Selenium-ligated palladium(II) complexes as highly active catalysts for carbon-carbon coupling reactions: The Heck reaction

Article abstract of DOI:10.1021/ol0486977

(Equation Presented) Three selenium-ligated Pd(II) complexes were readily synthesized and shown to be extremely active catalysts for the Heck reaction of various aryl bromides, including deactivated and heterocyclic ones. The catalytic activity of the selenide-based Pd(II) complexes not only rivals but vastly outperforms that of the corresponding phosphorus and sulfur analogues. Practical advantages of the selenium-based catalysts include their straightforward synthesis and high activity in the absence of any additives as well as the enhanced stability of the selenide ligands toward air oxidation.

Full text of DOI:10.1021/ol0486977

ORGANIC
LETTERS
2004
Vol. 6, No. 17
2997-2999
Selenium-Ligated Palladium(II)
Complexes as Highly Active Catalysts
for Carbon−Carbon Coupling Reactions:
The Heck Reaction
Qingwei Yao,* Elizabeth P. Kinney, and Chong Zheng
Department of Chemistry and Biochemistry, The Michael Faraday Laboratories,
Northern Illinois UniVersity, DeKalb, Illinois 60115-2862
qyao@niu.edu
Received July 8, 2004
ABSTRACT
Three selenium-ligated Pd(II) complexes were readily synthesized and shown to be extremely active catalysts for the Heck reaction of various
aryl bromides, including deactivated and heterocyclic ones. The catalytic activity of the selenide-based Pd(II) complexes not only rivals but
vastly outperforms that of the corresponding phosphorus and sulfur analogues. Practical advantages of the selenium-based catalysts include
their straightforward synthesis and high activity in the absence of any additives as well as the enhanced stability of the selenide ligands
toward air oxidation.
Selenium-containing organic molecules are extremely useful
compounds in synthetic organic chemistry.1 The arylselanyl
(ArSe-) and alkylselanyl (RSe-) moieties are two of the
most privileged functionalities that are of tremendous
synthetic utility by virtue of their versatile and well-behaved
transformations. The field is still being enriched by the recent
introduction of, among others, polymer-bound selenium
reagents,2 chiral organoselenium compounds,3 and fluorous
selenium reagents.4 However, one of the most important
facets of the electronic properties of selenium, its strong
electron-donating ability, has rarely been explored in the field
of transition metal catalysis. We report here that organose-
lenides, when used as ligands in catalytically active species,
offer great potential in transition metal-catalyzed C-C bond-
forming reactions such as the palladium-mediated Heck
reaction.
The Heck reaction is a powerful tool for C-C bond
formation in organic synthesis.5 Outstanding catalyst systems
developed to date include those that employ sterically bulky
and electron-rich phosphine ligands6 or phosphine mimics
(4) Crich, D.; Neelamkavil, S.; Sartillo-Piscil, F. Org. Lett. 2000, 2, 4029
and references therein.
(1) Selected monographs: (a) Paulmier, C. Selenium Reagents and
Intermediates in Organic Synthesis; Pergamon: Oxford, 1986. (b) Orga-
noselenium Chemistry; Black, T. G., Ed.; Oxford: New York, 1999. (c)
Organoselenium Chemistry; Wirth, T., Ed.; Springer: Berlin, 2000.
(2) Selected papers: (a) Nicolaou, K. C.; Pastor, J.; Barluenga, S.;
Winssinger, N. Chem. Commun. 1998, 1947. (b) Ruhland, T.; Andersen,
K.; Pedersen, H. J. Org. Chem. 1998, 63, 9204. (c) Uehlin, L.; Wirth, T.
Org. Lett. 2001, 3, 2931.
(5) (a) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: London, 1985. (b) Tsuji, J. Palladium Reagents and Catalysts;
Wiley: Chichester, 1995. (c) Crisp, G. T. Chem. Soc. ReV. 1998, 27, 427.
(d) Beletskaya, I. P.; Cheprokov, A. Chem. ReV. 2000, 100, 3009.
(6) Leading examples: (a) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc.
2001, 123, 6989. (b) Shaughnessy, K. H.; Kim, P.; Hartwig, J. F. J. Am.
Chem. Soc. 1999, 121, 2123. (c) Ehrentraut, A.; Zapf, A.; Beller, M. Synlett
2000, 1589.
(3) For a recent review, see: Wirth, T. Tetrahedron 1999, 55, 1.
10.1021/ol0486977 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/30/2004

