Cu(I) catalysed cyclopropanation of olefins: Stereoselectivity studies with Arylid-Box and Isbut-Box ligands

Article abstract of DOI:10.1016/j.jorganchem.2007.06.068

In our quest to find new ligands for highly stereoselective reactions, we tested a variety of chiral non-racemic pseudo-C₂ symmetric bis-oxazolines derived from malonic acid containing an arylidene bridge unit (and appropriately termed Arylid-B

Full text of DOI:10.1016/j.jorganchem.2007.06.068

Journal of Organometallic Chemistry 692 (2007) 4863–4874
www.elsevier.com/locate/jorganchem
Cu(I) catalysed cyclopropanation of olefins: Stereoselectivity
studies with Arylid-Box and Isbut-Box ligands
a,
a
a
Anthony J. Burke *, Elisabete da Palma Carreiro , Serghei Chercheja ,
Nuno M.M. Moura , J.P. Prates Ramalho a,b, Ana Isabel Rodrigues ,
a
c
a
Carla I.M. dos Santos
a
´
Departamento de Quı´mica and Centro de Quı´mica de Evora, Universidade de Evora, Rua Roma˜o Romalho 59, 7000 Evora, Portugal
´
´
b
Centro de Fı´sica Teo´rica e Computacional, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal
Departamento de Tecnologia de Indu´strias Quı´micas, Instituto Nacional de Engenharia, Tecnologia e Inovac¸a˜o, Estrada do Pac¸o do Lumiar,
Edifı´cio F, 1649-038 Lisboa, Portugal
c
Received 25 May 2007; received in revised form 29 June 2007; accepted 29 June 2007
Available online 18 July 2007
Abstract
In our quest to find new ligands for highly stereoselective reactions, we tested a variety of chiral non-racemic pseudo-C2 symmetric
bis-oxazolines derived from malonic acid containing an arylidene bridge unit (and appropriately termed Arylid-Box) in the catalytic
asymmetric cyclopropanation (CAP) reaction of styrene and ethyl diazoacetate using between only 1–2 mol% of a Cu(I) pre-catalyst.
Some very good e.e.s (up to 86%), were obtained. It was possible to isolate 10a0-[Cu(CH3CN)4]PF6 which existed as a bench stable solid
that proved to be more efficient than the catalyst prepared in situ. Cu(I) pre-catalysts were used for the first time in the CAP reaction with
the Isbut-Box ligands 13a and 13b and, although, the e.e.s were better for ligand 13a using these pre-catalysts, in the case of ligand 11b
1
this was not the case. Spectroscopic studies (UV–Vis and H NMR) were carried out to gain an insight into the nature of the catalytic
species at work so that the conditions could be optimised giving better results. Some theoretical studies were conducted to try to explain
the better enantioselectivities obtained using Evans’ tert-Box–Cu(I) complex over our complex.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Bis-oxazolines; Catalytic asymmetric synthesis; Cyclopropanation; Counter-ion effect
1. Introduction
Isbut-Box (Fig. 1) [2]. These ligands showed some satisfac-
tory e.e.s and d.e.s (up to 70% and 72%, respectively) for
catalytic asymmetric olefin cyclopropanations. A second
generation series of Boxs with an arylidine bridge between
the rings (and termed Arylid-Box) was then developed
(Fig. 1) and also tested in a number of benchmark catalytic
asymmetric olefin cyclopropanations [3]. In this paper, we
wish to report: (1) our full experimental results for the
synthesis of our Arylid-Box ligands; (2) our findings on
the effect of the counter-ion on the reaction selectivity;
(3) our results for comparative studies on the stereoselectiv-
ity of the Isbut-Box system with the Arylid-Box system in
this reaction (4) studies probing the structure of the cata-
lytic species involved and (5) a comparative DFT study
of a particular Arylid-Box–Cu(I) complex with Evans’
Over the last 15 years chiral bis-oxazoline (Box) com-
pounds have been shown to be very useful ligands for cat-
alytic asymmetric synthesis demonstrating good to high
enantioselectivities in a number of catalytic asymmetric
reactions [1]. Despite the large number of Box ligands
already known, only a handful have been shown to be
highly stereoselective, and there is a need to develop new
Box ligands with novel structures which have potential to
meet the aforementioned requirement. For this purpose
we recently introduced a new family of Box ligands called
*
Corresponding author. Tel.: +351 266745310; fax: +351 266745303.
E-mail address: ajb@dquim.uevora.pt (A.J. Burke).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.06.068

4864
A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
using para-methoxy substituted Arylid-Boxs 10e and 10f
X
due to the expected reduction in the metallocarbene inter-
mediate electrophilicity. Such trends were not observed
[3] and we postulated that this was due to slight tilting of
the arylidene phenyl unit out of the plane as suggested by
a density functional theory (DFT) study conducted at a
B3LYP level [6] that would prevent any significant resso-
nace effects in such complexes. The DFT study was con-
ducted on the Cu(I)–Arylid-Box 10b complex (for reasons
of simplicity, in our model no other ligands were consid-
ered nor any counter-ions) by using the GAMESS-US [7]
package with the 6-31G* basis-set for C, N, O and H and
applying the Stuttgart RSC 1997 [8] effective core potential
for Cu (some selected measurements appear in the caption
for Fig. 2). Geometry was optimized without symmetry
constraints and the stationary point was subsequently con-
firmed to be a minimum by frequency calculations carried
out at that level.
O
O
O
O
N
N
N
N
R
R
R
R
Arylid-Box
Isbut-Box
R = i Pr,Bn, Ph, t Bu
X = H, Cl, OMe
R = i Pr,Bn, Ph, t Bu
Fig. 1. Isbut-Box and Arylid-Box ligands.
tert-But-Box–Cu(I) complex and their corresponding
metallocarbene complexes.
2. Results and discussion
The series of Arylid-Box ligands1 5a, 5b, 10a–f and 10a0
were prepared in satisfactory yields using the synthetic
pathways shown in Schemes 1 and 2. For the synthesis of
the Arylid-Boxs 5a, 5b, 10a–f and 10a0 (Schemes 1 and 2)
the key starting material was the arylidene malonic acid
obtained either by hydrolysis of the corresponding com-
mercially available diethyl benzylidenemalonate ester [2]
or the known dimethyl arylidene malonate esters [4] or
via the procedure of Neustadt et al. [5] using a simple
Knoevenagel condensation with malonic acid (both benzyl-
idene malonic acid and its para-substituted methoxy ana-
logue were prepared by this procedure).
A standard synthetic procedure was subsequently used
to transform the acids to the corresponding Boxs via their
acid chloride intermediates. It must be noted that all the
acid chloride intermediates were not purified and used in
the proceeding step like so, due to their unstable nature.
Satisfactory yields could be obtained in all cases. To ascer-
tain the stability of these Arylid-Box ligands in solution, a
1H NMR spectrum was recorded for the Arylid-Box 10f in
CDCl3 and after 8 days of standing in this solvent at room
temperature there was no change in the 1H NMR
spectrum.
These ligands were subsequently used in a series of Cu(I)
catalysed olefin cyclopropanations with ethyl diazoacetate
[3] using the Cu(I) pre-catalyst, [Cu(CH3CN)4]PF6 and
the Cu(II) pre-catalyst, Cu(II)(OTf)2 (Scheme 3) which
was reduced in situ to Cu(I).
The highest e.e. recorded (89%) was obtained using
ligand 10d with [Cu(CH3CN)4]PF6 in toluene [3], and the
best d.e. (40%) using 10e with Cu(II)(OTf)2 in CH2Cl2.
At the outset we expected some correlation between the
type of substituted Arylid-Box ligand used and the reaction
stereoselectivity, given that both inductive and resonance
effects were expected, like for instance, decreased reactivity
˚
The two N–Cu bond lengths were equivalent (1.931 A
˚
and 1.932 A) and the N–Cu–N bite angle was calculated
to be 107.5ꢁ. The calculated N–Cu bond lengths are close
˚
to the values of 1.90–1.91 A reported by Rasmussen et al.
˚
[9] and compares well with the observed value of 1.88 A
for a N–Cu bond determined for an oxazoline–Cu(I) com-
plex by X-ray crystallographic analysis by Evans et al. [10].
Also the calculated bite angle value is close to the value of
110ꢁ 2ꢁ calculated by Rasmussen et al. [9].
The torsion angle of 28.6ꢁ calculated for the C(3)–C(1)–
C(2)–C(5) unit, showed the degree to which the phenyl ring
was forced out of the plane. A comparative study was also
carried out on the gem-dimethyl malonate derived ligand–
Cu(I) complex of Evans [11], namely tert-But-Box–Cu(I)
(see Supporting information) and bond lengths of 1.934
˚
and 1.932 A, respectively were calculated for both the N–
Cu bonds and a slightly smaller bite angle of 107.0ꢁ was
calculated. The fact that we were unable to obtain e.e.s
as high as those reported by Evans using the tert-But-
Box–Cu(I) complex led us to postulate that the slightly lar-
ger bite angle calculated for our complex should allow the
carbenoid carbon of the intermediate metallocarbene–
Cu(I) complex to move slightly away from the source of
asymmetric induction than in Evans’ complex. In fact, this
hypothesis was later substantiated by some DFT calcula-
tions on some Cu(I) metallocarbene model complexes (vide
infra).
In order to determine the extent of electron delocalisa-
tion between the phenyl backbone and the oxazoline ring,
Mayer bond orders were calculated for a number of key
bonds and are presented in Table 1. Relatively weak elec-
tron delocalisation about the p-bonded framework was
observed as implied by the magnitudes for the bond orders.
The bond order calculation predicts no uniform electron
delocalisation between the back-bone phenyl ring and the
oxazoline rings, because in some cases the bond order
approximates to 1 (for example, C1–C3, C2–C4 and
C2–C5), whilst in other cases it shows the typical bond
order of a delocalised system (C1–C2 and C4–N13). The
1
It must be noted that in our previous two papers [2,3] (R)-(+)-
phenylalaninol was inadvertently indicated and as a consequence the
configurations of the corresponding amides and Box ligands were wrongly
depicted in the relevant schemes.

