A versatile synthesis of electroactive stilbenoprismands for effective binding of metal cations

Article abstract of DOI:10.1021/jo802579f

A versatile synthesis of a new class of polyaromatic receptors (stilbenoprismands) containing a Δ-shaped cavity similar to that of the π-prisrnand together with an intimately coupled electroactive stilbenoid moiety was accomplished via an efficient intram

Full text of DOI:10.1021/jo802579f

A Versatile Synthesis of Electroactive Stilbenoprismands for
Effective Binding of Metal Cations
Paromita Debroy, Sergey V. Lindeman, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881
rajendra.rathore@marquette.edu
ReceiVed NoVember 20, 2008
A versatile synthesis of a new class of polyaromatic receptors (stilbenoprismands) containing a ∆-shaped
cavity similar to that of the π-prismand together with an intimately coupled electroactive stilbenoid moiety
was accomplished via an efficient intramolecular McMurry coupling reaction. The presence of the ∆-shaped
1
cavity in stilbenoprismands allows an efficient binding of a single silver cation as probed by H NMR
spectroscopy. Electron-rich stilbenoprismands undergo a ready oxidation to their highly robust
cation-radical and dicationic salts. X-ray structure determination of a representative dicationic
stilbenoprismand showed that the charges were largely localized on the tetraarylethylene moiety, which
results in a twisting of the ethylenic CdC bond by ∼35°. Moreover, the electronic coupling among the
stilbenoid and π-prismand moieties in various stilbenoprismands was briefly probed by optical methods.
Introduction
24 (see structures below) contain a ∆-shaped cavity that is
especially efficacious for binding a variety of metal cations, such
as Ag+, Tl+, Ga+, etc.3,4 Unfortunately, both 1 and 2 are neither
readily accessible, nor amenable to ready structural modifications
for practical applications.5
Polyaromatic receptors containing multiple aromatic groups
in (rigid) cofacially oriented arrays (such as cis-diarylalkenes,
cyclophanes, triptycenes, deltaphanes, cylinderophanes, etc.)1
continue to attract attention owing to their potential importance
in the areas of metal-ion sensing, electrical conductors, and
photoresponsive devices.2 Among the best known metal-ion
receptors, π-prismand (1)3 and a structurally analogous deltaphane
(1) (a) Heirtzler, F. R.; Hopf, H.; Jones, P. G.; Bubenitachek, P.; Lehne, V.
J. Org. Chem. 1993, 58, 2781. (b) Gano, J. E.; Subramaniam, G.; Birnbaum, R.
J. Org. Chem. 1990, 55, 4760. (c) Yang, R.-H.; Chan, W.-H.; Lee, A. W. M.;
Xia, P.-F.; Zhang, H.-K.; Li, K. J. Am. Chem. Soc. 2003, 125, 2884. (d) Chebny,
V. J.; Rathore, R. J. Am. Chem. Soc. 2007, 128, 8458. (e) Prodi, L.; Bolletta, F.;
Montalti, M.; Zaccheroni, N. Coord. Chem. ReV. 2000, 205, 59. (f) Ikeda, M.;
Tanida, T.; Takeuchi, M.; Shinkai, S. Org. Lett. 2000, 2, 1803. (g) Lindeman,
S. V.; Rathore, R.; Kochi, J. K Inorg. Chem. 2000, 39, 5707, and references
cited therein.
(2) (a) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga,
Y.; Sugimoto, K. Inorg. Chem. 1997, 36, 4903. (b) Munakata, M.; Wu, L. P.;
Ning, G. L.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Macao, Y. J. Am.
Chem. Soc. 1999, 121, 4968. (c) Szigeti, Z.; Malon, A.; Vigassy, T.; Csokai,
V.; Gru¨n, A.; Wygladacz, K.; Ye, N.; Xu, C.; Bitter, V. J.; Rathore, I.; Bakker,
R.; Pretsch, E. Anal. Chim. Acta 2006, 572, 1. (d) Chebny, V. J.; Banerjee, M.;
Rathore, R. Chem. Eur. J. 2007, 13, 6508, and references therein.
(3) (a) Pierre, J. L.; Baret, P.; Chautemps, P.; Armand, M. J. Am. Chem.
Soc. 1981, 103, 2986. (b) Pierre, G.; Baret, P.; Chautemps, P.; Pierre, J. L.
Electrochim. Acta 1983, 28, 1269.
It was conjectured that a polyaromatic receptor, referred to
as “stilbenoprismand” (SB), containing a cavity similar to that
of π-prismand 1 together with an intimately coupled electro-
active stilbenoid moiety can be easily assembled via a McMurry
coupling reaction of the conformationally mobile diketone A,
(4) (a) Kang, H. C.; Hanson, A. W.; Eaton, B.; Boelelheide, V. J. Am. Chem.
Soc. 1985, 107, 1979. (b) Schmidbaur, H.; Hager, R.; Huber, B.; Muller, G.
Angew. Chem., Int. Ed. 1987, 26, 338. (c) Probst, T.; Steigelmann, O.; Riede,
J.; Schmidbaur, H. Angew. Chem., Int. Ed. 1990, 29, 1397.
(5) See: Heirtzler, F. R.; Hopf, H.; Jones, P. G.; Bubenitschek, P. Tetrahedron
Lett. 1995, 36, 1239.
2080 J. Org. Chem. 2009, 74, 2080–2087
10.1021/jo802579f CCC: $40.75 2009 American Chemical Society
Published on Web 02/04/2009

Synthesis of ElectroactiVe Stilbenoprismands
SCHEME 1
SCHEME 2. A General Synthetic Scheme for the
Preparation of Diketones (6a-d) and Showing a
Representative X-ray Structure of 6d with the Transcoid
Geometry in the Solid State
i.e. Scheme 1.6 The proposed route for the synthesis of SBs in
Scheme 1 is based on the following observations: (i) there is
ample literature precedence for the efficient preparation of large
macrocycles using McMurry coupling7 and (ii) a facile inter-
conversion between the transcoid (A-trans) and ciscoid (A-cis)
diketone conformations (Scheme 1), which is not readily frozen
1
even at -80 °C as judged by variable-temperature H NMR
spectroscopy.8
Accordingly, herein we demonstrate that the approach in
Scheme 1 can be successfully employed for the preparation of
a number of SBs with both aliphatic and aromatic “R” groups,
and their structures were established by X-ray crystallography.
