The cyclopalladation reaction of benzylamine revisited

Article abstract of DOI:10.1016/S0277-5387(02)01338-4

The reaction of benzylamine and palladium(II) acetate in a one-to-one molar ratio in toluene or in glacial acetic acid at 60 °C for 24 h produced the acetato bridged cyclopalladated dimer of benzylamine (μ-OAc)₂[Pd(C₆H₄ CH₂NH₂)]₂ (1) in yields of 40-75% and of 20-40%, respectively. In addition, mixtures of compounds 1, 1d1 of formula [(3-DC₆H₃CH₂NH₂)Pd](μ- OAc)₂[Pd(C₆H₄CH₂NH₂)] and 1d2 of formula (μ-OAc)₂[Pd(3-DC₆H₃ CH₂NH₂)]₂ were obtained when benzylamine and palladium(II) acetate in a one-to-one molar were treated in monodeuterated acetic acid (DOAc) at 60 °C for 24 h. These mixtures were isolated in yields of 10-30% and deuterium contents in the range between 0.7 and 0.9. Furthermore, the treatment of benzylamine and palladium(II) acetate in a one-to-one molar ratio under reflux of glacial acetic acid for 45 min produced the acetato bridged endo-cyclopalladated dimer of benzyl-benzylidene-amine (μ-OAc)₂[Pd(C₆H₄ CH=NCH₂C₆H₅)] (compound 4), which was isolated in 14% yield relative to the initial palladium(II) acetate, together with other unidentified compounds. 2002 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0277-5387(02)01338-4

Polyhedron 22 (2003) 287Á291
/
www.elsevier.com/locate/poly
The cyclopalladation reaction of benzylamine revisited
Joan Albert ꢀ, Jaume Granell, Raquel Tavera
Departament de Qu´ımica Inorga´nica, Universitat de Barcelona, Mart´ı i Franque`s, 1-11, 08028 Barcelona, Spain
Received 7 June 2002; accepted 9 October 2002
Abstract
The reaction of benzylamine and palladium(II) acetate in a one-to-one molar ratio in toluene or in glacial acetic acid at 60 8C for
24 h produced the acetato bridged cyclopalladated dimer of benzylamine (m-OAc)2[Pd(C6H4CH2NH2)]2 (1) in yields of 40Á75% and
of 20Á40%, respectively. In addition, mixtures of compounds 1, 1d1 of formula [(3-DC6H3CH2NH2)Pd](m-
/
/
OAc)2[Pd(C6H4CH2NH2)] and 1d2 of formula (m-OAc)2[Pd(3-DC6H3CH2NH2)]2 were obtained when benzylamine and
palladium(II) acetate in a one-to-one molar were treated in monodeuterated acetic acid (DOAc) at 60 8C for 24 h. These mixtures
were isolated in yields of 10Á
benzylamine and palladium(II) acetate in a one-to-one molar ratio under reflux of glacial acetic acid for 45 min produced the acetato
bridged endo-cyclopalladated dimer of benzyl-benzylidene-amine (m-OAc)2[Pd(C6H4CHÄNCH2C6H5)] (compound 4), which was
/
30% and deuterium contents in the range between 0.7 and 0.9. Furthermore, the treatment of
/
isolated in 14% yield relative to the initial palladium(II) acetate, together with other unidentified compounds.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Cyclometallation; Benzylamine; Palladium(II) acetate
1. Introduction
et al. [4,5] showed that the treatment of benzylamine and
palladium(II) acetate in a one-to-one molar ratio in
benzene at 60 8C for 24 h gave place to the acetato
bridged cyclopalladated dimer of benzylamine (m-
OAc)2[Pd(C6H4CH2NH2)]2 (compound 1) in 53% yield.
Additionally, Vicente et al. [6] reported that both the
treatment of benzylamine and palladium(II) acetate in a
one-to-one molar ratio under reflux of acetonitrile and
the treatment of the coordination compound (m-
OAc)2[Pd(OAc)(benzylamine)]2 under reflux of acetoni-
Despite the fact that benzylamine is one of the least
elaborated N-donor ligands that can undergo the
cyclopalladation reaction [1], not until quite recently
have methods for its cyclopalladation been reported.