A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
4865
Ph
Ph
O
Ph
Ph
O
OH
OH
c
H
N
H
N
b
a
Cl
Cl
EtO
OEt
HO
OH
O
O
O
O
O
Bn
O
Bn
3 (100%)
1
2 (45%)
4b (43%)
d
e
Ph
Ph
Ph
O
N
O
OH
OH
e
H
H
N
O
N
O
N
N
N
O
O
5a (74%)
Bn
Bn
5b (64%)
4a (28%)
Scheme 1. Reagents and conditions: (a) NaOH, EtOH; (b) (COCl)2, DMF, CH2Cl2, 0 ꢁC; (c) (R)-(+)-phenylalaninol (2 equiv.), NEt3, CH2Cl2;
(d) (S)-valinol (2 equiv.), NEt3, CH2Cl2; (e) CH3SO2Cl (2.5 equiv.), NEt3 (6 equiv.), CH2Cl2.
X
X
X
X
OH
OH
a
b
c
H
N
H
N
MeO
OMe
Cl
Cl
HO
OH
O
O
O
O
O
O
R
O
O
R
6a X=Cl
7a X=Cl (39%)
8a X=Cl (100%)
9a X = H, R = Ph (54%)
9b X = H, R = tBu (67%)
9c X = Cl, R = Ph (51%)
6b X=OMe
7b X=OMe (55%)
8b X=OMe (95%)
X
9d X = Cl, R = tBu (60%)
9e X = OMe, R = Ph (67%)
9f X = OMe, R = tBu (59%)
d
O
O
e
N
N
OH
OH
H
R
R
H
N
N
10a (54%)
10b (67%)
10c (51%)
10d (60%)
10e (67%)
10f (59%)
Ph
O
O
Ph
9a' (39%)
e
O
N
O
N
Ph
Ph
10a'(46%)
Scheme 2. Reagents and conditions: (a) NaOH, EtOH; (b) (COCl)2, DMF, CH2Cl2, 0 ꢁC; (c) (S)-(+)-phenylglycinol or (S)-tert-leucinol, NEt3, CH2Cl2;
(d) (R)-(ꢀ)-phenylglycinol, NEt3, CH2Cl2; (e) CH3SO2Cl (2.5 equiv.), NEt3 (6 equiv.), CH2Cl2.
(a)
CO2Et
+
CO2Et
Scheme 3. Reagents and conditions [3]: (a) ethyl diazoacetate, [Cu(CH3CN)4]PF6 (1 or 2 mol%) or Cu(II)(OTf)2 (1 or 2 mol%), Arylid-Box ligand (1.1 or
2.2 mol%), CH2Cl2 or toluene, r.t.
calculation also predicts a stable coordinate bond between
N6, N13 and the metal (as the predicted bond orders of
0.475 and 0.463 show).
In our preliminary study [3], we used only two types of
pre-catalyst, one containing Cu(I) ([Cu(CH3CN)4]PF6) and
the other contained Cu(II) [Cu(II)(OTf)2]. It thus became

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A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
was generated in situ according to a modification of Evans’
procedure [12]. These reactions were carried out using 1
and 2 mol% of pre-catalyst, respectively, and styrene as
the olefin in all cases (Table 2).
It was obvious from this study that all the reactions
using [Cu(CH3CN)4]PF6 gave the highest e.e.s. Cu(I)SbF6
gave e.e.s comparable with Cu(I)OTf. The best d.e.s were
obtained using Cu(I)OTf, and in one case (Table 2, entry
6) a d.e. as high as 68% was obtained. CuCl was only used
once (Table 2, entry 3) and it gave poor e.e.s. This was
probably due to the poor coordinating ability of the coun-
ter-ion [11]. It would appear that the more coordinating tri-
fluoromethanesulfonate (OTf) [13] leads to greater
diastereoselection in this reaction. However, we are unsure
why the reaction with ligand 10a0 and Cu(I)OTf (Table 2,
entry 2) gives lower e.e.s than when PF6ꢀ or SbF6ꢀ are used
as counter ions (Table 2, entry 1 and 4).
Fig. 2. Chem-3D representation of the Cu(I)–Arylid-Box 10b model
complex (Hs are omitted for clarity) whose optimised structure was
We have also successfully isolated a Cu(I)–Arylid-
Box-10a0 complex by mixing [Cu(CH3CN)4]PF6 with Ary-
lid-Box 10a0. It existed as a bench stable metallic black
˚
determined using DFT. Selected bond lengths (A) and bond angles (ꢁ):
Cu–N13, 1.932; Cu–N6, 1.931; C1–C2, 1.372; N(13)–Cu(21), 1.932; C(4)–
C(2)–C(5), 121.2; N(13)–Cu(21)–N(6), 107.5; C(3)–C(1)–C(2)–C(5), 28.6.
1
powder, which was quite difficult to characterise (the H
NMR was very poorly resolved and furnished very little
information), however, mass spectrometric analysis would
appear to imply the existence of a di-coordinated complex.
We failed to obtain crystals of this compound that would
enable us obtain an X-ray crystal structure. It was used
to catalyse a cyclopropanation reaction between styrene
and ethyl diazoacetate giving a 43% yield of cyclopropane
diastereomers, with a diastereoselectivity of 36% and e.e.s
of 61% (trans-isomer) and 53% (cis-isomer) which was a
somewhat better result than that obtained when the com-
plex was generated in situ. The (1S,2S)-enantiomer was
the major trans enantiomer whilst the (1S,2R)-enantiomer
was the major cis-enantiomer.
Table 1
Mayer bond orders derived from the DFT study
Bond
Bond order
C1–C3
C1–C2
C2–C4
C2–C5
C4–N13
C5–N6
1.120
1.574
1.086
1.046
1.404
1.427
Cu(21)–N6
Cu(21)–N13
0.475
0.463
In each of these reactions dimerisation of the intermedi-
ate Cu(I)–metallocarbene giving a mixture of diethyl fuma-
rate and diethyl maleate occurred on a small scale (approx.
11% of the total product).
of importance to look at other Cu(I) pre-catalysts and
compare the results with those using [Cu(CH3CN)4]PF6.
For this reason, we decided to look at Cu(I)OTf, Cu(I)Cl
and Cu(I)SbF6. In the case of the latter, this pre-catalyst
Table 2
Study to obtain the most effective pre-catalyst for the catalytic asymmetric cyclopropanation of styrene using Arylid-Box ligands 10a, 10a0, 10b, 10d and
10fa
Entry
Pre-catalyst (mol%)
Ligand (mol%)
Yield (%)
trans:cisb
trans (% e.e.)c,d
cis (% e.e.)c,d
1
2
3
4
5
6
7
8
9
[Cu(CH3CN)4]PF6 (2)
Cu(I)OTf (2)
CuCl (2)
Cu(I)SbF6 (2)
[Cu(CH3CN)4]PF6 [3]
Cu(I)OTf (1)
[Cu(CH3CN)4]PF6 (2)
Cu(I)OTf (2)
Cu(I)SbF6 (2)
[Cu(CH3CN)4]PF6 (2)
Cu(I)OTf (2)
10a (2.2)
10a0 (2.2)
10a0 (2.2)
10a0 (2.2)
10b (1.1)
10b (1.1)
10d (2.2)
10d (2.2)
10d (2.2)
10f (2.2)
10f (2.2)
30
24
12
8
19
18
20
29
16
46
28
65:35
79:21
64:36
67:33
56:44
84:16
61:39
68:32
57:43
62:38
66:34
57(1R,2R)
12(1S,2S)
13(1S,2S)
45(1S,2S)
78(1R,2R)
74(1R,2R)
86(1R,2R)
63(1R,2R)
58(1R,2R)
83(1R,2R)
77(1R,2R)
45(1R,2S)
7(1S,2R)
10(1S,2R)
57(1S,2R)
69(1R,2S)
68(1R,2S)
78(1R,2S)
48(1R,2S)
52(1R,2S)
75(1R,2S)
62(1R,2S)
10
11
a
Ethyl diazoacetate, styrene, Cu(I) catalyst and Arylid-Box ligand, CH2Cl2, r.t.
Determined by GC analysis.
The % e.e. was determined by chiral GC analysis (on a cyclosil-B capillary column).
The major isomer is indicated in parenthesis.
b
c
d

