Synthesis of conformationally stable 1,8-diarylnaphthalenes: Development of new photoluminescent sensors for ion-selective recognition

Article abstract of DOI:10.1021/ja0358145

Highly constrained 1,8-diarylnaphthalenes exhibiting stability to isomerization have been prepared utilizing two consecutive CuO-promoted Stille cross-couplings of 1,8-dibromonaphthalene and 4-alkyl-9-trimethylstannylacridines, Screening of Pd catalysts Pd(PPh ₃)₄, PdCl₂dppf, or Pd₂(dba) ₃/t-Bu₃P and bases such as Cy₂NMe, t-BuOK, K₃PO₄, and Cs₂CO₃ in DME or DMF revealed superior results of Stille over Suzuki coupling with acridylboronic acids or pinacolate derivatives. The meso syn- and C₂-symmetric antiisomers of 1,8-bis(4,4′-dimethyl-9,9′-diacridyl)naphthalene, 2, and 1,8-bis(4,4′-diisopropyl-9,9′-diacridyl)-naphthalene, 3, did not show any sign of syn/anti-interconversion after heating to 180 °C for 24 h, Using the Eyring equation, we calculated the Gibbs standard activation energy for isomerization, ΔG^o?, to be higher than 180 kJ/mol. PM3 calculations of 2 and 3 suggest a highly congested structure exhibiting two parallel acridyl moieties perpendicular to the naphthalene ring. UV and fluorescence spectroscopy studies of 2 and 3 revealed remarkable quantum yields of these blue and green light emitters, Fluorescence titration experiments with the syn-isomer of 2 showed highly efficient quenching by Cu(II) ions, whereas almost no quenching effects were observed with Cu(I) and Zn(II) salts. The striking difference in fluorescence quenching was attributed to significant photoinduced electron transfer, resulting in nonradiative relaxation of excited Cu(II)-syn-2, Stern-Voelmer plots of syn-2 in the presence of CuCl₂ showed a sigmoidal quenching curve indicating cooperative recognition, whereas a linear response was observed with CuCl and ZnCl₂, Fluorescence experiments in the presence of various amounts of CuCl, CuBr, and Cu(ACN)₄BF₄ proved that the quenching is cation selective and independent of the nature of counteranions.

Full text of DOI:10.1021/ja0358145

Published on Web 07/29/2003
Synthesis of Conformationally Stable 1,8-Diarylnaphthalenes:
Development of New Photoluminescent Sensors for
Ion-Selective Recognition
Christian Wolf* and Xuefeng Mei
Contribution from the Department of Chemistry, Georgetown UniVersity,
Washington, D.C. 20057
Received April 27, 2003; Revised Manuscript Received June 16, 2003; E-mail: cw27@georgetown.edu
Abstract: Highly constrained 1,8-diarylnaphthalenes exhibiting stability to isomerization have been prepared
utilizing two consecutive CuO-promoted Stille cross-couplings of 1,8-dibromonaphthalene and 4-alkyl-9-
trimethylstannylacridines. Screening of Pd catalysts Pd(PPh3)4, PdCl2dppf, or Pd2(dba)3/t-Bu3P and bases
such as Cy2NMe, t-BuOK, K3PO4, and Cs2CO3 in DME or DMF revealed superior results of Stille over
Suzuki coupling with acridylboronic acids or pinacolate derivatives. The meso syn- and C2-symmetric anti-
isomers of 1,8-bis(4,4′-dimethyl-9,9′-diacridyl)naphthalene, 2, and 1,8-bis(4,4′-diisopropyl-9,9′-diacridyl)-
naphthalene, 3, did not show any sign of syn/anti-interconversion after heating to 180 °C for 24 h. Using
the Eyring equation, we calculated the Gibbs standard activation energy for isomerization, ∆G°q, to be
higher than 180 kJ/mol. PM3 calculations of 2 and 3 suggest a highly congested structure exhibiting two
parallel acridyl moieties perpendicular to the naphthalene ring. UV and fluorescence spectroscopy studies
of 2 and 3 revealed remarkable quantum yields of these blue and green light emitters. Fluorescence titration
experiments with the syn-isomer of 2 showed highly efficient quenching by Cu(II) ions, whereas almost no
quenching effects were observed with Cu(I) and Zn(II) salts. The striking difference in fluorescence quenching
was attributed to significant photoinduced electron transfer, resulting in nonradiative relaxation of excited
Cu(II)-syn-2. Stern-Vo¨lmer plots of syn-2 in the presence of CuCl2 showed a sigmoidal quenching curve
indicating cooperative recognition, whereas a linear response was observed with CuCl and ZnCl2.
Fluorescence experiments in the presence of various amounts of CuCl, CuBr, and Cu(ACN)4BF4 proved
that the quenching is cation selective and independent of the nature of counteranions.
Introduction
The unique stereochemical, electronic, and photochemical
properties of sterically congested aromatic compounds has
attracted considerable attention during recent years because they
afford promising optoelectronic devices, rotors, and chemical
sensors.1 The preparation of conformationally stable 1,8-
diarylnaphthalenes has been an unresolved challenge since
Clough and Roberts reported the synthesis of atropisomeric 1,8-
bis(2,2′-dimethyl-1,1′-diphenyl)naphthalene, 1, almost 30 years
ago (Figure 1).2 Exhibiting an energy barrier to isomerization
of 100 kJ/mol, 1 remains the most stable atropisomer of this
class of compounds reported to date. Despite the variety of cross-
coupling procedures that has been developed for the synthesis
of biaryls in recent years, coupling reactions using highly
hindered aromatic compounds has rarely been achieved.3 A
Figure 1. Structure of 1 exhibing antiparallel (anti-isomer) or parallel (syn-
isomer) 2-methylphenyl moieties.
number of aryl4 and hetaryl5 groups has been introduced into
the peri-positions of naphthalene by us and others to study the
energy barrier to rotation about the naphthyl-aryl bond. Incor-
(3) Yin, J.; Rainka, M. P.; Zhang, X.-X.; Buchwald, S. L J. Am. Chem. Soc.
