Imidazole and imidazoline derivatives as N-donor ligands for palladium-catalyzed Mizoroki-Heck reaction

Article abstract of DOI:10.1002/adsc.200600364

Imidazole and imidazoline (dihydroimidazole) derivatives can serve as efficient and simple Iigands for the palladium-catalyzed Mizoroki-Heck reaction. Among the imidazole and imidazoline derivatives in our investigations, the 2-methylimidazoline-palladium

Full text of DOI:10.1002/adsc.200600364

COMMUNICATIONS
DOI: 10.1002/adsc.200600364
Imidazole and Imidazoline Derivatives as N-Donor Ligands for
Palladium-Catalyzed Mizoroki–Heck Reaction
Satoshi Haneda,a Chigusa Ueba,a Kazuo Eda,a and Masahiko Hayashia,*
a
Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
Fax : (+81)-78-803-5688; e-mail: mhayashi@kobe-u.ac.jp
Received: July 19, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Imidazole and imidazoline (dihydroimida-
zole) derivatives can serve as efficient and simple li-
gands for the palladium-catalyzed Mizoroki–Heck
reaction. Among the imidazole and imidazoline de-
rivatives in our investigations, the 2-methylimidazo-
line-palladium(II) chloride complex exhibited the
highest catalytic activity.
Herein, we report on the palladium-catalyzed Miz-
oroki–Heck reactions that involve the simple catalytic
system of PdCl2–imidazole or PdCl2–imidazoline. Not
only are such complexes structurally simple, readily
available, and inexpensive, but the imidazole moiety
allows for the facile introduction of various substitu-
ents into the imidazole framework. Our report also
includes the single X-ray crystal structuresof the pal-
ladium-complexes, which, to the best of our knowl-
edge, is the first such report.
À
Keywords: catalyst design; C C coupling; ligand
effect; Mizoroki–Heck reaction; palladium
Initially, we investigated the reaction of 4-bromoto-
luene with methyl acrylate under the conventional
conditions in DMF at 1208C with K2CO3 as a base.
The highly reactive PdCl2 [compared to PdCAHTRE(UNG OAc)2]
Since the first report in 1971, the Mizoroki–Heck re- was chosen as the source of palladium for the reac-
action has been widely employed in the construction tion. Our results indicated that the ligands are neces-
of carbon-carbon bonds.[1] While phosphine and relat- sary for catalytic activity; in the absence of the imida-
ed compounds are often employed as ligands, elec- zole or imidazoline ligands, PdCl2 alone was not effec-
tron-rich and bulky alkylphosphines, such as PACHTREU(GN t-Bu)3, tive in catalyzing the reaction (only 9% yield). Vari-
have been recently developed as highly effective li- ous imidazoles and imidazolines, with substituents at
gands.[2] Furthermore, because of recent advances in the 1- or 2-position (see Table 1), were evaluated as
the field of palladacycles,[3] N-heterocyclic carbenes ligands for the reactions, which were run under identi-
have also been utilized as ligands.[4] Additionally, Miz- calconditions, inculding the reaction time of 24 h.
oroki–Heck reactions involving various catalytic sys- Among the ligands, 2-methylimidazoline (1e) exhibit-
tems such as palladium and activated aryl chlorides ed the highest reactivity (entry 5).
[e.g., (CH3CN)2PdCl2–PPh4Cl],[5] Pd
A
Next, our investigations examined the efficiency of
excess PACHTREUNG
(OEt)3 and heterogeneous Pd/C[7] have also this PdCl2–1e catalytic system. Reactions involving
[6]
been developed.[8,9] To date, however, the use of imi- various haloarenes and olefins, as listed in Table 2,
dazole moieties is generally limited to palladium-car- were carried out in the presence of PdCl2 (1 mol%)
bene complexes that possess a carbon-palladium and 1e (2 mol%). Among the haloarenes, the substi-
bond.
tuted bromobenzenes, which possess either electron-
In contrast to phosphine-type ligands, the low toxic- withdrawing groups or electron-donating groups af-
ity and stability of nitrogen-based ligands, such as pyr- forded the corresponding Mizoroki–Heck products in
idine or imidazole derivatives, have attracted the in- high yields. Decreasing the amount of the catalysts
terest of synthetic organic chemists. As examples, (PdCl2, 0.1 mol% and 1e, 0.2 mol%) did not signifi-
Rüther, Cavell, et al. have reported the catalytic be- cantly affect the yield (entry 4). In the case of PdCl2
havior of cationic and neutralPd(II) compelxes that
(0.01 mol%) and 1e (0.02 mol%), a turnover number
feature imidazole-based ligands.[10] Welton et al. have (TON) of 2,200 was attained (22% yield). In the
reported on the Suzuki coupling reactions catalyzed cases where styrene was used as an olefin (entries 6–
by (CH3CN)2PdCl2–imidazole and its derivatives.[11]
8), the products were obtained as mixtures of E/Z iso-
mers, inatios of 97/3–99/1. For the coupling reaction
Adv. Synth. Catal. 2007, 349, 833 – 835
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
833