Scheme 1. Synthesis of Selenium-Ligated Palladium
Complexes
Figure 1. Palladium(II) complexes for Heck reaction.
such as N-heterocyclic nucleophilic carbenes (NHCs).7
Phosphorus-, nitrogen-, and sulfur-containing palladacycles
have also emerged recently as powerful catalysts for this
reaction.8 In view of the strong donating properties of
organoselenides but an obvious lack of any precedents in
their utilization in the Heck reaction, we synthesized a
number of selenide-based Pd complexes including structures
1-3 (Figure 1). Each of these complexes represents one of
the three major types of Pd(II) catalysts5b-d,8 (types I-III,
respectively, with L being broadly defined as the ligating
atom) that have been used in the Heck chemistry. These
complexes, which proved to be remarkably stable toward
air and moisture, were readily synthesized as described in
Scheme 1 and isolated by standard means. Their structures
were fully characterized by various NMR techniques and
elemental analysis, and in the case of complex 1, by X-ray
crystallographic determination. Complex 1 adopts a distorted
square planar geometry at the metal center with the selenide
ligands disposed trans relative to each other.
To test their activity in the Heck reaction, we initially
examined the reaction of bromobenzene (PhBr) with n-butyl
acrylate in N,N-dimethylacetamide (DMA) in the presence
of the “pincer” complex 3. We were pleased to find that the
reaction proceeded smoothly with only 0.05 mol % 3 when
Na2CO3 was used as the base. Thus, after 22 h at 140 °C,
the product n-butyl trans-cinnamate was isolated in 71%
yield, corresponding to a turnover number (TON) of 1420.
This is especially interesting since a sulfur analogue of 3
(type III: L ) S, R ) Ph, X ) Cl, R′ ) NHAc) has
previously been shown to be completely inactive for the Heck
coupling of aryl bromides under similar conditions.8i Other
bases, including NaOAc, K2CO3, and KOAc, gave much
lower yield (39% with NaOAc, 16% with K2CO3 and 19%
with KOAc). No coupling product was isolated when NEt3
was used as the base. Aryl chlorides were found to be
inactive under these conditions. The Heck coupling of a
variety of aryl bromides including activated, deactivated, and
heterocyclic ones with n-butyl arylate was subsequently
examined using these complexes (Table 1).
As can be readily seen from the data in Table 1, all three
selenium-ligated Pd complexes show excellent catalytic
activity, giving rise to extremely high TONs, with complex
3 exhibiting the highest activity. It is noteworthy that the
simple bis(selenide)-Pd(II) complex 1 can act as an excellent
catalyst for the Heck reaction (TON up to 223 000 for PhBr)
and is significantly more reactive than the analogous pal-
ladium complex derived from organosulfide, PdCl2(SEt2)2,9
which gives much lower TON even in the presence of nBu4-
NBr as an additive.10 Similarly, the Se-palladacycle 2, which
gave a TON of up to 120 000 for PhBr (entry 8) and 1.7 x
106 for 4-bromobenzaldehyde (entry 18), outperforms its
sulfur analog8h (type II: L ) S, R ) tBu, R′ ) Me, X )
Cl) (maximum TON up to 33 000 for 4-bromobenzaldehyde
in the presence of nBu4NBr). Furthermore, 2 is even more
reactive than its phosphine analogues, including Herrmann’s
P-palladacycle8e and Gibson’s P-palladacycle (type II: L )
P, R ) Ph2, R′ ) H, X ) Br)8f for similar Heck
(7) For selected recent reviews, see: (a) Bourissou, D.; Guerret, O.;
Gabba¨ı, F. P.; Bertrand, C. B. Chem. ReV. 2000, 100, 39. (b) Herrmann,
W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(8) Leading reviews: (a) Albrecht, M.; van Koten, G. Angew. Chem.,
Int. Ed. 2001, 40, 3750. (b) Dupont, J.; Pfeffer, M. Spencer, M. Eur. J.
Inorg. Chem. 2001, 1917. (c) Bedford, R. B. Chem. Commun. 2003, 1787.
(d) van der Boom M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759. Selected
recent examples: (e) Herrmann, W. A.; Brossmer, C.; Reisinger, C.-P.;
Priermeier, T. H.; O¨ fele, K.; Beller, M. Chem. Eur. J. 1997, 3, 1357. (f)
Gibson, S.; Foster, D. F.; Eastam, G. R.; Tooze, R. P.; Cole-Hamilton, D.
J. Chem. Commun. 2001, 779. (g) Ohff, M.; Ohff, A.; van der Boom, M.
E.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 11687. (h) Gruber, A. S.;
Zim, D. Z.; Ebeling, G.; Monteriro, A. L.; Dupont, J. Org. Lett. 2000, 2,
1287. (i) Bergbreiter, D. E.; Osburn, P. L.; Wilson, A.; Sink, E. J. Am.
Chem. Soc. 2000, 122, 9058.
(9) Gruber, A. S.; Pozebon, D.; Monteriro, A. L.; Dupont, J. Tetrahedron
Lett. 2001, 42, 7345.
(10) See Supporting Information for a detailed comparison in activity
between catalysts 1-3 and their sulfur and phosphorus analogues.
2998
Org. Lett., Vol. 6, No. 17, 2004