A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
4867
Although our primary interest was the acquisition of
high enantio- and diastereoselection in this reaction, and
this is where we focused our efforts, we later became con-
scious of the generally poor isolated yields that were
obtained, no matter what reaction condition was used. This
was hard to explain due to the fact that we obtained only
cyclopropane isomers as the main products. However, care-
ful examination of our experimental procedure revealed
that as we used an excess of styrene (10 equiv. relative to
EDA) the excess styrene at the end appeared to form a col-
loidal type layer that impeded the isolation of all the cyclo-
propane product at the pre-column purification stage
(removal of the catalyst, see Experimental Part). To verify
this, we carried out a cyclopropantion using 10 equiv. of
styrene, with ligand 10a0 (2.2 mol%) and [Cu(CH3CN)4]PF6
(2 mol%) and at the end of the reaction the crude product
was filtered to remove the catalyst, but this time instead
of washing with just CH2Cl2 (as was always done before),
both EtOAc and MeOH were used. Using this procedure,
the cyclopropane products were isolated in a combined iso-
lated yield of 64% after column chromatography. We also
discovered that there was basically no change in the stere-
oselectivities from before (see Table 2, entry 1) as the
trans-isomer was obtained with an e.e. of 62% (in favour
of the 1S,2S-enantiomer) and the cis-isomer with an e.e.
of 43% (in favour of the 1S,2R-enantiomer). The trans:cis
ratio was also the same 62:38.
absorption maximum at 231 nm. Observation of the
[Cu(CH3CN)4]PF6–10a0 complex over a 3 h period, showed
that there was in fact no change in the UV–Vis spectrum of
the complex, thus ruling out the possibility of structural
re-organization occurring. It has to be noted that if one
considers the bathochromic shift from 285 to 330 nm, an
indication of complexation, then complex formation was
immediate. A study was then carried out to investigate
the kinetics of decomposition of the EDA with the
[Cu(CH3CN)4]PF6–10a0 complex. The [Cu(CH3CN)4]-
PF6–10a0 complex was treated with 1 equivalent of ethyl
diazoacetate (EDA). In this case, we observed a very slight
bathochromic shift of 5 nm (kmax = 335 nm). The spectrum
remained more or less constant through out the duration of
this study. The only difference was that the absorption
maximum for the EDA (kmax = 245 nm) disappeared after
about 5 hours. This implies that it takes the complex about
this time to decompose the EDA forming the transitory
metallocarbene copper complex. We also attempted isolat-
ing the metallocarbene copper (I)-complex which results
from the reaction of [Cu(CH3CN)4]PF6–10a0 complex with
EDA (by mixing [Cu(CH3CN)4]PF6–10a0 complex with
one equivalent of EDA) giving a green metallic solid. This
1
was analysed by both variable temperature H NMR and
mass spectrometry. The variable temperature 1H NMR
study was uninformative, and there was very little differ-
ence between the spectrum at room temperature and that
at ꢀ30 ꢁC, in fact, the resolution deteriorated at the lower
temperature. Likewise mass spectrometric analysis of this
solid was also uninformative. The solid was also mixed
with styrene in CH2Cl2 but no reaction was observed even
after several days. All indications being that this solid was
not the metallocarbene complex an assumption backed up
by literature precedent [14,17] as these species are very
elusive.
The proposed mechanism for the decomposition of the
short-lived electrophilic copper–carbene intermediates
derived from diazoacetates by olefins [9] (or the cycloprop-
anation step) is that shown in Scheme 4.
To obtain a better view of the nature of the catalytic spe-
cies at work in this reaction and the kinetics for its forma-
tion, we conducted some spectroscopic studies on the
1
[Cu(CH3CN)4]PF6–10a0 complex. The H NMR study on
the [Cu(CH3CN)4]PF6–10a0 complex was carried out to
verify if in fact the complex was undergoing some struc-
tural re-organization in the early stage of this reaction.
Unfortunately this study was inconclusive. We thus turned
to UV–Vis spectroscopy for some insight. Soon after mix-
ing the ligand 10a0 with the [Cu(CH3CN)4]PF6 complex an
aliquot was removed at hourly intervals. The spectra were
the same in all cases with absorption maxima of 231 and
330 nm, respectively. The ligand itself presented two
absorption maxima of 231 and 285 nm, the first most prob-
ably a p–p* transition was also observed in the complex.
Interestingly, [Cu(CH3CN)4]PF6 also presented an
In an attempt to gain an insight into the lower e.e.s that
were obtained for our tert-butyl substituted ligands com-
pared to those obtained with Evans’ gem-dimethyl malon-
ate derived ligand, we conducted some DFT calculations
(at the same level of theory as previously [6–8,15–18]), on
the model Cu(I)–metallocarbene complexes 11 and 12
(Fig. 3) which are basically approximations of the Cu(I)–
metallocarbene species derived from our Cu(I)–Arylid-
Box ligands and Evans’ gem-dimethyl malonate derived
ligands. It was observed that for both Cu(I)–metallocar-
bene complexes 11 and 12 the Cu–N bond lengths were
longer than in the Cu(I)–Arylid-Box 10b, meaning that
coordination with the metal was weaker in the metallocar-
bene complexes. There was also a greater difference in the
Cu–N bond lengths for either metallocarbene complex,
for example in the case of Cu(I)–Arylid-Box 10b, the differ-
R1O2C
R
CuL*
˚
ence was only 0.001 A, but in the case of 11 and 12 it was
˚
˚
0.023 A and 0.009 A, respectively. These bond lengths,
particularly those for 11 are close to the values calculated
by Fraile et al. [18] for the Cu(I) metallocarbene derived
R
CO2R1
CuL*
from
2,20-methylenebis[(4S)-methyl-2-oxazoline]
and
Scheme 4. Mechanism for the cyclopropanation step.
methyl diazoacetate. The fact that the calculated Cu–N

4868
A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
Fig. 3. Chem-3D representations (with the Hs omitted for clarity) of the Cu(I)–metallocarbene complexes 11 and 12 model complexes whose optimised
˚
structure was determined using DFT. Selected bond lengths (A) and bond angles (ꢁ): For complex 11: Cu–N18, 2.005; Cu–N24, 1.982; Cu–C2, 1.813; C19–
C8–C28, 116.3, N18–Cu–C2, 128.7; N24–Cu–C2, 140.5; N18–Cu–N24, 90.8; N18–Cu–C2–C3, 74.2. For complex 12: Cu–N30, 2.000; Cu–N24, 2.009; Cu–
C2, 1.818; C32–C8–C25, 112.0; N30–Cu–C2, 144; N24–Cu–C2, 124.3; N30–Cu–N24, 91.6; N24–Cu1–C2–C3, 67.9.
bond lengths for 11 were shorter than for 12 would seem to
indicate the former has the greater inherent stability, which
is reinforced by the observation that the calculated Cu–C
bond length for 11 was just slightly shorter than that for
12. The bite angles calculated for 11 and 12 were very close,
but somewhat smaller than that calculated in the Cu(I)
metallocarbene studied by Fraile et al. [18] (95ꢁ). The car-
bonyl group for both 11 and 12 was calculated to be
roughly perpendicular with the Cu–C bond, with 11 having
the greater dihedral angle of 74.2ꢁ as opposed to 67.9ꢁ for
12. An angle of 65.8ꢁ was reported by by Fraile et al. for
their system [18]. In our case, this would imply that the
carbenoid carbon of 11 should be the harder acid centre.
On the basis of Pearson’s HSAB theory [19] if one consid-
ers styrene a soft base then metallocarbene 12 should be the
more reactive of the two. What was most interesting was
the finding that there were significant differences between
the N–Cu–C angles for both 11 (128.7ꢁ and 140.5ꢁ) and
12 (124.3ꢁ and 144ꢁ) meaning that the Cu–carbenoid car-
bon bond deviates away from the symmetry axis of the
metallocarbene complex (this asymmetry was also observed
in the system studied by Fraile et al. [18]) and it has impor-
tant stereochemical implications as pointed out by Fraile
et al. [18]. This calculation suggests that when one ignores
electronic effects, complex 12 probably gives higher ees in
the cyclopropanation reaction due to the closer proximity
of the carbenoid carbon to one of the stereogenic centres.
Although we have shown that when Cu(II)(OTf)2 is used
as pre-catalyst with the Arylid-Box ligand system generally
the enantioselectivities are better than with our first gener-
ation Isbut-Box ligand system [3], we became curious to
know: (1) would there be a similar trend if a Cu(I) pre-cat-
alyst was used and (2) would a Cu(I) pre-catalyst improve
the enantioselectivities when Isbut-Box ligands were used.
We thus carried out a number of cyclopropanations with
the Isbut-Box ligands 13a [2] and 13b [2] (Fig. 4) and both
([Cu(CH3CN)4]PF6) and Cu(I)OTf as the pre-catalysts
(Table 3, the results obtained for the equivalent Arylid-
Box ligands are included for comparative purposes as are
2
the results for the Isbut-Box 13a with CuðIIÞðOTfÞ2).
This study demonstrated that on using both Cu(I)OTf
and [Cu(CH3CN)4]PF6 with 13a the e.e.s were much better
than when Cu(II) (OTf)2 was used (compare entry 1 with
entries 2 and 3). Ligand 10b gave slightly better e.e.s than
13a when [Cu(CH3CN)4]PF6 was used. In the case of
ligand 13b using Cu(II)(OTf)2 (entry 6) the best e.e. was
O
O
O
O
N
N
N
N
13a
13b
Ph
Ph
Fig. 4. Isbut-Box ligands used for cyclopropanations with Cu(I) pre-
catalysts (Table 3).

