1,2-Dicarba-closo-dodecaboran-1-yl naphthalene derivatives

Article abstract of DOI:10.1021/ic0509736

1,2-Dicarba-closo-dodecaboranes (o-carboranes) and naphthalenes have potential value as components or building blocks for supramolecular systems. We have efficiently synthesized 1-(1,2-dicarba-closo-dodecaboran-1-yl)naphthalene and 2-(1,2-dicarba-closo-do

Full text of DOI:10.1021/ic0509736

Inorg. Chem. 2005, 44, 8569−8573
1,2-Dicarba-closo-dodecaboran-1-yl Naphthalene Derivatives
Kiminori Ohta, Tokuhito Goto, and Yasuyuki Endo*
Faculty of Pharmaceutical Sciences, Tohoku Pharmaceutical UniVersity, 4-4-1 Komatsushima,
Aoba-ku, Sendai 981-8558, Japan
Received June 15, 2005
1,2-Dicarba-closo-dodecaboranes (o-carboranes) and naphthalenes have potential value as components or building
blocks for supramolecular systems. We have efficiently synthesized 1-(1,2-dicarba-closo-dodecaboran-1-yl)naphthalene
and 2-(1,2-dicarba-closo-dodecaboran-1-yl)naphthalene derivatives by employing three preparative methods:
cyclization of the corresponding acetylenes with decaborane(14), an Ullmann-type coupling reaction of carboranes
with aryl halide, and the aromatic nucleophilic substitution (SNAr) reaction of aryl-o-carboranes with nitrophenyl
halide. The optimum conditions of each method for synthesis of the title compounds were also investigated.
Introduction
analyses of the effects of π-bonding interactions and
hydrogen-bonding interactions of acidic CH in the o-
carborane cage.7
1,2-Dicarba-closo-dodecaborane (o-carborane; 1)1 has
unique properties and is also a useful chemical building block
in the materials and biomedical sciences. For example, the
high boron content and remarkable thermal and chemical
stability of o-carborane have been utilized in the preparation
of thermostable polymers2 and carrier molecules for boron
neutron capture therapy (BNCT),3 and the delocalization of
the 26 skeletal electrons in the o-carborane cage has been
utilized in nonlinear optics.4 We have studied the electronic
properties of carboranes from the viewpoint of physical
organic chemistry5 and have taken advantage of the spherical
geometry and the hydrophobic character of o-carborane to
utilize it as a hydrophobic pharmacophore in the field of
medicinal chemistry.6 Further, the potential of o-carboranes
as components or building blocks for supramolecular systems
has been explored. Recent studies in this area include
Naphthalene derivatives are used in various scientific
fields, such as metal complexation;8 materials chemistry;9
medicinal chemistry;10 structural chemistry;11 molecular
recognition;12 macromolecular chemistry;13 supramolecular
chemistry;14 fluorescence chemistry;15 and asymmetric syn-
thesis16 as backbones, platforms, or functional devices.
Parallel substituents at the 1 and 8 positions of naphthalene
show a significant steric interaction, which can provide novel
(6) Endo, Y.; Iijima, T.; Ohta, K.; Kagechika, H.; Kawachi, E.; Shudo,
K. Chem. Pharm. Bull. 1999, 47, 585-587. (b) Endo, Y.; Iijima, T.;
Yaguchi, K.; Kawachi, E.; Inoue, N.; Kagechika, H.; Kubo, A.; Itai,
A. Bioorg. Med. Chem. Lett. 2001, 11, 1307-1311. (c) Endo, Y.;
Iijima, T.; Yamakoshi, Y.; Fukasawa, H.; Miyaura, C.; Inada, M.;
Kubo, A.; Itai, A. Chem. Biol. 2001, 8, 341-355. (d) Ohta, K.; Iijima,
T.; Kawachi, E.; Kagechika, H.; Endo, Y. Bioorg. Med. Chem. Lett.
2004, 14, 5913-5918. (e) Fujii, S.; Hashimoto, Y.; Suzuki, T.; Ohta,
S.; Endo, Y. Bioorg. Med. Chem. Lett. 2005, 15, 227-230. (f) Endo,
Y.; Yoshimi, T.; Ohta, K.; Suzuki, T.; Ohta, S. J. Med. Chem. 2005,
48, 3941-3944.
(7) Davidson, M. G.; Hibbert, T. G.; Howard, J. A. K.; Mackinnon, A.;
Wade, K. Chem. Commun. 1996, 2285-2286. (b) Godfrey, P. D.;
Grigsby, W. J.; Nichols, P. J.; Raston, C. L. J. Am. Chem. Soc. 1997,
119, 9283-9284. (c) Harakas, G.; Vu, T.; Knobler, C. B.; Hawthorne,
M. F. J. Am. Chem. Soc. 1998, 120, 6405-6406. (d) Mac´ıas, R.; Rath,
N. P.; Barton, L. J. Organomet. Chem. 1999, 581, 39-44. (e) Lee,
H.; Knobler, C. B.; Hawthorne, M. F. Chem. Commun. 2000, 2485-
2486. (f) Alekseyeva, E. S.; Batsanov, A. S.; Boyd, L. A.; Fox, M.
