Copper(I)-imine complexes: Synthesis and catalytic activity in olefin cyclopropanation

Article abstract of DOI:10.1016/j.ica.2009.03.038

Different imine-type ligands, prepared by the condensation of anilines or of α-methylbenzylamine with 2-pyridinecarboxaldehyde (pyim^1,2) or 2-quinolinecarboxaldehyde (quim^1,2) were prepared. These species act as N,N′-bidentate, chela

Full text of DOI:10.1016/j.ica.2009.03.038

Inorganica Chimica Acta 362 (2009) 3507–3512
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Copper(I)-imine complexes: Synthesis and catalytic activity
in olefin cyclopropanation
*
G. Attilio Ardizzoia , Stefano Brenna, Fulvio Castelli, Simona Galli
Dipartimento di Scienze Chimiche e Ambientali, Università degli Studi dell’Insubria, Via Valleggio, 11 – 22100 Como, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Different imine-type ligands, prepared by the condensation of anilines or of a-methylbenzylamine with
2-pyridinecarboxaldehyde (pyim1,2) or 2-quinolinecarboxaldehyde (quim1,2) were prepared. These spe-
cies act as N,N0-bidentate, chelating ligands upon coordination to Cu(I): treatment of [Cu(PPh3)3Cl] with
an equimolar amount of the ligands resulted in the displacement of two molecules of PPh3, giving rise to
the formation of [Cu(pyim1,2)(PPh3)Cl] (1–2) and [Cu(quim1,2)(PPh3)Cl] (3–4), respectively. The copper
derivatives 1–4 proved to be highly active catalysts in olefin cyclopropanation in the presence of ethyl
diazoacetate, even using deactivated olefins (namely, 2-cyclohexen-1-one) as substrate. The X-ray struc-
ture of complex 2, [Cu(pyim2)(PPh3)Cl], is also reported.
Received 2 December 2008
Received in revised form 16 March 2009
Accepted 24 March 2009
Available online 31 March 2009
Keywords:
Copper
Cyclopropanation
Imines
Ó 2009 Elsevier B.V. All rights reserved.
Homogeneous catalysis
1. Introduction
scopic characterization of the [Cu(pyim1,2)(PPh3)Cl] (1–2) and
[Cu(quim1,2)(PPh3)Cl] (3–4) complexes, obtained by reacting
Transition metal complexes-catalyzed cyclopropanation of al-
kenes has attracted enormous research interest [1]. In fact, cyclo-
propanes are versatile intermediates that can be converted into a
variety of useful products by cleavage of the strained three-mem-
bered ring [2]. Moreover, the occurrence in nature of molecules
presenting interesting physiological properties and containing
the cyclopropane moiety [3] is a continuous stimulus to develop
new synthetic routes to functionalized cyclopropanes. One of the
most effective methods to convert olefins into the corresponding
cyclopropane derivatives is the metal-assisted decomposition of
diazocompounds [4]. Among the others, copper complexes cover
a crucial role in this field [5] and both experimental [6] and theo-
retical [7] studies have been accomplished in order to elucidate the
mechanism of olefin cyclopropanation promoted by Cu-based cat-
alysts. A large variety of sp2-nitrogen based ligands, such as C2-
symmetric semicorrins [8], bis(oxazolines) [9], bipyridines [10],
polypyrazolylborates [11] and diiminophosphoranes [12], have
been used with the aim of enhancing the selectivity. Recently, high
enantioselectivities were reached with binaphthyldiimines [13]
and chiral bispidines [14]. We previously reported on copper(I)
species containing pyrazolate [15] or triazolate [16] bridging li-
gands, which proved to be extremely selective towards the forma-
tion of the trans isomer in both terminal and internal olefin
cyclopropanation. Here we report on the synthesis and spectro-
[Cu(PPh3)3Cl] with different imines, containing a pyridine
(pyim1ꢀ2) or a quinoline (quim1ꢀ2) core (Chart 1), and coordinating
as neutral, bidentate ligands. These compounds showed high activ-
ity and acceptable diastereoselectivity in the conversion of termi-
nal and internal olefins into cyclopropanes. Moreover, they
proved to be highly active towards a deactivated substrate like 2-
cyclohexen-1-one. Finally, the X-ray crystal structure of compound
2, [Cu(pyim2)(PPh3)Cl], is described.
