Orthogonal C-N plus C-C tandem reaction of iodoanilines leading to styrylguanidines catalyzed by supported palladium nanoparticles

Article abstract of DOI:10.1002/chem.201202823

Two birds, one nanoparticle: A highly efficient and selective tandem reaction comprising C-N and C-C cross-coupling of iodoanilines to form styrylguanidines by using a recyclable, heterogeneous, supported palladium nanoparticle catalyst is reported (see s

Full text of DOI:10.1002/chem.201202823

DOI: 10.1002/chem.201202823
À
À
Orthogonal C N Plus C C Tandem Reaction of Iodoanilines Leading to
Styrylguanidines Catalyzed by Supported Palladium Nanoparticles
Abdessamad Grirrane,[a] Hermenegildo Garcia,*[a] Avelino Corma,*[a] and
Eleuterio ꢀlvarez[b]
One trend in catalysis is to combine two or more reac-
tions into a single tandem process without compromising
the overall selectivity of the final product.[1–3] Tandem reac-
tions exhibit several advantages with respect to the corre-
sponding consecutive, independent processes, including min-
imization of workup, process intensification, and reduction
of the number of purification steps.[4–7] Herein, we report
this work, it has been reported that PdII salts can promote
this guanylation reaction under homogeneous conditions.[31]
The reaction that we combined with the guanylation of ar-
À
omatic amines is the C C cross-coupling of aryl halides with
vinylsilanes, leading to styrene derivatives, the so-called
Hiyama coupling.[32] Simple styrenes are difficult to prepare
by Heck coupling because it would require the use of
ethene as the substrate, making the process impractical.
Styrenes are among the most important monomers for the
formation of a large variety of functionalized and cross-
linked copolymers[33] and cannot be obtained by convention-
al Heck reactions. An alternative for the preparation of sub-
stituted styrenes that has been employed in this case is
based on the use of vinylsilanes (see the reaction of 3a–d
with 4a in Table 1). Typically, this process is catalyzed under
homogenous conditions by palladium salts, such as PdCl2
À
the combination of two orthogonal coupling reactions (C N
À
and C C bond formation) into one tandem process, leading
in the formation of N-styrylguanidines in one step by using
supported palladium nanoparticles as the catalyst.[8–10] This
À
À
orthogonal C N and C C tandem reaction is the equivalent
to performing two independent catalytic reactions consecu-
tively.
One of the tandem reactions studied herein is the guan
ACHTUNGERTNyNUNG l-
ACHTUNGTRENNUNGation of aromatic amines by carbodiimides (see the reaction
of 1a–d with 2 in Table 1). Substituted guanidines are an im-
portant type of compound with a wide range of applications
as fungicides,[11] antifouling agents,[12–14] DNA-binding
motifs,[15,16] and building blocks for the synthesis of biologi-
cally relevant compounds.[17–20] For many applications, it
would be of interest to have a styrylguanidine that could
easily be used to provide the corresponding polystyrene.
Guanylation of anilines by carbodiimides is a catalytic reac-
tion. The various catalysts reported for the guanylation of
amines include rare-earth metals[21] and inorganic imido
and Pd
(PPh3)4].[32]
In view of these precedents and considering that palladi-
um salts can act as homogenous catalysts for both reactions
(Hiyama coupling and guanylation), it would be interesting
to combine these two processes into a single tandem reac-
tion from iodoanilines to styryl guanidines, which are versa-
tile monomers, by using a solid Pd catalyst. In addition, we
have developed a novel heterogeneous catalyst for this
tandem process.
ACHTUNGRTENUN(NG OAc)2, or palladium(0) complexes, such as [Pd-
AHCTUNGTRENNUNG
(Ti=N[22] and V=N[23,24]) or amino (M N; M=Li, Ti, Ca, Al,
In the first stage of our work, we prepared a series of pal-
ladium catalysts and tested their activity for the two inde-
À
and lanthanides) compounds.[25–30] More closely related to
À
À
pendent processes, that is, the C N and C C coupling reac-
tions. As a support for the Pd nanoparticles (NPs), we
tested activated carbon, as well as three nanoparticulate
metal oxides with either acidic (TiO2 or CeO2) or basic
(MgO) natures (see the Experimental Section for details).