A.; Hibbert, T. G.; Howard, J. A. K.; MacBride, J. A. H.; Mackinnon,
A.; Wade, K. J. Chem. Soc., Dalton Trans. 2003, 475-482.
(8) Sun, S.; Dullaghan, C. A.; Sweigart, D. A. J. Chem. Soc., Dalton Trans.
1996, 24, 4493-4507. (b) Oh, M.; Reingold, J. A.; Carpenter, G. B.;
Sweigart, D. A. Coord. Chem. ReV. 2004, 248, 561-569.
* Author to whom correspondence should be addressed. E-mail: yendo@
tohoku-pharm.ac.jp. Phone: +81-22-234-4181. Fax: +81-22-275-2013.
(1) For a recent review, see: Bregadze, V. I. Chem. ReV. 1992, 92, 209-
223.
(2) Williams, R. E. Pure Appl. Chem. 1972, 29, 569-583.
(3) Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1993, 32, 950-984.
(b) Feakes, D. A.; Shelly, K.; Hawthorne, M. F. Proc. Natl. Acad.
Sci. U.S.A. 1995, 92, 1367-1370. (c) Soloway, A. H.; Tjarks, W.;
Barnum, B. A.; Rong, F. G.; Barth, R. F.; Codogni, I. M.; Wilson, J.
G. Chem. ReV. 1998, 98, 1515-1562. (d) Tjarks, W. J. Organomet.
Chem. 2000, 614-615, 37-47.
(4) Murphy, D. M.; Mingos, D. M. P.; Haggit, J. L.; Powell, H. R.;
Westcott, S. A.; Marder, T. B.; Taylor, N. J.; Kanis, D. R. J. Mater.
Chem. 1993, 3, 139-148. (b) Lamrani, M.; Hamasaki, R.; Mitsuishi,
M.; Miyashita, T.; Yamamoto, Y. Chem. Commun. 2000, 1595-1596.
(c) Hamasaki, R.; Ito, M.; Lamrani, M.; Mitsuishi, M.; Miyashita, T.;
Yamamoto, Y. J. Mater. Chem. 2003, 13, 21-26.
(5) Endo, Y.; Taoda, Y. Tetrahedron Lett. 1999, 40, 9073-9076. (b) Endo,
Y.; Sawabe, T.; Taoda, Y. J. Am. Chem. Soc. 2000, 122, 180-181.
(c) Endo, Y.; Taoda, Y. Tetrahedron Lett. 2001, 42, 6327-6331.
(9) Staab, H. A.; Saupe, T. Angew. Chem., Int. Ed. Engl. 1988, 27, 865-
879. (b) Sun, Q.; Wang, H.; Yang, C.; Li, Y. J. Mater. Chem. 2003,
13, 800-806. (c) Svoboda, J.; Novotna´, V.; Kozm´ık, V.; Glogarova´,
M.; Weissflog, W.; Diele, S.; Pelzl, G. J. Mater. Chem. 2003, 13,
2104-2110.
10.1021/ic0509736 CCC: $30.25
Published on Web 10/01/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 23, 2005 8569

Ohta et al.
of the preparation of 1-(o-carboranyl)naphthalene,21 but
details are not available.21
Here, we describe efficient syntheses of 1,2-dicarba-closo-
dodecaboranyl naphthalenes and their 2-aryl derivatives by
employing three different methods, that is, the well-known
cyclization of acetylene compounds with decaborane(14) in
the presence of Lewis bases, the Ullmann-type coupling
reaction using an o-carboranyl copper derivative, and the
aromatic nucleophilic substitution (SNAr) reaction of aryl-
o-carboranes with 4-nitrofluorobenzene (Figure 1).22
Experimental Section
General Considerations. Melting points were determined on a
Yanaco micro melting point apparatus without correction. 1H NMR,
13C NMR, and 10B NMR spectra were recorded with JEOL JNM-
EX-270, JNM-LA-400, and JNM-LA-600 spectrometers. Chemical
shifts for 1H NMR spectra were referenced to tetramethylsilane (0.0
ppm) as an internal standard. Chemical shifts for 13C NMR spectra
were referenced to residual 13C present in deuterated solvents.
Chemical shift values for 11B spectra were referenced relative to
external BF3‚OEt (0.0 ppm with negative values upfield). Mass
spectra were recorded on a JEOL JMS-DX-303 spectrometer.
Elemental analyses were performed with a Perkin-Elmer 2400 CHN
analyzer.