A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
4869
Table 3
Study to compare the efficiency and stereoselectivity of Isbut-Box 13a and 13b with the Arylid-Box ligand 10b using Cu(I) pre-catalystsa
Entry
Pre-catalyst
Ligand
Yield (%)
trans:cisb
trans (% e.e.)c,d
cis (% ee)c,d
1
2
3
4
5
6
7
8
a
Cu(II) (OTf)e2 [2]
Cu(I)OTf
[Cu(CH3CN)4]PF6
Cu(I)OTf
[Cu(CH3CN)4]PF6 [3]
Cu(II) (OTf)e2 [2]
Cu(I)OTf
13af
13a
13a
10b
10b
13b
13b
13b
61
61
38
18
19
54
20
22
58:42
59:41
56:44
84:16
56:44
68:32
62:38
54:46
46(1R,2R)
79(1R,2R)
67(1R,2R)
74(1R,2R)
78(1R,2R)
47(1R,2R)
38(1R,2R)
25(1R,2R)
42(1R,2S)
73(1R,2S)
63(1R,2S)
78(1R,2S)
69(1R,2S)
22(1R,2S)
22(1R,2S)
7(1R,2S)
[Cu(CH3CN)4]PF6
Styrene, ethyl diazoacetate, Cu(I) catalyst (1 mol%) and ligand (1.1 mol%), CH2Cl2, r.t.
Determined by GC analysis.
The % e.e. was determined by chiral GC analysis (on a cyclosil-B capillary column).
The major isomer is indicated in parenthesis.
1.7 mol% pre-catalyst was used.
1.8 mol% of 13a was used.
b
c
d
e
f
obtained for the trans cyclopropane product. Overall,
ligand 13a seemed to give the better stereoselectivities.
oxybenzylidene)malonic acid 7b were prepared as reported
previously [3–5,20,21].
All reagents were obtained from Aldrich, Fluka, Alfa
Aesar or Acros. Solvents were dried using common labora-
tory methods.
3. Conclusions
In summary, we have carried out a number of detailed
experiments to determine the most suitable Cu(I) pre-cata-
lyst for the catalytic asymmetric cyclopropanation of sty-
rene with ethyl diazoacetate using our Arylid-Box
ligands. Our results indicate that it is [Cu(CH3CN)4]PF6
which gives the best e.e.s, whilst Cu(I)OTf gives the best
d.e.s. When the Isbut-Box ligands 13a and 13b were tested
with Cu(I) pre-catalysts, it was ligand 13a which gave the
best results as it gave e.e.s close to those obtained with
the equivalent Arylid-Box ligand 10b. A DFT study at a
B3LYP level [6] predicts a great similarity between our
ligand system and the Evans ligand system, when a com-
parative study was made between Cu(I)–Arylid-Box 10b
and Evans’ tert-But-Box–Cu(I) complex. The UV–Vis
spectroscopy study seems to indicate that the EDA decom-
posed relatively slowly. The DFT study of a model Cu(I)–
metallocarbene complex of the Cu(I)–metallocarbene com-
plex which we suspect to be active in our reactions and that
of the Cu(I)–metallocarbene complex suspected to be pres-
ent when Evans’ tert-But-Box–Cu(I) complex is used
showed that the carbenoid carbon in the latter veers more
to one of the oxazoline ring stereogenic centres implying
greater enantiofacial discrimination. We are currently eval-
uating these ligands and other analogues in this reaction
and other transition metal promoted catalytic asymmetric
reactions and at immobilising these ligand systems to
appropriate solid supports.
Column chromatography was carried out on silica gel
(sds, 70–200 lm) and flash column chromatography
(Merck, 40–63 lm and sds, 40–63 lm). TLC was carried
out on aluminium backed Kisel-gel 60 F254 plates (Merck).
Plates were visualised either by UV light or phosphomolyb-
dic acid in ethanol.
Gas chromatographic (GC) analyses of the products
were performed on a Hewlett–Packard (HP) 6890 series
instrument equipped with a flame ionization detector
(FID). The chromatograph was fitted with a cyclosil-B cap-
illary column (30 m, 250 lm, 0.25 lm) (Agilent 112-2532).
The melting points were recorded on a Barnstead
Electothermal 9100 apparatus and are uncorrected. The
1H NMR spectra were recorded on either a Bruker
AMX300, Bruker Avance 400 or a Bruker Avance 500
instrument using CDCl3 as solvent and TMS as internal
standard. In some cases, the signals for the NH and OH
protons could be observed, but in other cases they were
not observed possibly due to overlap with other peaks.
Mass spectra were recorded on a VG Autospec M
(Waters-Micromass) spectrometer using the FAB tech-
nique. Infra-red spectra were measured with a Perkin–
Elmer Paragon 1000 model.
4.2. Ligand synthesis
General procedure for the synthesis of acid chlorides (3,
8a and 8b): A dry two-necked round bottom flask
(50 mL) equipped with a magnetic stir bar was charged
with benzylidenemalonic acid (1.86 g, 9.6 mmol), dimethyl-
formamide (0.1 mL, 1.25 mmol) and CH2Cl2 (25 mL). The
solution was cooled to 0 ꢁC, and oxalyl chloride (2.5 mL,
29 mmol) was added dropwise over 30 min and the solution
was stirred at room temperature until the evolution of gas
ended. The solution was evaporated in vacuo to give benzy-
4. Experimental
4.1. General remarks
Dimethyl p-chlorobenzylidienemalonate 6a, dimethyl
p-metoxybenzylidienemalonate 6b, benzylidenemalonic
acid 2, 2-(4-chlorobenzylidene)malonic acid 7a, 2-(4-meth-