The SBs contain ∆-shaped cavities of similar shape and size as
1 and 2 and thus bind a silver cation with efficiency similar to
that of 1 or 2. Moreover, stilbenoprismands undergo reversible
electrochemical oxidations and form stable cation-radicals and
dicationic salts. The X-ray crystallography of a representative
dicationic salt of a stilbenoprismand shows that the double bond
in the stilbenoid moiety undergoes a twist of ∼35° around the
CdC bond despite being tied into the π-prismand macrocycle.
The details of these findings are described herein.
SCHEME 3. An Alternative Synthetic Scheme for the
Preparation of Diketone 6b and X-ray Structure of an
Unexpectedly Formed Borate Ester 8
Results and Discussion
Synthesis of Stilbenoprismands (SBs). The essential build-
ing blocks containing a one-aryl (5) and a three-aryl unit (3)
for the preparation of diketones (6) can be assembled from
commercially available starting materials via Friedel-Crafts
acylation and Suzuki coupling reactions, respectively (Scheme
2).
For example, 2,2′′-dibromo-p-terphenyl (3) was obtained in
excellent yield (∼90%) by a Suzuki coupling reaction9 of
1-bromo-2-iodobenzene10 with 1,4-benzenediborate ester in
refluxing dioxane in the presence of aqueous sodium carbonate
and a catalytic amount of Pd(PPh3)4. The structure of 3 was
established by 1H/13C NMR spectroscopy and was further
confirmed by X-ray crystallography (see the Supporting Infor-
mation). A reaction of 2,2′′-dibromo-p-terphenyl (3) with
n-butyllithium in tetrahydrofuran at -78 °C followed by
treatment with trimethylborate and hydrolysis with hydrochloric
acid afforded the corresponding diboronic acid 4 as a white solid
in 66% isolated yield. The structure of 4 was confirmed by its
conversion to the corresponding diborate ester (see the Sup-
porting Information). Having 4 in hand, the syntheses of
diketones 6a-d were readily accomplished via a standard Suzuki
coupling reaction of 4 with various 4-bromophenylketones,
5a-d, to produce the corresponding diketones 6a-d in moder-
ate to good yields (see Scheme 2).
To improve the yields of diketones 6a (R ) methyl) and 6b
(R ) n-pentyl), we sought an alternative approach. It was
envisioned that 6a and 6b could also be accessed by the Suzuki
coupling of 2,2′′-dibromo-p-terphenyl (3) with the boronic acids
of the corresponding bromophenylketones (e.g., 5b), i.e.,
Scheme 3.
Thus, a freshly prepared Grignard reagent from the protected
4-bromohexanophenone (i.e., 7) was reacted with trimethylbo-
rate in tetrahydrofuran at -78 °C for 12 h, followed by
quenching the resulting reaction mixture with dilute hydrochloric
acid at 0 °C to afford, to our surprise, the borate ester 8 in
excellent yield. The structure of the borate ester 8 was confirmed
by 1H/13C NMR spectroscopy and further established by X-ray
crystallography (see Scheme 3). This observation suggests that
the deprotection of the ketal and the esterification of the
corresponding boronic acid occurs in a single step leading to
(6) Compare: Emond, S. J.; Debroy, P.; Rathore, R. Org. Lett. 2008, 10, 389.
(7) (a) Baumstark, A. L.; McCloskey, C. J.; Witt, K. E. J. Org. Chem. 1978,
43, 3609. (b) Shukla, R.; Lindeman, S. V.; Rathore, R. J. Org. Chem. 2006, 71,
6124, and references cited therein.
(8) (a) Fujioka, Y. Bull. Chem. Soc. Jpn. 1985, 58, 481. (b) Ozasa, S.; Hatada,
N.; Fujioka, Y.; Ibuki, E. Bull. Chem. Soc. Jpn. 1980, 53, 2610.
(9) (a) Suzuki, A. Chem. Commun. 2005, 4759. (b) Hassan, J.; Sevignon,
M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. ReV. 2002, 102, 1359, and
references cited therein.
(10) Tunney, S. E.; Stille, J. K. J. Org. Chem. 1987, 52, 748.
J. Org. Chem. Vol. 74, No. 5, 2009 2081

Debroy et al.
SCHEME 4. A General Synthetic Scheme for the
Preparation of SBs and the Molecular Structures
Determined by X-ray Crystallography
the formation of the borate ester 8 in 84% yield. As such, this
unprecedented observation may hold potential for the one-pot
preparation of borate esters from protected haloarylketones and
aldehydes! A Suzuki coupling reaction of the borate ester 8 with
2,2′′-dibromo-p-terphenyl 3 in refluxing dioxane in the presence
of aqueous sodium carbonate and a catalytic amount of
Pd(PPh3)4 for 17 h, indeed, afforded 6b in a much improved
yield (77%).
The molecular structures of diketones 6a-d were easily
established by 1H and 13C NMR spectroscopy and further
confirmed by mass spectrometry. Furthermore, the molecular
structure of a representative diketone 6d, obtained by X-ray
crystallography (see Scheme 2), showed that the two carbonyl
groups lie on the opposite faces of the terphenyl framework in
the solid state (vide supra).