Figure 1. Synthetic route to monoaryl-o-carboranes and diaryl-o-carbo-
ranes.
properties, for example, a new type of intramolecular CT
complex17 and a stable dication with two positive rings fixed
in a face-to-face conformation.18
Therefore, the combination of naphthalene with the
o-carborane cage might provide a range of molecules with
unprecedented features. In general, monoaryl-o-carborane
derivatives can be synthesized by an Ullmann-type coupling
reaction of the C-copper derivative of o-carborane with a
variety of aryl iodides19 or cyclization of aryl acetylenes with
decaborane(14) in the presence of Lewis bases, such as
acetonitrile, amines, and dialkyl sulfides.20 The latter reaction
can also be used to prepare diaryl-o-carborane derivatives
from the corresponding diaryl acetylenes. There is a report
2-(Ethynyl)naphthalene (3a). To a solution of 2-(trimethylsi-
lylethynyl)naphthalene23 (2.0 g, 8.9 mmol) in 25 mL of MeOH was
added K2CO3 (1.38 g, 10 mmol), and the mixture was stirred for
14 h at room temperature. The mixture was poured into an aqueous
2 N HCl solution, and the solution was extracted with AcOEt,
washed with brine, dried over MgSO4, and then concentrated. The
residue was purified by column chromatography on silica gel with
n-hexane to give 1.35 g (99%) as a colorless solid: brown plates
(n-hexane). mp 40 °C (lit.23 mp 41 °C). 1H NMR (270 MHz,
CDCl3): δ 3.14 (s, 1 H), 7.46-7.54 (m, 3 H), 7.77-7.83 (m, 3
H), 8.02 (s, 1 H). MS (EI) m/z: 152 (M+, 100%). Anal. Calcd for
C12H8: C, 94.70; H, 5.30. Found: C, 94.45; H, 5.38.
(10) Rodriguez, A. L.; Tamrazi, A.; Collins, M. L.; Katzenellenbogen, J.
A. J. Med. Chem. 2004, 47, 600-611. (b) Ji, T.; Lee, M.; Pruitt, S.
C.; Hangauer, D. G. Bioorg. Med. Chem. Lett. 2004, 14, 3875-3879.
(c) Maya, A. B. S.; Pe´rez-Melero, C.; Mateo, C.; Alonso, D.;
Ferna´ndez, J. L.; Gajate, C.; Mollinedo, F.; Pela´ez, R.; Caballero, E.;
Medarde, M. J. Med. Chem. 2005, 48, 556-568.
(11) Helson, H. E., Vollhardt, P. C.; Yang, Z.-Y. Angew. Chem., Int. Ed.
Engl. 1985, 24, 114-115. (b) Shepherd, M. K. J. Chem. Soc., Perkin
Trans. 1 1994, 1055-1059.
(12) Liu, Y.; Han, B.-H.; Li, B.; Zhang, Y.-M.; Zhao, P.; Chen, Y.-T.;
Wada, T.; Inoue, Y. J. Org. Chem. 1998, 63, 1444-1454. (b)
Mallinson, P. R.; Smith, G. T.; Wilson, C. C.; Grech, E.; Wozniak,
K. J. Am. Chem. Soc. 2003, 125, 4259-4270.
(13) Francis, C. G.; Morand, P. D.; Spare, N. J. Organometallics 1985, 4,
1958-1964. (b) Bush, B. F.; Lagowski, J. J. J. Organomet. Chem.
1990, 386, 37-50.
(14) Tuna, F.; Hamblin, J.; Clarkson, G.; Errington, W.; Alcock, N. W.;
Hannon, M. J. Chem. Eur. J. 2002, 8, 4957-4964. (b) Toda, F.;
Senzaki, M.; Kuroda, R. Chem. Commun. 2002, 16, 1788-1789. (c)
Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.;
Champness, N. R.; Beton, P. H. Nano Lett. 2003, 3, 9-12.
(15) Tamura, M.; Gao, D.; Ueno, A. Chem. Eur. J. 2001, 7, 1390-1397.
(b) Wu¨rthner, F.; Ahmed, S.; Thalacker, C.; Debaerdemaeker, T. Chem.
Eur. J. 2002, 8, 4742-4750. (c) Lee, D. H.; Kim, S. Y.; Hong, J.-I.
Angew. Chem., Int. Ed. 2004, 43, 4777-4780.