4870
A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
22
lidenemalonyl chloride 3 as an orange oil (Due to the
689.55 cmꢀ1. ½aꢁD ¼ ꢀ16:64 (c = 0.3, CHCl3). FAB-MS
unstable nature of this compound it was stored in the free-
m/z: 363.27 [M+H]+.
1
zer at ꢀ10 ꢁC). Yield: 2.00 g (100%). H NMR (300 MHz,
4b: The same procedure as described previously was
used in the reaction of benzylidenemalonyl chloride 3
(0.89 g, 3.89 mmol) with (R)-(+)-phenylalaninol (1.18 g,
7.78 mmol) and dry triethylamine (1.63 mL, 11.7 mmol)
to give (R,R)-N,N0-bis-(1-benzyl-2-hydroxyethyl)-2-benzy-
lidenemalonamide 4b as a white solid after purification
by column chromatography (silica gel, EtOAc); Yield:
CDCl3) d = 7.96 (s, 1H, ArHC@C), 7.60–7.56 (m, 2H,
Har), 7.52–7.46 (m, 3H, Har) ppm.
8a: Using the same procedure as described previously 2-
(4-chlorobenzylidene)malonic acid 7a (4.0 g, 9.6 mmol) was
reacted with dimethylformamide (0.17 mL, 1.25 mmol) and
oxalyl chloride (3.8 mL, 44 mmol) to give 2-(4-chloroben-
zylidene)malonyl chloride 8a as an orange oil. Yield:
1
0.77 g (43%). m.p.: 167.2–168.1 ꢁC; H NMR (500 MHz,
1
4.80 g (100%). H NMR (300 MHz, CDCl3): d = 7.89 (s,
CD3OD): d = 7.45 (s, 1H, ArHC@C), 7.33–7.16 (m, 15H,
Har), 4.47–4.42 (m, 1H, CH), 4.28–4.23 (m, 1H, CH),
3.69 (dd, J = 11.5, 4.3 Hz, 1H, CHHOH), 3.61 (dd,
J = 11.5, 5 Hz, 1H, CHHOH), 3.57 (dd, J = 11, 6 Hz, 1H
CHHOH), 3.44 (dd, J = 11, 8 Hz, 1H, CHHOH), 2.89
(dd, J = 14, 6.5 Hz, 1H, CHHAr), 2.82 (dd, J = 14,
6.5 Hz, 1H, CHHAr), 2.77 (dd, J = 14, 8.5 Hz, 1H,
CHHAr), 2.66 (dd, J = 14, 8.5 Hz, 1H, CHHAr). 13C
NMR (100 MHz, CD3OD): d = 170.09, 166.05, 139.69,
139.44, 139.06, 134.75, 132.67–127.62, 127.43, 64.99,
1H, ArHC@C), 7.54–7.42 (m, 4H, Har) ppm.
8b: Using the same procedure as described preveliously
2-(4-methoxybenzylidene)malonic acid 7b (0.8 g, 3.6
mmol) was reacted with dimethylformamide (34.2 mg,
0.47 mmol) and oxalyl chloride (0.94 mL, 10.8 mmol) to
give 2-(4-methoxybenzylidene)malonyl chloride 8b as a
yellow oil. Yield: 0.89 g (95%). 1H NMR (300 MHz,
CDCl3): d = 7.86 (s, 1H, ArHC@C), 7.58 (d, J = 9 Hz,
2H, Har), 6.99 (d, J = 8.7 Hz, 2H, Har), 3.90 (s, 3H,
–OCH3) ppm.
64.00, 55.02, 54.48, 38.02, 37.39 ppm. IR (KBr) tmax
:
General Procedure for the synthesis of malonamides (4a,
4b, 9a–f and 9a0): A two necked round bottom flask
(50 mL) fitted with a magnetic stirring bar was charged
with a solution of (S)-valinol (0.90 g, 8.7 mmol) and dry
CH2Cl2 (10 mL) and the solution was cooled to 0 ꢁC using
an ice bath. Dry triethylamine (1.83 mL, 13.0 mmol) was
added via syringe. A solution of benzylidenemalonyl chlo-
ride 3 (1.2 g, 5.2 mmol) in CH2Cl2 (5 mL) was slowly added
via syringe to the vigorously stirred reaction mixture over
30 min. The ice bath was removed, and the reaction mix-
ture was stirred at room temperature for 4 h. The reaction
mixture was washed with 2 M HCl (8 mL), saturated aque-
ous NaHCO3 (10 mL) and the aqueous layer was back-ex-
tracted with CH2Cl2 (10 mL). The combined organic
extracts were washed with brine (10 mL), and the aqueous
layer was back-extracted with CH2Cl2 (10 mL). The com-
bined organic extracts were dried over anhydrous MgSO4,
filtered and concentrated in vacuo to give (S,S)-N,N0-bis-(1-
hydroxymethyl-2-methylpropyl)-2-benzylidenemalonamide
4a as an orange solid. The crude product was purified by
column chromatography (silica gel, EtOAc) to afford the
diamide 4a as a white solid. Yield: 0.44 g (28%); m.p.:
129.5–130.7 ꢁC; 1H NMR (400 MHz, CDCl3): d = 7.54
(s, 1H, ArHC@C), 7.43 (d, J = 1.6 Hz, 2H, Har), 7.42–7.27
(m, 4H, Har and N–H), 6.95 (d, 1H, J = 8.8 Hz, N–H),
3.90–3.82 (m, 2H, CH2OH), 3.73–3.68 (m, 2H, CH2OH),
3.54–3.45 (m, 2H, CH), 1.81–1.76 (m, 1H, CH(CH3)2),
1.73–1.68 (m, 1H, CH(CH3)2), 0.90 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 0.88 (d, J = 6.8 Hz, 3H, CH(CH3)2), 0.84 (d,
J = 6.8 Hz, 3H, CH(CH3)2), 0.77 (d, J = 6.8 Hz, 3H,
CH(CH3)2) ppm. 13C NMR (100 MHz, CDCl3):
d = 169.23, 165.11, 138.51, 133.52, 131.84, 129.67, 129.43,
128.65, 63.51, 63.16, 57.72, 57.32, 29.44, 28.90, 19.52,
19.43, 18.92, 18.56 ppm. IR (KBr) tmax: 3319.04, 3258.48,
3060.09, 2957.40, 2874.08, 1736.58, 1651.97, 1640.28,
1616.31, 1547.04, 1465.44, 1289.53, 1071.85, 749.37,
3257.77, 3058.18, 2920.82, 1736.40, 1643.81, 1611.29,
1538.60, 1449.51, 1288.84, 1068.52, 745.65, 695.68 cmꢀ1
.
22
½aꢁD ¼ ꢀ71:70 (c = 0.22, acetone). FAB-MS m/z: 459.28
[M+H]+.
9a: The same procedure as described previously was
used in the reaction of benzylidenemalonyl chloride 3
(1.0 g, 4.37 mmol) with (S)-(+)phenylglycinol (1.2 g,
8.74 mmol) and dry triethylamine (1.82 mL, 13 mmol) to
give (S,S)-N,N0-bis(2-hydroxy-1-phenylethyl)-2-benzylide-
nemalonamide 9a as a white solid after purification by col-
umn chromatography (silica gel, EtOAc); Yield: 0.77 g
1
(41%). m.p.: 74.6–75.2 ꢁC; H NMR (500 MHz, CDCl3):
d = 8.00 (d, 1H, J = 8 Hz, N–H), 7.66 (s, 1H, ArHC=H),
7.49 (d, 1H, J = 8 Hz, N–H), 7.38–7.24 (m, 10H, Ar),
7.16–7.12 (m, 5H, Ar), 5.29–5.25 (m, 1H, CH), 5.24–5.20
(m, 1H, CH), 3.90 (dd, 1H, J = 4.3, 12.3 Hz, CHHOH),
3.83–3.75 (m, 3H, CH2OH and CHHOH), 2.18 (s, 2H,
CH2OH) ppm. 13C NMR (100 MHz, CDCl3): d = 168.38,
164.86, 139.58, 138.38, 137.54, 132.83, 130.75–126.73,
126.55, 65.74, 65.44, 56.16, 55.99 ppm. IR (KBr) tmax
3269.24, 3059.14, 2874.53, 1736.39, 1659,84, 1612.97,
1529.28, 1451.82, 1055.29,
757.13, 697.47 cmꢀ1
:
.
22
½aꢁD ¼ þ73:46 (c = 0.26, CHCl3). FAB-MS m/z: 431.24
[M+H]+.
9a0: The same procedure as described previously was
used in the reaction of benzylidenemalonyl chloride 3
(1.1 g, 4.8 mmol) with (R)-(ꢀ)-phenylglycinol (1.04 g,
7.6 mmol) and dry triethylamine (1.3 mL, 9.5 mmol) to
give (R,R)-N,N0-bis(2-hydroxy-1-phenylethyl)-2-benzylide-
nemalonamide 9a0 as a white solid after purification by col-
umn chromatography (silica gel, EtOAc); Yield: 0.64 g
22
(39%). m.p.: 74.3–75.4 ꢁC; ½aꢁD ¼ ꢀ95:82 (c = 0.91,
CHCl3).
9b: The same procedure as described previously was
used in the reaction of benzylidenemalonyl chloride 3
(1.12 g, 4.9 mmol) with (S)-tert-leucinol (1.148 g,