The Pd loading of the catalysts ranged from 0.8 to
1.1 wt% with an average particle size of 1.9 to 3.5 nm. Ac-
cording to X-ray photoelectron spectroscopy (XPS), the oxi-
dation state of Pd depends upon the support, reflecting dif-
ferences in the interaction of the Pd NPs with the acidic or
basic support (see Table S1, Figure S1 (XPS), and Figur-
es S2–S4 (TEM images) in the Supporting Information).
We initially confirmed that the same Pd catalyst can pro-
[a] Dr. A. Grirrane, Prof. Dr. H. Garcia, Prof. Dr. A. Corma
Instituto Universitario de Tecnologꢀa Quꢀmica CSIC-UPV
Univ. Politꢁcnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia (Spain)
Fax : (+34) 963877809
E-mail: hgarcia@qim.upv.es
acorma@itq.upv.es
[b] Dr. E. ꢂlvarez
Instituto de Investigaciones Quꢀmicas CSIC-US
Departamento de Quꢀmica Inorgꢃnica
Av. Amꢁrico Vespucio 49, 41092 Sevilla (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202823. It contains full de-
tails of the preparation and characterization of guanidines 3a–d and
5a–d, as well as details of the isolation procedures and full characteri-
zation of trans-dichlorobis(4-iodoaniline-kN)palladium(II) (6) and
trans-dichlorobis[1,3-diisopropyl-2-(4-iodophenyl)guanidine-kN-
À
À
mote the C N and C C coupling reactions separately with
high efficiency. To advance towards the tandem process, we
performed both reactions consecutively in one pot with the
same catalyst, but adding the reagents stepwise (see
ACHTUNGTRENNUNG(aryl)]palladium(II) (7).
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Chem. Eur. J. 2012, 18, 14934 – 14938

COMMUNICATION
[a]
À
À
Table 1. Results of the consecutive one-pot, one-catalyst C N and C C coupling starting from iodoanilines.
esses. The tandem reaction con-
sists of the formation of N-sty-
À
rylguanidines by orthogonal C
À
N and C C cross-coupling cata-
lyzed by supported Pd NPs (see
Table 2).
Table 2 shows that in spite of
preliminary results in which the
two reactions were catalyzed ef-
ficiently, when the two process-
es are combined the final selec-
tivity for and yield of the de-
sired product 5a are far from
satisfactory (Table 2, entry 1).
A time–conversion plot shows
that the main reason for this is
Catalyst ([wt%])
Pd (0.87)/MgO
Reagent
Coupling step
t [h]
Conv. [%]
Sel. [%]
Yield [%]
1a
C N
11
1
10
1
98[b]
99[c]
98[b]
100[c]
98[b]
100[c]
96[b]
100[c]
97[b]
98[c]
81[b]
99[c]
97
99
91
98
92
90
80
99
93
98
92
70
50
98
92
96 (3a)
90 (5a)
95 (3a)
91 (5a)
88 (3a)
77 (5a)
94 (3b)
92 (5b)
94 (3c)
89 (5c)
57 (3c)
45 (5c)
93 (3d)
90 (5d)
À
À
C C
À
Pd (1)/C
1a
1a
1b
1c
1c
1d
C N
À
C C
À
Pd (1.1)/CeO2
Pd (1)/C
C N
9
À
C C
18
17
4
15
7
15
7
18
5
À
C N
À
the faster C C coupling reac-
À
C C
tion, which leads to the forma-
tion of a 4-aminostyrene inter-
mediate that reacts much more
slowly with carbodiimide 2 than
does 4-iodoaniline (1a) and
thus accumulates without giving
rise to the expected product 5a.