2-(Phenylethynyl)naphthalene (3b). To a solution of 2 (2.07
g, 10 mmol), ethynylbenzene (1.22 g, 12 mmol), Pd(PPh3)2Cl2 (280
mg, 0.4 mmol), and CuI (38 mg, 0.2 mmol) in 15 mL of THF was
added diisopropylmine (2.8 mL, 20 mmol), and the mixture was
refluxed for 12 h. The precipitate was removed by filtration and
washed with AcOEt, and the filtrate was washed with brine, dried
over MgSO4, and then concentrated. The residue was purified by
column chromatography on silica gel with 1:100 AcOEt/n-hexane
to give 2.11 g (93%) of the title compound: colorless prisms (n-
1
hexane). mp 118-119 °C (lit.24 mp 114-115 °C). H NMR (270
MHz, CDCl3): δ 7.34-7.39 (m, 3 H), 7.46-7.53 (m, 2 H), 7.56-
7.60 (m, 3 H), 7.79-7.84 (m, 3 H), 8.06 (d, J ) 1.1 Hz, 1 H). MS
(EI) m/z: 228 (M+, 100%). Anal. Calcd for C18H12: C, 94.70; H,
5.30. Found: C, 94.34; H, 5.30.
2-(1,2-Dicarba-closo-dodecaboran-1-yl)naphthalene (4a). Et2S
Method. A mixture of decaborane(14) (883 mg, 7.22 mmol), 3a
(1.0 g, 6.57 mmol), and Et2S (1.63 mL, 15.16 mmol) in 30 mL of
dry toluene was heated at 100 °C for 24 h. The solvent was removed
under reduced pressure. The residue was purified by silica gel
(16) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345-350. (b) Genet,
J.-P. Acc. Chem. Res. 2003, 36, 908-918. (c) Berthod, M.; Mignani,
G.; Woodward, G.; Lemaire, M. Chem. ReV. 2005, 105, ASAP.
(17) Komatsu, K.; Abe, N.; Takahashi, K.; Okamoto, K. J. Org. Chem.
1979, 44, 2712-2717. (b) Tsuji, R.; Komatsu, K.; Inoue, Y.; Takeuchi,
K. J. Org. Chem. 1992, 57, 636-641.
(18) Komatsu, K.; Arai, M.; Okamoto, K. Tetrahedron Lett. 1982, 23, 91-
94. (b) Komatsu, K.; Arai, M.; Hattori, Y.; Fukuyama, K.; Katsube,
Y.; Okamoto, K. J. Org. Chem. 1987, 52, 2183-2192.
(21) Ermaganbetov, B. T.; Kazantsev, A. V. Eksp. Metody Issled. Khim.
Biol. 1975, 2, 121-126.
(19) Coult, R.; Fox, M. A.; Gill, W. R.; Herbertson, P. L.; MacBride, J. A.
H.; Wade, K. J. Organomet. Chem. 1993, 462, 19-29.
(22) Ohta, K.; Goto, T.; Endo, Y. Tetrahedron Lett. 2005, 46, 483-485.
(23) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627-630.
(20) Heying, T. L.; Ager, J. W., Jr.; Clark, S. L.; Mangold, D. J.; Goldstein,
H. L.; Hillman, M.; Polak, R. J.; Szymanski, J. W. Inorg. Chem. 1963,
2, 1089-1092.
(24) Okuro, K.; Furuune, M.; Enna, M.; Miura, M.; Nomura, M. J. Org.
Chem. 1993, 58, 4716-4721.
8570 Inorganic Chemistry, Vol. 44, No. 23, 2005

1,2-Dicarba-closo-dodecaboran-1-yl Naphthalene DeriWatiWes
column chromatography with n-hexane to give 1.19 g (67%) of
the title compound as a colorless solid.
1-(1,2-Dicarba-closo-dodecaboran-1-yl)naphthalene (7a). Et2S
Method. Compound 7a was prepared in a manner similar to that
described for 4a (30% yield).
CH3CN Method. A mixture of decaborane(14) (883 mg, 7.22
mmol) and 3a (1.0 mg, 6.57 mmol) in a mixture of 2 mL of CH3-
CN and 8 mL of dry benzene was refluxed for 48 h. The solvent
was removed under reduced pressure. The residue was purified by
silica gel column chromatography with n-hexane to give 1.19 g
(86%) of the title compound as a colorless solid: colorless cubes
(n-hexane). mp 99-100 °C. 1H NMR (270 MHz, CDCl3): δ 1.0-
4.0 (m, 10 H), 4.08 (s, 1 H), 7.52-7.57 (m, 3 H), 7.78-7.85 (m,
3 H), 7.97 (d, J ) 2.1 Hz, 1 H). 13C NMR (68 MHz, CDCl3): δ
60.35, 76.67, 124.30, 127.34, 127.46, 127.52, 127.69, 128.35,
128.71, 130.59, 132.44, 133.28. 11B NMR (192 MHz, CDCl3): δ
-12.89, -11.34, -10.82, -9.05, -4.48, -2.22. MS (EI) m/z: 270
(M+, 100%). Anal. Calcd for C12H18B10: C, 53.31; H, 6.71.
Found: C, 53.24; H, 6.52.
CH3CN Method. Compound 7a was prepared in a manner
similar to that described for 4a (52% yield): colorless cubes (n-
1
hexane). mp 137-138 °C. H NMR (270 MHz, CDCl3): δ 1.0-
4.0 (brm, 10 H), 4.63 (s, 3 H), 7.40 (t, J ) 7.9 Hz, 1 H), 7.49-
7.62 (m, 2 H), 7.79 (dd, J ) 1.0 Hz, 7.8 Hz, 1 H), 7.89 (d, J ) 9.2
Hz, 2 H), 8.70 (d, J ) 9.1 Hz, 1 H). 13C NMR (150.8 MHz,
CDCl3): δ 61.14, 77.37, 124.37, 124.51, 126.11, 127.19, 128.20,
128.67, 129.79, 129.88, 131.73, 134.77. 11B NMR (192 MHz,
CDCl3): δ -13.32, -9.95, -8.90, -2.72. MS (EI) m/z: 270 (M+,
100%). Anal. Calcd for C12H18B10: C, 53.31; H, 6.71. Found: C,
53.14; H, 6.70.