2002, 124, 1162-1163 and references therein.
(1) (a) Qiao, X.; Padula, M. A.; Ho, D. M.; Vogelaar, N. J.; Schutt, C. E.;
Pascal, R. A., Jr. J. Am. Chem. Soc. 1996, 118, 741-745. (b) Dominguez,
Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc.
2002, 124, 7719-7727. (c) Charbonniere, L. J.; Ziessel, R.; Montalti, M.;
Prodi, L.; Zaccheroni, N.; Boehme, C.; Wipff, G. J. Am. Chem. Soc. 2002,
124, 7779-7788. (d) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-
F.; Lin, Y.-T.; Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su,
Y. O.; Lee, G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576-
11577. (e) Rathore, R.; Deselnicu, M. I.; Burns, C. L. J. Am. Chem. Soc.
2002, 124, 14832-14833.
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28, 2403-2406. (b) House, H. O.; Bashe, R. W. J. Org. Chem. 1965, 30,
2942-2947. (c) House, H. O.; Bashe, R. W. J. Org. Chem. 1967, 32, 784-
791. (d) House, H. O.; Campbell, W. J.; Gall, M. J. Org. Chem. 1970, 35,
1815-1819. (e) House, H. O.; Koepsell, D. G.; Campbell, W. J. J. Org.
Chem. 1972, 37, 1003-1011. (f) Cozzi, F.; Cinquini, M.; Annunziata, R.;
Dwyer, T.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 5729-5733. (g)
Cozzi, F.; Cinquini, M.; Annunziata, R.; Siegel, J. S. J. Am. Chem. Soc.
1993, 115, 5330-5331. (h) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini,
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Ernst, L.; Sakhaii, P. Magn. Reson. Chem. 2000, 38, 559-565.
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9
10.1021/ja0358145 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 10651-10658
10651

A R T I C L E S
Wolf and Mei
purification. Flash chromatography was carried out on silica gel (particle
size 0.032-0.063 mm). NMR spectra were obtained at 300 MHz (1H
NMR) and 75 MHz (13C NMR), using CDCl3 as the solvent. Chemical
shifts are reported in ppm relative to TMS. Elemental analysis data
were collected using a Perkin-Elmer 2400 CHN. UV measurements
were performed on an Agilent 8453 UV-visible spectrometer. Extinc-
tion coefficients were determined from five different measurements in
CH2Cl2. Absorption and emission spectra were collected under nitrogen.
Fluorescence experiments were conducted using a Fluoromax-2 from
Instruments S. A., Inc. Quantum yields of 2 and 3 were determined in
benzene following literature procedures.10 1,8-Bis(4,4′-dialkyl-9,9′-
diacridyl)naphthalenes were excited at 381 nm, and relative integrated
intensities of the emission spectra were compared to anthracene. The
quantum efficiency of anthracene in benzene (25.6%) was taken from
the literature.
poration of ortho- or meta-substituted aryl moieties into both
peri-positions of naphthalene results in two chiral anti-isomers
and one meso syn-isomer. Computational studies and X-ray
analysis have shown that the peri-aryl rings are coplanar and
almost perpendicular to the naphthalene moiety in the ground
state. In contrast to the significant steric hindrance to isomer-
ization suggested by CPK models, 1,8-diarylnaphthalenes such
as 1 are not stable to interconversion at room temperature. The
development of a synthetic route toward conformationally stable,
atropisomeric 1,8-diarylnaphthalenes would facilitate studies of
intramolecular interactions between stacked aryl groups6 and
has been considered an entry to a new class of chiral ligands
for asymmetric catalysis.7
To date, all attempts to prepare 1,8-diarylnaphthalenes
exhibiting conformational stability have been unsuccessful
because of the severe steric repulsion that one can expect during
the construction of such a highly constrained framework. On
the basis of ab initio calculations, Thirsk et al. recently predicted
a rotational energy barrier of approximately 160 kJ/mol to
isomerization for ortho-substituted 2,7-diisopropoxy-1,8-di-
arylnaphthalenes. However, their attempts to synthesize such
conformationally stable atropisomers using Suzuki cross-
coupling were not successful.8
Herein, we wish to report the synthesis of 1,8-diarylnaph-
thalenes exhibiting conformational stability to isomerization
even at high temperatures. We assumed that introduction of
acridyl moieties into the peri-positions of naphthalene would
result in a rigid scaffold that renders rotation of the aryl rings
about the acridylnaphthalene axis impossible. Careful incorpora-
tion of substitutents into the fluorescent acridyl rings was
expected to afford bidentate selectors exhibiting well-defined
pockets for coordination to Lewis or Bronsted acids. Selective
interactions between this new class of chemosensors and metal
ions or other substrates would be measurable by sensitive
fluorescence spectroscopy and allow real-time monitoring of
trace analytes. Although a variety of fluorescent sensors for
alkali, alkaline earth, and transition metals has been developed,
high ion selectivity and the ability to differentiate between
different oxidation states remain a challenge.9
Synthetic Procedures. 1,8-Bis(4,4′-dimethyl-9,9′-diacridyl)naph-
thalene, 2. A mixture of 1,8-dibromonaphthanene 1 (92 mg, 0.32
mmol), tetrakis(triphenylphosphine)palladium(0) (0.11 g, 0.10 mmol,
30 mol %), and CuO (51 mg, 0.64 mmol) in 10 mL DMF was stirred
at 140 °C. After 5 min, a solution of 4-methyl-9-trimethylstannanyl
acridine 12 (0.46 g, 1.29 mmol) dissolved in 2 mL of DMF was added
in one portion. After 16 h, the reaction mixture was quenched with
10% aqueous ammonium hydroxide, extracted with diethyl ether, dried
over MgSO4, and concentrated in vacuo. Purification of the orange
residue by flash chromatography (100:5:1 hexanes/ethyl acetate/
triethylamine) afforded 2 (41 mg, 25%) as a yellow solid. The
diastereoisomers were separated on a phenylglycine column (250 mm
× 4.6 mm) using hexanes/EtOH (98:2) as the mobile phase. The anti-
1
and syn-conformation of the two isomer of 2 was determined by H
NMR spectroscopy using 1.2 mol equivalents of (+)-Eu(tfc)3 as a chiral
shift reagent.11
anti-Isomer: 1H NMR δ ) 2.72 (s, 6H), 6.50 (d, J ) 8.8 Hz, 1H),
6.52 (d, J ) 8.8 Hz, 1H), 6.58-6.78 (m, 6H), 7.18 (d, J ) 6.6 Hz,
2H), 7.22-7.27 (m, 2H), 7.35 (ddd, J ) 1.7 Hz, J ) 7.8 Hz, J ) 8.8
Hz, 2H), 7.68-7.73 (m, 4H), 8.25 (dd, J ) 1.3 Hz, J ) 8.8 Hz, 2H).