1
FIGURE 1. Left: Partial H NMR spectra of SB2 (9 mM) in CDCl3
obtained upon an incremental addition of Ag+CF3SO3 (90 mM) in
CDCl3-CD3OD at 22 °C. Right: A plot of changes in the chemical
shifts of p-phenylene protons (labeled “c”) against the added equivalents
-
of Ag+CF3SO3
.
-
changes in its 1H NMR spectrum in chloroform-d by an
incremental addition of a concentrated solution of silver
trifluoromethanesulfonate in a mixture of chloroform-d and
methanol-d4. The addition of the substoichiometric increments
of the Ag+ solution showed considerable shifts of the aromatic
signals of SB2 up to the addition of 1 equiv of Ag+, as shown
in Figure 1. It is noteworthy that the 1H NMR spectra remained
unchanged upon further addition of Ag+ solution (i.e., beyond
1 equiv). Moreover, a 1:1 complexation stoichiometry for [SB2,
Ag+] was further confirmed from a plot of the changes in the
chemical shift of one of the signals in 1H NMR spectra against
the equivalents of added Ag+ solution (Figure 1, right).
Unfortunately, an accurate binding constant for the formation
of [SB2, Ag+] could not be determined by an NMR method as
it simply showed complete capture of the Ag+ and suggested
that the binding constant is too large to be measured by NMR
spectroscopy.11
An intramolecular McMurry coupling7 of the diketones 6a-d
under mild dilution afforded the desired SB1-4 in excellent
yields (Scheme 4). For example, a solution of the diketone 6a
in tetrahydrofuran was added slowly during a 12-h period to a
refluxing slurry of TiCl4, zinc dust, and a catalytic amount of
pyridine in tetrahydrofuran, and the resulting reaction mixture
was refluxed for an additional 12 h. A standard aqueous workup
followed by purification of the crude reaction mixture by column
chromatography afforded the SB1 in >90% yield. The molecular
structures of SB1-4 thus obtained were easily established by
1H/13C NMR spectroscopy and further confirmed by X-ray
crystallography (see Scheme 4).
The excellent yield of various stilbenoprismands in Scheme
4 clearly highlights the versatility of intramolecular McMurry
coupling, which occurs easily via a rotation of the carbonyl
bearing units about the terphenyl framework, thereby leading
to the desired stilbenoprismands in excellent yields (see Scheme
1). A comparison of the geometrical parameters of SB2-4 with
those for π-prismand 1 or deltaphane 2 indicated that the sizes
of the ∆-shaped cavity in various SBs were characteristically
similar, i.e., an average distance from the center of the ∆-shaped
cavity to the center of the three benzene rings is ∼2.6 Å in all
SBs as well as in 1 and 2. This structural similarity suggested
that SB1-4 should bind a silver cation with more or less similar
efficiency as that of π-prismand 1 or deltaphane 2.
A similar 1H NMR spectral titration of the solutions of SB3
and SB4, containing two phenyl or anisyl groups, respectively,
with Ag+ (under identical conditions) could not be carried out
as they underwent chemical oxidation to the corresponding
cation radicals in the presence of Ag+.12
Electrochemical Oxidations of SBs and Generation of
Their Cation Radicals. Ease of the oxidation of SB3 and SB4
(11) With the exception of the slight broadening in the first spectrum in Figure
1 (with 0.2 equiv of Ag+, which may arise due to the low concentration of [SB2,
Ag+]), the other spectra in Figure 1 did not show any significant line broadening
to allow the extraction of a reliable Kd value. Compare: Rathore, R.; Chebny,
V. J.; Abdelwahed, S. H. J. Am. Chem. Soc. 2004, 127, 8012. Also see: Connors,
K. A. Binding Constants; Wiley: New York, 1987.
Silver Cation Binding to Stilbenoprismands. The binding
of silver cation to readily soluble SB2 was monitored by the
2082 J. Org. Chem. Vol. 74, No. 5, 2009

Synthesis of ElectroactiVe Stilbenoprismands
TABLE 1. Oxidation Potentials of Various Stilbenoprismands and
e
the Corresponding Model Electron Donors in CH2Cl2
a V vs SCE at 22 °C. In dichloromethane containing 0.2
tetra-n-butylammonium hexafluorophosphate at 200 mV s-1
M
.
FIGURE 2. Cyclic voltammograms of 1 mM SB2, SB3, and SB4 in
dichloromethane at a scan rate of ν ) 200 mV s-1 (22 °C). The
corresponding square-wave voltammograms are shown as red curves.
b Irreversible wave. c Second oxidation wave was not observed.
d Dianisylditolylethylene (10) consisted of a ∼1:1 cis/trans mixture. e ph
) phenyl, Tol ) 4-methylphenyl, An ) 4-methoxyphenyl.
with Ag+ prompted us to evaluate their electron-donor strengths
and the initial indication of their cation radical stability by cyclic
voltammetry as follows.
The electrochemical reversibility of SB4 and its relatively
low oxidation potential prompted us to carry out its oxidation
to the corresponding cation radical using a stable aromatic
cation-radical salt MA+• SbCl6 (i.e., [1,4:5:8]dimethano-
-
Thus, the electron donor strengths of SBs in comparison with
the model compounds [such as tetraphenylethylene (9)13 and
dianisylditolylethylene (10)13] were evaluated by electrochemi-
cal oxidation at a platinum electrode as 1 × 10-3 M solutions
in dichloromethane containing 0.2 M n-Bu4NPF6 as the sup-
porting electrolyte. The cyclic voltammograms of various SBs
are compiled in Figure 2. It is notable that while SB2 and SB3
showed only quasireversible oxidation waves, SB4 displayed
two completely reversible oxidation waves owing to the presence
of electron-donating methoxy groups. Furthermore, the first
oxidation wave of SB3 (i.e., SB3 f SB3+•) is reversible if the
scanning is terminated before the start of the second oxidation
event (i.e., SB3+• f SB32+), see Figure 2. A quantitative
evaluation of the CV peak currents with added ferrocene (as an
internal standard, Eox ) 0.45 V vs SCE) revealed that the
oxidation potentials of SB3 (Eox ) 1.27 and 1.59 V vs SCE)
and SB4 (Eox ) 0.98 and 1.13 V vs SCE) were comparable to
those of the model donors tetraphenylethylene (9) and diani-
sylditolylethylene (10), respectively (see Table 1).