1-(2-Phenyl-1,2-dicarba-closo-dodecaboran-1-yl)naphtha-
lene (7b). Et2S Method. Compound 7b was prepared in a manner
similar to that described for 4a (8% yield).
2-(2-Phenyl-1,2-dicarba-closo-dodecaboran-1-yl)naphtha-
lene (4b). Et2S Method. Compound 4b was prepared in a manner
similar to that described for 4a (65% yield).
CH3CN Method. Compound 7b was prepared in a manner
similar to that described for 4a (10% yield): colorless cubes (n-
1
hexane). mp 153-154 °C. H NMR (270 MHz, CDCl3): δ 1.0-
CH3CN Method. Compound 4b was prepared in a manner
similar to that described for 4a (42% yield): colorless cubes
4.0 (brm, 10 H), 6.97 (t, J ) 7.9 Hz, 2 H), 7.10 (t, J ) 7.7 Hz, 1
H), 7.14 (t, J ) 7.9 Hz, 1 H), 7.28 (d, J ) 7.6 Hz, 2 H), 7.48 (t,
J ) 7.7 Hz, 1 H), 7.62 (dt, J ) 1.6 Hz, 7.3 Hz, 1 H), 7.72 (d, J )
8.2 Hz, 1 H), 7.77 (d, J ) 8.1 Hz, 1 H), 7.90 (d, J ) 7.8 Hz, 1 H),
9.05 (d, J ) 9.0 Hz, 1 H). 13C NMR (68 MHz, CDCl3): δ 87.55,
88.89, 124.05, 124.96, 125.23, 125.74, 126.93, 128.07, 129.56,
129.95, 130.25, 130.79, 131.79, 132.59, 133.64, 134.48. 11B NMR
(192 MHz, CDCl3): δ -10.11, -9.23, -8.86, -2.91, -1.15. MS
(EI) m/z: 346 (M+), 230 (100%). HRMS Calcd for C18H22B10:
346.2725. Found: 346.2748.
Synthesis of 7a by Ullmann-type Coupling at High Concen-
tration. To a solution of 1 (2.88 g, 20 mmol) in 30 mL of DME
was added dropwise a 1.56 M solution of n-BuLi in n-hexane (27.7
mL, 44 mmol) at 0 °C under Ar. The mixture was stirred for 30
min; then, CuCl (5.15 g, 52 mmol) was added in one portion, and
the mixture was stirred at room temperature for 2 h. Pyridine (12.13
mL, 150 mmol) and 5 (3.5 mL, 24 mmol) were added in one
portion, and the resulting mixture was refluxed for 14 h. After
cooling, insoluble materials were removed by filtration through
Celite. The filtrate was washed with a 2 N HCl solution and water
and brine, dried over MgSO4, and then concentrated. The residue
was purified by silica gel column chromatography with 1:10 AcOEt/
n-hexane to give 4.55 g (84%) of 7a as a colorless solid.
1-(1-Naphthyl)-2-(4-nitrophenyl)-o-carborane (11). To a sus-
pension of KOtBu (136 mg, 1.2 mmol) in dry DMF was added 7a
(270 mg, 1 mmol), and then, 4-fluoronitrobenzene (0.13 mL, 1.2
mmol) was added within 1 min at 0 °C. After 40 min, the reaction
was poured into an aqueous 2 N HCl solution and extracted with
AcOEt. The organic layer was washed with water and brine, dried
over MgSO4, and concentrated. The residue was purified by column
chromatography on silica gel with 1:10 CH2Cl2/n-hexane to give
351 mg (90%) of the title compound as a pale yellow solid: pale
yellow prisms (AcOEt-n-hexane). mp 187-188 °C. 1H NMR (270
MHz, CDCl3): δ 1.0-4.0 (m, 10 H), 7.18 (t, J ) 7.9 Hz, 1 H),
7.44 (d, J ) 8.9 Hz, 2 H), 7.53 (t, J ) 7.4 Hz, 1 H), 7.66 (ddd, J
) 1.4 Hz, 8.2 Hz, 8.9 Hz, 1 H), 7.56-7.83 (m, 2 H), 7.82 (d, J )
9.1 Hz, 2 H), 7.91 (d, J ) 7.9 Hz, 1 H), 9.01 (d, J ) 9.1 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ 85.96, 88.06, 123.08, 124.14,
124.60, 124.67, 126.12, 127.38, 129.82, 131.31, 131.62, 133.24,
133.68, 134.54, 137.15, 148.36. 11B NMR (192 MHz, CDCl3): δ
-10.51, -9.52, -8.49, -1.90, -0.99. MS (EI) m/z: 391 (M+,
100%). Anal. Calcd for C18H21B10NO2: C, 55.23; H, 5.41; N, 3.58.
