Tridentate SCS palladium(II) complexes: New, highly stable, recyclable catalysts for the Heck reaction

Article abstract of DOI:10.1021/ja991099g

A new pincer-type SCS ligand containing Pd(II) is a simple, robust catalyst for Heck chemistry using a variety of alkene acceptors and aryl iodides. It is less active with aryl bromides. While certain palladium-(II) species insert slowly into the aryl C-H bond of an unsubstituted version of this ligand, the introduction of activating groups into the 5 position of the aromatic ring readily allows quantitative metal insertion. These ligands were synthesized and attached to soluble polymers by simple modification of inexpensive starting materials. For example, both 5-oxy and 5-amido SCS ligands were successfully appended to 5000 M_n poly-(ethylene glycol) via ether or amide linkages, respectively. Both the 5-oxo and 5-amido complexes are active as Heck catalysts in DMF solution in air. The PEG-bound 5-amido-SCS-Pd complex was recycled via solvent precipitation three times with no observed catalyst deactivation. While the 5-amido-SCS-Pd complexes are very robust, their 5-oxo counterparts decompose slowly under certain conditions. These SCS catalysts are analogous to PCP-type catalysts previously reported in the literature but avoid the requirement of an air-sensitive phosphine synthesis.

Full text of DOI:10.1021/ja991099g

J. Am. Chem. Soc. 1999, 121, 9531-9538
9531
Tridentate SCS Palladium(II) Complexes: New, Highly Stable,
Recyclable Catalysts for the Heck Reaction
David E. Bergbreiter,* Philip L. Osburn, and Yun-Shan Liu
Contribution from the Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012,
College Station, Texas 77842-3012
ReceiVed April 7, 1999
Abstract: A new pincer-type SCS ligand containing Pd(II) is a simple, robust catalyst for Heck chemistry
using a variety of alkene acceptors and aryl iodides. It is less active with aryl bromides. While certain palladium-
(II) species insert slowly into the aryl C-H bond of an unsubstituted version of this ligand, the introduction
of activating groups into the 5 position of the aromatic ring readily allows quantitative metal insertion. These
ligands were synthesized and attached to soluble polymers by simple modification of inexpensive starting
materials. For example, both 5-oxy and 5-amido SCS ligands were successfully appended to 5000 Mn poly-
(ethylene glycol) via ether or amide linkages, respectively. Both the 5-oxo and 5-amido complexes are active
as Heck catalysts in DMF solution in air. The PEG-bound 5-amido-SCS-Pd complex was recycled via solvent
precipitation three times with no observed catalyst deactivation. While the 5-amido-SCS-Pd complexes are
very robust, their 5-oxo counterparts decompose slowly under certain conditions. These SCS catalysts are
analogous to PCP-type catalysts previously reported in the literature but avoid the requirement of an air-
sensitive phosphine synthesis.
Introduction
greatly extends the overall utility of a precious transition metal
catalyst, care must often be taken to avoid a steady deactiva-
tion of the catalyst, generally via adventitious oxidation, over
several cycles. This problem is especially acute with phosphine-
ligated transition metal complexes.14,15 Therefore, an increas-
ingly important objective within this area is the synthesis of
more robust catalysts that can be incorporated into polymer
supports.
Palladacycles have been known for over 20 years, but have
only recently been investigated as catalysts. Denmark has
described chiral bis(oxazoline)palladium(II) catalysts for use in
cyclopropanation reactions.16 Although these catalysts were not
very stereoselective, they were very active and the use of the
oxazoline ligand avoids the problem of phosphine oxidation.
Herrmann and co-workers demonstrated that palladacycles
formed from palladium(II) acetate and phosphines such as tris-
(o-tolyl)phosphine are efficient, thermally stable catalysts for
the Heck olefination and Suzuki coupling reactions.17,18 How-
ever, while these phosphine-ligated palladacycles were thermally
stable, they were still sensitive to adventitious oxidation. More
recently Milstein and co-workers described a phosphorus-
carbon-phosphorus (PCP)-type tridentate ligand system useful
The development of efficient methods to facilitate the
recovery and reuse of transition metal complexes remains an
important goal in catalytic organic chemistry. Several catalyst
recovery techniques have appeared in the literature. Most involve
binding the catalyst to an insoluble support (e.g., cross-linked
polystyrene) to facilitate catalyst separation after the reaction.1-3
However, we and others have recently emphasized the utility
of soluble polymer supports to achieve this separation.4-6 Our
early work in this area centered on the use of functionalized
polyethylene oligomers in catalysis and synthesis.5-7 More
recently, we have employed soluble supports such as poly-
(ethylene oxide), poly(N-isopropylacrylamide), and polyfluoro-
acrylates.8-13 However, while the element of recyclability alone
(1) Clark, J. H.; Macquarrie, D. J. Org. Proc. Res. DeV. 1997, 1, 149-
162.
(2) (a) Angelino, M. D.; Laibinis, P. E. Macromolecules 1998, 31, 7581-
7587. (b) Chemin, A.; Deleuze, H.; Maillard, B. Eur. Polym. J. 1998, 34,
1395-1404. (c) Stansbury, J. W.; Liu, D. E.; Kim, S. I. Macromolecules
1997, 30, 4540-4543. (d) Kamahori, K.; Ito, K.; Itsuno, S. J. Org. Chem.
1996, 61, 8321-8324. (e) Caze, C. React. Funct. Polym. 1995, 26, 85-94.
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1992, 16, 233-259.
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(5) Gravert, D. J.; Janda, K. D. Chem. ReV. 1997, 97, 489-509.
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74, 409-419.
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179.
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Macromolecules 1998, 31, 6053-6062.
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7846.
(12) Bergbreiter, D. E.; Liu, Y.-S.; Furyk, S.; Case, B. L. Tetrahedron
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(13) (a) Bergbreiter, D. E.; Franchina, J. G. Chem. Commun. 1997, 16,
1531-1532. (b) Bergbreiter, D. E.; Franchina, J. G.; Case, B.; Koshti, N.;
Williams, L. K.; Frels, J. J. Am. Chem. Soc. Submitted for publication.