Hence, the significant decrease
in product selectivity of styryl-
guanidine 5a is due to the un-
expectedly poor reactivity of 4-
aminostyrene towards guanyla-
tion.
À
Pd (0.87)/MgO
Pd (1.09)/TiO2
Pd (0.87)/MgO
C N
À
C C
À
C N
À
C C
À
C N
À
C C
99
À
[a] Reaction conditions: C N coupling: 1a, 1b, 1c, or 1d (0.25 mmol), carbodiimide 2 (0.3 mmol), DMF
(1 mL), Ar (2 bar), 1308C, Pd/substrate ratio: 1 mol%; C C coupling: 4a (0.075 mmol), KF (0.6 mmol). Re-
agents 2 and 4a were added sequentially after the indicated time (see the Supporting Information for the syn-
thesis and a complete characterization of guanidines 3a–d and 5a–d, including ORTEP diagrams obtained
from single-crystal XRD). Conv.=conversion, Sel.=selectivity. [b] Based on the disappearance of 1a, 1b, 1c,
or 1d. [c] Based on the disappearance of 3a, 3b, 3c, or 3d.
À
À
Table 1). In particular, we initially proceeded with the C N
addition of 1a, 1b, 1c, or 1d to 2. Once 1,3-diisopropyl-2-(4-
iodophenyl)guanidine (3a),[34] 1,3-diisopropyl-2-(3-iodophen-
In an attempt to circumvent the problems derived from
the low reactivity of aminostyrenes (see footnote [c] in
Table 2), while driving the formation of the tandem products
A
5a–d with high selectivity, we used trimethylACTHGNUTERNNU(G vinyl)silane 4b
A
instead of tetravinylsilane 4a to slow the formation of the
À
G
aminostyrenes and give the C N coupling a chance to com-
ravinylsilane (4a) and KF were added and the reaction was
allowed to proceed further towards the formation of the ex-
pected 1,3-diisopropyl-2-(4-vinylphenyl)guanidines 5a–d.
As can be seen in Table 1 both Pd/MgO and Pd/C give
similar results that are significantly better than Pd/CeO2 and
Pd/TiO2. Under the optimal conditions, with the appropriate
Pd catalysts, almost complete conversions of iodoanilines
(1a–d) and iodoguanidines (3a–d) were achieved with high
selectivity toward formation of the target vinylguanidines
(5a–d).
The temporal evolution of substrate 1a, intermediate gua-
nidine 3a, and final guanidine 5a by using Pd/MgO as the
catalyst (Table 1, entry 1) shows that the two consecutive or-
thogonal reactions proceed with high yields and selectivity
(see Figure S5 in the Supporting Information). It is notable
pete (Table 2, entry 2). Under these conditions, we were
able to perform the tandem reaction with high selectivity to-
wards the products 5a–d, and minimize the amount of the
aminostyrenes formed (Table 2, entries 2, 6, 7, and 8). In
this way, the reactions were carried out with the Pd
(0.87 wt%)/MgO catalyst by adding all the reagents at the
same time, providing the desired products in good yield and
high selectivity. These results are significantly better than
those obtained with Pd (1 wt%)/C instead of Pd
(0.87 wt%)/MgO (Table 2, entry 5). Figure 1 (top) shows the
temporal evolution of substrate 1a, intermediate guanidine
3a, and the final guanidine 5a by using Pd/MgO as the cata-
lyst (Table 2, entry 2), showing that the tandem reaction
proceeds with good yields and selectivity.
One important issue in heterogeneous catalysis by Pd NPs
is to establish the possible contribution of Pd species leach-
ed from the solid to the liquid phase. To address this point,
we analyzed the Pd content of the solid catalyst at the end
of the reaction. Inductive coupled plasma (ICP) analysis es-
tablished that the Pd content of the catalyst was 0.828 wt%
after the first reuse and 0.812 wt% after the second reuse
À
À
that the C N coupling reaction is slower than the C C cou-
pling reaction, which requires shorter reaction times.