1,2,3,4,5,6,7,8-octahydro-9,10-dimethoxyanthracenium hexachlo-
roantimonate)14 as an oxidant (Ered ) 1.11 V vs SCE). Thus,
Figure 3A shows the spectral changes attendant upon the
reduction of MA+• (λmax ) 518 nm, log ε518 ) 3.86) by an
incremental addition of substoichiometric amounts of SB4 in
dichloromethane at 22 °C. The presence of a well-defined
isosbestic point at λmax ) 520 nm (see Figure 3A) attests to an
uncluttered character of the electron transfer form SB4 to MA+•.
Furthermore, a plot of formation of SB4+• (i.e., an increase in
the absorbance at 930 nm) against the increments of added SB4
(Figure 3B) established that MA+• was completely consumed
after the addition of 1 equiv of SB4; and the resulting absorption
spectrum of the blue SB4+• [λmax ) 357, 616, and 930 nm; log
ε930 ) 4.22] remained unchanged upon further addition of
neutral SB4, i.e., eq 1.
(12) (a) Note that an exposure of a solution of SB4 (or SB3) to solid
CF3SO3Ag in anhydrous dichloromethane (in the absence of methanol) produced
a colored solution of the SB4 cation radical as judged by spectrophotometric
analysis. (b) Owing to the poor solubility of SB1, it was not utilized in the
subsequent studies.
(13) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem.
Soc. 1998, 120, 6931.
It is noteworthy that the blue SB4+•, obtained according to
the eq 1, is highly persistent at ambient temperatures and did
not show any decomposition during a 12-h period at 22 °C, as
confirmed by UV-vis spectroscopy. Moreover, a reduction of
(14) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Synth. 2005, 82, 1.
Also see: Rathore, R.; Kochi, J. K. J. Org. Chem. 1995, 60, 4399.
J. Org. Chem. Vol. 74, No. 5, 2009 2083

Debroy et al.
FIGURE 3. (A) Spectral changes upon the reduction of 3 × 10-5 M MA+• by an incremental addition of a 2 × 10-3 M solution of SB4 in CH2Cl2
at 22 °C. (B) A plot of increasing absorbance of SB4+• (monitored at 930 nm) against the equivalents of added SB4. Note that the depletion of
absorbance of MA+• at λmax ) 518 nm could not be monitored due to the overlap of the absorption band of SB4+•. (C) A comparison of the
absorption spectra of SB4+• and the model dianisylditolylethylene cation radical (10+•) under identical conditions.
SB4+• with zinc dust regenerated the neutral SB4 quantitatively.
It is also noted that the spectral characteristics of SB4+• closely
resemble that of the cation radical of dianisylditolylethylene
(10+•: λmax ) 346, 545, and 902 nm; log ε930 ) 4.34), see Figure
3C.
Isolation and X-ray Crystallography of the SB4
Dication. We have earlier shown that a stepwise one-electron
oxidation of various tetraarylethylene donors (D) can be carried
out by electron exchange with stable aromatic cation radicals
as oxidants to selectively afford the cation radicals (D+•) and
the dications (D2+).13 Furthermore, it was shown that the
disproportionation of the cation radicals of the tetraarylethylenes
(where aryl ) phenyl, p-tolyl, p-anisyl, etc.) to the corresponding
dications and neutral tetraarylethylenes (i.e., 2D+• f D2+ + D)
are dependent on their disproportionation constants (Kdisp) which
can be easily estimated from the cyclic voltammetric data (e.g.,
colored needles, thus obtained, was shown to be the dicationic
[SB42+ (SbCl6-)2] by UV-vis spectroscopy (see Figure S1 in
the Supporting Information) and by X-ray crystallography as
follows.15
Expectedly, the crystallographic analysis of the above dark-
colored needles further confirmed that the crystals consisted of
-
a dicationic salt [SB42+ (SbCl6 )2]. Figure 4A shows a unit cell
-
of [SB42+ (SbCl6 )2], which is made up of columns of dicationic
SB42+ which are surrounded by SbCl6 counteranions and
-
several heavily disordered toluene and dichloromethane mol-
ecules. Despite the low experimental precision, owing to the
disordered toluene molecules, the bond length changes in SB42+
clearly showed that the stilbenoid CdC double bond [1.362(3)
Å in neutral SB4] loses its bonding electrons and essentially
2
2
changes into a single Csp -Csp bond (1.475(9) Å). The cationic
charges are largely delocalized onto the adjacent p-anisyl groups
as judged by the shortening of the exocyclic CsC bonds from
1.491(3) Å (in neutral SB4) to 1.393(9) Å and of the CsOCH3
bonds from 1.372(3) Å (in neutral SB4) to 1.311(9) Å.