A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
4871
9.8 mmol) and dry triethylamine (2.05 mL, 14.7 mmol) to
give (S,S)-N,N0-bis-(1-hydroxymethy-2,2-dimethylpropyl)-
2-benzylidenemalonamide 9b as white crystals after purifi-
cation by column chromatography (silica gel, EtOAc);
Yield: 0.96 g (50%); m.p.: 144.3–145.7 ꢁC; 1H NMR
(300 MHz, CDCl3): d = 7.61 (s, 1H, ArHC@C), 7.46–7.39
(m, 3H, N–H and Har), 7.32–7.30 (m, 3H, Har), 6.81 (d,
J = 9 Hz, 1H, N–H), 4.01–3.90 (m, 2H, CH), 3.88–3.80
(m, 2H, CH2OH), 3.53–3.45 (m, 2H, CH2OH), 0.94 (s,
9H, C(CH3)3), 0.84 (s, 9H, C(CH3)3) ppm. 13C NMR
(75 MHz, CDCl3): d = 169.74, 165.61, 138.0, 138.04,
133.58, 132.24, 129.44, 128.58, 62.10, 61.74, 60.28, 59.99,
33.87, 33.30, 26.97, 26.72 ppm. IR (KBr) tmax: 3271.95,
3064.49, 2962.35, 1737.25, 1651.76, 1616.35, 1540.15,
3067.45, 2962.86, 1738.34, 1658.74, 1642.28, 1616.21,
1536.19, 1489.12, 1273.44, 1098.18, 1017.94, 821.28 cmꢀ1
.
20
½aꢁD ¼ ꢀ8:30 (c = 0.47, CHCl3). FAB-MS m/z: 425.10
[M+H]+.
9e: The same procedure as described previously was
used in the reaction of 2-(4-methoxybenzylidene)malonyl
chloride 8b (1.06 g, 4.1 mmol) with (R)-phenylglycinol
(0.9 g, 6.56 mmol) and dry triethylamine (1.36 ml,
9.8 mmol) to give (R,R)-N,N0-bis(2-hydroxy-1-phenyl-
ethyl)-2-(4-methoxybenzylidene)malonamide 9e as an
orange solid. The crude product was purified by column
chromatography (silica gel, EtOAc) to afford diamide as
1
a white solid. Yield: 0.72 g (38%); m.p.: 69.5–70.2 ꢁC; H
NMR (300 MHz, CDCl3): d = 8.04–8.01 (m, 1H, N–H),
7.85 (d, J = 8.4 Hz, 1H, N–H), 7.45 (s, 1H, ArHC@C),
7.28–7.24 (m, 10H, Har), 7.12 (d, J= 8.7 Hz, 2H, Har),
6.53 (d, J = 8.7 Hz, 2H, Har), 5.37–5.34 (m, 1H, CH),
5.25–5.18 (m, 1H, CH), 3.91–3.85 (m, 4H, CH2OH), 3.73
(s, 3H, –OCH3) ppm. 13C NMR (100 MHz, CDCl3):
d = 166.87, 160.99, 141.47, 138.98, 129.45, 129.33, 128.87,
128.78, 127.85, 127.33, 126.92, 126.81, 126.75, 126.64,
117.75, 114.23, 66.63, 66.43, 56.26, 55.96, 55.24 ppm. IR
(KBr) tmax: 3292.24, 1736.65, 1654.84, 1603.58, 1513.26,
1473.16, 1273.08, 1051.11, 756.71, 693.89 cmꢀ1
.
19
½aꢁD ¼ ꢀ16:33 (c = 0.98, CHCl3). FAB-MS m/z: 391.17
[M+H]+.
9c: The same procedure as described previously was
used in the reaction of 2-(4-chlorobenzylidene)malonyl
chloride 8a (1.28 g, 4.84 mmol) with (S)-phenylglycinol
(1.04 g, 7.59 mmol) and dry triethylamine (1.6 mL,
11 mmol) to give (S,S)-N,N0-bis(2-hydroxy-1-phenyl-
ethyl)-2-(4-chlorobenzylidene)malonamide 9c as white
crystals after purification by column chromatography (sil-
ica gel, EtOAc). Yield: 0.80 g (46%); m.p.: 153.8–
155.1 ꢁC; 1H NMR (400 MHz, CDCl3): d = 7.97 (d,
J = 8.5 Hz, 1H, N–H), 7.91 (d, J = 8.5 Hz, 1H, N–H),
7.42 (s, 1H, ArHC@C), 7.38–7.23 (m, 10H, Har),
7.18–7.14 (m, 2H, Har), 7.05 (d, J = 8.5 Hz, 1H, Har),
6.96 (d, J = 8.5 Hz, 1H, Har), 5.32–5.26 (m, 1H, CH),
5.22–5.17 (m, 1H, CH), 3.88–3.69 (m, 4H, CH2OH) ppm.
13C NMR (100 MHz, CDCl3): d = 168.01, 164.69, 138.21,
138.16, 137.42, 135.71, 131.42, 131.31, 130.69, 128.97,
128.82, 128.78, 128.75, 128.31, 128.02, 127.85, 127.05,
126.72, 126.56, 65.96, 65.50, 56.21, 56.01 ppm. IR (KBr)
1253.19,
1177.72,
1028.10,
759.04, 699.78 cmꢀ1
.
21
½aꢁD ¼ ꢀ16:2 (c = 0.58, CHCl3). FAB-MS m/z: 461.16
[M+H]+.
9f: The same procedure as described previously was used
in the reaction of 2-(4-methoxy-benzylidene)malonyl chlo-
ride 8b (1.0 g, 3.85 mmol) with (S)-tert-leucinol (0.91 g,
7.7 mmol) and dry triethylamine (1.62 ml, 12.0 mmol) to
give (S,S)-N,N0-bis(1-hydroxymethyl-2,2-dimethylpropyl)-
2-(4-methoxybenzylidene)malonamide 9f as an orange
solid. The crude product was purified by column chroma-
tography (silica gel, EtOAc) to afford diamide as a white
1
solid. Yield: 0.65 g (40%); m.p.: 161.1–161.8 ꢁC; H NMR
tmax
:
3395.91, 3243.14, 1739.26, 1651.71, 1613.84,
(300 MHz, CDCl3): d = 7.55 (s, 1H, ArHC@C), 7.43 (d,
J = 8.7 Hz, 2H, Har), 7.34 (d, J = 9.5 Hz, 1H, N–H),
6.82 (d, J = 8.7 Hz, 2H, Har), 6.71 (d, J = 9.5 Hz, 1H,
N–H), 4.04–3.98 (m, 1H, CHHOH), 3.94–3.86 (m, 3H,
CH2OH and CHHOH), 3.81 (s, 3H, –OCH3), 3.55–3.48
(m, 2H, CH), 0.94 (s, 9H, C(CH3)3), 0.88 (s, 9H,
C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): d = 170.13,
166.14, 160.63, 137.50, 131.50, 129.61, 125.85, 113.90,
62.20, 61.65, 60.18, 60.06, 55.22, 33.94, 33.27, 26.85,
26.71 ppm. IR (KBr) tmax: 3332.96, 3062.63, 2961.16,
1736.50, 1642.58, 1605.79, 1529.58, 1469.67, 1256.75,
1541.79, 1472.30. 1265.57, 1058.75, 744.28, 699.27 cmꢀ1
;
19
½aꢁD ¼ 38:6 (c = 0.59, CHCl3). FAB-MS m/z: 465.10
[M+H]+.
9d: The same procedure as described previously was
used in the reaction of 2-(4-chlorobenzylidene)malonyl
chloride 8a (1.23 g, 5.33 mmol) with (S)-tert-leucinol
(1.00 g, 8.53 mmol) and dry triethylamine (1.78 mL,
12.7 mmol) to give (S,S)-N,N0-bis(1-hydroxymethyl-2,2-
dimethylpropyl)-2-(4-chlorobenzylidene)malonamide 9d as
an orange solid. The crude product was purified by
column chromatography (silica gel, EtOAc) to afford the
diamide 9d as a white solid. Yield: 0.69 g (33%). m.p.:
171.1–171.8 ꢁC; 1H NMR (400 MHz, CDCl3): d = 7.46
(d, J = 10 Hz, N–H), 7.33 (d, J = 5.4 Hz, 2H, Har) 7.30
(s, 1H, ArHC@C), 7.21–7.18 (m, 3H, Har and N–H),
4.05 (td, J = 10, 3.5 Hz, 1H, CH), 3.91–3.79 (m, 3H, CH
and CH2OH), 3.55–3.42 (m, 2H, CH2OH), 0.89 (s, 9H,
C(CH3))3, 0.88 (s, 9H, C(CH3)3) ppm. 13C NMR
(100 MHz, CDCl3): d = 169.41, 165.79, 136.25, 135.62,
132.61, 131.82, 130.81, 128.82, 62.23, 61.57, 60.41, 60.21,
33.94, 33.31, 26.86, 26.71 ppm. IR (KBr) tmax: 3333.61,
20
1178.30, 1032.38, 827.35 cmꢀ1. ½aꢁD ¼ ꢀ5:87 (c = 0.46,
CHCl3). FAB-MS m/z: 421.17 [M+H]+.
General procedure for the synthesis of Bis-oxazolines (5a,
5b, 10a–10f and 10a0): A solution of methanesulfonyl chlo-
ride (0.24 g, 2.07 mmol) in dry dichloromethane (1 mL)
was added dropwise over 20 min to a solution of
diamide 4a (0.3 g, 0.83 mmol) and dry triethylamine
(0.69 mL, 4.97 mmol) in dry dichloromethane (10 mL)
and the solution was stirred between ꢀ5 and ꢀ10 ꢁC. The
reaction mixture was allowed to warm to room tempera-
ture and stirring was continued for 3 days. The reaction