The target then was to develop a tandem reaction in
which two individual catalytic reactions take place simulta-
neously and are promoted by a single solid-phase catalyst by
mixing together all of the reagents involved in the two proc-
Chem. Eur. J. 2012, 18, 14934 – 14938
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14935

H. Garcia, A. Corma et al.
Table 2. Results of tandem reactions of iodoanilines, carbodiimide, and vinylsilanes catalyzed by supported Pd
catalysts.[a]
compared with 0.870 wt% Pd
for the unused Pd/MgO cata-
lyst. In addition, we performed
the conventional hot filtration
test [Figure 1 (bottom)] in
which, after starting the reac-
tion under normal conditions,
the suspension is filtered at the
reaction temperature and the
clear solution, in the absence of
the solid, is allowed to react
further. In this case, we found
that removal of the solid Pd
catalyst after 1 h results in only
Catalyst [(wt%)]
Reagents 1/4
t [h]
Conv.[b] [%]
Sel. [%]
Yield[c] [%]
Pd (0.87)/MgO
1a/4a
1a/4b
1a/4b
1a/4b
1a/4b
1b/4b
1c/4b
1d/4b
20
10
12
14
10
20
17
20
100
100
100
100
98
100
100
99
54
71
72
70
62
70
76
73
52; 5a (40)
70;[e] 5a (7)
71;[e] 5a (6)
69;[e] 5a (7)
60;[f] 5a (10)
68;[g] 5b (6)
74;[h] 5c (4)
71;[i] 5d (5)
Pd (0.87)/MgO (fresh)
Pd (0.83)/MgO (first reuse)[d]
Pd (0.82)/MgO (second reuse)[d]
Pd (1)/C
Pd (0.87)/MgO
Pd (0.87)/MgO
limited
conversion
[<8%,
Figure 1 (bottom)], indicating
that the reaction is mostly het-
erogeneous. In the literature,
there are precedents showing
that related supported Pd cata-
lysts undergo leaching when
using DMF as the solvent.[35]
However, these precedents also
show that the activity of the
leached Pd depends on the sub-
Pd (0.87)/MgO
[a] Reaction conditions: amine (0.25 mmol), carbodiimide 2 (0.35 mmol), vinylsilane 4a/4b (0.075/0.35 mmol),
KF (0.6 mmol), DMF (1 mL), Ar (2 bar), Pd/substrate ratio: 1 mol%. [b] Based on the disappearance of the
iodoanilines. [c] The numbers in parentheses correspond to the yield of aminostyrenes. [d] Pd (0.87 wt%)/
MgO catalyst was recovered, washed with water (pH 7), acetone, and diethyl ether, and then dried. [e] A 20%
yield of (Z)- and (E)-1,3-diisopropyl-2-{4-[2-(trimethylsilyl)vinyl]phenyl}guanidine and a 2% yield of (Z)- and
(E)-4-[2-(trimethylsilyl)vinyl]aniline were observed. [f] A 14% yield of (Z)- and (E)-1,3-diisopropyl-2-{4-[2-
(trimethylsilyl)vinyl]phenyl}guanidine and a 12% yield of (Z)- and (E)-4-[2-(trimethylsilyl)vinyl]aniline were
observed. [g] An 18% yield of (Z)- and (E)-1,3-diisopropyl-2-{3-[2-(trimethylsilyl)vinyl]phenyl}guanidine and
a 2% yield of (Z)- and (E)-3-[2-(trimethylsilyl)vinyl]aniline were observed. [h] A 16% yield of (Z)- and (E)- strates involved and the de-
2-{2-fluoro-4-[2-(trimethylsilyl)vinyl]phenyl}-1,3-diisopropylguanidine and a 2% yield of (Z)- and (E)-2-fluoro-
mands of the catalytic reac-
4-[2-(trimethylsilyl)vinyl]aniline were observed. [i] A 15% yield of (Z)- and (E)-2-{2,5-dimethyl-4-[2-(tri
ACHTUNGTNERmNUNG eth-
tion.[35] In our case, and com-
paring both graphs in Figure 1,
it appears clear that the
0.042% of the initial Pd catalyst
leached during the first use
ACHTUNGTRENNUNG
lyl)vinyl]aniline were observed.