Moreover, the loss of electron density from the stilbenoid
ethylenic bond in SB4+2 also induces a twist of ∼35° around
the CdC bond (Figure 4B) as compared to the observed twist
of ∼60° around the central CdC bond in various tetraaryleth-
ylene dications (e.g., Figure 4C).13 It is noteworthy that the
modest twist of ∼35° in SB4+2 (as opposed to ∼60° in acyclic
tetraarylethylene dications) causes significant conformational
deformation in the π-prismand macrocycle. For example, the
π-prismand macrocycles in neutral SB2-4 are essentially planar
(considering the p-phenylene bridges as linear) with only minor
deviations (i.e., +4.4° for SB2, +5.4° for SB3, +9.2° for SB4)
from the ideal value of the sum of the intracyclic bond angles
(i.e., 6 × 120° ) 720°)16 due to the slight distortions (∼3°) in
the planarity of the p-phenylene rings and the twist around the
stilbenoid CdC double bonds (∼4°). Interestingly, however,
the oxidation of neutral SB4 to its dication (and an accompany-
Table 1) by using the Nernstian expression Kdisp
)
exp(-0.039∆E°) at 298 K, where ∆E° ) E2° - E1° (mV).13
It is evident from the spectral results in Figure 3A/B that the
SB4 cation radical is the predominant species in a dichlo-
romethane solution and is also consistent with the low value of
the estimated disproportionation constant Kdisp ) [SB4+2][SB4]/
[SB4+•]2 ) 6 × 10-3, which is comparable to the Kdisp values
for the model dianisylditolylethylene donor (10).13 Interestingly,
despite the low values of disproportionation constants for various
tetraarylethylene cation radicals, they all undergo crystallization-
induced disproportionation to form the corresponding crystalline
dicationic salts.13 The X-ray crystallographic analysis of the
dicationic salts of various tetraarylethylenes (with H, Me, and
OMe substituents) as well as 10 showed that the central ethylenic
bond undergoes a twist of ∼60° around the central CdC bond.13
To experimentally determine whether SB4+2 can accommodate
a similar twist around the CdC bond, we sought to obtain its
single crystals as follows.
Thus, a treatment of a solution of SB4 with equimolar
-
NO+SbCl6 in anhydrous dichloromethane at ∼0 °C im-
mediately resulted in a dark blue solution of SB4+• according
(15) Note that the repeated attempts to obtain single crystals of monocationic
SB4+• SbCl6-, even in the presence of excess neutral SB4, were thus far
unsuccessful. However, a reviewer suggested that it may be feasible to obtain
single crystals of monocationic SB4+• using a larger counter anion, e.g., see:
Nelsen, S. F.; Ismagilov, R. F.; Powell, D. R. J. Am. Chem. Soc. 1997, 119,
10213.
to the stoichiometry in eq 2.
-
SB4+NO+SbCl6 fSB4+•SbCl6- + NOv
(2)
(16) The ideal values for the sum of the intracyclic bond angles for the planar
rings are the following: 3 × 60 ) 180° for 3-membered cycles, 4 × 90 ) 360°
for 4-membered cycles, 5 × 108 ) 540° for 5-membered cycles, 6 × 120 )
720° for 6-membered cycles, etc. A deviation from the ideal value of the sum
of the intracyclic bond angles results either in the nonplanar conformations (e.g.,
the chair conformation of cyclohexane) or distortion of the bonds (e.g., the
“banana” bonds in cyclopropane).
The highly colored dichloromethane solution of SB4+• SbCl6
salt, obtained in eq 2, was found to be stable for days at 22 °C
and was spectrally identical with that obtained above in eq 1.
The blue solution was carefully layered with toluene and stored
in a refrigerator at -10 °C for 2 days. A good crop of dark-
-
2084 J. Org. Chem. Vol. 74, No. 5, 2009

Synthesis of ElectroactiVe Stilbenoprismands
FIGURE 5. (A) A comparison of the UV-vis spectral profile of SB2
(λmax ) 247 nm, log ε247 ) 4.33), SB3 (λmax ) 252 nm, log ε252
)
4.50), SB4 (λmax ) 253 nm, log ε253 ) 4.47), 9 (λmax ) 236 and 305
nm, log ε236 ) 4.31), and 10 (λmax ) 252 and 319 nm, log ε252 ) 4.50)
as 2.6 × 10-5 M solutions in CH2Cl2. (B) Emission spectra of SB2
(λmax ) 392 nm), SB3 (λmax ) 392), and SB4 (λmax ) 398) as 2.6 ×
10-5 M solutions in CH2Cl2 (excitation wavelength 250 nm).
Absorption and Emission Spectroscopy of the
Stilbenoprismands. Tetraarylethylenes and stilbenes are known
to have exceedingly short singlet excited state lifetimes and
negligible fluorescence quantum yields in solution.17 Herein,
we explore the emission/optical characteristics of various
stilbenoprismands in which the stilbenoid moieties are intimately
coupled with the π-prismand framework, which not only restricts
the avenues available for the deactivation of the excited state
but also provides a highly emissive terphenyl site for the
intramolecular reorganization of the exciton energy (vide infra).
For example, the absorption spectra of stilbenoprismands SB3
and SB4 as 2.6 × 10-5 M solutions in dichloromethane showed
similar (general) spectral band shapes and absorption maxima
(see legend in Figure 5A) as the model tetraphenylethylene (9)
and dianisylditolylethylene (10) with poorly resolved vibrational
structures (Figure 5A). As expected, SB2 with the alkyl
substituents (rather than phenyls) displayed a relatively blue-
shifted absorption envelope similar to the one observed for the
stilbenes.18
The emission spectra of the parent tetraphenylethylene (9)
or the dianisylditolylethylene (10) showed no detectable emis-
sion;17 however, the SB2-4, under similar conditions, showed
significant emission (Figure 5B). As such, the observation of
emission from stilbenoprismands would suggest that the emis-
sion must be arising from a reorganization of the exciton energy
from the stilbenoid moiety to the terphenyl group of the
π-prismand framework.19 Indeed, a comparison of the emission
spectrum of terphenyl (TP)20 with that of SBs showed a
reasonable spectral similarity. Further studies using time-
resolved spectroscopy as well as theoretical calculations will
be required to pinpoint the origin of the observed emission in
various stilbenoprismands.21
FIGURE 4. (A) A unit cell of the crystal structure of SB4+2 (SbCl6
)
2
-
showing the disordered toluene molecules. (B) The individual and
superimposed structures of neutral and dicationic SB4 showing the twist
around the CdC double bond. (C) The individual and superimposed
structures of neutral and dicationic 10 showing the twist around the
CdC double bond.
ing twist of ∼35° around the stilbenoid CdC bond) leads to a
decrease in the angular deviation of +2.9° (as opposed to +9.2°
in neutral SB4) from the ideal value of the sum of the intracyclic
bond angles (i.e., 720°), but a significant increase in the
nonplanarity of the p-phenylene rings in the π-prismand
macrocycle (i.e., ∼5°, which is a ∼50% increase in the distortion
of the p-phenylene rings as compared to the neutral SB4).
