, 140.6, 140.76, 140.78, 140.83,
140.89, 143.8, 143.9, 144.1.
General Procedure for the Preparation of Tetraarylethylene
Derivatives. To a solution of diphenylmethane (2.02 g, 12 mmol)
in dry tetrahydrofuran (20 mL) was added 4 mL of a 2.5 M solution
of n-butyllithium in hexane (10 mmol) at 0 °C under an argon
atmosphere. The resulting orange-red solution was stirred for 30
min at that temperature. To this solution was added the appropriate
benzophenone (9 mmol) or bis-benzoyl derivative (4 mmol), and
the reaction mixture was allowed to warm to room temperature
with stirring during a 6 h period. The reaction was quenched with
the addition of an aqueous solution of ammonium chloride, the
organic layer was extracted with dichloromethane (3 × 50 mL),
and the combined organic layers were washed with a saturated brine
solution and dried over anhydrous MgSO4. The solvent was
evaporated, and the resulting crude alcohol (containing excess
diphenylmethane) was subjected to acid-catalyzed dehydration as
follows.
The crude alcohol was dissolved in about 80 mL of toluene in
a 100 mL Schlenk flask fitted with a Dean-Stark trap. A catalytic
amount of p-toluenesulphonic acid (342 mg, 1.8 mmol) was added,
and the mixture was refluxed for 3-4 h and cooled to room
temperature. The toluene layer was washed with 10% aqueous
NaHCO3 solution (2 × 25 mL) and dried over anhydrous
magnesium sulfate and evaporated to afford the crude tetraphenyl-
ethylene derivative. The crude product was purified by a simple
recrystallization from a mixture of dichloromethane and methanol
or acetonitrile or by column chromatography. The spectral data of
various TPE derivatives are summarized next.
1,4-Bis(triphenylethenyl)benzene (11). White solid, mp 242-
1
244 °C; H NMR (CDCl3, 300 MHz) δ 6.80 (s, 4H), 7.03-7.15
(m, 30H); 13C NMR (CDCl3, 75 MHz) δ 126.57, 126.65, 127.75,
127.81, 130.9, 131.54, 131.59, 141.0, 142.1, 143.7, 143.94, 143.97.
4,4′-Bis(triphenylethenyl)-1,1′-Biphenyl (12). White solid, mp
288-290 °C; 1H NMR (CDCl3, 400 MHz) δ 7.03-7.14 (m, 34H),
7.33 (d, J ) 8.44 Hz, 4H); 13C NMR (CDCl3, 100 MHz) δ 126.1,
126.6, 126.67, 127.8, 127.9, 128.0, 131.55, 131.6, 131.9, 138.4,
140.7, 141.2, 142.9, 143.92, 143.94, 143.97.
2,7-Bis(triphenylethenyl)-9,9-dihexylfluorene (13). Yellow solid,
1,1,2,2-Tetraphenylethylene (1). White solid, mp 222-224 °C;
1H NMR (CDCl3, 400 MHz) δ 7.05-7.08 (m, 8H), 7.10-7.13 (m,
12H); 13C NMR (CDCl3, 100 MHz) δ 126.6, 127.8, 131.5, 141.1,
143.9.
1
mp 174-176 °C; H NMR (CDCl3, 400 MHz) δ 0.26-0.33 (m,
4H), 0.84-0.95 (m, 10H), 0.99-1.06 (m, 4H), 1.14-1.22 (m, 4H),
1.48-1.52 (m, 4H), 6.91-6.94 (m, 4H), 7.00-7.12 (m, 30H),
7.36-7.38 (m, 2H).; 13C NMR (CDCl3, 100 MHz) δ 14.4, 22.9,
23.6, 29.8, 31.8, 40.5, 54.8, 119.1, 126.1, 126.56, 126.60, 127.74,
127.76, 127.86, 130.4, 131.6, 139.4, 140.7, 141.7, 143.0, 144.1,
144.2, 150.5.
1-(4-Methylphenyl)-l,2,2-triphenylethylene (2). White solid, mp
153-155 °C (lit.13b mp 146-148 °C); 1H NMR (CDCl3, 300 MHz)
δ 2.31 (s, 3H), 6.97 (s, 4H), 7.07-7.17 (m, 15H); 13C NMR (CDCl3,
75 MHz) δ 21.4, 126.48, 126.52, 127.80, 127.86, 128.6, 131.4,
131.51, 131.55, 136.2, 140.6, 140.9, 141.1, 144.10, 144.13, 144.14.
1,1-Diphenyl-2,2-di-p-tolylethylene (3). White solid, mp 161-
General Procedure for the Suzuki Coupling for the Prepara-
tion of Triads 14-16. A mixture of aqueous 2 M Na2CO3 (4 mL)
and dioxane (12 mL) was repeatedly degassed by evacuation and
purging with argon gas and into it, 1-(4-bromophenyl)-1,2,2-
triphenylethylene (822 mg, 2 mmol) and the appropriate diboronic
acid (1 mmol) along with 3 mol % Pd(PPh3)4 (70 mg) were added.
