Mechanistic insight into the preactivation of a modern palladium catalyst precursor in phosphine-free Heck reactions

Article abstract of DOI:10.1007/s00706-008-0035-0

The use of well-defined trans-dichlorobis(diethanolamine-N)palladium(II) complex (trans-[PdCl₂(DEA)₂]) as a new, modern, and effective Heck precatalyst is reported. The mechanism of the key, initial steps of the reaction, i.e., the f

Full text of DOI:10.1007/s00706-008-0035-0

Monatsh Chem (2009) 140:371–374
DOI 10.1007/s00706-008-0035-0
ORIGINAL PAPER
Mechanistic insight into the preactivation of a modern palladium
catalyst precursor in phosphine-free Heck reactions
Zorica D. Petrovi c´ Æ Svetlana Markovi c´ Æ
Du sˇ ica Simijonovi c´ Æ Vladimir Petrovi c´
Received: 28 May 2008 / Accepted: 2 July 2008 / Published online: 17 September 2008
Ó Springer-Verlag 2008
Abstract The use of well-defined trans-dichlorobis(dieth-
anolamine-N)palladium(II) complex (trans-[PdCl (DEA) ])
phosphine-free Heck reactions [17]. The recently syn-
thesized trans-dichlorobis(diethanolamine-N)palladium(II)
complex (trans-[PdCl (DEA) ], 1) [14] contains both the
2
2
as a new, modern, and effective Heck precatalyst is reported.
The mechanism of the key, initial steps of the reaction, i.e.,
the formation of the Pd(0) complex, is examined using
density functional theory.
2
2
necessary amine (diethanolamine, DEA) and Pd(II). For
this reason we assume that it can be used as a precatalyst in
phosphine-free Heck reactions in the presence of strong
and weaker bases. Herein, we report the use and a pre-
liminary mechanistic study of the preactivation step of 1.
More precisely, we investigate the possible intramolecular
reduction of Pd(II) to Pd(0) using experimental and com-
putational methods.
Keywords Reaction mechanism ꢀ Pd(II) precatalyst ꢀ
Amino alcohol ꢀ Quantum chemical calculations
Introduction
The palladium-catalyzed arylation of olefins, known as the
Heck reaction, deserves considerable attention due to its
synthetic versatility [1]. In the past decade, phosphine-free
Pd complexes have been introduced as a less complicated
and environmentally more desirable alternative to original
Pd–phosphine catalysts [2]. However, nitrogen- and sulfur-
based palladacycles [3, 4], Pd complexes of N,S-based
carbenes and N-heterocyclic compounds [5–7] have been
associated with high-cost multistep syntheses and consid-
erable sensitivity of the ligands towards air and moisture.
In addition, very little is known about the molecular
structure of Pd(0) complexes, which have been generally
accepted as the catalytically active form [8, 9].
Results and discussion
When a reaction is performed in the presence of a strong
base, it can be expected that a proton of the OH group will
be abstracted from 1 (Fig. 1). In this way, the anion 2 would
be formed. We assume that a hydrogen bonded to the car-
binol C atom can be coordinated to Pd(II) [9, 18], thus
forming 4 via transition state 3 (Fig. 1). This transformation
can be followed by the reductive elimination of HCl, pro-
ducing 5. Complex 5 contains Pd(0), and so it should be a
catalytically active species in the Heck reaction. To confirm
our assumptions, we performed preliminary computational
and experimental investigations.
Amino alcohols and amino acids have been widely used
as O,N-ligands for the synthesis of different metal-organic
compounds and catalysts [10–16]. Triethanolamine has
proved to be an efficient base and reaction medium for
The optimized geometries of 2 and 3 are presented in
Fig. 2, whereas some crucial bond distances for structures
1–5 are given in Table 1.
Our calculation of 1 yields a structure with bond dis-
tances and angles consistent with the crystallographic
experimental data (Table 1) [14]. Following the consider-
ations presented above (Fig. 1), H1 is removed from 1 as a
proton, and the structure thus obtained is optimized (2 in
Fig. 2). The atomic polar tensor (APT) [19] charge
Z. D. Petrovi c´ ꢀ S. Markovi c´ (&) ꢀ D. Simijonovi c´ ꢀ V. Petrovi c´
Faculty of Science, University of Kragujevac,
34000 Kragujevac, Serbia
e-mail: mark@kg.ac.yu
123

