First synthesis of bidentate NHC-Pd complexes with anthracene and xanthene skeletons

Article abstract of DOI:10.1016/j.tetlet.2007.08.064

We synthesized a series of new bidentate NHC (N-heterocyclic carbene) precursors with anthracene and xanthene skeletons. The corresponding NHC-Pd complexes were prepared from these precursors. The catalytic activity of these Pd complexes was examined.

Full text of DOI:10.1016/j.tetlet.2007.08.064

Tetrahedron Letters 48 (2007) 7498–7501
First synthesis of bidentate NHC–Pd complexes with
anthracene and xanthene skeletons
Shinichi Saito,* Hiromitsu Yamaguchi, Hiroki Muto and Takeshi Makino
Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
Received 26 July 2007; revised 10 August 2007; accepted 16 August 2007
Available online 22 August 2007
Abstract—We synthesized a series of new bidentate NHC (N-heterocyclic carbene) precursors with anthracene and xanthene skele-
tons. The corresponding NHC–Pd complexes were prepared from these precursors. The catalytic activity of these Pd complexes was
examined.
Ó 2007 Elsevier Ltd. All rights reserved.
R
N
R
N-Heterocyclic carbenes (NHCs) derived from imidazo-
lium salts and its derivatives have been widely employed
as ligands for the synthesis of a number of NHC–Pd
complexes.1 Subsequently, the catalytic activity of the
complexes was examined and some complexes turned
out to be better alternative catalysts for the commonly
employed phosphine–Pd catalysts. Various monoden-
tate NHC–Pd complexes1a–d as well as Pd complexes
with two tethered NHC moieties have been reported
to date. As for the tethered NHC–Pd complexes, the
two NHC components were connected with various
functional groups such as methylene (ethylene) group
(A),2 binaphthyl group (B),3 2,6-dimethylenepyridyl
group (C),4–6 m-dimethylenephenyl group (D),5,7 and
so on (Fig. 1).8 However, despite the observed unique
properties of the xanthene-based diphosphine ligands
in organometallic chemistry,9,10 the corresponding
bidentate NHC ligands with rigid anthracene or xan-
thene skeleton have not been reported. In this Letter
we report the first synthesis of the bidentate NHC pre-
cursors with anthracene and xanthene skeletons. The
corresponding NHC–Pd complexes were prepared, and
the catalytic activity of the complexes was examined.
X
X
N
Pd
n
N
N
X
N
X
R
N
R
n
Pd
N
N
n
A
n = 1-2
B
n = 0-1
R
N
R
N
X
R
N
R
X
X
N
Pd
Pd
N
N
n
N
N
N
n
D
C
n = 0-1
Figure 1. Examples of Pd catalysts with two tethered NHC moieties.
(3)14 gave bisimidazole 4, which was alkylated15 to pro-
vide imidazolium salts 5a–b in good yields. The biden-
tate NHC–Pd complex 6a16 was isolated in 66% yield
by the palladation17,18 of 5a with Pd(OAc)2. The meth-
ylated Pd NHC complex, however, was not isolated
under similar reaction condition: the isolation (purifica-
tion) of the product from the reaction mixture turned
out to be extremely difficult due to the low solubility
of the crude reaction mixture.
The results of the synthesis of a bidentate NHC–Pd
complex (6a) with anthracene skeleton are summarized
in Scheme 1. 1,8-Diiodoanthracene (1)11 reacted with
pinacolborane in the presence of Pd catalyst12 to yield
1,8-bis(pinacolboryl)anthracene (2). Suzuki–Miyaura
coupling13 of 2 with 1-(4-iodophenyl)-1H-imidazole
We also synthesized a series of bidentate NHC–Pd com-
plexes (12a–d) with xanthene skeleton and the results are
summarized in Scheme 2. By Ogino’s method, disilyl-
ation of 7 was accomplished in high yield by the reaction
of chlorotrimethylsilane with dilithiated xanthene.19
*
Corresponding author. Tel.: +81 3 3260 4271; fax: +81 3 5261
4631; e-mail: ssaito@rs.kagu.tus.ac.jp
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.064

S. Saito et al. / Tetrahedron Letters 48 (2007) 7498–7501
7499
SiMe3
SiMe3
1) n-BuLi
2) Me3SiCl
O
O
O
O
O
O
O
O
I
I
B
B
B
H
7
8 (86%)
cat. PdCl2(dppf)
Na2CO3
B(OH)2
B(OH)2
1) BBr3
2) H2O
1
O
3
2 (88%)
N
N
N
N
N
cat. Pd(PPh3)4
Na2CO3
9 (96%)
N
R
R
N
N
N
N
N
N
RI
3
I
N
N
RX
cat. Pd(PPh3)4
Na2CO3
4 (45%)
O
O
R
N
R
N
R
R
I
N
N
Pd
I
2 X
n-octyl, X = I)
b (R = Me, X = I)
c (R = Bn, X = I)
d (R = Bn, X = Br)
N
N
N
10 (72%)
N
11a (R =
Pd(OAc)2
R
R
N
X
Pd
X
N
(60% - quant.)