The first attempt to cyclopalladate benzylamine was
described by Cope and Friedrich in 1968 [2]. In this
paper, and with the aim of preparing the chloro bridged
cyclopalladated
dimer
of
benzylamine
(m-
Cl)2[Pd(C6H4CH2NH2)]2, benzylamine and Li2[PdCl4]
in a two-to-one molar ratio were treated in methanol at
room temperature for 4 h. However, the coordination
compound [PdCl2(benzylamine)2] was formed in these
reaction conditions. Avshu et al. [3] later found that
treating the coordination compound [PdI2(benzyla-
mine)2] with Ag[BF4] in ethyl acetate and a posterior
reaction with [Bu4N]I allowed the isolation of the iodo
bridged cyclopalladated dimer of benzylamine (m-
I)2[Pd(C6H4CH2NH2)]2 in 73% yield. Recently, Fuchita
trile, produced 1 in yields of 40Á50%. It should also be
/
noted that in 1985 Dyke et al. [7] published the synthesis
of the bromo bridged cyclopalladated dimer of benzy-
lamine (m-Br)2[Pd(C6H4CH2NH2)]2 by reacting 2-bro-
mobenzylamine with bis(dibenzylideneacetone)palla-
dium(0).
In view of these precedents and the interest in the
organic and bioorganic chemistry applications of cyclo-
palladated primary and secondary amines [8Á23], we
/
decided to undertake a detailed study on the reaction
between benzylamine and palladium(II) acetate in a one-
to-one molar ratio in different solvents and under
different reaction conditions. The results of this study
are reported below.
ꢀ Corresponding author. Tel.: ꢁ
490-7725
E-mail address: joan.albert@qi.ub.es (J. Albert).
/34-93-402-1274/1100; fax: ꢁ34-93-
/
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 3 3 8 - 4

288
J. Albert et al. / Polyhedron 22 (2003) 287Á291
/
Scheme 1. (i) Pd(OAc)2 (molar ratio benzylamine/Pd(OAc)2ꢂ
/
1), toluene or acetic acid, 60 8C, 24 h; (ii) Pd(OAc)2 (molar ratio benzylamine/
1), acetic acid, reflux, 45 min; (iv) py-
Pd(OAc)2ꢂ1), monodeuterated acetic acid, 60 8C, 24 h; (iii) Pd(OAc)2 (molar ratio benzylamine/Pd(OAc)2ꢂ
/
/
d5, CDCl3, room temperature; (v) LiCl and PPh3, acetone, room temperature.
2. Results and discussion
was concentrated under vacuum. Addition of a mini-
mum volume of acetone to the resulting residue
produced the precipitation of 1 as a pale yellow powder,
Scheme 1 shows the compounds prepared in this work
and Fig. 1 gives the numbering of the hydrogen and
carbon atoms of the palladated phenyl ring for the
discussion that follows.
The reaction of benzylamine and palladium(II) ace-
tate in a one-to-one molar ratio in toluene or in glacial
acetic acid at 60 8C for 24 h produced a yellow
suspension or a red solution, respectively. In both cases,
the reaction mixture was concentrated under vacuum
and the residue was eluted through a SiO2 column with a
solution of methanol in chloroform in a 6-to-100 volume
ratio. Compound 1 eluted in the most polar band, which
which was isolated in yields of 40Á
/
75% in the reaction in
toluene and in yields of 20Á40% in the reaction in acetic
/
acid. Compound 1 produced satisfactory elemental
analyses, FABꢁ and IR spectrum, but it was only
slightly soluble in CDCl3. However, when a suspension
of 1 in CDCl3 was treated with py-d5, a pale yellow
solution was formed, whose 1H NMR at 300 MHz
showed the signals corresponding to compound 2.