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K. Russ. J. Gen. Chem. 1997, 67, 53-57. (b) Lee, W. C.; Lee, J. S.; Cho,
N. S.; Kim, K. D.; Lee, S. M.; Oh, J. S. J. Mol. Catal. 1993, 80, 31-41.
(c) Halpern, J.; Pickard, A. L. Inorg. Chem. 1970, 9, 2798-2800. (d) Birk,
J. P.; Halpern, J.; Pickard, A. L. J. Am. Chem. Soc. 1968, 90, 4491-4492.
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2726-2730.
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J. Org. Chem. 1997, 62, 3375-3389.
(17) Herrmann, W. A.; Brossmer, C.; O¨ fele, K.; Reisinger, C.-P.;
Priermeir, T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. Engl. 1995,
34, 1844-1848.
(18) Beller, M.; Fischer, H.; Herrmann, W. A.; O¨ fele, K.; Brossmer, C.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1848-1849.
10.1021/ja991099g CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/04/1999

9532 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Bergbreiter et al.
catalyst. The complex catalyzed the reaction between iodoben-
zene and methyl acrylate in DMF using Na2CO3 as the base
with complete conversion of iodobenzene in 5 h at 105 °C (92%
isolated yield). Moreover, no visible decomposition of 2
occurred during this reaction even though it was carried out in
the presence of air using undistilled DMF.
Motivated by this result, activating groups were introduced
into the 5 position of the aromatic ring in an effort to achieve
quantitative yields of analytically pure tridentate, metal-inserted
species. The synthesis of a 5-acetamido-SCS ligand 5 and its
palladium(II) complex 6 is shown in Figure 3. Esterification of
commercially available 5-aminoisophthalic acid, followed by
reduction with LAH in THF yielded an amino diol. Subsequent
protection of the -NH2 group with acetic anhydride and
triethylamine resulted in the acetamido diol. The acetamido
dichloride was then formed using equal amounts of SOCl2 and
pyridine. Subsequent treatment of this dichloride with thiophenol
and K2CO3 in DMF yielded the acetamido-SCS ligand 5.
Treatment of the 5-acetamido-SCS ligand with Pd(PhCN)2Cl2
in refluxing MeCN for 14 h then yielded the complex 6 as a
yellow crystalline material. 13C NMR spectroscopic analysis of
this material showed clearly that the chemical shift of the
2-position aromatic carbon moved downfield from δ 138.2 to
δ 149.8, indicating completion of the metal insertion reaction.
Exact mass analysis of 6 showed that the M-Cl peak (484.0027)
agreed well with the calculated value (484.0029, 0.41 ppm
error).
The thermal, oxidative, acid/base, and water stability of 6
was studied under various conditions. After 20 h at 140 °C in
DMF in air, the complex was isolated unchanged. Exposure of
6 to 90% EtOH, 90% EtOH acidified to pH 1 with HCl, and
90% EtOH containing 5 equiv of triethylamine (relative to 6)
at 80 °C for 30 h with exposure to air, also did not affect 6.
Finally, exposure of 6 to a small amount of thiophenol in 90%
EtOH at 80 °C for 35 h resulted in no black precipitate of
palladium metal.
Figure 1. General structure of a tridentate SCS palladium(II) complex.
The functionalized complexes are synthesized from isophthalic acid
derivatives substituted in the 5 position. For clarity, this position is
consistently referred to as the 5 position in all the SCS complexes
presented here.
in Heck-type vinylation of aryl halides.19 These latter PCP-Pd
catalysts were both thermally and oxidatively stable.
Motivated by these reports, we reasoned that other general
XCX-type ligand systems might lead to new, highly stable
catalyst systems. Moreover, with appropriately functionalized
ligands, these complexes could be attached to polymer supports
to produce robust, easily recyclable catalysts.
Complexes such as those shown in Figure 1 were synthesized
and their chemistry explored as early as 1980 by Shaw and co-
workers.20 More recently, similar SCS-Pd complexes have been
used by Reinhoudt and co-workers as scaffolds to build self-
assembling metallodendrimers.21 In addition, similar NCN-type
complexes have also been described.22 However, these reports
generally do not examine the catalytic potential of these com-
plexes. This paper describes the synthesis of new functionalized
SCS-Pd(II) complexes, studies of their stability, and studies
of their competence as homogeneous catalysts for Heck
chemistry. Preliminary work shows that polymer immobilization
of these complexes is also successful; a poly(ethylene glycol)-
bound 5-amido SCS-Pd catalyst was readily recyclable.
Results and Discussion
Synthesis, Stability, and Catalytic Properties of Low
Molecular Weight SCS Complexes. Our first objective was
to explore the catalytic properties of tridentate (SCS) bis-
(phenylsulfide)-Pd(II) complexes similar to those reported by
Shaw. The necessary SCS ligand 1 was first synthesized from
R,R′-dibromo-m-xylene via nucleophilic substitution with thiophe-
nol.Our attempts at palladium insertion into the aryl C-H bond
of this ligand to prepare an SCS-Pd(II) catalyst were initially
frustrated by a very slow insertion reaction. Using Pd(PhCN)2Cl2
in refluxing acetonitrile led mostly to isolation of the bidentate
sulfur complex of PdCl2 due to ligand exchange without metal
C-H insertion. This problem was solved by pretreatment of
the Pd(PhCN)2Cl2 with an excess of AgBF4. Reaction of the
more electrophilic Pd(II) species so formed with the SCS ligand
under the same conditions resulted in the isolation of an aryl-
Catalytic reactions of 6 using various aryl iodides and
acceptor alkenes were conducted with DMF or NMP solutions
containing 0.1 mol % of 6 at 105-110 °C using either
triethylamine or Na2CO3 as the base (eq 1). All of these reactions
1
palladium complexed product as determined by H and 13C
NMR spectroscopy. Preliminary catalytic studies with the crude
2 formed in this chemistry showed it to be an effective Heck
were conducted in air without solvent purification or precautions
to avoid oxygen exposure and led to complete conversion of
the starting aryl iodide in 5-6 h based on GC analysis. Turnover
numbers (TONs) and isolated yields from these reactions are
summarized in Table 1.