Satoshi Haneda et al.
COMMUNICATIONS
Table 1. Mizoroki–Heck reaction catalyzed by PdCl2 and a
nitrogenous ligand system.[a]
[a]
All reactions were carried out in DMF at 1208C for 24 h
using 1 mol% of PdCl2 and 2 mol% of the nitrogen-
based ligand.
Isolated yield by silica gel column chromatography.
[b]
Figure 1. X-ray structures of the PdCl2–1a complex (above)
and the PdCl2–1e complex (below).
Table 2. Mizoroki–Heck reactions between 4-substituted halo-
arenes and olefins.[a]
reactants – the reactions were terminated when the
haroarenes were completely consumed. For the reac-
tions that proceeded smoothly, the formation of Pd
black (precipitate) was not observed.
The PdCl2–1e complex was prepared by modifying
the reported procedures for PdCl2–1a.[13,14] The coor-
dination between one Pd atom and two molecules of
1e was confirmed by structuralanaylsis of PdCl 2–1a
and PdCl2–1e complexes using X-ray crystallography
(Figure 1; depository numbers: CCDC 279171 and
Entry
X
R1
R2
Time [h] Yield [%][b]
1
2
Br Me
Br Me
Br COMe CO2-t-Bu
Br COMe CO2-t-Bu
Br OMe
Br OMe
Br Me
Br COMe Ph
ClCHO
ClCHO
CO2Me
CO2-t-Bu
24
3
1
7
3
3
79
77
99
95
80
91
3
4[c]
5
CO2-t-Bu
Ph
Ph
1
289173, respectively) and H NMR spectroscopy (see
6
7
8
(98/2)[d]
ACHTREUNG
Supporting information). It is worthy of note that
these palladium complexes are stable under ambient
atmosphere, and are soluble in DMF, DMA, and
DMSO, but only slightly soluble in acetonitrile.
8
3
94 (99/1)[d]
98 (97/3)[d]
18
9
CO
CO
2-t-Bu 24
2-t-Bu 24
10[e]
85
Additionalinvestigations were carried out for the
Mizoroki–Heck coupling between 4-bromotoluene
and methylacryal te in the presence of PdCl 2–1e com-
plex (Scheme 1). Using the palladium complex that
was generated in situ (1 mol%), the product was ob-
tained with a yield of 79%. In contrast, the use of the
isolated PdCl2–1e complex (1 mol%), as described
above, increased the yield to 91%. Presumably, the in
situ catalyst may consist of some unreacted PdCl2 re-
sulting in lower yield. Even the presence of 0.01
[a]
All reactions between halobenzene (2 mmol) and olefin
(2 mmol) were carried out in DMF at 1208C.
Isolated yields determined using silica gel column chro-
matography.
PdCl2 (0.1 mol%) was used.
E/Z ratio shown in parentheses.
[b]
[c]
[d]
[e]
n-Bu4NBr (2 equivs.) was added.
between an unreactive arylchol ride and tert-butylac-
rylate (entry 9), the PdCl2–1e catalyst system was im- mol% of the isolated PdCl 2–1e complex gave the
proved by the addition of two equivalents of n- product in 70% yield (TON=7,000). Among the
Bu4NBr (TBAB), so-called Jeffery conditions,[12] compatible solvents for the PdCl2–1e complex, DMF
(entry 10). Except for that of entry 9, the reaction resulted in the highest reactivity and was thus chosen
times in Table 2 depended on the substrates and the as the preferred solvent for the reactions.
834
www.asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 833 – 835