4872
A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
mixture was then poured into a saturated aqueous NH4Cl
solution (10 mL). The organic layer was separated and the
aqueous layer was extracted with CH2Cl2 (2 · 5 mL). The
combined organic layers were washed with brine, dried
(MgSO4), filtered, and concentrated to afford the crude
product. The crude product was purified by column chro-
matography (silica gel, EtOAc) giving the (ꢀ)-bis[(S)-
4-isopropyloxazoline-2-yl]-2-phenylethene 5a as a white
semi-solid. Yield: 0.20 g (74%). 1H NMR (300 MHz,
CDCl3): d = 7.57 (s, 1H, ArHC@C), 7.49–7.45 (m, 2H,
Har), 7.35–7.32 (m, 3H, Har), 4.4–4.29 (m, 2H, CH),
4.14–4.06 (m, 4H, CH2), 1.90–1.77 (m, 2H, CH(CH3)2),
0.99 (s, 3H, CH(CH3)2), 0.97 (s, 3H, CH(CH3)2), 0.94 (s,
3H, CH(CH3)2), 0.89 (s, 3H, CH(CH3)2) ppm. 13C NMR
(75 MHz, CDCl3): d = 161.76, 160.41, 140.75, 134.11,
129.46, 129.34, 128.36, 118.94, 72.83, 72.80, 70.26, 70.15,
32.88, 32.36, 18.96, 18.77, 18.46, 18.21 ppm. IR (NaCl):
30.89 ppm. IR (NaCl): tmax: 3340.47, 3065.00, 3011.92,
2968.06, 2927.78, 1654.97, 1565.50, 1453.84, 1210.95,
19
1029.02, 763.75, 737.37, 669.76 cmꢀ1
.
½aꢁD ¼ þ90:7
(c = 0.9, CHCl3). FAB-MS m/z: 395.06 [M+H]+, HRMS
(FAB) found, 395.1760; C26H23N2O2 requires 395.1757.
10a0: Using the same procedure as described previously,
malonamide 9a0 (0.40 g, 0.93 mmol) was reacted with
methanesulfonyl chloride (0.27 g, 2.32 mmol) and dry tri-
ethylamine (0.8 mL, 5.57 mmol) to give the (ꢀ)-bis[(R)-4-
phenyloxazoline-2-yl]-2-phenylethene 10a0 as a yellow
semi-solid after purification by column chromatography
(silica gel, EtOAc). Yield: 0.17 g (46%). 1H NMR
(300 MHz, CDCl3): d = 7.76 (s, 1H, ArHC@C), 7.54–7.53
(m, 2H, Har), 7.37–7.23 (m, 13H, Har), 5.45–5.4 (m, 2H,
CHAr), 4.82–4.77 (m, 2H, CH2), 4.32–4.22 (m, 2H,
19
CH2) ppm. ½aꢁD ¼ ꢀ115 (c = 0.92, CHCl3).
10b: Using the same procedure as described previously,
malonamide 9b (0.40 g, 1.02 mmol) was reacted with
methanesulfonyl chloride (0.29 g, 2.55 mmol) and dry
triethylamine (0.86 mL, 6.14 mmol) to give the (+)-bis[(S)-
4-tert-butyloxazoline-2-yl]-2-phenylethene 10b as a white
semi-solid after purification by column chromatography
(silica gel, EtOAc). Yield: 0.27 g (67%). 1H NMR
(300 MHz, CDCl3): d = 7.53 (s, 1H, ArHC@C), 7.48–7.45
(m, 2H, Har), 7.34–7.31 (m, 3H, Har), 4.37–4.13 (m, 4H,
CH2), 4.07–3.99 (m, 2H, CH), 0.97 (s, 9H, C(CH3)3), 0.91
(s, 9H, C(CH3)3) ppm. 13C NMR (75 MHz, CDCl3):
d = 161.76, 160.45, 140.41, 134.15, 129.44, 129.40, 128.38,
118.99, 68.84, 68.53, 34.04, 33.89, 26.130, 25.8 ppm. IR
(NaCl): tmax: 3350.01, 2959.32, 2874.08, 1658.37, 1570.50,
1473.35, 1362.03, 1244.89, 1193.47, 1083.22, 770.77,
tmax
:
3343.37, 2964.15, 2930.98, 2875.81, 1662.37,
1568.96, 1466.28, 1364.46, 1176.97, 774.47, 734.28,
19
669.74 cmꢀ1. ½aꢁD ¼ ꢀ12:46 (c = 0.35, CHCl3). FAB-MS
m/z: 327.13 [M+H]+; HRMS (FAB) found, 326.2072;
C20H27N2O2 requires 327.2073.
5b: Using the same procedure as described previously,
malonamide 4b (0.45 g, 0.98 mmol) was reacted with
methanesulfonyl chloride (0.28 g, 2.45 mmol) and dry
triethylamine (0.85 mL, 5.89 mmol) to give the (ꢀ)-bis[(R)-
4-benzyloxazoline-2-yl]-2-phenylethene 5b as a yellow
semi-solid after purification by column chromatography
(silica gel, EtOAc). Yield: 0.27 g (64%). 1H NMR
(300 MHz, CDCl3): d = 7.60 (s, 1H, ArHC@C), 7.43–7.41
(m, 2H, Har), 7.34–7.32 (m, 3H, Har), 7.29–7.19 (m,
10H, Har), 4.62–4.55 (m, 1H, CHHCH), 4.40–4.14 (m,
2H, CH2CH), 4.11–4.06 (m, 1H, CHHCH), 3.21–3.15
(m, 2H, CH2Ar), 2.78–2.67 (m, 2H, CH2Ar). 13C NMR
(75 MHz, CDCl3): d = 162.25, 160.83, 141.49, 137.74,
137.68, 133.93, 129.57–126.35, 118.44, 71.98, 71.61, 68.06,
20
733.94, 669.67 cmꢀ1. ½aꢁD ¼ 22:1 (c = 1, CHCl3). FAB-
MS m/z: 355.13 [M+H]+, HRMS (FAB) found,
355.2389; C22H31N2O2 requires 355.2386.
10c: Using the same procedure as described previously,
malonamide 9c (0.3 g, 0.64 mmol) was reacted with
methanesulfonyl chloride (0.18 g, 1.6 mmol) and dry
triethylamine (0.54 mL, 3.87 mmol) to give the (+)-bis[(S)-
4-phenyloxazoline-2-yl]-2-(4-chlorophenyl)ethene 10c as a
white semi-solid after purification by column chromatog-
raphy (silica gel, hexane:EtOAc (1:1)). Yield: 0.14 g
(51%). 1H NMR (300 MHz, CDCl3): d = 7.70 (s, 1H,
ArHC@C), 7.47 (d, J = 8.7 Hz, 2H, Har), 7.39–7.25 (m,
12H, HAr), 5.48–5.36 (m, 2H, CH2), 4.82–4.75 (m, 2H,
CH2), 4.30 (t, J = 8.4 Hz, 1H, CH), 4.22 (t, J = 8.3 Hz,
1H, CH); 13C NMR (75 MHz, CDCl3): d = 163.17,
161.51, 141.96, 141.59, 140.44, 135.81, 132.47, 130.68,
128.88, 128.68, 128.59, 127.57, 126.88, 126.68, 119.13,
68.00, 41.41, 40.93 ppm. IR (NaCl): tmax
: 3337.78,
3065.18, 3025.61, 3008.22, 2962.12, 2932.84, 1667.93,
1638.42, 1496.59, 1452.14, 1357.16, 1182.32, 1011.28,
20
962.91, 771.63, 701.76 cmꢀ1. ½aꢁD ¼ ꢀ10:79 (c = 0.38,
CHCl3). FAB-MS m/z: 423.07 [M+H]+. HRMS (FAB)
found, 423.2066; C28H27N2O2 requires 423.2073.
10a: Using the same procedure as described previously,
malonamide 9a (0.40 g, 0.93 mmol) was reacted with
methanesulfonyl chloride (0.27 g, 2.32 mmol) and dry tri-
ethylamine (0.8 mL, 5.57 mmol) to give the (+)-bis[(S)-4-
phenyloxazoline-2-yl]-2-phenylethene 10a as
a yellow
semi-solid after purification by column chromatography
(silica gel, EtOAc). Yield: 0.2 g (54%). 1H NMR
(300 MHz, CDCl3): d = 7.76 (s, 1H, ArHC@C), 7.54–7.53
(m, 2H, Har), 7.37–7.28 (m, 10H, Har), 7.25–7.23 (m,
3H, Har), 5.48–5.37 (m, 2H, CHAr), 4.83–4.75 (m, 2H,
CH2), 4.32–4.19 (m, 2H, CH2) ppm. 13C NMR
(100 MHz, CDCl3): d = 163.40, 161.84, 142.10, 141.84,
141.81, 134.00, 129.79, 129.45, 129.20, 128.76, 128.64,
128.58, 128.53, 127.79, 127.50, 127.43, 126.94, 126.82,
126.69, 118.50, 74.89, 74.88, 70.23, 70.12, 53.39, 37.51,
74.94, 74.91, 70.25, 70.08 ppm; IR (CHCl3) tmax
2976.57, 1671.34, 1636.87, 1520.87, 1492.31, 1356.21,
1244.89, 1044.89, 929.19, 848.57,
626.00 cmꢀ1
:
21
½aꢁD ¼ 16:2 (c = 0.58, CHCl3). FAB-MS m/z: 429.07
[M+H]+. HRMS (FAB) found, 429.1370; C26H22N2O2Cl
requires 429.1384.
10d: Using the same procedure as described previously,
malonamide 9d (0.2 g, 0.47 mmol) was reacted with
methanesulfonyl chloride (0.13 g, 21.17 mmol) and dry tri-