2. Experimental
2.1. General procedures
All reactions were carried out under purified nitrogen using
standard Schlenk techniques. The solvents were dried and distilled
according to standard procedures prior to use. Alkenes employed
in the catalytic reactions were taken from new bottles kept at
ꢀ20 °C and their purity grade was confirmed by GC–MS control
analysis. Ethyl diazoacetate, 2-pyridinecarboxaldehyde, 6-methyl-
pyridine-2-carboxaldehyde, 2-quinolinecarboxaldehyde, 4-chloro-
aniline, p-toluidine, (rac)a-methylbenzylamine (Aldrich) were
used as purchased. [Cu(PPh3)3Cl] was prepared as reported in the
literature [17].
Infrared spectra were recorded on a Shimadzu Prestige 21 FTIR,
NMR spectra were acquired on a Bruker 400 Avance instrument,
elemental analyses were obtained with a Perkin–Elmer CHN
Analyser 2400 Series II. Quantitative analyses of products were
* Corresponding author. Tel.: +39 312386470; fax: +39 312386449.
E-mail address: attilio.ardizzoia@uninsubria.it (G. Attilio Ardizzoia).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.03.038

3508
G. Attilio Ardizzoia et al. / Inorganica Chimica Acta 362 (2009) 3507–3512
1H NMR (CDCl3, RT): 2.42 (s, 3H), 7.27 (d, 2H, J = 8.0 Hz), 7.34 (d,
2H, J = 8.0 Hz), 7.62 (t, 1H, J = 7.5 Hz), 7.79 (t, 1H, J = 7.6 Hz), 7.89
(d, 1H, J = 8.1 Hz), 8.19 (d, 1H, J = 8.5 Hz), 8.27 (d, 1H, J = 8.6 Hz),
8.39 (d, 1H, J = 8.6 Hz), 8.84 (s, 1H, HC@N). 13C NMR (CDCl3, RT):
20.66 (CH3), 118.59, 120.56, 126.90, 127.48, 127.83, 129.50,
129.75, 130.13, 131.90, 136.77, 144.99, 147.79, 154.51, 163.67
(HC@N). Anal. Calc. for C17H14N2: C, 82.90; H, 5.73; N, 11.37. Found:
C, 82.60; H, 5.65; N, 11.29%.
2.5. Synthesis of quim2
To a solution of 0.50 g (3.18 mmol) of 2-quinolinecarboxalde-
hyde in 10 ml of toluene, 0.42 g (3.47 mol) of
a-methylbenzyl-
amine were added. The solution was stirred at 70 °C for 8 h, then
the solvent was removed under reduced pressure and the crude
product suspended in 2 ml of methanol. An orange solid was iso-
lated after filtration. Yield: 72%.
1H NMR (CDCl3, RT): 1.68 (d, 3H, J = 6.6 Hz), 4.74 (q, 1H,
J = 6.6 Hz), 7.29 (t, 1H, J = 8.1 Hz), 7.39 (t, 2H, J = 7.4 Hz), 7.50 (d,
2H, J = 7.6 Hz), 7.61 (t, 1H, J = 7.5 Hz), 7.76 (t, 1H, J = 7.6 Hz), 7.86
(d, 1H, J = 8.1 Hz), 8.14 (d, 1H, J = 8.5 Hz), 8.21 (d, 1H, J = 8.5 Hz),
8.29 (d, 1H, J = 8.5 Hz), 8.66 (s, 1H, HC@N). 13C NMR (CDCl3, RT):
24.43 (CH3), 54.76 (CH), 120.61, 125.67, 126.77, 126.84, 127.80,
128.39, 128.53, 129.54, 129.70, 136.72, 143.97, 147.84, 157.84,
162.96 (HC@N). Anal. Calc. for C18H16N2: C, 83.04; H, 6.19; N,
10.76. Found: C, 82.50; H, 6.22; N, 10.74%.
Chart 1.