Found: C, 55.01; H, 5.36; N, 3.72.
1
(AcOEt-n-hexane). mp 111 °C. H NMR (270 MHz, CDCl3): δ
1.5-3.6 (brm, 10 H), 7.05-7.18 (m, 3 H), 7.42-7.50 (m, 5 H),
7.59 (d, J ) 8.9 Hz, 1H), 7.71 (m, 2 H), 7.93 (d, J ) 1.7 Hz, 1 H).
13C NMR (68 MHz, CDCl3): δ 85.39, 85.23, 126.52, 127.28,
127.65, 127.96, 128.24, 128.57, 128.70, 130.13, 130.55, 130.61,
131.26, 132.19, 133.33. 11B NMR (192 MHz, CDCl3): δ -11.46,
-10.43, -9.12, -2.51. MS (EI) m/z: 346 (M+, 100%). Anal. Calcd
for C18H22B10: C, 62.40; H, 6.40. Found: C, 62.23; H, 6.29.
1-(Ethynyl)naphthalene (6a).25A mixture of 1-(trimethylsilyl-
ethynyl)naphthalene25 (2.0 g, 8.9 mmol) and K2CO3 (1.25 g, 9.0
mmol) in CH3OH was stirred at room temperature for 5 h. The
mixture was poured into water and extracted with AcOEt. The
organic phase was washed with brine, dried over MgSO4, and then
concentrated. The crude mixture was purified by column chroma-
tography on silica gel with n-hexane to give 1.33 g (98%) of the
1
title compound as a brown oil. H NMR (270 MHz, CDCl3): δ
3.47 (s, 1 H), 7.43 (dd, J ) 7.3 Hz, 8.2 Hz, 1 H), 7.56 (m, 2 H),
7.74 (d, J ) 7.2 Hz, 1 H), 7.86 (d, J ) 7.6 Hz, 2 H), 8.36 (d, J )
8.3 Hz, 1 H). MS (EI) m/z: 152 (M+, 100%). HRMS Calcd for
C12H8: 152.0626. Found: 152.0640.
1-(Phenylethynyl)naphthalene (6b).26To a mixture of 5 (2.07
g, 10 mmol), Pd(PPh3)2Cl2 (280 mg, 0.4 mmol), CuI (38 mg, 0.2
mmol), and ethynylbenzene (1.22 g, 12 mmol) in 15 mL of THF
was added diisopropylamine (2.8 mL, 20 mmol), and the mixture
was refluxed for 12 h under an argon (Ar) atmosphere. It was cooled
to room temperature, the solvent was removed under reduced
pressure, and the resulting residue was dissolved in AcOEt. This
solution was washed with water and brine, dried over MgSO4, and
then evaporated. The crude product was purified by column
chromatography on silica gel with 1:100 AcOEt/n-hexane to give
2.13 g (94%) of the title compound as a yellow oil. 1H NMR (270
MHz, CDCl3): δ 7.46 (dd, J ) 7.2 Hz, 8.2 Hz, 1 H), 7.36-7.42
(m, 3 H), 7.53 (ddd, J ) 1.7 Hz, 7.0 Hz, 7.9 Hz, 1 H), 7.60 (ddd,
J ) 1.7 Hz, 6.8 Hz, 7.6 Hz, 1 H), 7.62-7.67 (m, 2 H), 7.76 (dd,
J ) 1.2 Hz, 7.1 Hz, 1 H), 7.86 (t, J ) 7.8 Hz, 2 H), 8.44 (d, J )
8.1 Hz, 1 H). MS (EI) m/z: 228 (M+, 100%). HRMS Calcd for
C12H8: 228.0939. Found: 228.0962.
(25) Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489-
2496.
(26) Feuerstein, M.; Berthiol, F.; Doucet, H.; Santelli, M. Synthesis 2004,
8, 1281-1289.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8571

Ohta et al.
Table 1. o-Carborane Construction by the Reaction of 2-(Ethynyl)naphthalenes (3) with Decaborane(14)
entry
R
Lewis base
solvent
time (h)
temp (°C)
product
yield (%)
1
2
3
4
H (3a)
H (3a)
Ph (3b)
Ph (3b)
CH3CN
Et2S
CH3CN
Et2S
benzene
toluene
benzene
toluene
48
48
48
48
reflux
80
reflux
80
4a
4a
4b
4b
86
67
42
65
Table 2. o-Carborane Construction by the Reaction of 1-(Ethynyl)naphthalenes (6) with Decaborane(14)
entry
R
Lewis base
solvent
time (h)
temp (°C)
product
yield (%)
1
2
3
4
H (6a)
H (6a)
Ph (6b)
Ph (6b)
CH3CN
Et2S
CH3CN
Et2S
benzene
toluene
benzene
toluene
48
48
48
48
reflux
80
reflux
80
7a
7a
7b
7b
52
30
10
8
Scheme 1. Synthesis of 2-(o-Carboranyl)naphthalene Derivatives (4)
Scheme 2. Synthesis of 1-(o-Carboranyl)naphthalene Derivatives (7)
(a) Ethynylbenzene or ethynyltrimethylsilane, PdCl2(PPh3)2, CuI, diiso-
propylamine, THF. (b) K2CO3, MeOH. (c) Decarborane(14), Lewis base.