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Z. D. Petrovi c´ et al.
Table 1 Selected bond distances for the species under consideration
Dist. (nm)
1
2
3
4
5
Pd–Cl1
Pd–Cl2
Pd–N1
Pd–N2
C1–H2
Pd–H2
0.2376
0.2376
0.2116
0.2116
0.1098
0.2866
0.2405
0.2408
0.2111
0.2123
0.1117
0.2603
0.2396
0.2396
0.2453
0.2247
0.1201
0.2056
0.2392
0.2408
–
0.2385
–
–
0.2325
–
0.2191
–
–
0.1536
Fig. 1 Proposed mechanism of the preactivation process of trans-
PdCl (DEA) ] precatalyst. R = –CH CH OH
[
2
2
2
2
?
iminium ion (HOCH CH=NHCH CH OH) , via a transi-
2
2
2
tion state which requires significantly higher activation
energy. The complete reaction path, as well as solvent
effects, will be reported elsewhere.
distribution of 2 shows that positive charge is distributed
among palladium (0.765), carbons, and some hydrogens,
whereas negative charge is distributed among all other
atoms. The C1–H2 and C1–H3 bonds become particularly
polarized, with the APT charges on C1, H2, and H3 of
Our predictions, obtained by means of DFT calculations,
were confirmed by performing Heck reactions catalyzed
with 1 in the presence of a weak and a strong base. The
results of our experiments are presented in Table 2.
At first, the reactions are performed with the equimolar
ratio of the reactants in the presence of DEA as a weak
base and 1 mol% of complex 1 (synthesized according to
the recently established procedure [14]), at 110 °C. In
order to increase the reaction rate, NaOEt is used instead of
DEA. It is established that the use of this strong base sig-
nificantly decreases the reaction time from 8 to 4 h for
iodobenzene, and from 11 to 6 h for bromobenzene. The
desired coupling products are obtained in good yield
(Table 2).
0
.700, -0.127, and -0.231, respectively. In addition, the
greatest contributions to the HOMO of 2 come from O1,
H2, and H3 (Fig. 2). Thus, one can expect that H2 will
perform a nucleophilic attack on palladium, because its
position is the most favorable. This attack leads to the
formation of transition state 3 (Fig. 2), with an energy
barrier of 83.0 kJ/mol. Here, the C1–H2 and Pd–N1 bonds
are being broken whereas the Pd–H2 bond is being formed
(
Table 1). The negative charge on H2 is slightly increased
(
-0.167). Subsequently in the reaction, H2 coordinates
with palladium as a hydride ion (4 in Fig. 1; Table 1). The
Pd–N1 and C1–H2 bonds are completely broken, implying
that dehydrogenated DEA is eliminated. In the basic
environment, 4 undergoes reductive elimination, thus
forming 5.
The considerable stability of complex 1 towards air and
moisture and the simplicity of the reaction procedure make
the investigated reactions desirable alternatives to original
reactions catalyzed with Pd–phosphine. The obtained
reaction products confirm that 1 really acts as an effective
precatalyst in the examined Heck reactions. In addition, our
experimental results show that the formation of the new
C–C bond between the aromatic ring and olefins does
If a strong base is not present in the reaction mixture,
then the initial abstraction of a proton from an alcoholic
OH group does not occur. Our calculations show that a
transformation of 1 involves the formation of 4 and
Fig. 2 Optimized geometries of
crucial participants in the
preactivation process of trans-
[
PdCl
depicted HOMO (left), and 3
right)
2 2
(DEA) ]: 2 with the
(
1
23