performed on a Shimadzu GC-17A gas chromatograph with a
PS225 capillary column (25 m, 0.25 mm) equipped with
QP5000 mass selective detector.
a
2.6. Synthesis of copper(I) complexes 1–4
2.2. Synthesis of pyim1
In a typical experiment, a suspension of 0.50 g (0.565 mmol) of
[Cu(PPh3)3Cl] in diethylether was treated with an equimolar
amount of the corresponding imine. The resulting suspension
was stirred at room temperature for 18 h, after which time a
brick-red precipitate formed. The solid was filtered off and dried
in vacuum. Yield: 77% (1); 81% (2); 86% (3); 91% (4).
To a solution of 2.25 g (21 mmol) of 2-pyridinecarboxaldehyde
in 10 ml of methanol, 2.95 g (23 mmol) of 4-chloroaniline were
added. The solution was stirred at 40 °C for 8 h, then it was reduced
to half the volume and chilled in an ice-bath. A yellow solid was
formed, which was filtered and dried in vacuum. Yield: 78%.
1H NMR (CDCl3, RT): 7.23 (d, 2H, J = 6.6 Hz), 7.37 (d, 2H,
J = 6.7 Hz), 7.38 (t, 1H, J = 5.0 Hz), 7.81 (t, 1H, J = 7.7 Hz), 8.18 (d,
1H, J = 7.9 Hz), 8.58 (s, 1H, HC@N), 8.72 (d, 1H, J = 4.7 Hz). 13C
NMR (CDCl3, RT): 121.50, 123.71, 125.08, 129.16, 131.37, 137.48,
139.45, 151.12, 152.36, 162.02, 162.65 (HC@N). Anal. Calc. for
C12H9ClN2: C, 66.52; H, 4.19; N, 12.93. Found: C, 66.39; H, 3.97;
N, 12.79%.
[Cu(pyim1)(PPh3)Cl], 1: 1H NMR (CD2Cl2, RT): 7.31–7.44 (m,
19H), 7.61 (t, 1H, J = 6.2 Hz), 8.01 (t, 1H, J = 6.7 Hz), 8.14 (d, 1H,
J = 5.9 Hz), 8.68 (s, 1H, HC = N), 8.70 (d, 1H, J = 4.9 Hz). 13C NMR
(CD2Cl2, RT): 120.87, 123.22, 124.92, 128.16, 128.35, 129.54,
130.96, 133.49, 137.28, 137.56, 139.12, 150.85, 151.98, 161.38,
164.14 (HC@N). 31P NMR (CD2Cl2, RT): ꢀ4.08 ppm (s). Anal. Calc.
for C30H24N2Cl2PCu: C, 62.34; H, 4.19; N, 4.85. Found: C, 62.22;
H, 4.31; N, 4.64%.
[Cu(pyim2)(PPh3)Cl], 2: 1H NMR (CD2Cl2, RT): 2.38 (s, 3H), 2.60 (s,
3H), 7.13 (d, 2H, J = 7.4 Hz), 7.23–7.34 (m, 17H), 7.48 (d, 1H,
J = 5.6 Hz), 7.54 (d, 1H, J = 5.4 Hz), 7.75 (t, 1H, J = 5.5 Hz), 8.41 (s,
1H, HC@N). 13C NMR (CD2Cl2, RT): 20.46 (CH3), 25.98 (CH3),
118.73, 120.38, 124.76, 128.12, 128.38, 131.11, 133.43, 136.69,
137.22, 137.87, 147.99, 155.10, 158.01, 162.49 (HC@N). 31P NMR
(CD2Cl2, RT): ꢀ4.30 ppm (s). Anal. Calc. for C32H29N2ClPCu: C,
67.24; H, 5.11; N, 4.90. Found: C, 67.09; H, 5.28; N, 4.77%.
Brick-red crystals suitable for X-ray analysis of species 2 were
obtained by slow diffusion of diethylether into a dichloromethane
solution of the complex.