In order to characterize 1 fully, it was converted into
the mononuclear compound 3. Compound 3 afforded
satisfactory elemental analyses, IR, FABꢁ and NMR
data. The 1H NMR at 500 MHz and 1HÁ1
H COSY and
/
NOESY experiments at 500 MHz were consistent with
the proposed structure for 3 [24] and allowed the
unambiguous assignation of the signals of the H3ÁH6
/
aromatic protons. The signals of the H3 and H4 protons
were quite well separated and other aromatic protons
did not interfere with them. Thus, the ratio of the
integrals of these two signals should give insight into the
composition of samples C (see below).
When benzylamine and palladium(II) acetate in a
one-to-one molar ratio were treated in monodeuterated
acetic acid (DOAc) at 60 8C for 24 h, samples A were
isolated in yields of 10Á
of compounds 1,
/
20%. Samples A were a mixture
1d1 of formula [(3-
Fig. 1. Numbering of the hydrogen and carbon atoms of the
palladated phenyl ring of the compounds under discussion.

J. Albert et al. / Polyhedron 22 (2003) 287Á
/291
289
DC6H3CH2NH2)Pd](m-OAc)2[Pd(C6H4CH2NH2)] and
1d2 of formula (m-OAc)2[Pd(3-DC6H3CH2NH2)]2. This
was inferred from the 1H NMR at 300 MHz of solutions
B, which presented the signals corresponding to com-
The palladium(0) was separated by filtration and the
reaction solution was concentrated under vacuum.
Elution of the residue of the reaction solution through
a SiO2 column with a solution of methanol in chloro-
form in a 6-to-100 volume ratio, allowed the isolation of
the acetato bridged endo-cyclopalladated dimer of
benzyl-benzylidene-amine of formula (m-OAc)2-
pounds
2
and 2d of formula trans-N,N-[Pd(3-
DC6H3CH2NH2)(OAc)(py-d5)]. Solutions B were ob-
tained by treating suspensions of samples A in CDCl3
with py-d5. The 1H NMR in perdeuterated acetic acid of
benzylamine demonstrated that benzylamine was in the
C6H5CH2ND2 form in this solvent, but the 1H NMR of
solutions B showed that the amino groups of samples A
did not present deuterium. This latter result suggested
that the amino groups of the samples A exchanged D by
H with water or methanol molecules, which were present
in the eluant used for their isolation by chromatogra-
phy.
[Pd(C6H4CHÄ
/
NCH2C6H5)] (compound 4). It should
be noted that other compounds formed in this latter
reaction could not be identified.
Compound 4 was isolated in a 14% yield in relation to
the initial palladium(II) acetate and it was characterized
1
1
by its H NMR at 200 MHz in CDCl3. This H NMR
spectrum was identical to that of a sample of 4 prepared
by reacting benzyl-benzylidene-amine with palladiu-
m(II) acetate [26]. Furthermore, a CDCl3 solution of 4
reacted with py-d5 to form quantitatively compound 5,
which was characterized by its 1H NMR in CDCl3
solution (see Section 3). Experiments for clarifying the
mechanism of the transformation of benzylamine in
compound 4 when benzylamine is treated in refluxing
glacial acetic acid in presence of palladium(II) acetate
are currently in progress.
In order to characterize samples A fully, they were
1
converted into samples C. The H NMR spectra at 500
MHz of samples C proved that they were a mixture of
compounds 3 and 3d of formula trans-N,P-[Pd(3-
DC6H3CH2NH2)Cl(PPh3)]. This was inferred from the
decrease of the integral of the signal of the H3 proton in
relation to the integrals of the signals of the H4ÁH6
/
2
protons. In addition, the H NMR at 76.77 MHz of
solutions in CHCl3 of samples C presented a broad
signal centred at 6.96 ppm, which was coincident with
the chemical shift of the H3 proton of compound 3,
showing that deuterium was selectively located at the 3
3. Experimental
3.1. General comments and materials
1
position of the palladated phenyl ring. The H NMR at
500 MHz of mixtures C allowed the determination of
their molar composition by means of Eq. (1), where x3d
and r were, respectively, the mole fraction of 3d and the
ratio between the integrals of the H3 and H4 protons of
the sample C under study.