N
N
2 I
n-octyl, 78%)
Pd(OAc)2
5a (R =
b (R = Me, 65%)
6a (R = n-octyl, 66%)
12a (R = n-octyl, X = I)
Scheme 1. Synthesis of a bidentate NHC–Pd complex (6a) with
anthracene skeleton.
O
b (R = Me, X = I)
c (R = Bn, X = I)
d (R = Bn, X = Br)
(15-73%)
Subsequently, ipso substitution20 of 8 with BBr3 was car-
ried out, and the hydrolysis of the bis(dibromoboryl)
compound gave 9. Suzuki–Miyaura coupling of bis-
boronic acid 9 with 3 gave 10, which was alkylated and
palladated to provide the corresponding Pd complexes
12a–d.21 Unlike the corresponding anthracene complex,
methyl derivative 12b was isolated without difficulty.
The compounds were characterized by NMR and the
monomeric structure of the complex was confirmed by
an ESI-MS analysis of 12c.22
Scheme 2. Synthesis of bidentate NHC–Pd complexes (12a–d) with
xanthene skeleton.
Table 1. Palladium-catalyzed coupling reactions of aryl iodides with
phenylboronic acid (Suzuki–Miyaura coupling)
+
I
R
(HO)2B
cat. Pd (0.5 mol %)
K3PO4 (2.0 equiv)
The catalytic activity of these new Pd complexes for
some reactions was examined. The results of Suzuki–
Miyaura coupling reaction are summarized in Table 1.
The reactions of various aryl iodides and phenylboronic
acid were carried out in the presence of Pd catalyst
(0.5 mol %) and K3PO4 (2.0 equiv) in DMF. The cata-
lytic activity of the Pd complexes was compared by car-
rying out the reaction of 4-nitroiodobenzene with
phenylboronic acid at 80 °C.23 The catalytic activity of
6a was much lower compared to other catalysts we
examined, and the coupling product was isolated in
low yield even after the reaction mixture was heated
for 72 h (entry 1). On the other hand, the catalytic activ-
ity of the xanthene complexes with alkyl substituents
(12a, b) was much higher: the reaction completed in
19 h, and the products were isolated in high yields (en-
tries 2 and 3). The catalytic activity of 12c was, however,
much lower (entry 4). The results demonstrate that the
catalytic activity of these complexes could be controlled
by the bridging group (xanthene/anthracene skeleton) as
well as the substituents bound to the NHC moiety.
R
DMF, 80 ºC
Entry
R
Catalyst
Time (h)
Yield (%)
1
2
3
4
5
6
7
8a
NO2
NO2
NO2
NO2
6a
72
19
19
19
48
19
32
32
15
82
96
Trace
96
86
96
81
12a
12b
12c
12b
12b
12b
12b
CH3
OCH3
CO2CH3
CO2CH3
a A smaller amount (0.1 mol %) of 12b was used.
Since 12b was the most efficient catalyst, we examined
the reactions of various iodoarenes with phenylboronic
acid in the presence of 12b. The reactions of electron-
rich iodoarenes such as 4-iodotoluene and 4-iodoani-
sole, as well as ethyl 4-iodobenzoate proceeded
smoothly and the corresponding biphenyls were isolated

7500
S. Saito et al. / Tetrahedron Letters 48 (2007) 7498–7501
in high yields (entries 5–7). The reaction of ethyl 4-
iodobenzoate proceeded even in the presence of
0.1 mol % of 12b, and the product was isolated in 81%
yield (entry 8).24
Acknowledgment
We thank Professor Shinji Toyota (Okayama University
of Science) for providing the detailed procedures for the
synthesis of 1.
We next examined Mizoroki–Heck reaction of various
aryl halides and n-butyl acrylate, and the results are
summarized in Table 2. The reactions were carried out
in the presence of Pd catalyst (0.5 mol %), Bu4NBr
(1.0 equiv), and NaOAc (1.1 equiv) in DMF.25,26 Com-
pared to Suzuki–Miyaura reaction, higher temperature
(120 °C) was required for the Mizoroki–Heck reaction.