2.3. Synthesis of pyim2
To a solution of 0.50 g (4.13 mmol) of 6-methylpyridine-2-car-
boxaldehyde in 10 ml of ethanol, 0.48 g (4.48 mol) of p-toluidine
was added. The solution was stirred at 80 °C for 6 h, then it was re-
duced to half the volume and a yellow solid formed. It was filtered
and dried in vacuum. Yield: 81%.
1H NMR (CDCl3, RT): 2.38 (s, 3H), 2.64 (s, 3H), 7.21 (d, 1H,
J = 7.6 Hz), 7.23 (d, 2H, J = 8.6 Hz), 7.25 (d, 2H, J = 8.7 Hz), 7.68 (t,
1H, J = 7.7 Hz), 8.02 (d, 1H, J = 7.7 Hz), 8.62 (s, 1H, HC@N). 13C
NMR (CDCl3, RT): 21.46 (CH3), 24.78 (CH3), 119.31, 121.57,
125.09, 130.22, 137.02, 137.25, 148.85, 154.60, 158.76, 160.36
(HC@N). Anal. Calc. for C14H14N2: C, 79.67; H, 6.79; N, 13.32. Found:
C, 79.95; H, 6.72; N, 13.33%.
[Cu(quim1)(PPh3)Cl], 3: 1H NMR (CD2Cl2, RT): 2.39 (s, 3H), 7.16
(d, 2H, J = 8.1 Hz), 7.22 (t, 1H, J = 7.4 Hz), 7.28–7.37 (m, 16H),
7.53–7.56 (m, 4H), 7.58 (d, 1H, J = 7.6 Hz), 8.27 (d, 1H, J = 8.3 Hz),
8.65 (s, 1H, HC@N). 13C NMR (CD2Cl2, RT): 19.71 (CH3), 117.99,
121.06, 126.31, 126.76, 127.43, 128.09, 128.31, 129.15, 129.39,
130.81, 132.07, 133.51, 136.12, 137.18, 144.74, 146.89, 156.62,
165.07 (HC@N). 31P NMR (CD2Cl2, RT): ꢀ5.56 ppm (s). Anal. Calc.
for C35H29N2ClPCu (3): C, 69.19; H, 4.81; N, 4.61. Found: C, 69.57;
H, 4.98; N, 4.20%.
2.4. Synthesis of quim1
To a solution of 0.50 g (3.18 mmol) of 2-quinolinecarboxalde-
hyde in 10 ml of toluene, 0.37 g (3.45 mol) of p-toluidine were
added. The solution was stirred at 70 °C for 8 h, during which time
a yellow solid formed. It was filtered and dried in vacuum. Yield:
88%.
[Cu(quim2)(PPh3)Cl], 4: 1H NMR (CD2Cl2, RT): 1.81 (d, 3H,
J = 6.7 Hz), 4.91 (q, 1H, J = 6.6 Hz), 7.24–7.39 (m, 20H), 7.52 (d,
1H, J = 6.3 Hz), 7.65 (t, 1H, J = 6.8 Hz), 7.79 (t, 1H, J = 6.4 Hz), 7.84

G. Attilio Ardizzoia et al. / Inorganica Chimica Acta 362 (2009) 3507–3512
3509
Table 1
(d, 1H, J = 5.8 Hz), 8.24 (d, 1H, J = 7.9 Hz), 8.33 (d, 1H, J = 6.9 Hz),
8.48 (s, 1H, HC@N). 13C NMR (CD2Cl2, RT): 20.83 (CH3), 58.90
(CH), 119.82, 125.12, 126.88, 127.65, 127.92, 128.21, 128.38,
128.44, 128.47, 129.50, 129.85, 133.63, 135.30, 137.20, 144.17,
145.13, 158.91, 164.70 (HC@N). 31P NMR (CD2Cl2, RT):
ꢀ5.52 ppm (s). Anal. Calc. for C36H31N2ClPCu (4): C, 69.56; H,
5.03; N, 4.51. Found: C, 69.85; H, 4.97; N, 4.33%.
Crystallographic data and refinement parameters for species 2.