(0.016% during the second use) of the catalyst is not effi-
À
À
cient at promoting the C N or C C coupling reactions, and
that the majority of the products observed in the tandem re-
action are formed by the action of the solid catalyst.
Another important point of a heterogeneous catalyst is its
reusability for consecutive runs of the reaction. This point
was studied by recovering the catalyst at the end of the reac-
tion, washing it with water, acetone, and diethyl ether,
drying it under vacuum at 908C, and reusing it for subse-
quent reactions without additional treatment. The degree of
deactivation was determined by comparing the time–conver-
sion plots for the fresh and reused catalysts, in which a grad-
ual decrease in the catalytic activity was observed (see
Table 2, entries 2, 3, and 4). To achieve the same yield of 5a,
an additional 2 and 4 h of reaction time for the first and
second reuse, respectively, were required. To understand
this smooth deactivation, TEM images of the reused catalyst
were compared with the fresh material and show an increase
in the average size of the Pd NPs, with an average particle
size of 3.5 nm for the catalyst after one use and 3.9 nm after
the second use [see TEM images of the reused catalysts
(Figures S6 and S7) and Table S1 in the Supporting Informa-
tion].
Figure 1. Top: Time–conversion plot for the tandem reaction of 4-iodo-
ACHTUNGTRENNUNGaniline (1a), carbodiimide 2, and trimethylvinylsilane (4b) in the pres-
ence of Pd (0.87 wt%)/MgO. Bottom: Time–conversion plot for the
tandem reaction when the solid catalyst is removed by hot filtration after
^
1 h. : disappearance of 4-iodoaniline (1a); : yield of the aryl iodide
*
guanidine intermediate 3a; ꢅ: yield of 2-(4-vinylphenyl)guanidine (5a);
À
À
The most likely reaction mechanisms for the C N and C
C elementary reactions, based on literature data, as well as
*
and +: yields of (Z)- and (E)-1,3-diisopropyl-2-{4-[2-(trimethylsilyl)vi-
~
nyl]phenyl}guanidine; : yield of 4-vinylaniline.
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Chem. Eur. J. 2012, 18, 14934 – 14938

Tandem Reaction of Iodoanilines to Form Styrylguanidines
COMMUNICATION
2, and [PdCl2ACHTNUTRGENNUG(MeCN)2], which
corresponds to the homogenous
phase analogue of the tandem
reaction (see Scheme 2).
Pd complex 7 could also be
fully characterized, including by
single-crystal XRD (see the
Supporting Information for the
synthesis and a complete char-
acterization of complex 7). We
have observed that this ternary
complex (7) thermally decom-
poses to form the correspond-
ing N-arylguanidine. Based on
the structures of Pd complexes
6 and 7, isolated in the homoge-
nous phase, we propose an
analogous mechanism for the
À
heterogeneous C N coupling
reaction, in which the role of
the Pd atom in complexes 6
and 7 will be played by Pd NPs
on the support (see Scheme 1
for the proposal for the hetero-
genous reaction). In addition, in
the case of supported Pd cata-
lysts, basic supports like MgO
can contribute to the reaction
mechanism by polarization of
À
Scheme 1. Proposed mechanism for the C N coupling in the presence of heterogeneous Pd catalysts.
À
the N H bond, increasing ani-
line nucleophilicity and favor-
ing the formation of analogues
of complexes 6 and 7, but in-
volving Pd NPs.