(a) Ethynylbenzene or ethynyltrimethylsilane, PdCl2(PPh3)2, CuI, diiso-
propylamine, THF. (b) K2CO3, MeOH. (c) Decarborane(14), Lewis base.
Results and Discussion
Table 3. Synthesis of 1-(o-Carboranyl)naphthalene (7a) by
Ullmann-Type Coupling
First, we attempted to prepare 2-(o-carboranyl)naphthalene
derivatives 4 by the cyclization of acetylenes 3 with
decaborane(14) in the presence of a Lewis base (Scheme
1). The acetylene derivatives 3 were prepared by a palladium-
catalyzed Sonogashira reaction with the corresponding
acetylene units derived from 2-bromonaphthalene 2.23 The
trimethylsilyl group in the precursor of 3a was removed with
potassium carbonate in methanol.27 Compounds 3 were
converted into 2-(o-carboranyl)naphthalene derivatives 4 by
cyclization with decaborane(14) in the presence of a Lewis
base in moderate yields, in a manner similar to that used for
the o-carboranyl benzene derivatives (Table 1).20 We exam-
ined the effect of the Lewis base in this cyclization. It is
already known that the use of diethyl sulfide as a Lewis base
generates o-carborane derivatives more efficiently as com-
pared to acetonitrile.28 We found that acetonitrile is a better
Lewis base than diethyl sulfide in the cyclization of 2-(ethy-
nyl)naphthalene 3a with decaborane(14) and that 2-(phenyl-
ethynyl)naphthalene 3b reacts efficiently with decaborane(14)
in the presence of diethyl sulfide as a Lewis base to give
the desired product 4b in better yield.
Next, 1-(o-carboranyl)naphthalene derivatives 7 were
synthesized by means of a procedure similar to that used
for the preparation of 2-(o-carboranyl)naphthalene derivatives
4 (Scheme 2). We performed the Sonogashira reaction of
commercially available 1-iodonaphthalene 5 with ethynylt-
rimethylsilane or ethynylbenzene, followed by deprotection
of the trimethylsilyl group. The cyclization of 1-(ethynyl)-
naphthalene 6a with decaborane(14) in the presence of
acetonitrile or diethyl sulfide gave 1-(o-carboranyl)naphtha-
lene 7a in 52% or 30% yield, respectively (Table 2; entries
entry n-BuLi (eq) CuCl (eq) concentration (M) time (h) yield (%)
1
2
2.2
2.2
2.6
2.6
0.1
0.3
48
5
41
84
1 and 2). However, the yield of 1-(2-phenyl-o-carboranyl)-
naphthalene 7b was markedly reduced to less than 10%,
regardless of the kind of Lewis base used in the cyclization.
Moreover, the yield when acetonitrile was used as a Lewis
base was greater than that in the case of diethyl sulfide (Table
2; entries 3 and 4). There are significant structural disad-
vantages for the cyclization of 1-(phenylethynyl)naphthalene
6b with decaborane(14), because 1-(2-phenyl-o-carboranyl)-
naphthalene 7b has greater steric hindrance due to the
hydrogen at the 8 position in the naphthalene ring and the
two aromatic rings bound to the two carbon atoms in
o-carborane than does 2-(2-phenyl-o-carboranyl)naphthalene
4b.
The features of the cyclization of acetylenes and decabo-
rane(14) in the presence of Lewis bases can be summarized
as follows: (1) Diethyl sulfide is an effective Lewis base
for the cyclization of endo-acetylene derivatives. (2) Aceto-
nitrile is an effective Lewis base for the cyclization of exo-
acetylene derivatives. (3) Diethyl sulfide is less influenced
by steric effects as compared to acetonitrile. We have
previously reported the synthesis and structure of 1,2-bis-
(o-carboranyl)benzene, which has a distorted benzene ring
due to the enormous steric hindrance arising from two
neighboring o-carboranes.29 The procedure using acetonitrile
(27) Eaborn, C.; Walton, D. R. M. J. Organomet. Chem. 1965, 4, 217-
228.
(28) Teplyakov, M. M.; Khotina, I. A.; Gelashvili, T. L.; Kovshak, V. V.