13C NMR δ ) 18.87, 123.75, 124.67, 125.00, 125.70, 125.74, 128.18,
128.37, 129.72, 129.87, 130.58, 130.73, 134.37, 135.10, 135.90, 146.73,
146.81, 146.04, 146.32. LC/APCI/MS: m/z ) 511 (M + H).
syn-Isomer: 1H NMR δ ) 2.67 (s, 6H), 6.45-6.52 (m, 2H), 6.58-
6.70 (m, 4H), 6.80 (d, J ) 8.8 Hz 2H), 7.20-7.40 (m, 6H), 7.65-7.78
(m, 4H), 8.25 (dd, J ) 1.3 Hz, J ) 8.8 Hz, 2H). 13C NMR δ ) 18.94,
123.42, 124.67, 125.00, 125.73, 125.76, 128.21, 128.41, 129.73, 129.85,
130.58, 130.73, 134.37, 135.10, 135.90, 146.72, 146.81, 146.04, 146.32.
LC/APCI/MS: m/z ) 511 (M + H). Anal. Calcd for syn- and anti-
C38H26N2: C, 89.38; H, 5.13; N, 5.49. Found: C, 89.80; H, 5.57; N,
4.99.
Experimental Section
General Procedures. All reaction were carried out under nitrogen.
1,8-Bis(4,4′-diisopropyl-9,9′-diacridyl)naphthalene, 3. A mixture
1,8-dibromonaphthanene 1 (0.25 g, 0.89 mmol), tetrakis(triphenylphos-
Commercially available reagents and solvents were used without further
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Tetrahedron 1997, 53, 465-468. (d) Zoltewicz, J. A.; Maier, N. M.; Fabian,
W. M. F. J. Org. Chem. 1997, 62, 2763-2766. (e) Zoltewicz, J. A.; Maier,
N. M.; Fabian, W. M. F. J. Org. Chem. 1997, 62, 3215-3219. (f) Zoltewicz,
J. A.; Maier, N. M.; Lavieri, S.; Ghiviriga, I.; Abboud, K. A. Tetrahedron
1997, 53, 5379-5388. (g) Lavieri, S.; Zoltewicz, J. A. J. Org. Chem. 2001,
66, 7227-7230. (h) Wolf, C.; Ghebremariam, B. T. Synthesis 2002, 749-
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Soumillion, J.-P.; Valeur, B. Chem. Commun. 1999, 795-796. (d) Valeur,
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(g) Singh, A.; Yao, Q.; Tong, L.; Clark Still, W.; Sames, D, Tetrahedron
Lett. 2000, 41, 9601-9605. (h) Prodi, L.; Montalti, M.; Zaccheroni, N.;
Dallavalle, F.; Folesani, G.; Lanfranchi, M.; Corradini, R.; Pagliari, S.;
Marchelli, R. HelV. Chim. Acta 2001, 84, 690-706. (i) Burdette, S. C.;
Walkup, G. K.; Spingler, B.; Tsien, R. Y.; Lippard, S. J. Am. Chem. Soc.
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(11) See Supporting Information.
9
10652 J. AM. CHEM. SOC. VOL. 125, NO. 35, 2003

Synthesis of Stable 1,8-Diarylnaphthalenes
A R T I C L E S
phine)palladium(0) (0.31 g, 0.27 mmol, 30 mol %), and CuO (0.14 g,
1.78 mmol) in 18 mL DMF was stirred at 140 °C. After 5 min, a
solution of 4-isopropyl-9-trimethylstannanyl acridine 12 (1.45 g, 3.8
mmol) dissolved in 2 mL DMF was added in one portion. After 16 h,
the reaction mixture was quenched with 10% aqueous ammonium
hydroxide, extracted with diethyl ether, dried over MgSO4, and
concentrated in a vacuum. Purification of the orange residue by flash
chromatography (100:5:1 hexanes/ethyl acetate/trimethylamine) af-
forded 3 (126 mg, 25%) as a yellow solid. The diastereoisomers were
separated on a phenylglycine column (250 mm × 4.6 mm) using
hexanes/EtOH (98.4:1.6) as the mobile phase.