Elemental analyses of C, H and N were performed
with an Eager 1108 microanalyzer. IR spectra were
recorded on a Nicolet Impact-400 spectrophotometer
using pressed discs of diluted samples of compounds 1
and 3 in KBr. 1H NMR spectra in CDCl3 were recorded
at 500 MHz on a Bruker DMX 500 instrument, at 300
MHz on a Varian Unity 300 instrument and at 200 MHz
on a Varian Gemini-200 instrument. The 31P{1H} NMR
spectrum in CHCl3 was recorded at 101.26 MHz on a
Bruker DRX 250 instrument. 2H NMR spectra in
CHCl3 at 76.77 MHz were recorded on a Bruker
DMX 500 instrument. Chemical shifts are reported in
d values (ppm) relative to SiMe4 for 1H, 85% H3PO4 for
31P and C3H6O-d6 for 2H. FABꢁ mass spectra were
obtained with a VG-Quatro Fisions instrument, using 3-
nitrobenzylalcohol as matrix. All chemicals were of
commercial grade and used as received. Solvents were
distilled before use as follows: CHCl3 over CaO; C3H6O
and MeOH over CaCl2 and Et2O over Na and
benzophenone.
x3d ꢂ1ꢃr
(1)
Therefore, mole fractions of 3d in samples C in the range
between 0.7 and 0.9 were obtained. The mole fractions
of 3d in samples C were thereafter assumed to be the
deuterium contents of samples A; because the metathesis
reactions with LiCl and the splitting reactions with
PPh3, which transform samples A into samples C,
should affect neither the PdÃ
/
C s bonds nor the CÃ
/
D
s bonds of samples A.
The presence of deuterium atoms in the 3 position of
the phenyl rings of the molecules which constitute
samples A, shows that the CÃ
/
H bond activation in the
cyclopalladation reaction of benzylamine is a reversible
process in acetic acid. Furthermore, it is consistent with
the CÃ
/
H bond activation taking place in an intramole-
cular mode, in accordance with the general mechanism
accepted for the cyclometallation reaction [25].
3.2. Preparation of 1
In a last reaction, benzylamine and palladium(II)
acetate in a one-to-one molar ratio were treated under
reflux of glacial acetic acid for 45 min. In these reaction
conditions, the formation of palladium(0) was observed.
3.2.1. Method A
A suspension formed by 2.22ꢄ
Pd(OAc)2, 2.22ꢄ
10ꢃ3 mol (0.238 g) of benzylamine
and 30 cm3 of C6H5CH3 was stirred at 60 8C for 24 h.
/
10ꢃ3 mol (0.500 g) of
/

290
J. Albert et al. / Polyhedron 22 (2003) 287Á291
/
The resulting yellow suspension was concentrated under
vacuum and the residue was eluted through a silica gel
column with a solution of MeOH in CHCl3 in a 6-to-100
volume ratio. The most polar band was collected and
concentrated under vacuum. Addition of C3H6O (4 cm3)
to the residue produced the precipitation of 1 as a yellow
powder, which was filtered and dried under vacuum.
Yields were in the range between 40 and 75%.
powder, which was filtered and dried under vacuum.
Yields were in the range between 10 and 30% and
deuterium contents in the range between 0.7 and 0.9.
3.6. Preparation of solutions B
A suspension formed by 0.020 g of sample A and 0.7
cm3 of CDCl3 was treated with four drops of Py-d5. The
formation of a colourless solution was indicative of the
quantitative transformation of the solid sample A into
solution B.
3.2.2. Method B
A suspension formed by 2.22ꢄ
/
10ꢃ3 mol (0.500 g) of
Pd(OAc)2, 2.22ꢄ
/
10ꢃ3 mol (0.238 g) of benzylamine
and 30 cm3 of glacial AcOH was stirred at 60 8C for 24
h. The resulting red solution was concentrated under
vacuum and the residue was eluted through a silica gel
column with a solution of MeOH in CHCl3 in a 6-to-100
volume ratio. The most polar band was collected and
concentrated under vacuum. Addition of C3H6O (4 cm3)
to the residue produced the precipitation of 1 as a yellow
powder, which was filtered and dried under vacuum.