Mechanistic insight into preactivation of a modern palladium catalyst precursor
373
Table 2 Reactions between olefines and aryl halides catalyzed by
trans-[PdCl (DEA) ]
2 2
The crystal structure determination of trans-[PdCl2
DEA) ], as well as selected bond distances, bond angles
(
2
transꢁ½PdCl2ðDEAÞ ꢂ1mol%
2
and hydrogen bonds, are presented in [14].
C6H5X + CH2 ¼CHR ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ! C6H5CH = CHR
base
6
: R = C
6
H
5
; 7: R = COOCH ; 8: R = COOC
3
2
H
5
; 9: R = COOC
4
H
9
c
Entry Organic halide
R
Base
Time (h) Yield (%)
a
1
2
3
4
5
6
7
8
9
1
a
C
C
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
5
5
5
5
5
5
5
5
5
5
I
C
C
6
H
5
5
DEA
8
4
75
72
75
71
80
76
81
80
82
81
b
I
6
H
NaOEt
I
COOCH
COOCH
3
3
DEA
8
I
NaOEt
DEA
4
I
COOC
COOC
COOC
COOC
2
2
4
4
H
5
H
5
H
9
H
9
8
I
NaOEt
DEA
4
I
8
I
NaOEt
DEA
4
Br
Br
C
C
6
H
5
5
11
6
0
6
H
NaOEt
DEA (0.25 mmol) at 110 °C
b
NaOEt (0.2 mmol) at 100 °C
Isolated yield; only the trans product was detected by H NMR
c
1
Aryl halide (1 mmol), activated olefin (1 mmol) and
0.026 g DEA (0.25 mmol) are then added to the same
reaction flask. The reaction mixture is stirred and heated at
indeed occur faster in the presence of the strong base,
which is in agreement with our theoretical and computa-
tional predictions.
110 °C for 8 h in the case of iodobenzene, and 11 h in the
case of bromobenzene. After cooling the reaction mixture
3
to room temperature, CH Cl (7.0 cm ) is added to extract
2
2
the product. The organic phase is washed with water, and
Materials and methods
the aqueous layer is extracted with dichloromethane
3
2 9 10 cm ). The combined organic layers are dried over
(
GC analyses were performed with an Agilent (Palo Alto,
CA, USA) 6,890 N (G 1,530 N) instrument (Serial#
anhydrous sodium sulfate, and then the solvent is evapo-
rated in vacuo. When reactions are performed in the
presence of strong base, equimolar ratios of aryl halide
(1 mmol) and activated olefin (1 mmol), 1 mol% trans-
[PdCl (DEA) ], and 0.20 mmol NaOEt are mixed. The
1
13
CN10702033), with a capillary apolar column. H and
NMR spectra were run in CDCl on a Varian Gemini
C
3
2
00 MHz spectrometer. IR spectra were recorded on a
2
2
PerkinElmer (Waltham, MA, USA) Spectrum One FT-IR
spectrophotometer. Silica gel 60 (Merck, Darmstadt, Ger-
many; particle size 0.063–0.200 mm) was used for column
reactions are performed at 100 °C, in CH CN as solvent
3
3
(1 cm ), for a duration of 4 h for iodobenzene, and 6 h for
bromobenzene.
chromatography. The compounds: PdCl , DEA, aryl
2
The reactions were monitored and analyzed with GC
1
chromatography and H NMR spectroscopy. Reaction
halides and olefins were obtained from Aldrich Chemical
Co. (St. Louis, MO, USA).
products were purified with column chromatography (silica
gel; ethylacetate:dichloromethane = 1:1). The known cou-
pling products were analyzed and characterized on the basis
of their spectroscopic data, and by comparing these data to
the spectra of the commercially available compounds.
All calculations were conducted using Gaussian03 [20]
with the B3LYP hybrid functional [21, 22]. The 6–31G(d)
basis set was used for C, H, O, N, and Cl, and LANL2D-
Z ? ECP [23] was employed for the Pd center. Geometrical
parameters of all investigated species were optimized in
vacuum. All calculated structures were verified as being local
minima (all positive eigenvalues) for ground-state structures
or first-order saddle points (one negative eigenvalue) for
transition-state structures by frequency calculations.
The reactions of olefins and aryl halides in the presence
of trans-[PdCl (DEA) ] were performed in the following
way. To a magnetically stirred solution of 1 mol% of PdCl
2
2
2
3
in 2 cm of ethanol, 2 mol% of diethanolamine at room
temperature is added. Stirring is continued at 50–60 °C for
half an hour. After the evaporation of EtOH in vacuo, the
orange complex 1 is obtained. Crystallographic data have
been deposited with the Cambridge Crystallographic Data
Centre. Copies of the information may be obtained free of
charge from the Director, CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (Fax: ?44-1223-336033; E-mail:
deposit@ccdc.cam.ac.uk or URL: http://www.ccdc.cam.
ac.uk), quoting the deposition number CCDC299671.
123

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Z. D. Petrovi c´ et al.
Acknowledgments This work is supported by the Ministry of Sci-
ence and the Environment of Serbia, project No. 142013 B.
11. Lu G, Li X, Zhou Z, Chan WL, Chan ASC (2001) Tetrahedron
Asymm 12:2147
1
2. Wu X, Li X, McConville M, Saidi O, Xiao J (2006) J Mol Catal
A 247:153
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1
3. Patti A, Pedotti S (2003) Tetrahedron Asymm 14:597
4. Petrovi c´ ZD, Djuran MI, Heinemann FW, Rajkovi c´ S, Trifunovi c´
S (2006) Bioorg Chem 34:225
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