Isomer 1: 1H NMR δ ) 1.22 (d, J ) 6.9 Hz, 6H), 1.52 (d, J ) 6.9
Hz, 6H), 4.23 (sept, J ) 6.9 Hz, 2H), 6.60-6.70 (m, 6H), 6.85 (d, J
) 8.4 Hz, 2H), 7.18-7.30 (m, 6H), 7.60-7.78 (m, 4H), 8.26 (dd, J )
1.6 Hz, J ) 8.4 Hz, 2H). 13C NMR δ ) 24.38, 27.25, 27.33, 123.58,
123.96, 124.68, 124.74, 124.74, 125.54, 125.58, 125.87, 128.21, 129.72,
129.90, 130.92, 134.93, 135.10, 144.95, 145.01, 145.61, 145.97, 146.79.
LC/APCI/MS: m/z ) 567 (M + H).
Isomer 2: 1H NMR δ ) 1.22 (d, J ) 6.9 Hz, 6H), 1.52 (d, J ) 6.9
Hz, 6H), 4.23 (sept, J ) 6.9 Hz, 2H), 6.59-6.78 (m, 6H), 7.20-7.37
(m, 8H), 7.64-7.72 (m, 4H), 8.26 (dd, J ) 1.6 Hz, J ) 8.4 Hz, 2H).
13C NMR δ ) 24.62, 26.98, 27.08, 123.54, 123.95, 124.60, 124.71,
125.16, 125.52, 125.74, 128.09, 129.69, 129.93, 130.79, 134.59, 134.70,
144.95,145.01, 145.57, 145.77, 146.79. LC/APCI/MS: m/z ) 567 (M
+ H). Anal. Calcd for syn- and anti-C42H34N2: C, 89.01; H, 6.05; N,
4.94. Found: C, 89.38; H, 6.25; N, 4.67.
2-(2′-Methylphenylamino)benzoic Acid, 7. A mixture of 2-methy-
laniline (2.68 g, 25 mmol), 2-chlorobenzoic acid (3.8 g, 24 mmol),
K2CO3 (4.1 g, 30 mmol), Cu powder (0.05 g), Cu2O (0.05 g), and 5
mL of 2-methoxyethanol was refluxed for 2 h. The cooled reaction
mixture was poured into 30 mL of water. Charcoal was then added,
and the solution was filtrated through Celite. The crude product was
obtained by acidification of the filtrate with diluted HCl at ambient
temperature, and subsequent recrystallization from acetone/water (1:
8). The crystals were dissolved in 100 mL of 5% aqueous Na2CO3.
The solution was filtered through Celite, and the product was
precipitated by acidification to afford acid 7 (3.0 g, 55%) as a white
powder. 1H NMR δ ) 2.29 (s, 3H), 6.72 (bs, 1H), 6.85 (d, J ) 8.2 Hz,
1H), 7.12 (dd, J ) 7.2 Hz, J ) 7.4 Hz, 1H), 7.20-7.34 (m, 5H), 8.05
(d, J ) 7.2 Hz, 1H), 9.18 (bs, 1H). 13C NMR δ ) 18.96, 114.4, 117.3,
125.67, 125.98, 127.22, 127.49, 131.61, 131.85, 134.10, 135.79, 135.97,
139.16, 150.35.
2-(2′-Isopropylphenylamino)benzoic Acid, 8. A mixture of 2-iso-
propylaniline (3.4 g, 25 mmol), 2-chlorobenzoic acid (3.8 g, 24 mmol),
K2CO3 (4.1 g, 30 mmol), Cu powder (0.05 g), and Cu2O (0.05 g) in 5
mL of 2-methoxyethanol was refluxed for 2 h. The cooled reaction
mixture was poured into 30 mL of water. Charcoal was then added,
and the solution was filtrated through Celite. The crude product was
obtained by acidification of the filtrate with diluted HCl at ambient
temperature and by subsequent recrystallization from acetone/water (1:
8). The crystals were dissolved in 100 mL of 5% aqueous Na2CO3 and
filtered through Celite, and the crystals were recrystallized by acidifica-
tion to yield acid 8 (4.4 g, 73%) as a white powder. 1H NMR δ ) 1.22
(d, J ) 6.9 Hz, 6H), 3.21 (sept, J ) 6.9 Hz, 1H), 4.68 (bs, 1H), 6.68
(dd, J ) 7.2 Hz, J ) 7.4 Hz, 1H), 6.81 (d, J ) 8.2 Hz, 1H), 7.22-7.40
(m, 4H), 8.1 (dd, J ) 1.7 Hz, J ) 8.2 Hz, 1H), 9.18 (s, 1H). 13C NMR
δ ) 23.97, 28.83, 114.38, 117.09, 126.72, 126.90, 127.13, 127.31,
133.08, 135.68, 136.04, 137.84, 145.09, 151.20, 174.73.
9-Bromo-4-methylacridine, 9. 2-(2′-Methylphenylamino)benzoic
acid 7 (1.0 g, 4.4 mmol) was suspended in 11.0 g (38 mmol) of
phosphorus oxybromide, and the mixture was heated to 120 °C for 2
h. Excess phosphorus oxybromide was removed by distillation, and
the residual solution was poured into a 1:1 mixture of aqueous
ammonium hydroxide/CH2Cl2. The CH2Cl2 solution was separated,
dried, and filtered, and the combined organic layers were dried in vacuo
to give 9 (1.0 g, 85%) as a yellow powder. 1H NMR δ ) 2.94 (s, 3H),
7.53 (dd, J ) 8.5 Hz, J ) 8.8 Hz, 1H), 7.59-7.69 (m, 2H), 7.78 (ddd,
J ) 1.4 Hz, J ) 8.5 Hz, J ) 8.5 Hz, 1H), 8.28 (dd, J ) 8.5 Hz, J )
8.5 Hz, 2H), 8.43 (dd, J ) 1.4 Hz, J ) 8.8 Hz, 1H). 13C NMR δ )
19.48, 126.37, 128.49, 128.76, 129.07, 129.28, 129.72, 130.95, 131.15,
131.88, 135.87, 138.12, 148.43, 148.71. Anal. Calcd for C14H10NBr:
C, 61.79; H, 3.70; N, 5.15. Found: C, 61.40; H, 3.72; N, 5.05.