As such, the observed twist of ∼35° in SB4+2 does lend
confidence to our earlier observations13 that the olefinic cation
radicals and dications do undergo an increasing twist around
the central CdC bond owing to the loss of its bonding electrons
and these observations were not simply artifacts of the simple
crystal-packing forces.
In summary, we have successfully demonstrated a versatile
synthesis of π-prismand based receptors that can be readily
(17) (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.
(18) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Com-
pounds, 2nd ed.; Academic Press: New York, 1971.
(19) Compare:(a) Banerjee, M.; Emond, S. J.; Lindeman, S. V.; Rathore, R
J. Org. Chem. 2007, 72, 8054. (b) Gano, J. E.; Osborn, D. J., III; Kodali, N.;
Sekher, P.; Liu, M.; Luzik, E. D, Jr J. Org. Chem. 2003, 68, 3710.
(20) Banerjee, M.; Lindeman, S. V.; Rathore, R. J. Am. Chem. Soc. 2007,
129, 8070.
(21) For example: Momicchioli, F.; Bruni, M. C.; Baraldi, I. J. Phys. Chem.
1972, 26, 3983.
J. Org. Chem. Vol. 74, No. 5, 2009 2085

Debroy et al.
Pd(PPh3)4 (0.09 g, 0.08 mmol) was refluxed for 48 h. After cooling,
the reaction mixture was quenched with 5% hydrochloric acid (30
mL). A standard aqueous workup afforded a solid that was purified
by crystallization from a mixture of dichloromethane/acetonitrile
to furnish 6c as a white solid. Yield 1.94 g, 82%; mp 217-218
°C; 1H NMR (CDCl3) δ 7.02 (s, 4H), 7.23 (s, 2H), 7.40-7.57 (m,
16H), 7.65 (d, J ) 8 Hz, 4H), 7.74 (d, J ) 8 Hz, 4H); 13C NMR
(CDCl3) δ 127.8, 128.4, 129.8, 129.9, 130.1, 130.6, 130.9, 132.4,
135.6, 137.8, 139.5, 139.6, 140.3, 146.1, 196.5.
Following a similar procedure as above, reactions of 2,2′′-p-
terphenyldiboronic acid (4; 1.27 g, 4 mmol) with 4-bromoacetophe-
none (2.0 g, 10 mmol) or 4-bromohexanophenone6 (2.61 g, 10
mmol) or 4-bromo-4′-methoxybenzophenone (2.91 g, 10 mmol)
afforded the corresponding diketones 6a, 6b, and 6d, respectively,
and their yields and spectral data are summarized below:
tailored by varying the “R” groups for the desired applications.
The presence of the ∆-shaped cavity in SBs allows an efficient
binding of silver cation, which can be monitored with the aid
of 1H NMR spectroscopy. Furthermore, highly electroactive SB3
and SB4 undergo ready oxidation to their highly robust and
stable cation-radical salts. The isolation and X-ray crystal-
lography of the dicationic salt of SB4 allows us to demonstrate
that the charge is largely localized on the tetraarylethylene
moiety, which undergoes a somewhat limited twist (∼35°)
around the CdC bond as compared to the twist of ∼60° around
the CdC bond in various dicationic tetraarylethylenes.
Experimental Section
6a: yield 0.60 g, 30% (after column chromatography with 8:2
hexanes-ethyl acetate as eluent); mp 210-211 °C; 1H NMR
(CDCl3) δ 2.62 (s, 6H), 7.01 (s, 4H), 7.25 (d, J ) 8 Hz, 4H),
7.43-7.46 (m, 8H), 7.84 (d, J ) 8 Hz, 4H); 13C NMR (CDCl3) δ
26.8, 127.8, 128.1, 128.4, 129.7, 130.2, 130.6, 130.9, 135.4, 139.4,
139.6, 140.2, 146.7, 198.1. 6b: yield 1.11 g, 48% (after column
chromatography with 9:1 hexanes-ethyl acetate as eluent); mp
166-167 °C; 1H NMR (CDCl3) δ 0.89 (t, J ) 7 Hz, 6H), 1.33-1.36
(m, 8H), 1.70-1.75 (m, 4H), 2.94 (t, J ) 7 Hz, 4H), 6.97 (s, 4H),
7.21 (d, J ) 8 Hz, 4H), 7.40-7.43 (m, 8H), 7.81 (d, J ) 8 Hz,
4H); 13C NMR (CDCl3) δ 14.2, 22.8, 24.3, 31.8, 38.8, 127.9, 128.3,
129.7, 130.2, 130.6, 130.9, 135.4, 139.5, 139.6, 140.3, 146.4, 200.5.
6d: yield 1.64 g, 63% (after recrystallization from a mixture of
dichloromethane/acetonitrile); mp 151-152 °C; 1H NMR (CDCl3)
δ 3.85 (s, 6H), 6.91 (d, J ) 9 Hz, 4H), 7.02 (s, 4H), 7.24 (d, J )
8 Hz, 4H), 7.45 (s, 8H), 7.62 (d, J ) 8 Hz, 4H), 7.76 (d, J ) 9 Hz,
4H); 13C NMR (CDCl3) δ 55.6, 113.6, 127.8, 128.3, 129.6, 129.7,
129.8, 130.3, 130.6, 130.8, 132.6, 136.3, 139.65, 139.68, 140.3,
145.6, 163.2, 195.3.