The anthracene complex 6a turned out to be a good cat-
alyst for the reaction of 4-iodonitrobenzene with n-butyl
acrylate, and the product was isolated in 92% yield (en-
try 1). Moreover, the catalytic activity of xanthene com-
plexes was much higher, and the reactions completed in
shorter periods (entries 2–5). It is noteworthy that the
benzyl derivatives were also efficient catalysts for this
reaction (entries 4 and 5, see also Table 1, entry 4).
The reactions of various iodoarenes were examined in
the presence of 12b and the corresponding esters were
isolated in good yields (entries 6–8). The observed sub-
stituent effects on the reactivity of the iodoarenes were
in accordance with those reported in Table 1, and simi-
lar results were also reported in general: electron-defi-
cient haloarenes were more reactive. The reactions of a
bromoarene proceeded at 140 °C, while the reaction of
4-chloronitrobenzene was very slow (entries 9 and 10).27
Supplementary data
Experimental details for the synthesis of 2, 4–6 and 8–12
and spectral data of the compounds. Supplementary
data associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2007.08.064.
References and notes
1. (a) Reviews: N-Heterocyclic Carbenes in Synthesis; Nolan,
S. P., Ed.; Wiley-VCH: Weinheim, 2006; (b) N-Heterocy-
clic Carbenes in Transition Metal Catalysis; Glorius, F.,
Ed.; Springer: Berlin, 2006; (c) Kantchev, E. A. B.;
O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed.
2007, 46, 2768–2813; (d) Kantchev, E. A. B.; O’Brien, C.
J.; Organ, M. G. Aldrichim. Acta 2006, 39, 97–111; (e)
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2005, 105, 3978–4008; (g) Peris, E.; Crabtree, R. H. Coord.
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Xu, Q.; Shi, M. Org. Lett. 2007, 9, 865–868.
In summary, we synthesized new bidentate NHC precur-
sors with anthracene and xanthene skeletons and pre-
pared a series of bidentate NHC–Pd complexes. The
catalytic activity of the complexes for Mizoroki–Heck
reactions and Suzuki–Miyaura coupling was examined.
The bidentate NHC precursors prepared in this study
could be utilized for the synthesis of various bidentate
NHC–metal complexes. The study of the detailed struc-
ture of the complexes, as well as the examination of the
catalytic activity of related complexes, is on going.
Table 2. Palladium-catalyzed coupling reactions of aryl halides with
n-butyl acrylate (Mizoroki–Heck reaction)
+
X
R
COOn-Bu
cat. Pd (0.5 mol %)
-Bu4NBr (1.0 equiv)
n
NaOAc (1.1 equiv)
R
COOn-Bu
DMF
4. CNC type complexes (n = 0): (a) Peris, E.; Mata, J.; Loch,
J. A.; Crabtree, R. H. Chem. Commun. 2001, 201–202; (b)
Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Inorg. Chim. Acta 2002, 327, 116–125; (c) Loch, J. A.;
Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.; Crabtree,
R. H. Organometallics 2002, 21, 700–706.
5. CNC and CCC type complexes (n = 1): (a) Grundemann,
S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree, R.
H. Organometallics 2001, 20, 5485–5488; (b) Miecznikow-
ski, J. R.; Grundemann, S.; Albrecht, M.; Megret, C.;
Clot, E.; Faller, J. W.; Eisenstein, O.; Crabtree, R. H.
Dalton Trans. 2003, 831–838; (c) Danopoulos, A. A.;
Tulloch, A. A. D.; Winston, S.; Eastham, G.; Hursthouse,
M. B. Dalton Trans. 2003, 1009–1015.
Entry
R
X
Catalyst Temperature Time Yield
(°C)
(h)
(%)
1
2
3
4
5
6
7
8
9
NO2
NO2
NO2
NO2
NO2
CH3
OCH3
CO2CH3
NO2
I
I
I
I
I
I
I
I
6a
120
120
120
120
120
120
120
120
140
140
26
16
16
16
16
22
40
16
17
48
92
80
92
91
92
83
85
76
70
7
12a
12b
12c
12d
12b
12b
12b
Br 12b
Cl 12b
10
NO2

S. Saito et al. / Tetrahedron Letters 48 (2007) 7498–7501
7501
6. Hahn, F. E.; Jahnke, M. C.; Gomez-Benitez, V.; Morales-
Morales, D.; Pape, T. Organometallics 2005, 24, 6458–
6463.
7. Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics
2007, 26, 150–154.
20. (a) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 1772–1784; (b) Qin, Y.; Cheng, G.;
Sandararaman, A.; Ja¨kle, F. J. Am. Chem. Soc. 2002,
124, 12672–12673.
21. Selected analytical data: Compound 12b: Mp >300 °C; 1H
NMR (270 MHz, CDCl3) 7.72–7.69 (d, J = 8.6 Hz, 4H),
7.59–7.55 (d, J = 8.6 Hz, 4H), 7.44–7.40 (q, J = 3.2 Hz,
2H), 7.14–7.12 (m, 4H), 7.02–7.01 (d, J = 2.2 Hz, 2H),
6.98–6.97 (d, J = 1.9 Hz, 2H), 4.09 (s, 6H), 1.68 (s, 6H);
13C NMR (150 MHz, CDCl3) 167.7, 147.8, 138.9, 137.7,
131.3, 130.4, 130.1, 128.6, 125.2, 125.1, 123.4, 123.1, 122.9,
39.0, 34.6, 31.8; IR (KBr) 2923, 2852, 1519, 1469, 1433,
1395, 1216, 841, 790, 745, 696 cmÀ1. Anal. Calcd for
C35H34I2N4OPd: C, 47.40; H, 3.86; N, 6.32. Found: C,
8. (a) Magill, A. M.; McGuinness, D. S.; Cavell, K. J.;
Britovsek, G. J. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J.; White, A. H.; Skelton, B. W. J. Organo-
met. Chem. 2001, 617–618, 546–560; (b) Perry, M. C.; Cui,
X.; Burgess, K. Tetrahedron: Asymmetry 2002, 13, 1969–
1972; (c) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R.
Organometallics 2003, 22, 4384–4386; (d) Douthwaite, R.
E.; Houghton, J.; Kariuki, B. M. Chem. Commun. 2004,
698–699; (e) Marshall, C.; Ward, M. F.; Harrison, W. T.
A. Tetrahedron Lett. 2004, 45, 5703–5706; (f) Nielsen, D.
J.; Cavell, K. J.; Skelton, B. W.; White, A. H. Organo-
metallics 2006, 25, 4850–4856.
9. Recent examples: (a) Fujita, K.; Yamashita, M.; Pusch-
mann, F.; Alvarez-Falcon, M. M.; Incarvito, C. D.;
Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 9044–9045;
(b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc.
2006, 128, 12644–12645; (c) Ngassa, F. N.; DeKorver, K.
A.; Melistas, T. S.; Yeh, E. A. H.; Lakshman, M. K. Org.
Lett. 2006, 8, 4613–4616; (d) Klingensmith, L. M.;
Strieter, E. R.; Barder, T. E.; Buchwald, S. L. Organo-
metallics 2006, 25, 82–91.
1
47.20; H, 3.57; N, 6.25. Compound 12c: Mp >300 °C; H
NMR (300 MHz, CDCl3) 7.80–7.78 (d, J = 8.6 Hz, 4H),
7.66–7.63 (d, J = 8.6 Hz, 4H), 7.57–7.54 (m, 4H), 7.49–
7.46 (q, J = 2.8 Hz, 2H), 7.35–7.33 (d, J = 6.4 Hz, 6H),
7.21–7.19 (m, 4H), 7.04–7.03 (d, J = 1.9 Hz, 2H), 6.74–
6.73 (d, J = 1.9 Hz, 2H), 5.77 (s, 4H), 1.73 (s, 6H); 13C
NMR (75 MHz, CDCl3) 167.4, 147.8, 139.0, 137.6, 135.2,
131.3, 130.3, 130.0, 129.5, 128.8, 128.5, 128.4, 125.3, 125.2,
123.4, 121.1, 55.3, 34.6, 31.7; IR (KBr) 2924, 1731, 1520,
1470, 1432, 1393, 1280, 1215, 839, 792, 753, 725, 701 cmÀ1
;
HR-MS (ESI) Calcd for C47H38I2N4OPdNa ([M+Na]+):
1057.0079. Found: 1057.0051.
10. The xanthene motif has also been used for the rigid
bidentate ligands such as Xantphos. See: (a) Freixa, Z.;
van Leeuwen, P. W. N. M. Dalton Trans. 2003, 1890–1901;
(b) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J.