Yields were in the range between 20 and 40%.
3.7. Preparation of samples C
A suspension formed by 0.080 g of sample A (1.47ꢄ
10ꢃ4 mol), 0.025 g of LiCl (5.89ꢄ10ꢃ4 mol), 0.154 g of
PPh3 (5.89ꢄ
10ꢃ4 mol) and 15 cm3 of C3H6O was
/
/
/
stirred at r.t. for 15 min. The suspension formed was
concentrated under vacuum and the residue was eluted
through a silica gel column with a solution of MeOH in
CHCl3 in a 4-to-100 volume ratio. The pale yellow band
was collected and concentrated under vacuum. Addition
of Et2O (4 cm3) to the residue produced the precipitation
of sample C as a pale yellow powder, which was filtered
and dried under vacuum. Yields were in the range
between 50 and 65% and the mole fractions of 3d in the
range between 0.7 and 0.9.
3.3. Preparation of 2
A suspension formed by 0.020 g of 1 and 0.7 cm3 of
CDCl3 was treated with four drops of Py-d5. The
formation of a colourless solution was indicative of
the quantitative transformation of compound 1 into 2.
3.4. Preparation of 3
A suspension formed by 0.075 g of 1 (1.38ꢄ
mol), 0.023 g of LiCl (5.52ꢄ
10ꢃ4 mol), 0.144 g of PPh3
(5.52ꢄ
10ꢃ4 mol) and 15 cm3 of C3H6O was stirred at
/
10ꢃ4
3.8. Preparation of 4
/
/
A suspension formed by 2.22ꢄ
/
10ꢃ3 mol (0.500 g) of
room temperature (r.t.) for 15 min. The suspension
formed was concentrated under vacuum and the residue
was eluted through a silica gel column with a solution of
MeOH in CHCl3 in a 4-to-100 volume ratio. The pale
yellow band was collected and concentrated under
vacuum. Addition of Et2O (4 cm3) to the residue
produced the precipitation of 3 as a pale yellow powder,
which was filtered and dried under vacuum. Yield: 57%
(0.081 g).
Pd(OAc)2, 2.22ꢄ
/
10ꢃ3 mol (0.238 g) of benzylamine
and 30 cm3 of glacial AcOH was treated under reflux of
AcOH for 45 min. The reaction mixture was filtered
thorough a 2 cm path of celite and the red solution was
concentrated under vacuum. The resulting residue was
eluted through a silica gel column with a solution of
MeOH in CHCl3 in a 6-to-100 volume ratio. The orange
band was collected and concentrated under vacuum.
Addition of Et2O (5 cm3) to the residue produced the
precipitation of 4 as an orange powder, which was
filtered and dried under vacuum. Yield [relative to the
initial palladium(II) acetate]: 14% (0.112 g).
3.5. Preparation of samples A
A suspension formed by 2.22ꢄ
/
10ꢃ3 mol (0.500 g) of
Pd(OAc)2, 2.22ꢄ
/
10ꢃ3 mol (0.238 g) of benzylamine
and 5 cm3 of monodeuterated AcOH (DOAc) was
stirred at 60 8C for one day. The resulting red solution
was concentrated under vacuum and the residue was
eluted through a silica gel column with a solution of
MeOH in CHCl3 in a 6-to-100 volume ratio. The most
polar band was collected and concentrated under
vacuum. Addition of C3H6O (4 cm3) to the residue
produced the precipitation of sample A as a yellow
3.9. Preparation of 5
An amount of 0.020 g of 4 was dissolved in 0.7 cm3 of
CDCl3 and the resulting orange solution was treated
with four drops of Py-d5. A change of colour from
orange to yellow was indicative of the quantitative
transformation of compound 4 into 5.