Nitrosonium hexachloroantimonate was prepared from a reaction
of SbCl5 and NOCl in dichloromethane at -78 °C and the resulting
salt was recrystallized from a dichloromethane-hexanes mixture
and stored in an inert-atmosphere glovebox.22 (See the Supporting
Information for a General Experimental Section.)
Preparation of 2,2′′-Dibromo-p-terphenyl (3). A mixture of
1-bromo-2-iodobenzene (3.10 g, 24 mmol) and 1,4-benzenediborate
ester (3.62 g, 12 mmol) in dioxane (60 mL) in the presence of
degassed 2 M aqueous sodium carbonate (30 mL) and a catalytic
amount of Pd(PPh3)4 (0.27 g, 0.24 mmol) was refluxed for 12 h
under an argon atmosphere. A standard aqueous workup and
crystallization of the resulting solid from a mixture of dichlo-
romethane/hexanes afforded 3 as a crystalline white solid (4.1 g,
87%): mp 135-136 °C (lit.23 mp 137.4 °C); H NMR (CDCl3) δ
1
7.17-7.23 (m, 2H), 7.33-7.40 (m, 4H), 7.47 (s, 4H), 7.65-7.71
(m, 2H); 13C NMR (CDCl3) δ 122.8, 127.7, 129.1, 129.3, 131.7,
133.5, 140.5, 142.4.
Preparation of 2,2′′-p-Terphenyldiboronic Acid (4). A Schlenk
flask containing a solution of 2,2′′-dibromo-p-terphenyl (3; 4.03 g,
10.4 mmol) in anhydrous THF (30 mL) was cooled to -78 °C
under an argon atmosphere. To this solution was added n-BuLi
(10.4 mL, 26 mmol, 2.5 M in hexanes) dropwise, and the resulting
mixture was stirred for 2 h at -78 °C. Trimethylborate (5.2 mL,
45.7 mmol) was added dropwise (with the aid of a syringe) to the
above mixture and the resulting mixture was allowed to warm to
room temperature during a 12-h period with stirring. The reaction
was quenched by addition of 2 N HCl (40 mL) and the resulting
mixture was stirred for an additional 3 h and extracted with ether
(3 × 50 mL). The ether layer was washed with water (3 × 50 mL)
and brine (50 mL) and dried over anhydrous MgSO4. Concentration
in vacuo and subsequent trituration with hexane resulted in a white
solid diboronic acid 4, which was separated by filtration (2.2 g,
66%). The 4 was characterized by its conversion to the correspond-
ing diborate ester as follows.
Diboronic acid 4 (2.00 g, 6.3 mmol) was converted to the
corresponding diborate ester by a reaction with pinacol (2.23 g,
18.9 mmol) and a catalytic amount of p-toluenesulfonic acid (0.10
g) in refluxing benzene with use of a Dean-Stark apparatus. After
cooling, the benzene was removed under reduced pressure and the
resulting solid was dissolved in dichloromethane (100 mL) and
washed with aqueous NaHCO3 (2 × 25 mL) and brine (1 × 25
mL) and dried over MgSO4. Evaporation of the solvent and
crystallization of the resulting solid from a mixture of dichlo-
romethane/hexanes afforded the diborate ester of 4 as a white solid.
Yield 1.82 g, 60%; mp 219-220 °C; 1H NMR (CDCl3) δ 1.22 (s,
24H), 7.31-7.49 (m, 10H), 7.71-7.73 (m, 2H); 13C NMR (CDCl3)
δ 24.8, 83.9, 126.4, 128.8, 129.2, 130.2, 134.6, 142.0, 147.5.
Preparation of Diketones 6a-d: Typical Procedure. A solu-
tion of 2,2′′-p-terphenyldiboronic acid (4; 1.27 g, 4 mmol) in
dioxane (30 mL) in the presence of degassed 2 M aqueous sodium
carbonate (15 mL), 4-bromobenzophenone (2.61 g, 10 mmol), and
Alternate Preparation of the Diketone 6b with the Route
in Scheme 3. Preparation of acetal 7: Reaction of 4-bromohex-
anophenone6 (5b; 6.4 g, 25 mmol) with 2,2-dimethyl-1,3-pro-
panediol (7.82 g, 75 mmol) in the presence of a catalytic amount
of p-toluenesulfonic acid (0.2 g) in toluene (130 mL) under standard
conditions afforded pure acetal 7 (7.31 g, 86%) as a viscous liquid,
after column chromatography with 9:1 hexanes-ethyl acetate as
1
eluent. H NMR (CDCl3) δ 0.47 (s, 3H), 0.73 (t, J ) 7 Hz, 3H),
1.07-1.26 (m, 9H), 1.57-1.63 (m, 2H), 3.27 (s, 4H), 7.17 (d, J )
8 Hz, 2H), 7.40 (d, J ) 8 Hz, 2H); 13C NMR (CDCl3) δ 14.3,
22.0, 22.8, 23.1, 30.2, 32.1, 44.7, 71.7, 101.5, 121.8, 129.4, 131.7,
139.4. Preparation of borate ester 8: A freshly prepared solution
of Grignard reagent [from 7 (4.44 g, 13 mmol) and excess Mg
turnings (0.94 g, 39 mmol) in anhydrous THF (30 mL)] was
transferred with the aid of cannula to a Schlenk flask containing a
solution of trimethylborate (2.0 mL, 17 mmol) in anhydrous THF
(25 mL) at -78 °C. The resulting mixture was allowed to warm to
room temperature during a 12 h period and was then treated with
6 N HCl (15 mL) and stirred for an additional 4 h. The organic
layer was separated and the aqueous layer was extracted with diethyl
ether (3 × 50 mL). The combined organic extracts were dried over
anhydrous MgSO4, filtered, and evaporated. Purification of the crude
solid by column chromatography on silica gel with hexane-ethyl
acetate as eluent afforded the pure borate ester 8 as a white solid
1
(3.3 g, 84%): mp 52-53 °C; H NMR (CDCl3) δ 0.90 (t, J ) 7
Hz, 3H), 1.02 (s, 6H), 1.36 (s, 4H), 1.73 (t, J ) 7 Hz, 2H), 2.96 (t,
J ) 7 Hz, 2H), 3.78 (s, 4H), 7.86 (d, J ) 8 Hz, 2H), 7.91 (d, J )
8 Hz, 2H); 13C NMR (CDCl3) δ 12.7, 20.6, 21.3, 22.8, 30.3, 30.6,
37.5, 71.1, 125.7, 132.7, 137.3, 199.8. Preparation of diketone
6b: To a Schlenk flask was added borate ester 8 (3.0 g, 9.8 mmol)
and dioxane (60 mL) under an argon atmosphere and the flask was
subjected to three cycles of alternate evacuation and purging with
argon. Degassed 2 M aqueous sodium carbonate (20 mL), 2,2′′-
dibromo-p-terphenyl (3; 1.75 g, 4.5 mmol) and Pd(PPh3)4 (0.15 g,
0.13 mmol) were then added successively under an argon atmo-
(22) Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071–4074.