N-Donor Ligands for Palladium-Catalyzed Mizoroki–Heck Reaction
COMMUNICATIONS
Scheme 1. Mizoroki-Heck reactions between 4-bromotoluene and methyl acrylate in the presence of PdCl2–1e complex.
References
In summary, a simple catalytic system involving pal-
ladium-imidazole or -imidazoline was developed for
the Mizoroki–Heck reaction. Because the reaction
using only PdCl2 afforded the coupling product with a
[1] a) T. Mizoroki, K. Mori, A. Ozaki, Bull. Chem. Soc.,
Jpn. 1971, 44, 581; b) R. F. Heck, J. P. Nolley, Jr, J.
Org. Chem. 1972, 37, 2320–2322.
yield of merely 9%, it was evident that the presence
of imidazole or imidazoline ligands significantly im-
proves the effectiveness of the catalyst. The X-ray
structural analysis of the complex reveals that Pd
atom is coordinated to two imidazole or imidazoline
molecules via their nitrogen atoms. In the case of the
unreactive chlorobenzene reaction, the yield was im-
proved under Jeffery (n-Bu4NBr) conditions. A high
TON (up to 7,000) was achieved in the reaction of 4-
acetylbromobenzene with tert-butylacryal te using iso-
lated PdCl2–1e complex. Further investigations to-
wards other coupling reactions using our catalytic
system are currently in progress.
[2] a) A. F. Littke, G. C. Fu, J. Org. Chem. 1999, 64, 10–
11; b) A. F. Littke, G. C. Fu, J. Am. Chem. Soc. 2001,
123, 6989–7000.
[3] a) W. A. Herrmann, C. Brossmer, K. Öfele, C.-P. Rei-
singer, T. Priermeier, M. Beller, H. Fischer, Angew.
Chem. Int. Ed. Engl.,1995, 34, 1844–1847; b) W. A.
Herrmann, V. P. W. Bçhm, C.-P. Reisinger, J. Organo-
met. Chem. 1999, 576, 23–41.
[4] a) W. A. Herrmann, Angew. Chem. Int. Ed. 2002 41,
1290–1309; b) A. C. Hillier, G. A. Grasa, M. S. Viciu,
H. M. Lee, C. Yang, S. P. Nolan, J. Organomet. Chem.
2002, 653, 69–82.
[5] M. T. Reetz, G. Lohmer, R. Schwickardi, Angew.
Chem. Int. Ed. 1998, 37, 481–483.
[6] M. Beller, A. Zapf, Synlett 1998, 792–793.
[7] M. Julia, M. Duteil, M. C. Grard, E. Kuntz, Bull. Soc.
Chim. Fr. 1973, 2791.
[8] Recent excellent review: N. T. S. Phan, M. V. D. Sluys,
C. W. Jones, Adv. Synth. Catal. 2006, 348, 609–679.
[9] J. Tsuji, Palladium Reagents and Catalysts, John Wiley
& Sons Ltd., 2004, Chapter 3.
[10] M. C. Done, T. Rüther, K. J. Cavell, M. Kilner, E. J.
Peacock, N. Braussaud, B. W. Skelton, A. White, J. Or-
ganomet. Chem. 2000, 607, 78–92.
Experimental Section
General Procedure for Mizoroki–Heck Reaction
A mixture of PdCl2 (3.55 mg, 0.01 mmol), imidazole-based
ligand (0.02 mmol) and K2CO3 (2 equivs.) in DMF
(10.0 mL) was stirred at 508C for 1 h. To this mixture was
added 4-bromotoluene (2 mmol) and olefin (2 or 4 mmol),
then it was then stirred at 1208C. After allowing the reac-
tion mixture to coolto room temperature, the precipitates
were removed by filtration, and the products were extracted
with diethylether. The combined organic alyers were dried
over anhydrous magnesium sulfate, filtered, and evaporated
to afford the crude product, which was purified by silica gel
column chromatography to give the coupling products.
[11] C. J. Mathews, P. J. Smith, T. Welton, J. Mol. Catal. A.
2003, 206, 77–82; see also, a PdACHTRE(UNG OAc)2/2-aryl-2-oxazo-
lines-catalyzed Suzuki coupling: B. Tao, D. W. Boykin,
Tetrahedron Lett. 2002, 43, 4955–4957.
[12] a) T. Jeffery, J. Chem. Soc., Chem. Commun. 1984,
1287–1289; b) T. Jeffery, Tetrahedron 1996, 52, 10113–
10130.
[13] a) J. Reedijk, J. K. de Ridder, Inorg. Nucl. Chem. Lett.
1976, 12, 585–590; b) C. G. van Kralingen; J. K. de Rid-
der, J. Reedijk, Inorg. Chem. Acta 1979, 36, 69–77;
c) for powder diffraction data, see: M. C. N. Ranninger,
J. Appl. Cryst. 1979, 12, 420–421.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Areas “Advanced Molecular Transforma-
tions ofCarbon Resources” and No. B17340020 rfom the
Ministry ofEducation, Culture, Sports, Science and Technol-
ogy, Japan.
[14] For the X-ray structure of a bisCAHTRE(UNG pyridine)-based palladi-
um catalyst, see: M. R. Buchmeiser, T. Schareina, R.
Kempe, K. Wurst, J. Organomet. Chem. 2001, 634, 39–
46; see also ref.[10]
Adv. Synth. Catal. 2007, 349, 833 – 835
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
835