Since Pd(OAc)2 alone is an active catalyst for Heck reactions
when aryl iodides are used, it is important to show that complex
6 is more than just a source of Pd(II). When Heck chemistry
was carried out at 80 °C in DMF in air using 0.001 mol % of
6 for the reaction of iodobenzene with methyl acrylate using
triethylamine as the base, GC analysis (60 h) showed that this
reaction had a turnover number (TON, mol of product/mol of
(19) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem.
Soc. 1997, 119, 11697-11698.
(20) Errington, J.; McDonald, W. S.; Shaw, B. L. J. Chem. Soc., Dalton
Trans. 1980, 2312-2314.
(21) Huck, W. T. S.; van Veggel, F. C. J. M.; Kropman, B. L.; Blank,
D. M. A.; Keim, E. G.; Smithers, M. M. A.; Reinhoudt, D. M. J. Am. Chem.
Soc. 1995, 117, 8293-8294.
(22) (a) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove, D. M.;
van Koten, G. Chem. Eur. J. 1998, 4, 763-768. (b) Lagunas, M.-C.;
Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Eur. J. Inorg.
Chem. 1998, 2, 163-168. (c) Steenwinkel, P.; Kooijman, H.; Smeets, W.
J. J.; Spek, A. L.; Grove, D. M.; van Koten, G. Organometallics 1998, 17,
5411-5426.

Tridentate SCS Palladium(II) Complexes
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9533
Figure 2. Synthesis of the unsubstituted SCS-Pd catalyst 2.
Figure 3. Synthesis of an SCS-Pd complex functionalized para to the C-Pd bond with an acetamido group (6).
Table 1. Results from Heck Reactions Using SCS-Pd Catalyst 6a
zyloxy ligand 8 was prepared in a manner analogous to that
described for the 5-acetamido-SCS ligand, starting with the
commercially available 5-hydroxyisophthalic acid (Figure 4).
R
R′
product
yield (%)
-H
-H
-CO2Me
-CO2t-Bu
-CO2Me
-CO2t-Bu
-CO2Me
-CO2Me
-CO2 t-Bu
-CN
7a
7b
7c
7d
7e
7f
7g
7h
7i
92
93
95
96
94
95
96
91
94
96
92
93
Milder palladation conditions were sufficient for the synthesis
of 9; the insertion reaction was complete in 2.5 h using
palladium(II) trifluoroacetate at room temperature. When more
vigorous conditions were used (e.g., Pd(PhCN)2Cl2 in refluxing
acetonitrile), large amounts of black precipitate formed during
the reaction; the isolated product was a mixture of metal inserted
and noninserted products. The evident decomposition observed
under these conditions raised questions regarding the stability
of this complex to potential reaction conditions. Indeed, when
stability experiments analogous to those for 6 were performed
on pure 9, slow degradation of the complex did occur in
-NHAc
-NHAc
-OMe
-COOH
-COOH
-NHAc
-H
-C5H4N
-C5H4N
-Ph
-NHAc
-H
-OMe
7j
7k
7l
-Ph
a All reactions were carried out in air at 110 °C for 5-6 h using 0.1
mol % of catalyst 6. For all reactions, the turnover number (TON) is
1000 and the yield listed is for pure isolated product (based on 1H and
13C NMR spectroscopy). GC yields for all the products were quantita-
tive.
1
acetonitrile under both acidic and basic conditions. H NMR
spectra of the complex during these experiments showed the
steady disappearance of the singlet at δ 6.74 (2H, protons meta
to the C-Pd bond) and the appearance of signals at δ 6.91 and
6.79 in a 1:2 integrated ratio, indicating cleavage of the C-Pd
bond. In view of these results, catalytic experiments were not
performed using 9.
catalyst) of 70000. Under the same conditions, 0.007 mol % of
Pd(OAc)2 had a TON of 1400. A black solid, presumably
Pd(0), also precipitated from the reaction solution when using
Pd(OAc)2 under these conditions.
Synthesis and Catalytic Properties of Poly(ethylene glycol)-
Bound SCS-Pd Complexes. Efforts to bind SCS-Pd com-
plexes to a soluble polymer support were carried out concur-
rently with work on synthesizing low molecular weight
complexes. Thus, the first PEG-bound complex synthesized was
14, in which the SCS ligand is connected to the polymer via an
ether linkage. The synthetic scheme depicted in Figure 5 was
initially chosen as the most direct route to a poly(ethylene
glycol)-bound SCS catalyst. The synthesis paralleled that
outlined for the low molecular weight 5-amido counterpart 6.
Purification at each step of the process was achieved by
recrystallization of the polymer from 2-propanol. The soluble
However, while 6 was an effective catalyst for Heck
chemistry using aryl iodides, it was ineffective with aryl
bromides. After 10 h, no reaction was observed between 3,5-
dimethylbromobenzene and tert-butyl acrylate using 5 mol %
of 6 at 140 °C. Also, 4-bromoacetanilide failed to react with
either methyl acrylate or 4-vinylpyridine in the presence of 0.1
mol % of 6 within 14 h at 135 °C. After 20 h at 140 °C, GC
analysis showed only 30% conversion using 2 mol % of 6 and
p-bromoanisole and tert-butyl acrylate as substrates.
In an effort to build a more electron-rich palladium center
that would be more reactive toward aryl bromides, the 5-ben-
zyloxy-SCS-Pd complex 9 was synthesized. The free 5-ben-

9534 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Bergbreiter et al.
Figure 4. Synthesis of an SCS-Pd complex functionalized para to the C-Pd bond with a benzyloxy group (9).
Figure 5. Synthesis of the poly(ethylene glycol)-supported 5-oxo-SCS-Pd catalyst 14.
Table 2. Results from Heck Reactions Run with PEG-Bound
1
intermediate products were readily characterized by H NMR
spectroscopy.
Catalyst 14a
R
R′
product
yield (%)
The first step in this synthesis involved the coupling of the
ligand scaffold to a PEG support. Diethyl 5-hydroxy isophthalate
was treated with potassium tert-butoxide and the phenolate so
formed was allowed to react with mesylate-terminated 5000
molecular weight (Mn) poly(ethylene glycol) monomethyl ether.