The mixture was refluxed in an argon atmosphere under the
complete exclusion of light for 16 h after which it was quenched
with the addition of 10% aqueous HCl. The organic layer was
extracted with dichloromethane and washed consecutively with
water and brine before being dried over MgSO4 and concentrated.
The crude product was then chromatographed over silica gel, using
ethyl acetate/hexanes as the eluent to afford the coupled products.
The spectral data of various triads are summarized next.
163 °C (lit.24 mp 152-154 °C); H NMR (CDCl3, 300 MHz) δ
1
2.29 (s, 6H), 6.95 (s, 8H), 7.06-7.15 (m, 10H); 13C NMR (CDCl3,
75 MHz) δ 21.4, 127.8, 128.5, 131.45, 131.53, 136.1, 140.2, 141.0,
141.1, 144.3.
1-(4-Bromophenyl)-1,2,2-triphenylethylene (4). White solid,
mp 148-150 °C (lit.25 mp 151-152 °C); H NMR (CDCl3, 400
1
MHz) δ 6.88 (d, J ) 8.52 Hz, 2H), 6.99-7.03 (m, 6H), 7.07-
7.12 (m, 9H), 7.20 (d, J ) 8.52 Hz, 2H); 13C NMR (CDCl3, 100
MHz) δ 120.6, 126.8, 126.8, 126.9, 127.9, 128.0, 128.1, 131.0,
131.42, 131.44, 131.5, 133.2, 139.8, 141.8, 142.9, 143.4, 143.5,
143.6.
(24) Mills, N. C.; Tirla, C.; Benish, M. A.; Rakowitz, A. J.; Bebell, L.
M.; Hurd, C. M. M.; Bria, A. L. M. J. Org. Chem. 2005, 70, 10709.
(25) Forrester, A. R.; Hepburn, S. P. J. Chem. Soc. 1971, 20, 3322.
(26) Franco, M. L. T. M. B.; Herold, B. J.; Evans, J. C.; Rowlands, C.
C. J. Chem. Soc., Perkin Trans. 1 1998, 443.
8060 J. Org. Chem., Vol. 72, No. 21, 2007

Practical Synthesis of Unsymmetrical Tetraarylethylenes
4,4′′-Bis(triphenylethenyl)-1,1′:4′,1′′-terphenyl (14). White solid,
4H), 7.73-7.75 (m, 2H); 13C NMR (CDCl3, 100 MHz) δ 13.2,
21.8, 22.9, 28.9, 30.7, 39.7, 54.4, 119.1, 120.2, 125.0, 125.4, 125.7,
126.8, 126.9, 127.0, 130.6, 131.0, 138.5, 139.2, 139.7, 141.8, 143.0,
150.8.
1
mp 294-296 °C; H NMR (CDCl3, 400 MHz) δ 7.03-7.14 (m,
34H), 7.38 (d, J ) 8.48 Hz, 4H), 7.59 (s, 4H); 13C NMR (CDCl3,
100 MHz) δ 126.2, 126.63, 126.69, 126.71, 127.3, 127.84, 127.90,
127.98, 131.56, 131.63, 132.0, 138.5, 139.5, 140.7, 141.3, 143.0,
143.92, 143.97.
Acknowledgment. We thank the Petroleum Research Fund,
administered by the American Chemical Society, and the
National Science Foundation (CAREER Award) for financial
support.
4,4′′′-Bis(triphenylethenyl)-1,1′:4′,1′′:4′′,1′′′-quaterphenyl (15).
1
Pale yellow solid, mp 320-324 °C; H NMR (CDCl3, 400 MHz)
δ 7.03-7.14 (m, 38H), 7.39-7.41 (m 4H), 7.65-7.71 (m, 4H);
13C NMR (CDCl3, 100 MHz) δ 126.3, 126.7, 127.4, 127.85, 127.92,
127.98, 131.57, 131.6, 132.1, 138.5, 139.56, 139.77, 140.7, 141.4,
143.1, 143.92, 143.96.
Supporting Information Available: Experimental details,
product characterization data, and 1H and 13C NMR spectra for all
compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
2,7-Bis(tetraphenylethenyl)-9,9-dihexylfluorene (16). Yellow
1
solid, mp 207-209 °C; H NMR (CDCl3, 400 MHz) δ 0.71 (m,
4H), 0.79 (t, J ) 7.06 Hz, 6H), 1.04-1.17 (m, 12H), 2.02-2.06
(m, 4H), 7.09-7.20 (m, 34H), 7.47-7.49 (m, 4H), 7.56-7.58 (m,
JO701474Y
J. Org. Chem, Vol. 72, No. 21, 2007 8061