Suzuki-Miyaura cross-coupling of bulky anthracenyl carboxylates by using pincer nickel N-heterocyclic carbene complexes: An efficient protocol to access fluorescent anthracene derivatives

Article abstract of DOI:10.1039/c3cc46663a

A series of fluorescent (hetero)-aryl substituted anthracene derivatives were readily accessible from the corresponding bulky anthracen-9-yl carboxylates via Suzuki-Miyaura cross-coupling reactions by using pincer nickel N-heterocyclic carbene complex 1 even at the catalyst loading as low as 0.1 mol% in the presence of catalytic amounts of PCy₃.

Full text of DOI:10.1039/c3cc46663a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Suzuki–Miyaura cross-coupling of bulky anthracenyl
carboxylates by using pincer nickel N-heterocyclic
carbene complexes: an efficient protocol to access
fluorescent anthracene derivatives†
Cite this: Chem. Commun., 2013,
49, 11539
Received 31st August 2013,
Accepted 15th October 2013
DOI: 10.1039/c3cc46663a
Mizhi Xu, Xingbao Li, Zheming Sun and Tao Tu*
www.rsc.org/chemcomm
A series of fluorescent (hetero)-aryl substituted anthracene deri- hindered substrates, which remains a challenging task. Especially,
vatives were readily accessible from the corresponding bulky the study on the coupling with bulky anthracenyl carboxylates is
anthracen-9-yl carboxylates via Suzuki–Miyaura cross-coupling neglected.
reactions by using pincer nickel N-heterocyclic carbene complex 1
As strong s-donors and weak p-acceptors, robust N-hetero-
even at the catalyst loading as low as 0.1 mol% in the presence of cyclic carbenes (NHCs) have been regarded as suitable replace-
catalytic amounts of PCy3.
ments for the traditional air-sensitive phosphine ligands.11 The
catalysts derived from NHCs may also be applicable to the
Since the first observation of organic electroluminescence from selective C–O activation in the coupling of aryl carboxylates.
anthracene crystals in the 1960s, (hetero)-aryl substituted However, the examples of Ni–NHC catalyzed cross-coupling
anthracene compounds have attracted considerable attention reactions with inactive aryl carboxylates are still rare, in which
and constituted fascinating building blocks to fabricate various air-sensitive Ni(COD)2 has to be involved.10b,e Recently, our group
optoelectronic devices in organic light-emitting diodes (OLEDs).1 found that robust pyridine-bridged bis-benzimidazolylidene
Therefore, a great number of synthetic protocols have been pincer nickel complex 1 not only efficiently catalyzes the Suzuki–
developed,2–5 most of which focused on simple functionaliza- Miyaura cross-coupling reactions with various aryl halides, but
tion of the anthracene core. Besides the addition of Grignard or also well tolerates electron-rich, sterically demanding and
lithium reagents to anthraquinone,4 Suzuki–Miyaura cross- heterocyclic inert aryl tosylates and mesylates with a catalytic
coupling of halogenated anthracenes constituted a preferred amount of PPh3 as a reductant (Scheme 1).12f Following our
protocol.1,5,6
recent research interest in the synthesis of metal complexes and
Besides aryl halides, recently, phenolic sulfonates and sulf- their potential applications in catalysis and soft materials,12–14
amates have been successfully applied as electrophiles in the we would like to explore the applicability of robust complex 1 in
Suzuki–Miyaura cross-coupling reactions.6,7 Moreover, less Suzuki–Miyaura cross-coupling reactions with challenging
studied aryl carboxylates represent inactive electrophiles with sterically bulky anthracenyl carboxylates and further study the
better stability and less toxicity for cross-coupling reactions. fluorescence properties of the resulting (hetero)-aryl substituted
The essential obstacle for applicability of aryl carboxylates is anthracene derivatives.