The product of this reaction, a PEG-bound isophthalate diester,
was then subjected to the same synthetic protocol (reduction,
conversion to good leaving groups, and substitution with
thiophenol) as that previously described. The ligand 13 was then
treated with the palladium precatalyst Pd(PhCN)2Cl2 to give the
-NHAc
-NHAc
-NHAc
-H
-CO2Me
-CO2t-Bu
-C5H4N
-CO2Me
-CO2t-Bu
-C5H4N
7c
7d
7j
7a
7b
7i
95
71
93
83
>99
74
-H
-H
a All reactions were carried out in air at 110 °C for 5-6 h using 0.1
mol % of catalyst 14. For all reactions, the turnover number (TON) is
1000 and the yield listed is for pure isolated product (based on 1H and
13C NMR spectroscopy). GC yields for all the products were quantita-
tive.
1
final product 14. H NMR spectroscopy clearly showed the
disappearance of the signal from the C-2 proton of the central
aromatic ring (present at 6.82 δ in 13) and the shifting of the
singlet for the C-4 and C-6 protons from δ 6.71 in 13 to δ 6.59
in the metal-inserted product 14.
The synthesis of a more stable 5-amido SCS complex is
shown in Figure 6. The 5-acetamido-SCS ligand 5 was
hydrolyzed using NaOH in ethanol, yielding a 5-amino-SCS
species. Treatment of this amino SCS ligand with glutaric
anhydride yielded an acid-terminated SCS ligand. This ligand
was then converted to the Pd complex 17 by treatment with
Pd(PhCN)2Cl2. In addition to 1H NMR, 13C NMR, and HRMS
analyses, 17 was also characterized by X-ray crystallography.
The crystal structure of 17 is shown in Figure 7.
In catalytic experiments, 14 was an active Heck catalyst under
conditions similar to those used for 6. Several reactions were
run in DMF in air at 110 °C; completion times for the reactions
were similar to those for 6 (5-6 h). Isolated yields for these
reactions are summarized in Table 2. However, catalyst 14 did
slowly decompose like its low molecular weight analogue. A
black precipitate of palladium metal and reformation of the
The structure of 17 shows that the S-phenyl groups provide
1
a C2-chiral ligand environment for the Pd center. H NMR
1
original bis(phenyl sulfide) group based on H NMR spectro-
spectroscopy indicates that rapid interconversion of the two
scopic analysis indicated that 14 would not be recyclable.
chiral sulfur centers of 17 occurs. The signal for the four benzyl

Tridentate SCS Palladium(II) Complexes
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9535
Figure 6. Conversion of the 5-acetamido-SCS ligand 5 to an acid-terminated species 17.
1
workup, the pure (by H and 13C NMR spectroscopy) Heck
coupling products were isolated. Three cycles were run for each
reaction; the resulting isolated yields are summarized in Figure
9. No loss or deactivation of the catalyst is observed in this
recycling protocol. GC analysis of each cycle yielded a constant
rate of conversion over all three cycles for both reactions. This
suggests that the actual turnover number for catalyst 18 was
much higher than that represented by a single cycle. Whereas
a conventional catalyst turnover number represents a single
reaction, the effective TON for 18 was limited only by the
number of cycles run.
Conclusions
New tridentate SCS-type palladium(II) complexes are effec-
tive catalysts for the Heck reaction between aryl iodides and
alkene acceptors. These catalysts were synthesized from readily
available starting materials using straightforward chemistry.
Changing the group occupying the 5 position of the central
aromatic ring influenced the stability of the complex. While
both the 5-oxy- and 5-amido-SCS catalysts performed well
under homogeneous conditions in DMF, only the 5-amido
complexes such as 6 had long-term stability to the reaction
conditions. Thus, complex 17 was then synthesized and
subsequently coupled to an amine-terminated PEG support,
yielding a highly stable, recyclable catalyst. These new SCS
catalysts are attractive due to their simple, efficient synthesis
and modification as well as their high durability and easy
recyclability.
Figure 7. X-ray crystal structure of the acid-terminated SCS-Pd
complex 17; thermal ellipsoids are 50% probability. Selected bond
distances: C1-Pd1 (1.97 Å), Pd1-S1 (2.30 Å), Pd1-S2 (2.30 Å), Pd1-
Cl1 (2.41 Å). Additional crystallographic information is given in Table
3 in the Experimental Section.
protons of 17 was a broad singlet at room temperature, but this
singlet sharpened significantly at higher temperatures. This
structure and dynamic behavior suggest that other S-chiral
ligands such as 17 can be prepared. We are currently working
toward that goal.
To append the acid-terminated species 17 to a PEG support
via an amide linkage, the hydroxyl end group of the polymer
was first converted to an amine using a previously reported
procedure.23 As shown in Figure 8, the monomethoxy PEG was
converted to the mesylate and then reacted with potassium
phthalimide in DMF. The resulting PEG-phthalimide was
cleaved with hydrazine in ethanol to yield an amine-terminated
PEG. The coupling step was accomplished simply by activating
17 with dicyclohexylcarbodiimide (DCC) and then allowing it
Experimental Section
General. THF was distilled from sodium-benzophenone ketyl,
CH2Cl2 was distilled from calcium hydride, and DMF was distilled
from and stored over molecular sieves. All other reagents and solvents
were obtained from commercial sources and were generally used
without further purification. Gas chromatographic analyses were
performed on a Shimadzu instrument equipped with a 15-m SPB-5
(poly(5%-diphenyl-95%-dimethylsiloxane)) normal phase fused silica
capillary column (0.53 ID). 1H NMR spectra were recorded on Varian
VXR-300 or Unity p300 spectrometers at 300 MHz. Chemical shifts
are reported in ppm using either tetramethylsilane (TMS, 0.00 ppm)
or hexamethyldisiloxane (HMDS, 0.055 ppm) as internal standards.
13C NMR spectra were recorded at 75 MHz with CDCl3 or DMSO-d6
as the internal reference. Crystallographic data for 17 were collected
on a Siemens P4 instrument using graphite monochromated Mo KR
radiation (λ ) 0.71073 Å). The data were corrected for Lorentz and
polarization effects and absorption using semiempirical corrections from
ψ-scans. Minimum and maximum absorption corrections for 17 were
0.835 and 0.932, respectively. The structure of 17 was solved and
refined using the SHELX-97 package.24 Refinement was by full-matrix
least-squares on F2 using all data. The weighting scheme was w )
1
to react with the PEG-amine. H NMR spectroscopic analysis
of the resulting PEG-bound 5-amido-SCS-Pd complex 18
showed complete conversion of the amine group.