N. H. Acc. Chem. Res. 2001, 34, 895–904.
22. The attempted synthesis of the following Pd complexes
failed since the palladation of the corresponding imidazo-
lium salts did not proceed. The details will be reported in
due course.
11. (a) Goichi, M.; Segawa, K.; Suzuki, S.; Toyota, S.
Synthesis 2005, 2116–2118; (b) Lovell, J. M.; Joule, J. A.
Synth. Commun. 1997, 27, 1209–1215.
12. Murata, M.; Watanabe, S.; Masuda, Y. J. Org. Chem.
1997, 62, 6458–6459.
R
N
R
N
R
N
R
N
X
Pd
X
X
Pd
N
N
N
N
X
O
13. Manickam, G.; Schluter, A. D. Synthesis 2000, 442–446.
14. Walser, A.; Flynn, T.; Mason, C.; Crowley, H.; Maresca,
C.; O’Donnell, M. J. Med. Chem. 1991, 34, 1440–1446.
15. Vargas, V. C.; Rubio, R. J.; Hollis, T. K.; Salcido, M. E.
Org. Lett. 2003, 5, 4847–4849.
23. Lower catalytic activity of 12b was observed when aryl
bromides or chlorides were used as the substrate.
24. General procedure for Suzuki–Miyaura reaction: A mixture
of Pd complex (0.5 mol %), K3PO4 (424 mg, 2.0 mmol),
aryl halide (1.0 mmol), and phenylboronic acid (183 mg,
1.5 mmol) in DMF (4 mL) was heated under Ar, and the
course of the reaction was monitored by GC. After the
reaction completed, the mixture was extracted with Et2O,
and the organic layer was dried over MgSO4. The filtrate
was concentrated and the residue was purified by column
chromatography.
25. Grundemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.;
Crabtree, R. H. Organometallics 2001, 20, 5485–5488.
26. General procedure for Mizoroki–Heck reaction: To a
mixture of Pd complex (0.5 mol %), aryl halide
(1.0 mmol), AcONa (90 mg, 1.1 mmol) and tetra-n-butyl-
ammounium bromide (322 mg, 1.0 mmol) in DMF (4 mL)
was added n-butylacrylate (0.20 mL, 1.4 mmol), and the
mixture was heated under Ar. The mixture was worked up
as described for the Suzuki–Miyaura reaction.
16. Compound 6a: A gray solid; mp 271–274 °C; 1H NMR
(270 MHz, CDCl3) 8.89 (s, 1H), 8.55 (s, 1H), 8.07–8.04 (d,
J = 8.4 Hz, 2H), 7.96–7.93 (d, J = 8.1 Hz, 4H), 7.65–7.62
(d, J = 8.1 Hz, 4H), 7.57–7.51(t, J = 8.1 Hz, 2H), 7.42–
7.40 (d, J = 5.9 Hz, 2H), 7.13–7.12 (d, J = 1.9 Hz, 2H),
7.10–7.09 (d, J = 1.6 Hz, 2H), 4.73–4.67 (t, J = 7.7 Hz,
4H), 2.20–2.09 (qui, J = 7.7 Hz, 4H), 1.53–1.23 (m, 20H),
0.92–0.87 (t, J = 6.8 Hz, 6H); 13C NMR (150 MHz,
DMSO-d6) 162.4, 160.7, 137.8, 137.1, 129.9, 129.2, 127.1,
126.4, 124.7, 124.2, 123.1, 122.0, 121.4, 49.3, 34.2, 29.7,
27.9, 27.4, 27.1, 24.9, 20.6, 12.4; IR (KBr) 2923, 2852,
2360, 1669, 1513, 1455, 1421, 1266, 842, 747, 695,
419 cmÀ1. Anal. Calcd for C48H56I2N4OPd: C, 54.95; H,
5.38; N, 5.34. Found: C, 54.93; H, 5.14; N, 5.55.
17. Enders, D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J.
H. Chem. Ber. 1996, 129, 1483–1488.
18. Magill, A. M.; McGuinness, D. S.; Cavell, K. J.;
Britovsek, G. J. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J.; White, A. H.; Skelton, B. W. J. Organo-
met. Chem. 2001, 617–618, 546–560.
19. Tobita, H.; Hasegawa, K.; Minglana, J. J. G.; Luh, L. S.;
Okazaki, M.; Ogino, H. Organometallics 1999, 18, 2058–
2060.
27. It should be noted that the Pd black formed by the
decomposition of the NHC–Pd complexes at high
temperature (120–140 °C) could be the true catalytic
species.