the difficulty in achieving selective activation of inert Caryl–O
Initially, the reaction between phenyl-boronic acid and
bonds rather than Ccarbonyl–O bonds.8 Until very recently, Shi anthracen-9-yl pivalate 3 was selected to optimize the reaction
and Garg groups realized the first examples of the cross- conditions (Table 1). According to our previous results,12f
coupling between aryl pivalates and aryl boronic reagents a catalytic amount of PPh3 was applied as a reductant. However,
by using nickel catalysts containing air-sensitive phosphine in the presence of 2 mol% complex 1 and 4.5 equiv. of K3PO4ÁH2O,
ligands,9 which could be further extended to other carboxylates an unsatisfactory result was observed in 1.5 mL toluene at 100 1C
and carbamates.7,10 Besides these achievements, the applic- for 24 hours with 8 mol% PPh3 (4%, Table 1, entry 1). When PCy3
ability of the protocol is still hindered by: (1) high catalyst was applied instead, pivalate 3 was completely converted to
loadings (usually 20 mol% is required); (2) tedious handling
steps required for air-sensitive catalysts; and (3) sterically
Department of Chemistry, Fudan University, 220 Handan Road, 200433, Shanghai,
China. E-mail: taotu@fudan.edu.cn; Fax: +86-21-65102412
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization data of products. See DOI: 10.1039/c3cc46663a
Scheme 1 Pyridine-bridged pincer NHC nickel complexes 1 and 2.
Chem. Commun., 2013, 49, 11539--11541 11539
c
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
Table 1 Optimization of the reaction conditionsa
Table 2 Suzuki–Miyaura cross-coupling reactions of phenylboronic acids with
different esters derived from anthronea
Entry
[Cat]
Base
Yieldb (%)
Entry
–COR
Yieldb (%)
Entry
–COR
Yieldb (%)
1
2
3
4
5
6
7
8
1 (2 mol%)
1 (2 mol%)
1 (1 mol%)
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4Á3H2O
K3PO4
4c
97
96
96
93
1
2
97
5
6
75
89
1 (0.5 mol%)
1 (0.5 mol%)
1 (0.5 mol%)
1 (0.5 mol%)
1 (0.5 mol%)
1 (0.5 mol%)
1 (0.1 mol%)
1 (0.5 mol%)
—
>99
79
93
KOH
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4ÁH2O
K3PO4
97d
27d,e
82d
18d, f
n.r.d
n.r.d
93d,g
83d
3
71
86
7
8
31 (95c)
19c
9
10
11
12
13
14
15
4
a
The reactions were carried out in a 0.5 mmol scale at 100 1C for 6 h.
Isolated yield. With 5 mol% complex 1 and 20 mol% PCy3 for 24 h.
NiBr2 (0.5 mol%)
NiCl2(PCy3)2 (0.5 mol%)
2 (0.5 mol%)
b
c
K3PO4ÁH2O
a
obtained when other aliphatic carboxylates such as the iso-
butyrate, propionate, and acetate analogues, as well as benzoate,
were employed (97% vs. 71–89%, Table 2, entry 1 vs. 3–6). Bulkier
2,4,6-trimethylbenzoate resulted in a better yield with an
increased catalyst loading (95%, Table 2, entry 7). However,
even with 5 mol% catalyst, anthracen-9-yl phenylacetate only
resulted in a 19% yield along with a large amount of hydrolyzed
anthrone, which may suggest that selective activation of Caryl–O
bonds in the sterically hindered carboxylates was favourable.
From Table 2, sterically hindered pivalates and carbamate
are found to be most reactive. Therefore, the substrate scope of
various (hetero)-arylboronic acids with pivalate 3 was then
explored and the results are depicted in Scheme 2. However,
in the presence of 0.5 mol% catalyst 1, besides phenylboronic
acid, pivalate 3 was hardly converted to products when other
(hetero)-arylboronic acids were involved. Only moderate yields
were obtained when 4-tert-butyl and 4-methoxy phenyl sub-
stituted boronic acids were applied. To our delight, when the
catalyst loading was slightly increased to 1 mol%, full con-
version was achieved in most cases at 120 1C with extended
reaction time. In the case of arylboronic acids with electron-
donating groups, up to quantitative isolated yields were
obtained (5–8, Scheme 2). The relative position of substituents
hardly has an impact on the coupling efficiency; similar iso-
lated yields were observed with p, m, and o-tolylboronic acids
In a 0.5 mmol scale with PCy3 in 1.5 mL toluene at 100 1C for 6 h.