J. Albert et al. / Polyhedron 22 (2003) 287Á
/291
291
3.10. Characterization data
Acknowledgements
We are grateful to the Ministerio de Ciencia y
Tecnolog´ıa and to the Generalitat de Catalunya for
financial support (Grants BQU2000-0652 and
2001SGR-00054).
3.10.1. Compound 1
Anal. Calc. for C18H22N2O4Pd2: C, 39.8; H, 4.0; N,
5.2. Found: C, 39.8; H, 4.1; N, 5.0%. FABꢁ (most
intense peaks): 542 ([M]ꢁ), 483 ([Mꢃ
/
OAc]ꢁ), 212 ([M/
2ꢃ
/
OAc]ꢁ). IR (cmꢃ1): 3293, 3231 st NÃ
(carboxylato), 1410 st s (carboxylato).
/H, 1570 st as
References
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3.10.2. Compound 2
1H NMR (300 MHz, CDCl3): 6.97Á
6.93 (2H, H3 and
/
3
H4), 6.77 t JHH
3
6 Hz (1H, H5), 6.19 d JHH
ꢂ
/
ꢂ
/
6 Hz
6 Hz
[2] A.C. Cope, E.C. Friedrich, J. Am. Chem. Soc. 90 (1968) 909.
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3
(1H, H6), 5.20 br signal (2H, CH2), 4.12 t JHH
(2H, NH2), 1.90 s (3H, OAc).
ꢂ
/
3.10.3. Compound 3
Anal. Calc. for C25H23ClNPPd: C, 58.9; H, 4.5; N,
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2.7. Found: C, 59.6; H, 4.7; N, 2.6%. FABꢁ (most
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NÃH, 1094 q X-sensitive mode of the coordinated PPh3.
1H NMR (500 MHz, CDCl3): 7.71Á
7.30 (15H, PPh3),
/
Cl]ꢁ). IR (cmꢃ1): 3236, 3180 st
/
/
3
3
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Tetrahedron Lett. 42 (2001) 7791.
6.96 d JHH
ꢂ
/
7 Hz (1H, H3), 6.81 t JHH
ꢂ
/
7 Hz (1H,
JPH
H4), 6.39 t 3JHH
ꢂ
/
7 Hz (1H, H5), 6.34 t 3JHHꢂ4
7 Hz (1H, H6), 4.28 br signal (2H, CH2), 3.91 br signal
/
ꢂ
/
[10] J. Albert, R. Bosque, J.M. Cadena, J.R. Granell, G. Muller, J.I.
Ordinas, Tetrahedron: Asymmetry 11 (2000) 3335.
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(2H, NH2). 31P{1H} NMR (101.26 MHz, CHCl3): 40.87
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Sanudo, Eur. J. Inorg. Chem. (2000) 1286.
˜
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1H NMR (300 MHz, CDCl3): 6.93 d JHH
(1H, H4). The rest of the signals are coincident with
those of compound 2.
3
ꢂ6 Hz
/
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1H NMR (500 MHz, CDCl3) (selected data): 6.81 d
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3JHH
coincident with those of compound 3. H NMR (76.77
MHz, CHCl3): 6.96 br signal.
ꢂ7 Hz (1H, H4). The rest of the signals are
/
2
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3.10.6. Compound 4
Its 1H NMR is coincident with that previously
reported [26].
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2747.
3.10.7. Compound 5
1H NMR (300 MHz, CDCl3): 7.70 s (1H, CHÄ
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/
N),
3
7.35 (5H, PhCH2), 7.15 d JHH
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Organometallics 14 (1995) 1393.
7.40Á
/
ꢂ
/
7 Hz (1H, H3),
3
6.97 t JHH
3
7 Hz (1H, H4), 6.89 t JHH
ꢂ
/
ꢂ/7 Hz (1H,
[25] A.D. Ryabov, Chem. Rev. 90 (1990) 403.
[26] J. Albert, R. Bosque, J. Granell, R. Tavera, Polyhedron 20 (2001)
3225.
3
H5), 6.23 d JHH
1.91 s (3H, OAc).
ꢂ/7 Hz (1H, H6), 4.86 s (2H, CH2),