The PEG-bound catalyst 18 was readily recycled after a
typical reaction. The reactions shown in Figure 9 were run under
conditions similar to those previously described: 115 °C in DMF
in air, using TEA and 0.1 mol % 18. GC analysis showed that
both reactions were complete in under 7 h. Upon the completion
of each cycle, the reaction solution was poured into diethyl ether,
precipitating the catalyst, which was isolated and redissolved
in DMF for use in the next cycle. After a simple extractive
(23) Harada, A.; Li, J.; Kamachi, M. J. Am. Chem. Soc. 1994, 116, 3192-
3196.

9536 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Bergbreiter et al.
Figure 8. Attachment of complex 17 to a poly(ethylene glycol) support.
mmol) was added dropwise to the mixture, resulting in the eventual
dissolution of the starting material. The ensuing homogeneous solution
was stirred at room temperature for 18 h. After the reaction, the organic
phase was washed with water (3 × 30 mL), 1 N HCl (2 × 10 mL),
and brine (2 × 10 mL) and then dried over MgSO4. The solvent was
removed under reduced pressure and the isolated yellowish solid was
dried under vacuum, yielding 2.10 g (89%) of the 5-acetamido
dichloride: 1H NMR (CDCl3) δ 2.10 (s, 3H), 4.51 (s, 4H), 7.12 (s,
1H), 7.57 (s, 2H).
Synthesis of N-Acetyl-3,5-bis(phenylthiomethyl)aniline (5). A
solution of 4 (2.10 g, 8.60 mmol) in 30 mL of DMF was prepared. To
this solution was added potassium carbonate (2.97 g, 21.5 mmol) and
thiophenol (1.78 mL, 17.3 mmol). The reaction mixture was heated at
70 °C for 16 h. After this time, it was then poured into 400 mL of H2O
and chilled at 5 °C for 24 h to induce precipitation of the product as
an off-white solid material. The product was collected by filtration and
washed with saturated Na2CO3 (2 × 20 mL), 1 N HCl (1 × 10 mL),
and water (2 × 20 mL). The solid was then dried under vacuum,
yielding 2.99 g (89%) of the product 5-acetamido SCS ligand: 1H NMR
(CDCl3) δ 2.15 (s, 3H), 4.01 (s, 4H), 6.95 (s, 1H), 7.10-7.32 (m, 8H),
7.34 (s, 2H), 7.43 (m, 2H).
Figure 9. Recycling of the PEG-bound 5-amido-SCS-Pd catalyst 18.
The catalyst was recycled after each reaction by solvent precipitation
(diethyl ether).
Palladation of N-Acetyl-3,5-bis(phenylthiomethyl)aniline (6). The
ligand 5 (2.0 mmol, 0.70 g) was dissolved in 10 mL of MeCN under
N2 in a flame-dried flask. Then Pd(PhCN)2Cl2 (2.0 mmol, 760 mg)
was added and the mixture was first stirred for 1 h at room temperature
and then refluxed for 14 h. The hot solution was filtered rapidly to
remove any insoluble material. The filtrate was allowed to cool to 25
°C and the precipitated yellow crystals were collected, washed with
hexane (2 × 5 mL), and dried under vacuum to yield 0.88 g (85%) of
the pure product 6: 1H NMR (DMSO-d6) δ 2.04 (s, 3H), 4.83 (s, 4H),
7.30 (s, 2H), 7.49-7.53 (m, 6H), 7.87-7.90 (m, 4H), 9.86 (s, 1H);
13C NMR (DMSO-d6) δ 23.88, 49.91, 113.08, 129.40, 129.53, 130.66,
132.22, 132.76, 136.36, 149.78 (C-Pd), 168.06; HRMS m/z 484.0027
(calcd for C22H20NOS2Pd (M-Cl) 484.0029, (0.41 ppm error)).
General Procedure for Using 6 in Homogeneous Heck Reactions
(7a-l). A 20-mL vial was charged with 2 mmol of aryl iodide, 3 mmol
of olefin, 3 mmol of triethylamine or Na2CO3, 0.002 mmol of the SCS-
Pd catalyst (6), and 10 mL of DMF or NMP. The mixture was stirred
at 105-110 °C in air and the reaction progress was analyzed by GC.
The conversion was generally greater than 98% after 5 h and the average
isolated yield was 93%. The product was isolated by pouring the
reaction mixture into excess water and either collecting the precipitate
or washing the aqueous phase several times with diethyl ether. The
structures of the isolated products were confirmed by 1H and 13C NMR
analysis. All of the products are known compounds.28
1/[σ2(Fo ) + 28.58P ] where P ) (Fo2 + 2Fc2)/3. Hydrogen atoms were
2
included in the calculated positions and treated as riding atoms. The
ORTEP plot in Figure 7 was generated using ORTEP3 for Windows.25
For simplicity associated solvent molecules (3/2CH3CN) were removed
from the plot. Accurate mass measurements of compounds 6, 8, and
17 were made using a VG Analytical 70S high resolution, double
focusing, magnetic sector (EB) mass spectrometer in the +FAB
ionization mode; all determinations resulted in <1 ppm error from the
theoretical accurate mass values.
Synthesis of N-Acetyl-3,5-bis(hydroxymethyl)aniline (3). 3,5-Bis-
(hydroxymethyl)aniline was synthesized in two steps from 5-ami-
noisophthalic acid according to previously reported procedures.26,27
Triethylamine (2.02 mL, 14.5 mmol) and acetic anhydride (1.37 mL,
14.6 mmol) were then added to a solution of this material (2.40 g,
14.5 mmol) in 40 mL of THF. The reaction solution was allowed to
stir at room temperature; within 10 min, large amounts of a white
precipitate, the product acetamido diol, had formed. The reaction was
allowed to stir for an additional 3 h. The product was isolated by
filtration, washed with portions of THF, and dried under vacuum,
yielding 2.56 g (85%) of a white solid: 1H NMR (DMSO-d6) δ 2.00
(s, 3H), 4.41 (s, 4H), 5.18 (br s, 2H), 6.90 (s, 1H), 7.40 (s, 2H), 9.92
(s, 1H).