Formula
C32H29CuN2ClP
571.53
298(2)
0.71073
monoclinic
P21
Formula weight (g molꢀ1
)
T (K)
k (Å)
Crystal system
Space group
a (Å)
8.712(3)
14.877(8)
10.612(3)
87.20(2)
1383(1)
b (Å)
c (Å)
2.7. Olefin cyclopropanation
b (°)
V (Å3)
Z
In
a standard procedure, to a solution of species 1–4
2
(0.01 mmol) in dichloromethane (10 ml) at room temperature
and under inert atmosphere, EDA and the olefin were added in
one portion (catalyst:EDA:olefin molar ratio 1:100:250). The con-
sumption of EDA was monitored by infrared spectroscopy. The
mixture was then worked up by removing the solvent and the
crude product was purified by column chromatography (dichloro-
methane:hexane = 6:4). All the cyclopropanes obtained were char-
acterized by 1H NMR and GC–MS. Diastereoselectivity (trans:cis
ratio) was measured by GC–MS analysis.
qcalc (Mg mꢀ3
)
1.382
0.974
592
l
(Mo K
a
, mmꢀ1
)
F(000)
Sample size (mm3)
h range (°)
0.25 ꢁ 0.25 ꢁ 0.10
3.0–25.3
ꢀ10 6 h 6 10
0 6 k 6 17
0 6 l 6 12
2604, 2418
0.012, 0.0
2604, 0, 334
1.070
hkl range
Unique, observed reflections
Rint = 0.012, R(
Data, restrains, parameters
(F2)a
r
)
v
R(F), wR(F2) for I > 2
r
(I)a
0.025, 0.059
0.030, 0.061
0.31, ꢀ0.16
2.8. X-ray crystallography
R(F), wR(F2) for all reflectionsa
Highest peak, deepest hole (e Åꢀ3
)
The single crystal X-ray diffraction data for 2 were acquired on
an Enraf Nonius CAD-4 automated diffractometer using graphite-
a
v
(F2) = [
R
w(F2o ꢀ F2c )2/(n ꢀ p)]1/2 where n is the number of reflections, p the
number of parameters and w = 1/[
R(F) =
r
2(F2o) + (0.019P)2 + 1.88P] with P = (Fo2 þ 2F2c )/3.
monochromated Mo Ka radiation (k = 0.71073 Å). A brick-red
R
||Fo| ꢀ |Fc||/
R
|Fo| and wR(F2) = [
R
w(F2o ꢀ F2c )2/
R .
wF4o]1/2
platelet single crystal of approximate 0.25 ꢁ 0.25 ꢁ 0.10 mm
dimensions was mounted on top of a goniometer head. The unit
cell was determined on the basis of the setting angles of 25 ran-
domly distributed reflections in the 9.0° < h < 11.0° range. The data
collection was performed in the 3.0° < h < 25.3° range by applying
the stretching at about 1600 cmꢀ1 in the infrared spectra of the
solids.
the
and 2418 observed [I > 2
0.012, R( ) = 0.020], and used for the structure solution and the
x
-scan mode [
D
x
= 1.4 + (0.35tanh)]. A total of 2604 unique
3.2. Synthesis of copper(I) complexes
rI)] reflections were collected [Rint
=
r
Treatment of a suspension of [Cu(PPh3)3Cl] in diethylether with
an equimolar amount of one of the previously isolated iminic li-
gands (pyim1ꢀ2–quim1ꢀ2), at room temperature, resulted in the
formation of an orange suspension, from which it was possible to
isolate a brick-red solid (species 1–4, respectively). The relatively
long reaction time (18 h) was due to the slow substitution of two
molecules of triphenylphosphine by the ligand in the heteroge-
neous system. However, the high solubility of PPh3 in diethylether
favoured its dissociation, and the subsequent replacement by the
imine. On the basis of spectroscopic data and elemental analysis,
the copper(I) species 1–4 were formulated as [Cu(N–N)(PPh3)Cl]
(where N–N represents the chelating imine ligand). The presence
of a remaining PPh3 molecule in 1–4 was confirmed by their 31P
NMR spectra, showing a single resonance in the range between
ꢀ5.56 and ꢀ4.08 ppm (see Section 2 for details). As described for
structure refinement (against 334 parameters).The data were cor-
rected for absorption and Lorenz-polarization effects. The structure
was solved by direct methods [18] and refined by full-matrix least-
squares on F2 [19]. All the non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were made riding their parent atoms
with an isotropic temperature factor 1.2 times that of their parent
atoms. Details on the crystal data and on the refinement results are
collected in Table 1.