9-Bromo-4-isopropyl-acridine, 10. 2-(2′-Isopropylphenylamino)-
benzoic acid 8 (1.0 g, 3.9 mmol) was suspended in 11.0 g (38 mmol)
of phosphorus oxybromide, and the mixture was heated to 120 °C for
2 h. Excess phosphorus oxybromide was removed by distillation, and
the residual solution was poured into a 1:1 mixture of aqueous
ammonium hydroxide/CH2Cl2. The CH2Cl2 solution was separated,
dried, and filtered, and the combined organic layers were dried in a
vacuum to give 10 (1.2 g, 79%) as yellow powder. 1H NMR δ ) 1.45
(d, J ) 6.9 Hz, 6H), 4.56 (sept, J ) 6.9 Hz, 1H), 7.56-7.70 (m, 3H),
7.78 (ddd, J ) 1.5 Hz, J ) 6.6 Hz, J ) 6.6 Hz, 1H), 8.24 (d, J ) 8.4
Hz, 1H), 8.29 (dd, J ) 1.5 Hz, J ) 8.5 Hz, 1H), 8.41 (d, J ) 8.8 Hz,
1H). 13C NMR δ ) 24.00, 28.21, 125.20, 125.62, 125.86, 126.49,
127.38, 127.64, 129.90, 130.58, 130.83, 135.69, 147.44, 148.01, 148.14.
Anal. Calcd for C16H14NBr: C, 64.02; H, 4.70; N, 4.67. Found: C,
64.23; H, 4.78; N, 4.59.
4-Methyl-9-trimethylstannanylacridine, 11. A solution of 9-bromo-
4-methylacridine 9 (1 g, 3.7 mmol) in 50 mL of anhydrous diethyl
ether was cooled to -78 °C under nitrogen. To the solution were added
1.6 M n-BuLi in hexanes (0.74 mmol, 0.46 mL) dropwise over a period
of 15 min and a 1.0 M solution of Me3SnCl in hexanes (0.81 mL, 0.81
mmol). The reaction solution mixture was allowed to warm to room
temperature, stirred for 18 h, and concentrated in vacuo. Purification
of the orange residue by flash chromatography (100:10:1 hexanes/ethyl
acetate/triethylamine) afforded 11 (1.1 g, 84%) as a yellow solid. GC-
MS revealed contamination of the product with 5-10% 4-methylacri-
dine that could not be separated by chromatography. The stannane was
therefore employed in the Stille coupling with 1,8-dibromonaphthalene
1
without further purification. H NMR δ ) 0.67 (s, 9H), 2.95 (s, 3H),
7.41 (dd, J ) 6.9 Hz, J ) 8.8 Hz, 1H), 7.52 (ddd, J ) 1.4 Hz, J ) 6.5
Hz, J ) 6.5 Hz, 1H), 7.61 (d, J ) 6.9 Hz, 1H), 7.74 (ddd, J ) 1.4 Hz,
J ) 7.4 Hz, J ) 7.4 Hz, 1H), 7.97 (d, J ) 7.9 Hz, 1H), 8.12 (d, J )
9.3 Hz, 1H), 8.28 (d, J ) 7.9 Hz, 1H). 13C NMR δ ) -4.63, 19.51,
125.33, 125.56, 128.42, 128.94, 129.30, 129.93, 131.38, 133.49, 133.62,
138.58, 147.36, 147.53, 156.78.
4-Isopropyl-9-trimethylstannylacridine, 12. Stannane 12 (1.4 g,
3.5 mmol) was obtained in 90% yield using 9-bromo-4-isopropyl-
acridine 10 (1.2 g, 3.9 mmol), 1.6 M n-BuLi in hexanes (2.6 mL, 4.2
mmol), and a 1.0 M solution of Me3SnCl in hexanes (4.5 mL, 4.5
mmol), following the procedure described for the preparation of 11.
GC-MS revealed contamination of the product with 5-10% 4-iso-
propylacridine that could not be separated by chromatography. The
stannane was therefore employed in the Stille coupling with 1,8-
1
dibromonaphthalene without further purification. H NMR δ ) 0.71
(s, 9H), 1.51 (d, J ) 6.9 Hz, 6H), 4.69 (sept, J ) 6.9 Hz, 1H), 7.48-
7.60 (m, 2H), 7.67 (d, J ) 6.4 Hz, 1H), 7.76 (ddd, J ) 1.4 Hz, J ) 6.0
Hz, J ) 6.0 Hz, 1H), 8.02 (dd, J ) 1.1 Hz, J ) 8.6 Hz, 1H), 8.16 (d,
J ) 8.5 Hz, 1H), 8.33 (d, J ) 8.8 Hz, 1H). 13C NMR δ ) -4.05,
24.05, 28.02, 124.77, 125.37, 125.50, 128.05, 129.05, 129.95, 131.71,
133.33, 133.72, 146.16, 147.18, 148.72, 156.59.