Synthesis of N-Acetyl-3,5-bis(chloromethyl)aniline (4). A suspen-
sion of 3 (2.00 g, 9.66 mmol) in 50 mL of methylene chloride was
prepared. Pyridine (1.72 mL, 21.3 mmol) was added and the resulting
mixture was cooled to 0 °C. Then, thionyl chloride (1.55 mL, 21.3
Synthesis of O-Benzyl-3,5-bis(phenylthiomethyl)phenol (8). O-Ben-
zyl-3,5-bis(phenylthiomethyl)phenol was prepared in five steps from
5-hydroxyisophthalic acid in a manner analogous to that previously
reported:21 1H NMR (CDCl3) δ 3.97 (s, 4H), 4.83 (s, 2H), 6.75 (s, 2H),
6.81 (s, 1H), 7.17-7.42 (m, 15H); 13C NMR (CDCl3) δ 38.77, 69.78,
113.99, 121.84, 126.24, 127.40, 127.85, 128.43, 128.74, 129.72, 136.11,
(24) Sheldrick, G. M. SHELX-97, release 97-2, 1997.
(25) Farrugia, L. J. J. Appl. Crystallogr. 1997, 565.
(26) Cunliffe, D.; Leason, M.; Parkin, D.; Lea, P. J. Phytochemistry 1983,
22, 1357-1370.
(27) Behrens, C.; Egholm, M.; Buchardt, O. Synthesis 1992, 12, 1235-
1236.
(28) Yoshino, T.; Nagata, Y.; Itoh, E.; Hashimoto, M.; Katoh, T.;
Terashima, S. Tetrahedron Lett. 1996, 37, 3475-3478.

Tridentate SCS Palladium(II) Complexes
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9537
136.67, 139.04, 158.80; HRMS m/z 428.1268 (calcd for C27H24OS2
428.1264 (0.93 ppm error)).
temperature for 20 h. Afterward, the reaction solution was filtered
through Celite and the filtrate concentrated to approximately 20 mL.
The filtrate was then poured into 200 mL of diethyl ether, precipitating
the product dimesylate. This material was isolated by filtration and
then dissolved in 90 mL of acetone. Dry lithium bromide (2.09 g, 24.0
mmol) was added and the resulting mixture was refluxed for 21 h.
The reaction solution was then filtered to remove insoluble material
and the filtrate was poured into 350 mL of diethyl ether, precipitating
the product. This material was filtered, recrystallized from 2-propanol,
and dried under vacuum, yielding 16.4 g (91%) of the PEG-dibromide
12: 1H NMR (CDCl3) δ 3.28-3.85 (PEG, 440H), 4.17 (t, 2H), 4.41
(s, 4H), 6.85 (s, 2H), 6.99 (s, 1H).
Synthesis of MeO-PEG5000-oxy-SCS Ligand (13). A solution of
thiophenol (0.986 mL, 9.60 mmol) in dry, freshly distilled THF was
prepared. To this solution was added potassium tert-butoxide (1.33 g,
9.62 mmol). A white precipitate formed instantly. The resulting mixture
was stirred at room temperature for 45 min. Then the solvent was
removed under reduced pressure and the residual powdery salt was
redissolved in 10 mL of DMF. This solution was then added to a
solution of PEG-dibromide (12; 5.0 g, 2.0 mmol) in 50 mL of DMF in
one portion. The resulting homogeneous solution was stirred at 75 °C
for 14 h. The reaction solution was then concentrated under reduced
pressure. This concentrated solution was poured into 250 mL of diethyl
ether, precipitating the product. This material was isolated by filtration,
recrystallized from 2-propanol, and dried under vacuum, yielding 5.0
g (>99%) of the PEG-SCS ligand 13: 1H NMR (CDCl3) δ 3.28-3.85
(PEG, 440H), 3.85 (t, 2H), 4.01 (s, 4H), 6.71 (s, 2H), 6.82 (s, 1H),
7.12-7.24 (m, 10 H).
Palladation of O-Benzyl-3,5-bis(phenylthiomethyl)phenol (9). A
solution of 8 (0.502 g, 1.17 mmol) in 3 mL of dry, freshly distilled
DMF was added to a flame-dried flask and the system sealed. The
oxygen in the flask was removed with a vacuum pump and replaced
with dry N2; the flask was allowed to purge with nitrogen for 15 min.
Three pump/purge cycles were performed. Then, palladium(II) trifluo-
roacetate (388 mg, 1.17 mmol) dissolved in 1 mL of DMF was added
via syringe. The resulting solution took on the deep purple color of
the dissolved Pd(TFA)2. The reaction was stirred at room temperature
and its progress monitored by TLC (heptane). The reaction was
complete after 2.5 h, after which the reaction solution was poured into
10 mL of diethyl ether, precipitating the product as a sticky purple
material. This mixture was cooled at -5 °C for 16 h to solidify the
product. The supernatant was poured off and the product washed with
further portions of ether (to remove traces of byproduct TFA) and then
dried under vacuum to yield 0.647 g (97%) of the product 9: 1H NMR
(CDCl3) δ 4.71 (br s, 4H), 4.99 (s, 2H), 6.73 (s, 2H), 7.30-7.41 (m,
5H), 7.22 (m, 6H), 7.82 (m, 4H); 13C NMR (CDCl3) δ 50.62, 70.14,
109.60, 127.39, 128.09, 128.61, 129.66, 130.18, 131.63, 132.90, 136.53,
149.47 (C-Pd), 156.98. Due to the observed instability of 9, spectral
data were obtained as quickly as possible after isolation. Even NMR
samples of 9 gradually formed a black precipitate (presumably Pd metal)
on standing. Additionally, HRMS analysis was not performed on 9;
the immediate precursor, ligand 8, was, however, characterized by this
method.
Synthesis of MeO-PEG5000-Isophthalic Acid Diethyl Ester (10).
To a solution of diethyl 5-hydroxy isophthalate (1.43 g, 6.00 mmol) in
freshly distilled THF was added potassium tert-butoxide (0.674 g, 6.00
mmol). Upon addition of the base, the solution turned a bright yellow-
orange color. The solution was stirred at room temperature for 30 min.