Dokl. Akad. Nauk. SSSR 1980, 271, 874-877. (b) Jiang, W.; Knobler,
C. B.; Hawthorne, M. F. Inorg. Chem. 1996, 35, 3056-3058.
8572 Inorganic Chemistry, Vol. 44, No. 23, 2005

1,2-Dicarba-closo-dodecaboran-1-yl Naphthalene DeriWatiWes
Table 4. Synthesis of Various Aryl Carboranes by Ullmann-Type Reaction at High Concentration
substrate
carboranes
R
Ar
time (h)a
product
yield (%)a
1
8a
8b
o-carborane
p-carborane
p-carborane
H
H
3-NO2-Ph
Ph
3-CN-Ph
8 (42)
8 (20)
4 (24)
9
10a
10b
70 (37)
64 (52)
70 (19)
-CH2OTBS
a Yield and time using the original procedure of Ullmann-type coupling in parentheses.19
Scheme 3. Efficient Synthesis of
1-[(4-Nitrophenyl)-o-carboranyl]naphthalene Derivatives (11)
proceeds smoothly under mild conditions and is a powerful
tool for the synthesis of 1,2-diaryl-o-carboranes from monoar-
yl-o-carboranes. To obtain 1-[2-(4-nitrophenyl)-o-carboranyl]-
naphthalene 11 bearing a nitro group, which could readily
be transformed into various substituents, we applied the SN-
Ar reaction to 1-(o-carboranyl)naphthalene 7a. The reaction
of 1-(o-carboranyl)naphthalene 7a with 4-nitrofluorobenzene
in the presence of KOtBu in DMF at 0 °C proceeded
smoothly to give 1-[2-(4-nitrophenyl)-o-carboranyl]naphtha-
lene 11 within 40 min in 90% yield (Scheme 3).
(a) n-BuLi, CuCl, 1-iodonaphthalene, pyridine, DME. (b) KOtBu,
4-fluoronitrobenzene, DMF.
as a Lewis base worked effectively in the preparation of 1,2-
bis(o-carboranyl)benzene, whereas the desired product was
not obtained in the presence of diethyl sulfide as a Lewis
base. Acetonitrile is a more effective Lewis base for the
cyclization with decaborane(14) in the cases of exo-
acetylenes and sterically hindered acetylenes.
Conclusion
In conclusion, we have efficiently synthesized 2-(o-
carboranyl)naphthalene derivatives 4 and 1-(o-carboranyl)-
naphthalene derivatives 7 by employing three methods. 2-(o-
Carboranyl)naphthalene derivatives 4 are effectively synthe-
sized by employing cyclization of 2-ethynylnaphthalenes,
even if a second aryl substituent exists on the acetylene
group. However, the synthesis of 1-(o-carboranyl)naphthalene
derivatives 7 has some difficulties. Although unsubstituted
1-(o-carboranyl)naphthalene 7a could be prepared by cy-
clization of 1-(ethynyl)naphthalene in moderate yield, it was
better to use the Ullmann-type coupling reaction. In the case
of synthesis of sterically hindered 1-(2-phenyl-o-carboranyl)-
naphthalene 7b, the cyclization of 1-(phenylethynyl)naph-
thalene 6b with decaborane(14) afforded a poor result. The
combination of an Ullmann-type coupling reaction followed
by an SNAr reaction seems to be more suitable for the
preparation of 1-(2-aryl-o-carboranyl)naphthalene. Thus, a
variety of molecules in which naphthalene is linked with the
o-carborane cage can be easily prepared. The results de-
scribed here should provide a basis for synthesizing a variety
of sterically hindered molecules and supramolecules on the
basis of host-guest interactions involving the combination
of naphthalene and o-carborane.
We also attempted a direct synthesis of 1-(o-carboranyl)-
naphthalene 7a by means of the Ullmann-type coupling
reaction of 1-iodonaphthalene 5 with an o-carboranyl copper
derivative (Table 3). Under the conditions of the original
procedure for Ullmann-type coupling,19 compound 7a was
obtained in low yield (41%), after prolonged reaction.
However, when the reaction was conducted at a high
concentration, such as 0.3 M, it proceeded smoothly within
5 h to afford the desired compound, 7a, in excellent yield
(84%), which was much better than that of the cyclization
of 1-(ethynyl)naphthalene 6a with decaborane(14). Various
aryliodides were examined in the Ullmann-type coupling
reaction at high concentrations (Table 4). This approach
resulted in marked improvements of the reaction rate and
the yield with all of the substrates examined, affording the
corresponding arylated carboranes 9, 10a, and 10b in high
yields. The concentration of the reaction solution is likely
to be an extremely important factor in controlling the reaction
rate and yield of this coupling reaction.
We have recently reported a novel synthetic method of
1,2-diaryl-o-carboranes via an SNAr reaction.22 This reaction
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (B) (No. 16390032) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
(29) Endo, Y.; Songkram, C.; Ohta, K.; Kaszynski, P.; Yamaguchi, K.
Tetrahedron Lett. 2005, 46, 699-702. (b) Endo, Y.; Songkram, C.;
Ohta, K.; Yamaguchi, K. J. Organomet. Chem. 2005, 690, 2750-
2756.
IC0509736
Inorganic Chemistry, Vol. 44, No. 23, 2005 8573