Results and Discussion
Retrosynthetic analysis of 1,8-bis(4,4′-dimethyl-9,9′-diacridyl)-
naphthalene, 2, and 1,8-bis(4,4′-diisopropyl-9,9′-diacridyl)-
naphthalene, 3, suggested Stille or Suzuki cross-coupling of 1,8-
dibromo- or 1,8-diiodonaphthalene with a 4-substituted-9-acridyl
stannane or boronic acid derivative, which can be formed via
ring construction from 2-substituted anilines. We have found
that 1,8-bis(4,4′-dialkyl-9,9′-diacridyl)naphthalenes 2 and 3 can
be synthesized from readily available 2-chlorobenzoic acid, 4,
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A R T I C L E S
Wolf and Mei
Scheme 1. Synthesis of 1,8-Diacridylnaphthalenes 2 and 3
Table 1. Optimization Results of the Pd-Catalyzed
Cross-Coupling Reactions
find less than 10% of coupling byproducts 13, 14, and 15 under
these conditions (Table 1). Notably, Suzuki coupling of 1,8-
dibromonaphthalene and 4-isopropyl-9-acridylboronic acid or
its pinacolate derivative employing Pd(PPh3)4, PdCl2dppf, or
Pd2(dba)3/t-Bu3P as the catalyst as well as t-BuOK, K3PO4, or
Cs2CO3 as the base in DME and DMF, respectively, did not
result in the formation of the desired coupling product.
The observed cross-coupling byproducts are indicative of the
steric hindrance that occurs during the Pd-catalyzed reaction
between the intermediate 1-(4-isopropyl-9-acridyl)-8-bromonaph-
thalene, 16, and another stannane, 12 (Scheme 3). Oxidative
addition of 16 to the Pd catalysts provides a reactive Pd complex
17 that can undergo transmetalation, followed by reductive
elimination to yield Stille products 3 and 13 or intramolecular
coupling through Pd-activation of a peri-acridyl C-H bond to
form 14 and 15. The formation of carbon-carbon bonds via
metal-mediated C-H bond activation has been reported by
others and used as a powerful strategy for the synthesis of
complex compounds.14 Because of the steric hindrance that can
be expected between Pd complex 17 and a second stannyl
reagent 12, the transfer of a methyl group instead of the acridyl
moiety becomes a competitive side reaction that results in the
formation of the undesired coupling product 13. Since the
transmetalation between 17 and 12 is considered to be slow,
C-H insertion of 17 and subsequent reductive elimination afford
14 and 15.
b
c
a DMF, 140 °C, 18 h. DMF, 100 °C, 18 h. DMF, 140 °C, 5 h.
Scheme 2. Stille Cross-Coupling Products Obtained from
Stannane 12 and 1,8-Dihalonaphthalenes
Because of the severe steric hindrance and possible side
reactions inherent to their synthesis, highly constrained 1,8-
diarylnaphthalenes exhibiting conformational stability have not
been reported previously. The preparation of 2 and 3 in 25%
yield using our CuO-promoted Stille coupling procedure is quite
remarkable since it affords two consecutive coupling steps.
and anilines 5 and 6, respectively (Scheme 1). Commercially
available acid, 4, was converted to 4-alkyl-9-trimethylstannyl-
acridine, 11 and 12, in three steps with high overall yield. First,
we attempted Stille cross-coupling of stannane 12 and 1,8-
diiodonaphthalene using Pd(PPh3)4 as the catalysts. However,
14 and 15 were obtained as the only cross-coupling products
in addition to degradation products of stannane 11 (Scheme 2).12
We therefore decided to use 1,8-dibromonaphthalene13 in the
Stille reaction. Screening of various catalysts such as Pd(PPh3)4,
PdCl2dppf, or Pd2(dba)3/t-Bu3P and optimization of reaction
conditions revealed that employing Pd(PPh3)4 and CuO in DMF
at 140 °C affords 1,8-bis(4,4′-dialkyl-9,9′-diacridyl)naphthalenes
2 and 3 via Stille coupling of stannanes 11 or 12 with 1,8-
dibromonaphthalene in remarkable yields. We were pleased to
We were pleased to find that the diastereoisomers of 2 and
3 can be separated by HPLC on a (S)-phenylglycine column.15
Semipreparative separation allowed us to determine the anti/
syn ratio of 2 and 3 as 4.6:1 and 1:1, respectively.16 Isomer-
ization via rotation of one acridyl ring about the chiral
(14) (a) Dyker, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 103-105. (b) Reetz,
M. T.; Wanninger, K.; Hermes, M. J. Chem. Soc., Chem. Commun. 1997,
535-536. (c) Jia, C.; Kitamura, T.; Fujiwara, Y.; Acc. Chem. Res. 2001,
34, 633-639. (d) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002,
102, 1731-1769. (e) Dangel, B. D.; Godula, K.; Youn, S. W.; Sezen, B.;
Sames, D. J. Am. Chem. Soc. 2002, 124, 11856-11857.
(12) The structures of 14 and 15 were determined by NMR and LC/APCI/MS.
(13) Seyferth, D.; Vick, S. C. J. Organomet. Chem. 1977, 141, 178-187.
(15) Using a variety of chiral columns, we were not able to separate the
enantiomers of anti-2 and anti-3, respectively.