The THF was removed under reduced pressure and the salt residue
was dissolved in 10 mL of DMF. This solution was added to a solution
of PEG-mesylate (10.0 g, 2.0 mmol; synthesized using a procedure
analogous to that previously reported23) in 50 mL of DMF. The resulting
homogeneous solution was stirred at 75 °C for 20 h. After the reaction,
the solution was filtered through Celite to remove insoluble materials
and the filtrate was acidified to pH 4 with concentrated HCl. The solvent
was then removed from the filtrate under reduced pressure. The residue
was redissolved in 75 mL of MeOH. This solution was poured into
300 mL of diethyl ether, with stirring, to precipitate the product as a
white solid. This solid was isolated, recrystallized from 2-propanol,
and washed with further portions of cold 2-propanol before being dried
under vacuum to yield 9.8 g (98%) of the product 10: 1H NMR (CDCl3)
δ 1.40 (t, 6H), 3.28-3.85 (PEG, 440H), 4.21 (t, 2H), 4.39 (q, 4H),
7.75 (s, 2H), 8.25 (s, 1H).
Synthesis of MeO-PEG5000-Diol (11). A solution of 10 (9.62 g, 3.84
mmol) in dry, freshly distilled THF (150 mL) was prepared. To this
solution was added lithium aluminum hydride (800 mg, 21.08 mmol)
in several small portions. After the addition was complete, the reaction
mixture was refluxed for 8 h. The mixture was then cooled and
neutralized by the slow addition of 10 M aqueous NaOH. Neutralization
was assumed to be complete upon the disappearance of all gray color
from the solution and the appearance of a white, sticky precipitate.
The mixture was then filtered through Celite and the solvent removed
from the filtrate under reduced pressure. The resulting residue was then
dissolved in 40 mL of acetone and filtered again through Celite. The
filtrate was poured into 500 mL of diethyl ether to precipitate the
product polymer as a white solid. This material was isolated by
filtration, recrystallized from 2-propanol, and dried under vacuum to
yield 9.52 g (99%) of the product 11: 1H NMR (CDCl3) δ 2.75 (br s,
2H), 3.28-3.85 (PEG, 440H), 4.13 (t, 2 H), 4.60 (s, 4H), 6.81 (s, 2H),
6.95 (s, 1H).
Synthesis of MeO-PEG5000-oxy-SCS-PdCl (14). A solution of 13
(100 mg, 0.02 mmol) in 3 mL of CH3CN was prepared in an oven-
dried flask. The flask was connected to a condenser and the system
sealed. The air in the system was removed by vacuum and replaced
with dry N2 and the system was allowed to purge with N2 for 30 min.
Three pump/purge cycles were performed. Then, Pd(PhCN)2Cl2 (7.6
mg, 0.02 mmol Pd) was quickly added to the flask. The reaction mix-
ture was allowed to stir for 1 h at room temperature and then re-
fluxed for 14 h. The reaction solution was then cooled and poured
into 10 mL of diethyl ether, precipitating the product as a slightly
golden yellow solid. This material was isolated by centrifugation,
recrystallized from 2-propanol, and dried under vacuum, yielding 100
mg (>99%) of 14: 1H NMR (CDCl3) δ 3.28-3.85 (PEG, 440H), 4.05
(t, 2H), 4.50-4.59 (br s, 4 H), 6.59 (s, 2H), 7.33-7.38 (m, 6H), 7.78-
7.82 (m, 4H).
General Procedure for Using 14 in Homogeneous Heck Reac-
tions. The procedure for using 14 in homogeneous Heck reactions was
analogous to that previously described for 6. The products were isolated
in 71 to >99% yield and were pure by 1H and 13C NMR spectroscopy.
Synthesis of 3,5-Bis(phenylthiomethyl)aniline (15). To a solution
of 5 (0.487 g, 1.25 mmol) in 15 mL of 95% EtOH was added NaOH
(0.150 g, 3.75 mmol). This mixture was refluxed for 20 h. Afterward,
the reaction solution was neutralized with concentrated HCl and the
majority of the solvent removed under reduced pressure. The remaining
residue was taken up in methylene chloride. The resulting solution was
washed with saturated Na2CO3 (3 × 15 mL) and then dried over
MgSO4. The solvent was removed under reduced pressure and the
resulting oily residue was recrystallized from cyclohexane/ THF to yield
0.398 g (92%) of a yellow crystalline material: 1H NMR (CDCl3) δ
3.59 (br s, 2H), 3.94 (s, 4H), 6.49 (s, 2H), 6.62 (s, 1H), 7.10-7.19 (m,
2H), 7.20-7.29 (m, 8H).
Synthesis of N-(3,5-Bis(phenylthiomethyl))phenylglutaramide
(Monoamide) (16). To a solution of 15 (0.371 g, 1.10 mmol) in dry,
freshly distilled THF was added glutaric anhydride (0.125 g, 1.10 mmol)
and triethylamine (0.209 mL, 1.50 mmol). The resulting solution was
refluxed for 30 h. After cooling, the THF was removed under reduced
pressure. The residue was taken up in 20 mL of ethanol and 2 mL of
8 M NaOH was added. The solvent was then removed under reduced
pressure, yielding an off-white solid material. This solid was washed
with diethyl ether (3 × 40 mL) and then added to 5 mL of 6 N HCl.
The solution was then extracted with methylene chloride (2 × 50 mL).
The organic extracts were dried over MgSO4 and filtered, and the
solvent was removed from the filtrate under reduced pressure. The
Synthesis of MeO-PEG5000-Dibromide (12). A solution of 11 (18.0
g, 7.2 mmol) in 90 mL of methylene chloride was cooled to 0 °C.
Triethylamine (3.01 mL, 21.6 mmol) was added and the solution was
stirred at 0 °C for 10 min. Then, methane sulfonyl chloride (1.66 mL,
21.6 mmol) was added to the reaction solution dropwise, with stirring.
