Isolation and X-ray characterization of palladium-N complexes in the guanylation of aromatic amines. Mechanistic implications

Article abstract of DOI:10.3762/bjoc.9.165

In the context of palladium-catalyzed guanylation of anilines herein, we have been able to characterize and isolate bis(anilino) and bis(guanidino)Pd(II) complexes using reaction conditions under which stoichiometric amounts of palladium salts are used. Characterization of these palladium complexes strongly supports a mechanistic proposal for the catalytic guanylation of anilines using PdCl₂ (NCCH₃)₂ as catalyst that involves the intermediacy of these Pd(II) complexes.

Full text of DOI:10.3762/bjoc.9.165

Isolation and X-ray characterization of
palladium–N complexes in the guanylation of
aromatic amines. Mechanistic implications
Abdessamad Grirrane*1, Hermenegildo Garcia*1 and Eleuterio Álvarez2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1455–1462.
1Instituto Universitario de Tecnología Química CSIC-UPV,
Universidad Politécnica de Valencia, Av. De los Naranjos s/n, 46022
Valencia, Spain and 2Instituto de Investigaciones Químicas CSIC-US,
Departamento de Química Inorgánica, Av. Américo Vespucio 49,
41092 Sevilla, Spain
doi:10.3762/bjoc.9.165
Received: 03 April 2013
Accepted: 21 June 2013
Published: 22 July 2013
Email:
This article is part of the Thematic Series "New reactive intermediates in
organic chemistry".
Abdessamad Grirrane* - grirrane@itq.upv.es; Hermenegildo Garcia* -
hgarcia@qim.upv.es
Guest Editor: G. Bucher
* Corresponding author
© 2013 Grirrane et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
coordination chemistry; guanylation reaction; homogeneous catalysis;
palladium complexes; reactive intermediates
Abstract
In the context of palladium-catalyzed guanylation of anilines herein, we have been able to characterize and isolate bis(anilino) and
bis(guanidino)Pd(II) complexes using reaction conditions under which stoichiometric amounts of palladium salts are used. Charac-
terization of these palladium complexes strongly supports a mechanistic proposal for the catalytic guanylation of anilines using
PdCl2(NCCH3)2 as catalyst that involves the intermediacy of these Pd(II) complexes.
Introduction
N-Arylguanidines are important compounds with interesting magnesia can be a solid catalyst for this process [20]. Working
biological activities [1,2] such as fungicides [3] and also in with PdCl2(NCCH3)2 in dichloromethane we were able to
supramolecular chemistry as complementary partners of isolate two types of palladium complexes with iodoaniline and
carboxylate and nitro groups [4-7]. Some of these guanidines guanidine, respectively, (see Scheme 1) that give some clue
are commercially used as antifouling agents in marine paints about the reaction mechanism of the catalytic process.
and in the formulation of protective surface coatings [8-10].
N-Arylguanidines can be obtained by aniline insertion into the Results and Discussion
corresponding carbodiimide [11-18]. This nucleophilic addition In order to provide further support to the mechanistic proposal
can be efficiently catalyzed by palladium salts [19], such as for the C–N insertion promoted by palladium(II) suggested by
PdCl2 or Pd(OAc)2 in homogenous phase. Also recently we us [20], in the present report we describe the study of palla-
have reported that palladium nanoparticles supported on dium-catalyzed guanylation of three additional anilines (1a–c)
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Scheme 1: Isolation of trans-dichlorobis(4-iodoanilino-ĸN)palladium(II) and trans-dichlorobis[1,3-diisopropyl-2-(4-iodophenyl)guanidino-ĸN(aryl)]palla-
dium(II) complexes formed by reaction of PdCl2(MeCN)2 with 4-iodoaniline (upper row) or with 4-iodoaniline and N,N’-diisopropylcarbodiimide (lower
reaction).
with N,N’-diisopropylcarbodiimide (2). For these reactions CH2Cl2, evolution at initial reaction times of a solid precipitate
we have been able to characterize three palladium complexes is observed (Scheme 2). Filtration of these precipitates and
of the type of bis(anilino)Pd(II) (3a–c) as well as three palla- subsequent washing with CH2Cl2 renders three solids whose
dium complexes of the type of bis(guanidino)Pd(II) (4a–c) combustion analysis is in accordance with the percentages
(Scheme 2) whose structures have been characterized by single- expected for dichlorobis(anilino-ĸN)palladium(II) (3a–c) (see
crystal X-ray structural analysis, as well as to obtain evidence Supporting Information File 1, experimental section). IR spectra
for the intermediacy of these complexes in the catalytic process. of complexes 3a–c show the characteristic absorption peaks due
Overall the present data reinforce the previous proposal for the to the coordinated anilines 1a–c (see Supporting Information
mechanism of aniline guanylation.