Isolated yield. With PPh3 for 24 h. In 3 mL toluene. At 80 1C.
Without PCy3. For 24 h.
b
f
c
d
e
g
product 4 and resulted in a nearly quantitative yield (97%,
Table 1, entry 2). When the catalyst was reduced to 0.5 mol%,
no obvious decrease of the yield was found (Table 1, entries 3
and 4). The trace amount of water played a crucial role in the
coupling process: K3PO4Á3H2O resulted in a slightly lower yield,
whereas, anhydrous K3PO4 led to an inferior yield (96% vs. 93%
and 79%; Table 1, entry 4 vs. 5 and 6). To our surprise, a good
yield was still obtained when KOH was selected as a base,15 and
only a small amount of anthrone hydrolyzed from esters was
detected (Table 1, entry 7). Other inorganic and organic bases
all resulted in unsatisfactory yields (ESI†). After screening
various solvents, toluene was still the best choice (ESI†).
A slightly higher yield was observed in the diluted case (3 mL,
97%, Table 1, entry 8). Lower temperature led to a worse result
(27%, Table 1, entry 9). Under the optimal conditions, even
with 0.1 mol% 1 and 0.4 mol% PCy3, an 82% isolated yield was
still obtained (Table 1, entry 10). A very poor yield was observed
without PCy3, and even no reaction occurred in the absence of
pincer complex 1 (Table 1, entries 11 and 12). When NiBr2 was
utilized instead, no product was detected by GC-MS (Table 1,
entry 13). All these results indicate that NHC ligands improve
the catalytic activation of the nickel center. The known catalytic
system (NiCl2(PCy3)2, K3PO4, toluene)9b developed by Grag led
to a slightly inferior yield, and 3 is hardly converted even after
24 h (93%, Table 1, entry 14). Other reductants such as Zn and
phenylboronic acid pinacol ester revealed much poor results
(ESI†). Moreover, in comparison with complex 1, its imid-
azolium analogue 2 only resulted in a moderate yield (83%,
Table 1, entry 15), which confirmed that the extended aromatic
rings of the NHC ligand lead to higher catalytic efficiency.12,13
With these exciting results in hand, the role of the carboxyl-
ate part was examined with a variety of esters (Table 2). The
carbamate analogue was also proved to be an effective substrate
under optimal reaction conditions, and an almost quantitative
yield was observed (>99%, Table 1, entry 2). In contrast to the
Scheme 2 Suzuki–Miyaura cross-coupling reactions of
3 with various aryl
coupling of anthracen-9-yl pivalate, slightly inferior yields were boronic acidsa.
c
11540 Chem. Commun., 2013, 49, 11539--11541
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
Suzuki–Miyaura cross-coupling reactions between anthracen-
9-yl pivalate and (hetero)-arylboronic acids via selective C–O
bond activation. Pyridine-bridged bis-benzimidazolylidene
pincer nickel complex 1 was proved to be an effective catalyst
in this transformation with the catalyst loading as low as
0.1 mol%. Furthermore, our new developed protocol is also
applicable to the coupling of other challenging inactive
anthracenyl carboxylates. All these results indicate that NHCs
are also suitable ligands in the selective C–O bond activation
coupling reactions.
Fig. 1 The fluorescence spectra of compounds 4–12, An and DPA, measured
in CH2Cl2 (2 Â 10À5 mol LÀ1) at room temperature, excitation wavelength at
263 nm.