Table 1. Heck Reaction of Aryl Bromides and n-Butyl
Acrylate Using Pd Complexes 1-3a
Table 2. Heck Reaction of Styrene Using Catalyst 2a
entry
1c
ArBr (mmol)
PhBr (2.0)
mol % Pd time (h) % yieldb (TON)
3.9 × 10-3
3.9 × 10-3
2.0 × 10-3
3.9 × 10-3
3.9 × 10-2
21
21
20
42
90
21
20
24
20
24
21
24
43
27 (6900)
2d PhBr (2.0)
4e (1000)
3
4
5
6
7
8
9
10
11
12
13
PhBr (4.0)
4-Me-PhBr (2.0)
2-Me-PhBr (2.0)
97 (49 000)
56 (14 000)
41 (1000)
4-MeO-PhBr (2.0) 3.9 × 10-3
4-OHC-PhBr (2.0) 3.3 × 10-3
4-OHC-PhBr (2.0) 9.0 × 10-4
51 (13 000)
95 (29 000)
92 (100 000)
99e (30 000)
96 (110 000)
85 (22 000)
77e (81 000)
40 (10 000)
4-NC-PhBr (2.0)
4-NC-PhBr (2.0)
3.3 × 10-3
9.0 × 10-4
4-CF3-PhBr (2.0) 3.9 × 10-3
4-CF3-PhBr (2.0) 9.0 × 10-4
3-BrPyr (2.0)
3.9 × 10-3
a Unless otherwise noted, all reactions were performed with 1 equiv of
ArBr, 1.2 equiv of styrene, and 1.2 equiv of NaOAc. b Average of isolated
yields of two runs. c Na2CO3 (1.12 equiv) was used as the base. d K3PO4
(1.2 equiv) was used as the base. e Single run.
In conclusion, we have shown that the selenium-ligated
Pd(II) complexes 1-3 are excellent catalysts for Heck
reaction. These catalysts not only rival but, in most cases,
outperform each of their respective phosphorus and sulfur
analogues for similar Heck reactions of aryl bromides. The
high activity of the catalysts reported here introduces a new
approach to transition metal catalysis. The repertoire of their
application in various coupling reactions and their mechanism
of action only remains to be further explored.
a Unless otherwise noted, all reactions were performed with 1 equiv of
ArBr, 1.2 equiv of n-butyl acrylate, and 1.12 equiv of Na2CO3. b Average
of isolated yields of two runs. c Single run. d Performed with 1.2 equiv of
Na2CO3.
Acknowledgment. We thank the NIH (GM63522) and
the donors of the Petroleum Research Fund, administered
by the American Chemical Society (36466-G1), for partial
support of this work.
reactions.10 The SeCSe-pincer complex 3 (TON up to 240 000
for PhBr, entry 12) is also a more powerful catalyst than
both the phenyl-containing SCS-pincer complex (type III:
L ) S, R ) Ph, X ) Cl, R′ ) NHAc, vide supra)8i and an
electron-rich SCS-type pincer complex (type III: L ) S, R
) tBu, X ) Cl, R′ ) H),8h which were shown to be active
only toward aryl iodides. It is interesting to note that 3 (TON
up to 243 000 for PhBr, entry 12) is almost twice as reactive
as Milstein’s PCP-pincer complex (TON up to 133 000) for
a very similar reaction.8g The high reactivity of the Se-
palladacycle 2 was further established by its excellent
performance in the Heck reaction of styrene with various
aryl bromides using NaOAc as the base11 (Table 2).
Supporting Information Available: Synthesis and full
characterization of 1-3; general procedures for the Heck
reactions; tabulated data for reactivity comparison. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0486977
(11) Interestingly, other bases such as Na2CO3 and K3PO4 gave much
lower yield for this reaction, cf. entries 1-3, Table 2.
Org. Lett., Vol. 6, No. 17, 2004
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