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Synthesis of Stable 1,8-Diarylnaphthalenes
A R T I C L E S
Scheme 3. Possible Transmetalation Pathways during the Second Catalytic Cycle of the Pd-Catalyzed Stille Coupling of
1-(4-Isopropyl-9-acridyl)-8-bromonaphthalene 16 and Stannane 12
acridylnaphthalene axis is highly restricted because of steric
hindrance. According to PM3 calculations, diacridyls 2 and 3
afford compact structures in which both acridyl rings are roughly
perpendicular to the mean plane of the naphthyl ring, with
dihedral angles ranging from 85° to 95°. The acridyl moieties
are slightly twisted away from each other and undergo enforced
π-stacking (Figure 2).17
The isolated isomers were heated in acetonitrile in a closed
vessel to 180 °C for 24 h. As expected, rotation of either acridyl
moiety about the chiral acridylnaphthalene axis of 2 and 3 is
highly sterically hindered, and HPLC analysis did not show any
sign of isomerization. Utilizing reversible first-order kinetics
and the Eyring equation, we calculated the Gibbs standard
activation energy, ∆G°q, for the isomerization of 2 and 3 to be
Figure 2. Space-filling model of anti-2 calculated by PM3.
higher than 180 kJ/mol.18
Investigation of the photochemical properties of 2 and 3
revealed UV absorption maxima at 246 nm (lg ꢀ ) 8.02 and
8.06, respectively). The UV spectra proved to be reminiscent
of isolated, nonconjugated naphthyl and acridyl chromophores
as one would expect for a structure exhibiting both hetaryl rings
perpendicular to the naphthalene plane. The UV data are thus
in agreement with the results of our PM3 calculations. The
fluorescence spectra of 2 and 3 were found to be significantly
red-shifted compared to that of acridine, which exhibits an
emission maximum around 400 nm in benzene. This may be
attributed to the enhanced π-stacking of the two acridyl rings.
Fluorescence studies of both isomers of diacridine 3 revealed
only one maximum at approximately 530 nm and a quantum
yield of 18%. In contrast, the fluorescence of the isomers of 2
(16) The isomer ratios of 2 and 3 determined after semipreparative HPLC
separation are in very good agreement with 1H NMR integration results of
the product mixtures. The syn- and anti-conformations of 2 were determined
by 1H NMR, using a chiral Lanthanide shift reagent. The formation of
diastereomeric complexes of the C2-symmetric anti-isomers of 2 exhibiting
anisochronous signals was observed in the presence of (+)-Eu(tfc)3, whereas
the signals of the methyl protons of the meso form are downfield-shifted
but are still isochronous; see Supporting Information. The determination
of the conformation of the two isomers of 3 using NMR shift reagents or
NOESY experiments was not successful.
(17) (a) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc.,
Perkin Trans. 2 2002, 651-669. (b) Rashkin, M. J.; Waters, M. L. J. Am.
Chem. Soc. 2002, 124, 1860-1861.
(18) Decomposition of 2 and 3 was observed at 200 °C.
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A R T I C L E S
Wolf and Mei
Figure 3. PM3 computation of the ground state of anti- (left) and syn-3 (right).
proved to be significantly different. We found that syn-2 is a
blue light emitter with a fluorescence maximum at 460 nm,
whereas excited anti-2 emits green light at approximately 540
nm. Notably, syn-2 was found to be more fluorescent than its
diastereoisomeric anti-isomer. The quantum yields for syn- and
anti-2 were determined as 22% and 13%, respectively. The
striking difference in the flourescence behavior of the isomers
of 2 may be attributed to increased π-π interactions between
the anti-parallel acridyl rings of anti-2. The distance between
the acridyl nitrogens of syn-2 was determined as 3.6 Å on the
basis of PM3 optimization of the ground state. By contrast, PM3
calculations suggested a N-N distance of only 3.4 Å for anti-
2. We assume that the close proximity of the acridyl rings
facilitates nonradiative relaxation of excited anti-2, resulting in
a significantly lower quantum yield than its syn-isomer.
Computational studies of the ground state of syn- and anti-3
provided a N-N distance of 3.8 Å for both isomers, which
explains their indistinguishable fluorescence maximum and
quantum yield (Figure 3).
We chose to investigate the usefulness of this new class of
conformationally stable, bidentate 1,8-dihetarylnaphthalenes,
combining unique stereochemical and photoluminescent features
as selective sensors. The majority of fluorosensors developed
to date are macrocyclic structures exhibiting a chelating group
and a fluorophore physically separated by a spacer.19 However,
small sensors afford advantageous cell permeability properties
and are therefore particularly promising for biomedical trace
analysis. Because of its higher quantum yield, we decided to
employ the syn-isomer of 2 in metal ion-sensing studies.
Fluorescence emission titrations with syn-2 and CuCl and CuCl2
were performed in acetonitrile at room temperature. We did not
observe any significant red or blue shifts in the emission spectra
of syn-2 in the presence of the metal ions. Addition of CuCl
did not result in significant quenching even at high excess
(Figure 4). By contrast, titration of the same sensor with CuCl2
revealed highly efficient quenching (Figure 5).
Figure 4. Fluorescence quenching of syn-2 using various concentrations
of CuCl. Diacridylnaphthalene syn-2 was dissolved in acetonitrile at a
concentration of 10-6 M.
the diacridylnaphthalene sensor to Cu(II) exhibiting a d9-
electronic configuration is likely to result in considerable
fluorescence quenching because of photoinduced electron
transfer. Interestingly, we observed nonlinear Stern-Vo¨lmer
quenching of syn-2 by Cu(II) chloride. A fluorescence titration
experiment with CuCl2 gave a sigmoidal quenching curve,
indicating cooperative recognition such as formation of less
fluorescent agglomerates at a high Cu(II)/sensor ratio (Figure
6). Chemical sensing based on cooperative recognition has rarely
been observed and is believed to result in higher selectivity
compared to noncooperative sensing exhibiting a linear fluo-
rescence response.20 By contrast, photoinduced electron transfer
is not significant in Cu(I)-syn-2 because of the d10-electronic
configuration of the metal ion. Further titration experiments
revealed that the selector is also capable of differentiating
between CuCl2 and ZnCl2. Stern-Vo¨lmer plots of the two salts
show that Zn(II) barely induces fluorescence quenching of syn-2
(Figure 7). This may also be attributed to negligible photoin-
duced electron transfer of excited Zn(II)-syn-2. We conducted
UV titration experiments of syn- and anti-2 using Cu(II) chloride
for the determination of the stoichiometry of the metal-sensor
Because of its considerably different fluorescence response
to Cu(I) and Cu(II) ions, the syn-isomer of 2 may be used as a
highly selective sensor for real-time qualitative and quantitative
analysis of the oxidation state of copper ions in solution. Binding
to a metal ion opens new vibrational or electronic pathways
for nonradiative relaxation of excited syn-2. Coordination of
(19) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-
1566.