Notes and references
1 J. Huang, J.-H. Su and H. Tian, J. Mater. Chem., 2012, 22, 10977,
and references therein.
2 R. Sivasakthikumaran, M. Nandakumar and A. K. Mohanakrishnan,
J. Org. Chem., 2012, 77, 9053, and references therein.
3 (a) Y. Zou, D. D. Young, A. Cruz-Montanez and A. Deiters, Org. Lett.,
2008, 10, 4661; (b) W. Huang, X. Zou, K. Kanno and T. Takahashi,
Org. Lett., 2004, 6, 2429; (c) T. Takahashi, M. Kitamura, B. Shen and
K. Nakajima, J. Am. Chem. Soc., 2000, 122, 12876.
4 (a) C.-J. Zheng, W.-M. Zhao, Z.-Q. Wang, D. Huang, J. Ye, X.-M. Ou,
X.-H. Zhang, C.-S. Lee and S.-T. Lee, J. Mater. Chem., 2010, 20, 1560;
(b) S. Tao, S. Xu and X. Zhang, Chem. Phys. Lett., 2006, 429, 622.
5 (a) S. R. S. Sarsah, M. R. Lutz Jr., M. Zeller, D. S. Crumrine and
D. P. Becker, J. Org. Chem., 2013, 78, 2051; (b) S. Kotha, A. K. Ghosh
and K. D. Deodhar, Synthesis, 2004, 549.
6 Metal-Catalyzed Cross-Coupling Reactions, ed A. de Meijere and
F. Diederich, Wiley-VCH, New York, 2004.
7 For reviews, see: (a) B.-J. Li, D.-G. Yu, C.-L. Sun and Z.-J. Shi,
Chem.–Eur. J., 2011, 17, 1728; (b) B. M. Rosen, K. W. Quasdorf,
D. A. Wilson, N. Zhang, A.-M. Resmerita, N. K. Garg and V. Percec,
Chem. Rev., 2011, 111, 1346.
8 T. Yamamoto, J. Ishizu, T. Kohara, S. Komiya and A. Yamamoto,
J. Am. Chem. Soc., 1980, 102, 3758.
9 (a) B.-T. Guan, Y. Wang, B.-J. Li, D.-G. Yu and Z.-J. Shi, J. Am. Chem.
Soc., 2008, 130, 14468; (b) K. W. Quasdorf, X. Tian and N. K. Garg,
J. Am. Chem. Soc., 2008, 130, 14422.
(5a–c, 94–99%, Scheme 2). The protocol is also suitable for
3,5-dimethylphenyl-, 4-tert-butylphenyl and 4-methoxyphenyl-
boronic acids (6–8, 89–98%, Scheme 2); in some cases, the
reaction time had to be extended to 24 h to achieve satisfactory
yields. However, when more sterically hindered 2,6-dimethyl-
phenylboronic acid was applied, no product was detected and
most of 3 was recovered. Delightedly, the arylboronic acids
containing fluoridated electron-withdrawing groups were also
well tolerated when 2 mol% complex 1 was applied (9 and 10,
Scheme 2). In the case of substituted groups with coordination
ability, such as 4-cyano- and 4-formylphenylboronic acids, no
products were observed. Furthermore, 2-naphthylboronic acid
resulted in an almost quantitative yield, whereas the 1-naphthyl
analogue only revealed a moderate outcome (>99% vs. 46%,
11a vs. 11b, Scheme 2). Finally, the heteroarylboronic acids were
also involved in the substrate scope test. Furan-2-boronic acid
resulted in a moderate yield (12a, 56%, Scheme 2), whereas only 10 (a) Q. Zhou, H. D. Srinivas, S. Dasgupta and M. P. Watson, J. Am.