The solution was stirred for an additional 20 min at 0 °C and then
allowed to warm to room temperature. It was then stirred at room

9538 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Bergbreiter et al.
isolated white solid product was then dried under vacuum to yield 0.438
g (92%) of the product 16: 1H NMR (CDCl3) δ 2.03 (m, 2H), 2.43 (t,
2H), 2.49 (t, 2H), 4.03 (s, 4H), 6.97 (s, 1H), 7.17-7.24 (m, 10H), 7.36
(s, 1H), 7.39 (s, 2H); 13C NMR (CDCl3) δ 20.36, 32.72, 36.11, 38.68,
118.90, 125.19, 126.37, 128.87, 129.69, 136.05, 137.99, 138.69, 170.80,
177.74.
Table 3. Summary of Crystal Data, Details of Intensity Collection,
and Least-Squares Refinement Parameters for 17
empirical formula
Mr
C28H24ClNO3PdS2‚3/2CH3CN
690.07
crystal size, mm
crystal class
space group
temp, K
a, Å
b, Å
c, Å
0.10 × 0.30 × 0.30
monoclinic
P21/n
Palladation of N-(3,5-Bis(phenylthiomethyl))phenylglutaramide
(Monoamide) (17). A solution of 16 (0.310 g, 0.687 mmol) in 8 mL
of acetonitrile was prepared in an oven-dried flask. The flask was
connected to a condenser and the system sealed. The oxygen in the
system was removed with a vacuum pump and replaced with dry N2;
the system was allowed to purge with nitrogen for 20 min. Three pump/
purge cycles were performed. Then, Pd(PhCN)2Cl2 (0.263 g, 0.687
mmol) dissolved in 2 mL of CH3CN was added via syringe. The
resulting solution was stirred for 1 h at room temperature and then
refluxed for 14 h. The reaction solution was filtered while hot to remove
any insoluble material. Upon cooling, yellow crystals of pure 17 formed
from the solution; the mixture was cooled at 0 °C for several hours to
induce further crystallization. Finally, the product was isolated by
filtration and dried under vacuum to yield 0.394 g (97%) of the product
17: 1H NMR (DMSO-d6) δ 1.76 (m, 2H), 2.25 (t, 2H), 2.29 (t, 2H),
4.78 (s, 4H), 7.26 (s, 2H), 7.45 (m, 6H), 7.81 (m, 4H), 9.80 (s, 1H);
13C NMR (DMSO-d6) 20.45, 32.91, 35.34, 49.80, 113.22, 129.44,
129.60, 130.65, 132.24, 136.30, 149.90 (C-Pd), 170.58, 174.14; HRMS
m/z 556.0232 (calcd for C25H24NO3S2Pd (M-Cl) 556.0230 (0.35 ppm
error)). Selected crystallographic data for 17 are given in Table 3.
Detailed X-ray crystallographic tables of 17 are available as Supporting
Information.
193(2)
5.6046(11)
30.464(6)
17.146(4)
97.295(19)
2903.8(10)
4
1.451
90.40
1284
â, deg
V, Å3
Z
D
calc, g cm-3
µ(Mo KR), cm-1
F(000)
ω scan width, deg
0.69
2.34-24.99
5078
3841
0.0609
0.1641
range Θ collected, deg
no. of independent reflcns
no. of obsd data [I > 2σ(I)]
R1 [I > 2σ(I)]a
wR2 (all data)
goodness of fit
1.178
359
1.050
no. of parameters refined
max peak in final ∆F map, eÅ-3
a Definition of R indices: R1 ) ∑(Fo - Fc)/∑(Fo); wR2 ) [∑[w(Fo
2
2
- Fc2)2]/∑[w(Fo )2]]1/2
.
material was washed with further portions of diethyl ether, and the
washes combined with the initial filtrate. The catalyst was redissolved
in DMF for use in the next cycle. The combined ether phases were
washed several times with water (to remove traces of salt byproduct)
then dried over MgSO4. After filtration and removal of the solvent,
the product was dried under vacuum. Three cycles were run for each
reaction, achieving isolated yields of 85%, 87%, and 95% for 7a and
91%, 95%, and 92% for 7k. The isolated products 7a and 7k are known
compounds.28
Synthesis of MeO-PEG5000-amido-SCS-PdCl (18). To a solution
of 17 (0.118 g, 0.200 mmol) in 5 mL of DMF was added dicyclo-
hexylcarbodimide (41.2 mg, 0.200 mmol). This solution was stirred at
room temperature for 1 h. Then, a solution of amine-terminated PEG
(1.02 g, 0.204 mmol; prepared using a previously reported procedure23)
in 5 mL of DMF was added to the first solution in one portion. The
resulting homogeneous solution was stirred at room temperature for
14 h. After the reaction, the solution was poured into 50 mL of diethyl
ether, yielding a slightly pale yellow solid. This solid was collected by
filtration and digested in 15 mL of methylene chloride. The insoluble
urea byproduct was removed by filtration. The filtrate was poured into
75 mL of diethyl ether, precipitating the pure polymer product as a
faintly yellow solid. This material was filtered, recrystallized from
excess 2-propanol, and dried under vacuum to yield 1.00 g (98%) of
the product 18: 1H NMR (CDCl3) δ 2.15 (m, 2H), 2.31 (t, 2H), 2.79
(t, 2H), 3.38 (t, 2H), 4.62 (s, 4H), 5.42 (t, 1H), 6.71 (s, 2H), 7.40 (m,
6H), 7.82 (m, 4H).
Acknowledgment. We thank Dr. Joe Reibenspies (TAMU)
for the X-ray crystallographic analysis of complex 17. Support
of this work by the National Science Foundation (CHE-
9707710) and the Robert A. Welch Foundation is gratefully
acknowledged. P.L.O. also thanks the National Science Founda-
tion for support from a Graduate Research Fellowship Award.
Supporting Information Available: Tables of crystal data,
structure solution and refinement, atomic coordinates, bond
lengths, and angles, and anisotropic thermal parameters for 17
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
General Procedure for Using 18 in Homogeneous Heck Reac-
tions. A solution of the aryl iodide (10.0 mmol), olefin (15.0 mmol),
triethylamine (15.0 mmol), and the catalyst 18 (50 mg, 0.1 mol %) in
8 mL of DMF was heated at 115 °C under an air atmosphere. The
reaction progress was monitored by GC. After the reaction (2.5 or 6.5
h for 7a and 7k, respectively), the solution was poured into 30 mL of
diethyl ether, precipitating the catalyst as a pale yellow solid. This
JA991099G