File 1, Figures S1–S3); these are compatible with the proposed
structure for these intermediates. Complex 3a derived from 1a
When a stoichiometric (2:2:1) mixture of anilines 1a–c and has been recently characterized by single-crystal X-ray diffrac-
carbodiimide 2 with PdCl2(MeCN)2 is stirred at 60 °C in tion [21] showing similar coordination to trans-dichlorobis(4-
Scheme 2: Isolation of trans-dichlorobis[1,3-diisopropyl-2-(aryl)guanidino-ĸN(aryl)]palladium(II) complexes (4a–c) by reaction of PdCl2(MeCN)2 with
anilines 1a–c and N,N’-diisopropylcarbodiimide (2). Dashed lines indicate the formation of trans-dichlorobis(anilino-ĸN)palladium(II) complexes (3a–c)
as primary intermediates during the reaction, and their subsequent reaction with 2 to led 4a–c.
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iodoaniline-ĸN)palladium(II) complex recently published by us Besides X-ray crystal structure analysis, palladium complexes
[20]. Compounds 3a–c were also characterized by solid state 4a–c were also characterized by NMR spectroscopy, ESIMS
13C NMR spectroscopy that gave spectra showing carbon peaks and combustion analysis (see experimental section in
compatible with the proposed structure (see Supporting Infor- Supporting Information File 1). 1H, 13C and 19F NMR spec-
mation File 1, Figures S4–S6).
troscopy of 4a–c in CD2Cl2 solution provides evidence showing
that under the reaction conditions the starting reagents
After prolonging the reaction time, the initially evolved precipi- (PdCl2(MeCN)2, 1a–c and 2) including intermediates bis(ani-
tate undergoes dissolution indicating that it has been trans- lino-ĸN)palladium(II) (3a–c) are completely converted into the
formed under the reaction conditions. At this stage, filtering of corresponding bis(guanidino-ĸN)palladium(II) (4a–c) (see
the transparent orange-red (4a,b) and red (4c) solutions fol- Supporting Information File 1, Figures S8 and S9 for 4a,
lowed by subsequent addition of ethyl ether or toluene and slow Figures S12 and S13 for 4b and Figures S16–S18 for 4c).
solvent evaporation at ambient temperature allows the forma- ESIMS of a solution obtained after dissolving complexes 4a–c
tion of crystals with suitable quality for a crystallographic in CH2Cl2/CH3CN (1:1) shows single positive MS peaks at
diffraction study. The structures of these intermediates solved 639.3, 667.2 and 867.1 attributable, respectively, to the com-
by X-ray analysis showed that these compounds corresponding plexes [C28H46Cl2N6O2Pd (4a) − Cl−]+, [C28H42Cl2N6O4Pd
to trans-dichlorobis[arylguanidino-ĸN(aryl)]palladium(II). (4b) − Cl−]+ and [C26H38Cl2F2I2N6Pd (4c) − Cl−]+. Also
Figure 1, Figure 2 and Figure 3 present ORTEP views of com- negative MS shows single peaks at 711.2, 739.1 and 938.9
plexes 4a–c as well as selected views along some crystallo- attributable, respectively, to the complexes [C28H46Cl2N6O2Pd
graphic axes (see Tables S1–S3 and also Figures S7, S11 and (4a) + Cl−]−, [C28H42Cl2N6O4Pd (4b) + Cl−]− and
S15 in Supporting Information File 1, and for full details of the [C26H38Cl2F2I2N6Pd (4c) + Cl−]− (see Supporting Information
crystallographic data see Supporting Information File 2).
File 1, Figures S10, S14 and S19).
Figure 1: (Top) ORTEP view of the centrosymmetric molecule 4a. (Bottom) Crystal packing detail of 4a viewed along the a-axis showing the pres-
ence of inter- and intramolecular hydrogen bonds between Cl and H (NH groups) atoms.
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Figure 2: (Left) ORTEP representation of 4b. (Right) Crystal packing detail of 4b viewed along the a-axis showing the presence of intermolecular
hydrogen bonds between O (from the dioxole moieties) and N (NH groups) atoms.
Figure 3: (Left) ORTEP representation of 4c. (Right) Crystal-packing detail of 4c viewed along the a-axis showing the presence of intermolecular
hydrogen bonds between Cl and N (NH groups) atoms.