Chem. Soc., 2013, 135, 3307; (b) M. R. Harris, L. E. Hanna,
M. A. Greene, C. E. Moore and E. R. Jarvo, J. Am. Chem. Soc., 2013,
135, 3303; (c) A. R. Ehle, Q. Zhou and M. P. Watson, Org. Lett., 2012,
a 16% yield of 12b was obtained when benzo[b]furan-2-boronic
acid was applied in the reaction (Scheme 2). A poor yield was
observed in the case of 1-methyl-1H-indol-5-yl boronic acid
(7%, ESI†). As expected, no corresponding products formed for
other heteroarylboronic acids with strong coordination ability,
such as (benzo)thiophen-2-yl or 4-pyridinyl boronic acid.
With a series of (hetero)-aryl substituted anthracene deriva-
tives in hand, the study on their fluorescence properties in
solution was carried out. Fig. 1 shows the fluorescence spectra
of the anthracene derivatives 4–12 along with the well-known
fluorescent compounds anthracene (An) and 9,10-diphenyl-
anthracene (DPA). Upon excitation at 263 nm, all 9-arylanthracenes
(4–11) exhibited similar blue emissions of An and DPA ranging
from 382 nm to 432 nm. In comparison with 4, a relative red-shift
14, 1202; (d) K. Muto, J. Yamaguchi and K. Itami, J. Am. Chem. Soc.,
2012, 134, 169; (e) T. Shimasaki, M. Tobisu and N. Chatani, Angew.
Chem., Int. Ed., 2010, 49, 2929.
11 N. Marion and S. P. Nolan, Acc. Chem. Res., 2008, 41, 1440.
12 (a) W. Fang, Q. Deng, M. Xu and T. Tu, Org. Lett., 2013, 15, 3678;
(b) Z. Liu, N. Dong, M. Xu, Z. Sun and T. Tu, J. Org. Chem., 2013,
78, 7436; (c) W. Fang, J. Jiang, Y. Xu, J. Zhou and T. Tu, Tetrahedron,
2013, 69, 673; (d) T. Tu, Z. Sun, W. Fang, M. Xu and Y. Zhou, Org.
Lett., 2012, 14, 4250; (e) T. Tu, W. Fang and J. Jiang, Chem. Commun.,
¨
2011, 47, 12358; ( f ) T. Tu, H. Mao, C. Herbert, M. Xu and K. H. Dotz,
Chem. Commun., 2010, 46, 7796.
13 (a) Z. Wang, X. Feng, W. Fang and T. Tu, Synlett, 2011, 951; (b) T. Tu,
X. Feng, Z. Wang and X. Liu, Dalton Trans., 2010, 39, 10598; (c) T. Tu,
¨
J. Malineni, X. Bao and K. H. Dotz, Adv. Synth. Catal., 2009,
¨
351, 1029; (d) T. Tu, J. Malineni and K. H. Dotz, Adv. Synth. Catal.,
2008, 350, 1791.
¨
was observed with anthracene derivatives 7, 8 and 11a, whereas 14 (a) T. Tu, W. Fang, X. Bao, X. Li and K. H. Dotz, Angew. Chem.,
Int. Ed., 2011, 50, 6601; (b) T. Tu, X. Bao, W. Assenmacher,
a slight blue-shift was found with analogues 5c and 11b. The
¨
H. Peterlik, J. Daniels and K. H. Dotz, Chem.–Eur. J., 2009,
heterocyclic substitutions affected the emission: 2-furanyl and
2-benzofuranyl derivatives 12a,b exhibited rather different
emission peaks, which were obviously red-shifted.
In summary, a series of strong fluorescent (hetero)-aryl
substituted anthracene derivatives were synthesized through
15, 1853; (c) T. Tu, W. Assenmacher, H. Peterlik, G. Schnakenburg
¨
and K. H. Dotz, Angew. Chem., Int. Ed., 2008, 47, 7127; (d) T. Tu,
W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger and
¨
K. H. Dotz, Angew. Chem., Int. Ed., 2007, 46, 6368.
15 K. W. Anderson, T. Ikawa, R. E. Tundel and S. L. Buchwald, J. Am.
Chem. Soc., 2006, 128, 10694.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11539--11541 11541