3. Results and discussion
3.1. Synthesis of ligands
When 2-pyridinecarboxaldehyde or 6-methylpyridine-2-car-
boxaldehyde was treated with a slight excess of the appropriate
amine, the formation of the corresponding ligands pyim1ꢀ2 took
place (Scheme 1). The reaction was performed in alcohol (metha-
nol or ethanol) and was monitored via 1H NMR. Actually, the occur-
rence of the desired product was confirmed by the disappearing of
the signal associated to the aldehyde and the parallel appearance
of the typical imine resonance, at about 8–9 ppm. The infrared
spectra (nujol mull) of the ligands showed the expected absorp-
tions in the range 1550–1690 cmꢀ1 usually attributed to the C@N
stretching.
previously reported complexes of general formula [M2(CO)6(
a-dii-
mine)] (M = Fe, Ru;
a-diimine = pyridine-2-carbaldehyde-imines)
[20], the pyim1ꢀ2 and quim1ꢀ2 ligands in compounds 1–4 reason-
ably chelate the copper(I) centre by means of the two nitrogen do-
nor atoms (Scheme 3). These features find proper support in the
crystal structure characterization of complex 2 (see below).
3.3. Crystal structure of [Cu(pyim2)(PPh3)Cl], 2
Compound 2 crystallizes in the monoclinic P21 space group and
is composed by mononuclear complexes of [Cu(pyim2)(PPh3)Cl]
formula. In each complex, the Cu(I) metal centre possesses a
slightly distorted tetrahedral geometry of the CuClN2P kind. The
two nitrogen atoms of the coordination sphere belong to one pyim2
ligand, exhibiting a N,N0-endo-bidentate coordination mode. The
metal ion coordination is completed by one chlorine atom and by
Similarly, when 2-quinolinecarboxaldehyde was treated with a
slight excess of p-toluidine or a-methylbenzylamine in toluene, it
was possible to isolate pure, crystalline yellow (quim1) or orange
(quim2) imines, as confirmed by spectroscopic data (Scheme 2). In-
deed, the formation of the C@N iminic moiety was proved both by
the resonances at 8.84 and 8.66 ppm in the 1H NMR (CDCl3) and by

3510
G. Attilio Ardizzoia et al. / Inorganica Chimica Acta 362 (2009) 3507–3512
Scheme 1. Synthesis of ligands pyim1ꢀ2
.
Scheme 2. Synthesis of ligands quim1ꢀ2
.
Scheme 3. Formation of the copper(I) complexes 1–4.
Table 2
Significant bond distances (Å) and angles (°) at the metal centre in species 2.
Cu–Cl
Cu–N
Cu–P
2.310(1)
2.101(3), 2.134(3)
2.204(1)
Cl–Cu–N
Cl–Cu–P
N–Cu–N
N–Cu–P
107.53(8), 109.34(8)
111.78(4)
78.9(1)
118.35(8), 126.15(8)
Fig. 1. Ortep representation (at
a 20% probability level) of the mononuclear
complex present in the crystal structure of 2. Carbon, grey; hydrogen, light grey;
chlorine, light green; copper, magenta; nitrogen, light blue; phosphorous, orange.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
the phosphorous atom of one PPh3 ligand. A search through the
Cambridge Structural Database (Version August 2008) evidenced
that only a few Cu(I) monomeric complexes possessing a N,N0-che-
lating ligand and showing, on the whole, a CuClN2P coordination,
have been structurally characterized. As expected, their bond dis-
tances and angles at the metal are nicely met by those present in
species 2. Significant bond distances and angles at the metal centre
are gathered in Table 2 (see Fig. 1).