(20) Glass, T. E. J. Am. Chem. Soc. 2000, 122, 4522-4523 and references
therein.
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Synthesis of Stable 1,8-Diarylnaphthalenes
A R T I C L E S
Figure 8. UV titration of syn-2 with CuCl2. Inset: Job plot recorded at
225 nm. Sum of concentrations was fixed at 1.5 × 10-5 M.
Figure 5. Fluorescence quenching of syn-2 using various concentrations
of CuCl2. Diacridylnaphthalene syn-2 was dissolved in acetonitrile at a
concentration of 10-6 M.
Figure 9. UV titration of anti-2 with CuCl2. Inset: Job plot recorded at
225 nm. Sum of concentrations was fixed at 1.5 × 10-5 M.
Figure 6. Stern-Vo¨lmer plot of syn-2 in the presence of Cu(I) and Cu(II)
chloride. The concentration of syn-2 was 10-6 M.
Figure 10. Counterion effect on the fluorescence quenching of syn-2 in
acetonitrile. The concentration of syn-2 was 5 × 10-6 M.
of Cu(II) chloride and syn-2 or anti-2 at a total concentration
of 1.5 × 10-5 M revealed the existence of one maximum at a
molar ratio of 0.5, which is in agreement with the formation of
an equimolar complex (Figures 8 and 9).21 Notably, a job plot
affords the sensor/metal ratio but does not differentiate between
1:1 and 2:2 complexation or formation of even higher aggre-
grates, which would explain the cooperative recognition of Cu-
(II) observed with syn-2.
The selectivity between copper and zinc ions is quite
important for bioanalytical and environmental studies. Cu(II)
and Zn(II) are essential trace elements that occur in metallo-
proteins with various biological functions in bacteria, plants,
and mammals. Cu(II) is also a significant environmental
pollutant. An important requirement for a useful cation-selective
Figure 7. Stern-Vo¨lmer plot of syn-2 in the presence of Cu(II) and Zn-
(II) chloride. The concentration of syn-2 was 10-6 M.
complexes formed in acetonitrile (Figures 8 and 9). In ac-
cordance with our fluorescence experiments, we did not observe
any significant red or blue shifts in the absorption spectra of
the diacridylnaphthalenes in the presence of Cu(II). Job analysis
(21) Connors, K. A. Binding Constants, The Measurement of Molecular Complex
Stability; Wiley: New York, 1987.
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A R T I C L E S
Wolf and Mei
sensor is the absence of a significant fluorescence response to
anions also present in solution. We therefore selected three
different Cu(I) salts for fluorescence titration experiments to
determine any counteranion effects on the photochemical
properties of the sensor. Indeed, syn-2 did not show any
significant fluorescence quenching due to the presence of
chloride, bromide, or tetrafluoroborate (Figure 10). The Stern-
Vo¨lmer plots obtained with CuCl and CuBr are almost super-
imposable, whereas quenching by Cu(ACN)4BF4 was found to
be approximately 7% more effective.
electron-transfer pathways for nonradiative relaxation. The
fluorescence quenching was found to be cation-selective and
almost independent of counteranions present in solution. The
high sensitivity inherent to fluorescence spectroscopy, combined
with the remarkable ion selectivity of this new class of
chemosensors, opens new venues for probing small traces of
metal ions.
We believe that the development of a synthetic route toward
C2-symmetric and conformationally stable 1,8-diarylnaphtha-
lenes such as anti-2 and anti-3 will allow the exploration of a
new class of compounds for enantioselective sensing of chiral
molecules. The synthesis and enantioseparation of new axially
chiral 1,8-dihetarylnaphthalenes that are capable of participating
in diastereomeric interactions that can be quantitatively mea-
sured by fluorescence quenching are currently underway in our
laboratories.
Conclusions
We have synthesized conformationally stable 1,8-diarylnaph-
thalenes via CuO-promoted Stille cross-coupling of 1,8-dibro-
monaphthalene and 4-alkyl-9-trimethylstannylacridines. The
syn- and anti-isomers of 1,8-bis(4,4′-dimethyl-9,9′-diacridyl)-
naphthalene, 2, and its diisopropyl analogue, 3, have been
isolated for investigation of their stereodynamic properties. No
sign of syn/anti-isomerization was observed even at high
temperatures, indicating a rotational energy barrier above 180
kJ/mol. Fluorescence titration experiments with the syn-isomer
of 2 revealed highly efficient quenching by Cu(II) ions, which
was attributed to cooperative recognition. Job analysis based
on UV titration experiments revealed formation of a complex
exhibiting equimolar amounts of the sensor and Cu(II). Almost
no quenching effects were observed with Cu(I) and Zn(II) salts,
which is probably a consequence of negligible photoinduced
Acknowledgment. We thank Mark Olsen of GlaxoSmith-
Kline Pharmaceuticals for LC/APCI/MS analysis of compounds
2, 3, 14, and 15.
Supporting Information Available: NMR, UV, and fluores-
cence spectroscopy data of compounds 2 and 3 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0358145
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10658 J. AM. CHEM. SOC. VOL. 125, NO. 35, 2003