In this communication, we report the development of a
new method for synthesis of multiarylanthracenes by means
of the RuH2(CO)(PPh3)3-catalyzed arylation of anthraquinone
with arylboronates. This method consists of short and
straightforward sequences starting with an easily accessible
anthraquinone5 and was applicable to the syntheses of various
multiarylanthracenes, including those bearing twisted back-
bones.
Table 1. Tetraarylation of Anthraquinone with Arylboronatesa
When the reaction of anthraquinone 1 with 10 equiv of
p-tolylboronate 2a (2.5 equiv to each ortho C-H bond) was
carried out in the presence of RuH2(CO)(PPh3)3 (3) as a
catalyst in refluxing pinacolone, tetraarylation product 4a was
obtained in 77% isolated yield (eq 1). Triarylanthraquinone
5a and its reduction product 6a6 were also isolated in 5 and
3% yields, respectively.
a Reaction conditions: anthraquinone (1) (0.25 mmol), arylboronate (2)
(2.5 mmol), pinacolone (0.5 mL), RuH2(CO)(PPh3)3 (3) (0.05 mmol), reflux,
24 h. b Isolated yield.
afforded tetraarylanthraquinones 4c,d in 74 and 89% isolated
yields, respectively (entries 2 and 3). Electronically diverse
substituents (n-hexyl, Me2N, OMe, and CF3 groups) at the
4-position on the benzene ring in the arylboronates were
tolerated, and the corresponding tetraarylated anthraquinones
(4e-h) were isolated in 83, 33, 55, and 48% isolated yields,
respectively (entries 4-7). The reaction employing a boronic
acid ester (2i) containing a p-(trans-4-alkylcyclohexyl)phenyl
moiety, found in many liquid crystals,7 also provided the
1:4 coupling product (4i) in 39% yield (entry 8). Differential
scanning calorimetry (DSC) analysis of 4i showed three
endothermic peaks between 150 and 280 °C during heating.
Reduction of tetra(p-tolyl)anthraquinone 4a with aqueous
HI and acetic acid at 140 °C in a sealed tube afforded 1,4,6,9-
tetra(p-tolyl)anthracene (7) in 83% yield (eq).5d,8 This result
illustrates that the ruthenium-catalyzed tetraarylation of
anthraquinone with arylboronates and the reduction with HI
constitutes a convenient two-step procedure for the synthesis
of tetraarylanthracenes.9
A variety of arylboronates can be used for the tetraarylation
of 1 (Table 1). In the case of phenylation, the corresponding
product (4b) was precipitated during the reaction due to the
low solubility of 4b in pinacolone (entry 1). Washing the
precipitate with dichloromethane to remove the impurity
provided analytically pure 4b in 28% yield. The reaction
using 3-methylphenyl- and 3,5-dimethylphenylboronates
(5) For reported syntheses of highly functionalized anthracenes bearing
five or more carbon substituents using Diels-Alder Reaction, see: (a) Hart,
H.; Lai, C.-Y.; Nwokogu, G.; Shamouilian, S.; Teuerstein, A.; Zlotogorski,
C. J. Am. Chem. Soc. 1980, 102, 6649. (b) Hart, H.; Ok, D. J. Org. Chem.
1986, 51, 979. (c) 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. (d) Lu,
J.; Zhang, J.; Shen, X.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc.
2002, 124, 8035. (e) Rigaudy, J.; Ricard, M. M. Tetrahedron 1968, 24,
3241. For the syntheses using nucleophilic addition to carbonyl groups of
oxidized anthracene frameworks, see: (f) Lepage, Y.; Pouchot, O. Bull. Soc.
Chim. Fr. 1965, 2342. (g) Heller, C. A.; Henry, R. A.; McLaughlin, B. A.;
Bliss, D. E. J. Chem. Eng. Data 1974, 19, 214. (h) Godinez, C. E.; Zepeda,
G.; Mortko, C. J.; Dang, H.; Garcia-Garibay, M. A. J. Org. Chem. 2004,
69, 1652. For the syntheses using metal-catalyzed couplings of substrates
involving alkynes, see: (i) Takahashi, T.; Hara, R.; Nishihara, Y.; Kotora,
M. J. Am. Chem. Soc. 1996, 118, 5154. (j) Takahashi, T.; Li, Y.; Stepnicka,
P.; Kitamura, M.; Liu, Y.; Nakajima, K.; Kotora, M. J. Am. Chem. Soc.
2002, 124, 576. (k) Huang, W.; Zhou, X.; Kanno, K.-i.; Takahashi, T. Org.
Lett. 2004, 6, 2429. (l) Hsieh, J.-C.; Cheng, C. H. Chem. Commun. 2008,
2992. (m) Umeda, N.; Tsurugi, H.; Satoh, T.; Miura, M. Angew. Chem.,
Int. Ed. 2008, 47, 4019. For the syntheses using Friedel-Crafts reaction,
see: (n) Marks, V.; Gottlieb, H. E.; Melman, A.; Byk, G.; Cohen, S.; Biali,
S. E. J. Org. Chem. 2001, 66, 6711. (o) Yamato, T.; Sakaue, N.; Shinoda,
N.; Matsuo, K. J. Chem. Soc., Perkin Trans. 1 1997, 1193. For the syntheses
using another approach, see: (p) Li, S.; Xiang, J.; Mei, X.; Xu, C.
Tetrahedron Lett. 2008, 49, 1690. For the related palladium-catalyzed
triarylation of anthrone, see: (q) Terao, Y.; Kametani, Y.; Wakui, H.; Satoh,
T.; Miura, M.; Nomura, M. Tetrahedron 2001, 57, 5967–5974.
Conversion of 4 to hexaarylanthracenes was then examined
using a reported strategy for transformation of anthraquinone
moieties to diarylanthracenes (Scheme 1).10 First, tetraphe-
nylanthraquinone 4b was reacted with phenyllithium in THF.
Diol 8a was obtained by arylation of both carbonyl groups
(6) Formation of 6a was observed by ESI-MS, but the structure could
not be determined by NMR analysis.
1952
Org. Lett., Vol. 11, No. 9, 2009