À
The mechanism of C C cou-
pling of aryl iodides with vinyl-
silanes is proposed to involve
the oxidative addition of Pd0 to
Scheme 2. Isolation of trans-dichlorobis(4-iodoaniline-kN)palladium(II) (6) and trans-dichlorobis[1,3-diiso-
propyl-2-(4-iodophenyl)guanidine-kN
1a and 2.
ACHUTNGTRENUNG(aryl)]palladium(II) (7) by reaction of [PdCl2ACHTUNGTERN(NUNG MeCN)2] with 1a or with
À
the C I bond in the arenes,
subsequent nucleophilic attack
the data collected herein, are shown in Scheme 1 and
Scheme S1 (in the Supporting Information), respectively.
With respect to the guanylation (Scheme 1), our mechanistic
proposal is based on our isolation of a tetracoordinated Pd
complex 6, obtained from bis(acetonitrile)dichloropalladi-
of the vinyl anion, and a final reductive elimination. Vinyl
À
À
anions will be formed by cleavage of the C Si bond by F
ions (Scheme S1, in the Supporting Information).
In summary, an orthogonal tandem reaction in which sty-
AHCTUNGTREGrNNNU ylguanidines can be formed with high yield and selectivity
A
in one pot has been developed by using supported Pd NPs
as a heterogeneous catalyst. These guanidines are interesting
monomers for the preparation of polymeric antifouling
agents and as scaffolds in supramolecular chemistry. Besides
optimization of the reaction conditions, the key point of the
tandem process is the nature of the solid in which the Pd
NPs are supported and also the kind of vinylsilane used as
the reagent. Isolation of Pd complexes with the substrates
has provided evidence for a mechanistic proposal in which
the Pd is coordinated to the basic N atoms of the aniline
and guanidine, respectively.
characterized, including by single-crystal XRD (see the Sup-
porting Information for the synthesis and a complete charac-
terization of complex 6). Complex 6 shows similar coordina-
tion to the Pd complex isolated for the reaction of 4-meth-
ACHTUNGTRENNUNGoxyaniline and H2PdCl4, which has previously been reported
in the literature.[36] When Pd complex 6, containing two ani-
line molecules as ligands, is reacted with N,N’-diisopropyl-
carbodiimide (2), a ternary aniline–carbodiimide–Pd com-
plex 7 was isolated by us. Complex 7 can also be obtained in
a single step upon mixing 4-iodoaniline (1a), carbodiimide
Chem. Eur. J. 2012, 18, 14934 – 14938
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14937

H. Garcia, A. Corma et al.
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Experimental Section
Catalyst preparation: Pd/TiO2, Pd/CeO2, and Pd/MgO were prepared by
impregnation of commercially available TiO2, CeO2, and MgO, respec-
tively (2 g, calcined at 4008C for 4 h), with a mixture of deionized water
(15 mL) and a solution of PdCl2 (35 mg, Aldrich, purum, 60% palladium
content) in acetone (8 mL). The slurry was stirred for 15 h at room tem-
perature, then the liquid phase was evaporated and the solid was dried
under vacuum at 1008C for 2 h, and reduced with 1-phenylethanol at
433 K for 3 h. The catalyst was then filtered, washed (acetone and diethyl
ether), and dried under vacuum at 1008C for 12 h. The final Pd content
was found to be 1.09, 1.10, and 0.87 wt%, respectively, by atomic absorp-
tion analysis.
X-Ray structure analysis: CCDC-881452 (3a), CCDC-881453 (5a),
CCDC-891374 (5b), CCDC-881454 (6), and CCDC-881455 (7) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgement
Financial support by the Spanish Ministry of Science and Innovation
(Consolider MULTICAT and CTQ 2012–32315) is gratefully acknowl-
edged. The Generalidad Valenciana is also thanked for partial financial
support (Prometeo Grant).
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Inorg. Chim. Acta 2009, 362, 2814–2819.
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