Similar reactions were carried out mixing anilines 1a–c and precipitates were observed. In contrast, in the presence of
N,N’-diisopropylcarbodiimide (2), but in this case in the pres- catalytic amounts of palladium, formation of the corresponding
ence of only a catalytic amount of PdCl2(NCCH3)2 (4 mol %) N-arylguanidines was observed in almost quantitative yield.
(Scheme 3). Under these conditions no evidence for the forma- These guanidines 5a–c formed by nucleophilic attack of
tion of palladium complexes (3a–c) and (4a–c) could be anilines 1a–c to N,N’-diisopropylcarbodiimide (2) catalyzed by
obtained due to the low amount of palladium and no solid palladium were fully characterized by analytical and spectro-
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Scheme 3: Guanylation reactions of anilines 1a–c by N,N’-diisopropylcarbodiimide (2) catalyzed by Pd(II) salt.
scopic data (see Supporting Information File 1, experimental two kinds of palladium complexes have been detected and
section).
isolated, with the formation of guanidines under conditions in
which a catalytic amount of palladium is present, allows us to
Structure of guanidine 5a was confirmed by single-crystal make reasonable mechanistic proposals. Thus, upon contacting
X-ray analysis, Figure 4 shows the corresponding ORTEP for anilines 1a–c and PdCl2(MeCN)2, a rapid formation of di-
compound 5a as well as some views of the crystal packing (see chlorobis(anilino-ĸN)palladium(II) (3a–c) complexes should
also Supporting Information File 1, Table S4 and Figure S20 take place. These palladium complexes will interact with
and for full details of crystallographic data see Supporting the N,N’-diisopropylcarbodiimide (2) giving rise to the di-
Information File 2). Beside X-ray crystal analysis of guanidine chlorobis(guanidino-ĸN)palladium(II) (4a–c) complexes.
5a, guanidines 5a–c were also characterized by 1H, 13C and When Pd is used in catalytic amounts, cleavage of this
19F NMR spectroscopy and combustion analysis (see Figures bis(guanidino-ĸN) complex by aniline will form another di-
S21 and S22 for 5a, Figures S24 and S25 for 5b, our recent chlorobis(anilino-ĸN)palladium(II) (3a–c) completing one cycle
published work for 5c [20] and experimental section in and liberating guanidines 5a–c as free products of this catalytic
Supporting Information File 1). ESIMS and GC–MS of reaction with high yields and selectivities (see Scheme 4). In
solutions obtained respectively after dissolving guanidines 5a this mechanism the rate determining step will be the attack of
and 5b in CH2Cl2/MeOH (1:1) and CH2Cl2 shows a single dichlorobis(anilino-ĸN)palladium(II) (3a–c) to the N,N’-diiso-
positive MS peak at 250.2 and 263.2 Da attributable, respective- propylcarbodiimide (2) (Scheme 4).
ly, to the complexes [C14H23N3O (5a) + H+]+ and C14H21N3O2
(5b) (see Figure S23 and Figure S26 in Supporting Information In this process, coordination of nitrogen to palladium should
File 1).
strongly reduce the nucleophilicity of the corresponding
nitrogen atom and, therefore, the attack at the carbodiimide
Overall the information obtained from the experiments would be significantly slowed down compared to free uncoordi-
performed in the presence of a large palladium excess, in which nated aniline. However, this negative effect of palladium coor-
Figure 4: (Left) ORTEP representation of 5a. (Right) Crystal packing details of 5a viewed along the a-axis showing the presence of intermolecular
hydrogen bonds between N atoms.
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Scheme 4: Possible mechanisms for the C–N coupling catalyzed by PdCl2(NCMe)2 in homogeneous phase.
dination to aniline should be overcompensated by coordination and difficult to isolate under the reaction conditions due the
of carbodiimide to palladium in close proximity to aniline in presence of an aniline excess, and for this reason we have been
fixed geometry as indicated in the presumed intermediates unable to isolate labile intermediates 6a–c. This preassociation
6a–c. There are precedents in the literature [22] showing that between complexes 3a–c and carbodiimide 2 to form intermedi-
palladium(II) can interact weakly with accumulated carbodi- ates 6a–c would make easier the key step of C=N insertion
imide bonds, but this type of complex is typically very labile leading to guanidines (Scheme 4).
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Acknowledgements
Financial support by the Spanish Ministry of Economy and
competitiveness (Severo Ochoa and CTQ2012-36351) and
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