(23) Fujioka, Y. Bull. Chem. Soc. Jpn. 1984, 57, 3494.
2086 J. Org. Chem. Vol. 74, No. 5, 2009

Synthesis of ElectroactiVe Stilbenoprismands
sphere. The resulting mixture was refluxed for 17 h, cooled to room
temperature, and quenched with 5% aqueous hydrochloric acid (50
mL). The organic layer was separated and the aqueous layer was
extracted with dichloromethane (3 × 50 mL). The combined organic
extracts were dried over anhydrous magnesium sulfate and filtered.
Evaporation of the solvent afforded crude 6b, which was purified
by crystallization from a mixture of chloroform/methanol to give
6b as a white crystalline solid (1.85 g, 71%), which was identical
with the sample of 6b obtained above according to the Scheme 1.
Preparation of Stilbenoprismands SB1-4: Typical
Procedure. To a chilled (∼0 °C) Schlenk flask containing
anhydrous tetrahydrofuran (150 mL) was added TiCl4 (1.2 mL, ∼11
mmol) dropwise with the aid of a dropping funnel under an argon
atmosphere. To this mixture was then added Zn dust (1.0 g, 15.3
mmol) and dry pyridine (0.05 mL, 0.6 mmol) and a black
suspension thus obtained was warmed to room temperature and
refluxed for an additional 2 h. A solution of the diketone 6b (1.04
g, 1.80 mmol) in THF (80 mL) was added dropwise to the above
mixture during a course of 20 h while refluxing, and the resulting
mixture was refluxed for an additional 12 h. The reaction mixture
was cooled to room temperature and quenched with 10% aqueous
K2CO3 (50 mL). The organic layer was separated, and the aqueous
suspension was extracted with dichloromethane (5 × 25 mL). The
combined organic extracts were dried over anhydrous MgSO4,
filtered, and evaporated to afford a syrupy liquid that was purified
by flash chromatography on silica gel by using a 1:9 mixture of
ethyl acetate/hexanes to afford pure SB2 (0.91 g, 93%) as a white
(CDCl3) δ 14.4, 22.8, 28.3, 32.1, 34.4, 127.0, 127.1, 128.9, 129.06,
129.09, 129.2, 129.5, 139.0, 139.1, 139.7, 141.8, 141.9, 142.3.
Following a similar procedure as above, SB1, SB3, and SB4
were prepared and their spectral data are summarized below:
SB1: yield 0.27 g, 91% (after chromatography with hexanes-ethyl
1
acetate as eluent); H NMR (CDCl3) δ 2.15 (s, 6H), 6.81-6.87
(m, 8H), 6.92 (s, 4H), 7.37-7.39 (m, 8H); 13C NMR (CDCl3) δ
21.2, 127.1, 128.4, 129.1 129.2, 129.3, 129.5, 134.0, 139.2, 139.8,
142.0, 142.2, 143.2. SB3: yield 0.93 g, 98% (after crystallization
from a mixture of dichloromethane/acetonitrile); mp 308-309 °C;
1H NMR (CDCl3) δ 6.89 (s, 8H), 7.02 (s, 4H), 7.11 (s, 10H),
7.38-7.41 (m, 8H); 13C NMR (CDCl3) δ 126.8, 127.1, 127.2, 127.9,
129.1, 129.3, 129.4, 129.5, 130.4, 131.3, 139.6, 140.0, 141.7,
142.03, 142.09, 142.5, 142.8. SB4: yield 0.89 g, 96% (after
crystallization from a mixture of dichloromethane/acetonitrile); mp
303-304 °C; 1H NMR (CDCl3) δ 3.76 (s, 6H), 6.68 (d, J ) 8 Hz,
4H), 6.87 (s, 8H), 7.02 (d, J ) 7 Hz, 8H), 7.37-7.40 (m, 8H); 13
C
NMR (CDCl3) δ 55.3, 113.3, 127.0, 127.2, 129.1, 129.3, 129.5,
130.5, 132.5, 135.6, 139.4, 140.0, 140.2, 142.0, 142.1, 143.0, 158.3.
Acknowledgment. We thank the National Science Founda-
tion (Career Award) for financial support.
Supporting Information Available: General experimental
1
methods, H/13C NMR spectra, the ORTEP diagrams, and the
X-ray crystallographic data for various compounds. This mate-
rialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
1
crystalline solid. Mp 142-143 °C; H NMR (CDCl3) δ 0.92 (t, J
) 5 Hz, 6H), 1.36-1.43 (m, 12H), 2.54 (t, J ) 5 Hz, 4H),
6.82-6.88 (m, 8H), 6.96 (s, 4H), 7.41-7.45 (m, 8H); 13C NMR
JO802579F
J. Org. Chem. Vol. 74, No. 5, 2009 2087