3.4. Cyclopropanation reactions
When complexes 1–4 were dissolved in dichloromethane under
inert atmosphere, in the presence of ethyl diazoacetate (EDA), the

G. Attilio Ardizzoia et al. / Inorganica Chimica Acta 362 (2009) 3507–3512
3511
conversion of olefins into the corresponding cyclopropane deriva-
tives took place. Together with the desired products, diethyl male-
ate and diethyl fumarate, deriving from EDA self-coupling [21],
were always detected. In order to minimize these side-reactions,
strates to coordinate to copper with a favoured approaching direc-
tion, thus reducing diastereoselectivity. To improve the selectivity,
the synthesis of some new imine-type ligands and corresponding
copper(I) complexes is under investigation.
addition of
a large excess of olefin (namely, a Cu:EDA:ole-
fin = 1:100:250 molar ratio) was employed. As reported in Table
3, the yields in cyclopropane products were excellent in all the
runs performed. Complexes 1–4 showed to be highly active cata-
lysts in the cyclopropanation of both terminal and internal olefins.
As reported in the literature, in the conversion of styrene into the
corresponding cyclopropane esters, most of the copper(I) catalysts
provided the trans isomer as the main product [22], the common
trans:cis ratio ranging from 50:50 to 75:25. Increasing the size of
R on N2CHCOOR enhances enantiocontrol, while the diastereose-
lectivity is not usually affected by steric effects, except when very
bulky substituents are used, as in the case of 2,6-di-tert-butyl-4-
methylphenyl (BDA) diazoacetate [23]. Species 1–4 confirmed this
trend in the catalytic cyclopropanation of styrene (Table 3, entries
1–4), in accord with some previously reported Cu(Schiff-base) cat-
alysts [24]. A similar tendency was noted using different terminal
4. Conclusions
The syntheses and characterization of copper(I) complexes con-
taining chelating imines have been reported. Complexes 1–4, of
general formula [Cu(N–N)(PPh3)Cl] (where N–N represents the
bidentate imine), proved to be active catalysts in olefin cycloprop-
anation in the presence of EDA, under very mild conditions. These
compounds also converted internal deactivated olefins such as
2-cyclohexen-1-one into the corresponding esters derivatives.
The X-ray crystal structure of one of these copper(I) species,
[Cu(pyim2)(PPh3)Cl], (2) (pyim2 = [1-(6-methyl-pyridin-2-yl)-
methylene]-p-tolyl-amine) has also been described.
Finally, the preliminary results achieved using complex 4 as cat-
alyst, encourage us to investigate the potential enantioselectivity
of similar species containing the imines obtained by means of
olefins, such as
a-methylstyrene or 1-hexene, as substrates (Table
enantiopure a-benzylamine.
3, entries 5–12).
Compounds 1–4 exhibited to be active in internal olefin cyclo-
propanation as well. In fact, the conversion of cyclohexene into
its corresponding cyclopropylic derivatives took place in high
yields (Table 3, entries 13–16).
Moreover, as reported by Zhang and co-workers [25], cyclopro-
panes containing electron-withdrawing groups have shown to be
valuable synthetic intermediates for various applications. Accord-
ing to this, the cyclopropanation of 2-cyclohexen-1-one (present-
ing a deactivated C@C bond because of the presence of the
carbonyl group) was also investigated. All catalysts 1–4 provided
high conversion yields, together with slightly better diastereose-
lectivities when compared to other substrates.
Unfortunately, despite their elevated catalytic activity, com-
plexes 1–4 showed an ordinary selectivity when compared to our
previously reported copper(I) species [15,16]. This is probably
attributable to the absence of steric effects: the first step after dis-
solution in CH2Cl2 is the dissociation of the triphenylphosphine (as
evidenced by 31P NMR), which reduces the steric hindrance around
the metal centre. The imine ligands alone cannot force the sub-
Appendix A. Supplementary data
CCDC 710251 contains 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. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2009.
03.038.
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a
Reaction conditions: CH2Cl2, RT, catalyst:EDA:olefin molar ratio 1:100:250.

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