A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
4873
ethylamine (0.4 mL, 2.82 mmol) to give the (+)-bis[(S)-4-
tert-butyloxazoline-2-yl]-2-(4-chlorophenyl)ethene 10d as
a white semi-solid after purification by column chromatog-
raphy (silica gel, hexane:EtOAc (1:1)). Yield: 0.109 g
(60%). 1H NMR (400 MHz, CDCl3): d = 7.45 (s, 1H,
ArHC@C), 7.39 (d, J = 8.7 Hz, 2H, Har), 7.28 (d,
J = 8.8 Hz, 2H, Har), 4.31 (dd, J = 10.4, 8.6 Hz, 1H,
CH), 4.26–4.19 (m, 2H, CH2), 4.13 (t, J = 8.1 Hz, 1H,
CH), 4.03–3.98 (m, 2H, CH2), 0.95 (s, 9H, C(CH3)3), 0.88
(s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3):
d = 161.57, 160.15, 138.96, 135.39, 132.62, 130.63, 128.68,
119.70, 76.56, 76.49, 68.92, 68.62, 34.04, 33.90, 26.12,
25.80 ppm. IR (NaCl): tmax: 3329.61, 2963.04, 2909.13,
1736.84, 1673.79, 1633.30, 1483.82, 1402.05, 1364.06,
1249.46, 1190.29, 1092.14, 1016.36, 823.07, 775.96,
m/z: 385.17 [M+H]+. HRMS (FAB) found, 385.2491;
C23H33N2O3 requires 385.2490.
4.3. Formation of a [Cu(CH3CN)4][PF6]–Box-10a0
complex
[Cu(CH3CN)4][PF6] (47 mg, 0.126 mmol) was added to
a two-neck round-bottomed flask containing the ligand
10a0 (50 mg, 0.126 mmol) in CH2Cl2 (5 ml), the mixture
was stirred for 6 h at room temperature. The solvent was
removed under vacuo giving a dark blue solid. Yield:
0.043 g (75%); IR (KBr) tmax: 3417.46, 3031.57, 2924.21,
2362.18, 1647.04, 1592.19, 1489.38, 1453.39, 1386.53,
1238.81, 1063.68, 944.98, 839.45, 755.35, 697.48,
554.94 cmꢀ1. FAB-MS m/z: 474.94 (M+1).
20
669.16 cmꢀ1. ½aꢁD ¼ 45:75 (c = 0.37, CHCl3). FAB-MS
m/z: 389.12 [M+H]+. HRMS (FAB) found, 389.1996;
C22H30N2O2Cl requires 389.1996.
4.4. Cyclopropanation reactions
10e: Using the same procedure as described previously,
malonamide 9e (0.45 g, 0.98 mmol) was reacted with
methanesulfonyl chloride (0.28 g, 2.44 mmol) and dry tri-
ethylamine (1.36 ml, 5.86 mmol) to give the (ꢀ)-bis[(S)-4-
phenyloxazoline-2-yl]-2-(4-methoxyphenyl)ethene 10e as a
white semi-solid after purification by column chromatogra-
phy (silica gel, hexane:EtOAc (1:1)). Yield: 0.28 g (67%).
1H NMR (300 MHz, CDCl3): d = 7.70 (s, 1H, ArHC@C),
7.52 (d, J = 8.7 Hz, 2H, Har), 7.41–7.26 (m, 10H, Har),
6.87 (d, J = 8.7 Hz, 2H, Har), 5.47–5.41 (m, 2H, CH2),
4.86–4.75 (m, 2H, CH2), 4.33 (t, J = 8.4 Hz, 1H, CH),
4.21 (t, J = 8.2 Hz, 1H, CH) ppm; 13C NMR (100 MHz,
CDCl3): d = 164.63, 160.75, 142.36, 140.00, 128.96,
128.65, 127.86, 127.46, 126.58, 114.24, 112.53, 74.26,
69.90, 55.26 ppm. IR (CHCl3): tmax: 2975.91, 2927.43,
1667.12, 1608.51, 1516.87, 1424.21, 1043.45, 928.38,
4.4.1. Cyclopropanations using [Cu(OTf)]2(C6H6) and
[Cu(CH3CN)4]PF6 pre-catalysts
Catalyst (0.014 mmol, 1 mol%) or (0.028 mmol, 2 mol%)
was added to a two-neck round-bottomed flask containing
the chiral ligand (0.015 mmol, 1.1 mol%) or (0.030 mmol,
2.2 mol%) in CH2Cl2 (1 ml) and the solution was stirred
at room temperature for 15 min under a nitrogen atmo-
sphere. Alkene (14 mmol) and a solution of ethyl diazoac-
etate (0.159 g, 1.4 mmol) in CH2Cl2(1 ml) or toluene (1 ml)
was then added to the reaction mixture over a period of
16 h using a syringe pump. After the addition of ethyl dia-
zoacetate, the mixture was stirred for 16 h. The reaction
mixture was firstly passed through a short pad of silica
gel (washed with CH2Cl2) to remove the catalyst complex,
the products were then isolated by column chromatogra-
phy (hexane/EtOAc 9:1). All cyclopropane products were
obtained as a mixture of cis and trans diastereomers. Iso-
lated yields, diastereoselectivities, and enantioselectives
are given in Tables 2 and 3.
23
626.58 cmꢀ1. ½aꢁD ¼ ꢀ4:7 (c = 0.51, CHCl3). FAB-MS
m/z: 425.13 [M+H]+. HRMS (FAB) found, 425.1865;
C27H25N2O3 requires 425.1883.
10f: Using the same procedure as described previously,
malonamide 9f (0.40 g, 1.02 mmol) was reacted with
methanesulfonyl chloride (0.20 g, 1.78 mmol) and dry tri-
ethylamine (0.3 g, 0.71 mmol) to give the (+)-bis[(S)-4-
4.4.2. Cyclopropanations using Cu(I)SbF6 generated in situ
CuCl (4 mg, 0.04 mmol) was added to a two-neck round-
bottomed flask containing the ligand 10a0or 10d
(0.044 mmol) in CH2Cl2, the mixture was stirred for 1.5 h
at room temperature. AgSbF6 (27.7 mg, 0.08 mmol) was
added to the mixture and stirred for 3 h without light. The
mixture was transferred by cannula to another flask con-
taining CH2Cl2 (1 ml), and was added styrene (1.91 g,
18.4 mmol) and stirred for 0.5 h. A solution of ethyl dia-
zoacetate (0.210 g, 1.84 mmol) in CH2Cl2 (1 ml) was then
added to the reaction mixture over a period of 4 h using a
syringe pump. After the addition of ethyl diazoacetate, the
mixture was stirred for 16 h. The reaction mixture was
firstly passed through a short pad of silica gel (washed with
CH2Cl2) to remove the catalyst complex, the products were
then isolated by column chromatography (hexane/EtOAc
9:1). All cyclopropane products were obtained as a mixture
of cis and trans diastereomers. Reaction temperatures,
tert-butyloxazoline-2-yl]-2-(4-methoxyphenyl)ethene
10f
as a white semi-solid after purification by column chroma-
tography (silica gel, hexane:EtOAc (1:1)). Yield: 0.16 g
(59%). 1H NMR (400 MHz, CDCl3): d = 7.44 (s, 1H,
ArHC@C), 7.40 (d, J = 8.8 Hz, 2H, Har), 6.83
(d, J = 8.8 Hz, 2H, Har), 4.33 (dd, J = 10, 8 Hz, 2H,
CH2), 4.22 (t, J = 9 Hz, 1H, CH), 4.11 (t, J = 8 Hz, 1H,
CH), 4.06–3.96 (m, 2H, CH2), 3.78 (s, 3H, –OCH3), 0.96
(s, 9H, C(CH3)3), 0.88 (s, 9H, C(CH3)3) ppm. 13C NMR
(100 MHz, CDCl3): d = 162.01, 160.78, 160.62, 139.92,
131.25, 126.71, 116.43, 113.88, 76.48, 76.38, 68.76, 68.43,
55.20, 34.02, 33.89, 26.14, 25.78 ppm. IR (NaCl): tmax
:
3339.29, 2962.33, 2911.21, 1637.63, 1511.62, 1471.64,
1357.68, 1254.29, 1182.30, 1022.12, 830.30, 751.34,
20
668.48 cmꢀ1. ½aꢁD ¼ 63:0 (c = 0.50, CHCl3). FAB-MS

4874
A.J. Burke et al. / Journal of Organometallic Chemistry 692 (2007) 4863–4874
[5] B.R. Neustadt, E.M. Smith, T.L. Nechuta, A.A. Bronnenkant, M.F.
Haslanger, R.W. Watkins, C.J. Foster, E.J. Sybertz, J. Med. Chem.
37 (1994) 2461.
isolated total yields, diastereoselectivities, and enantio-
selectives are given in Table 2.
[6] (a) C. Lee, W. Yang, R. Parr, Phys. Rev. B 37 (1988) 785;
(b) A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[7] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S.
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14 (1993) 1347.
Acknowledgements
We thank the Fundac¸ao para a Cieˆncia e a Tecnologia
˜
and the Programma Opercional para Cieˆncia Tecnologia
[8] (a) A. Bergner, M. Dolg, W. Kuechle, H. Stoll, H. Preuss, Mol.
Phys. 80 (1993) 1431;
e Inovac¸ao (POCTI) for generous financial support in the
˜
form of a research grant (POCTI/QUI/38797/2001),
including a PhD grant to EPC (SFRH/BD/27768/2006)
which was partly funded by the European Community
fund FEDER. The personnel of the C.A.C.T.I (University
of Vigo, Spain) are acknowledged for analytical data.
M. Kaupp, P.v.R. Schleyer, H. Stoll, H. Preuss, J. Chem. Phys. 94
(1991) 1360;
(b) M. Dolg, H. Stoll, H. Preuss, R.M. Pitzer, J. Phys. Chem. 97
(1993) 5852.
[9] J.F. Rasmussen, N. Jensen, N. Østergaard, D. Tanner, T. Ziegler,
P.-O. Norrby, Chem. Eur. J. 8 (2002) 177.
[10] D.A. Evans, K.A. Woerpel, M.J. Scott, Angew. Chem. Int. Ed. Engl.
31 (1992) 430.
Appendix A. Supplementary material
[11] D.A. Evans, K.A. Woerpel, M.M. Hinman, M.M. Faul, J. Am.
Chem. Soc. 113 (1991) 726.
[12] D.A. Evans, C.S. Burgey, N.A. Paras, T. Vojkovsky, S.W. Tregay, J.
Am. Chem. Soc. 120 (1998) 5824.
[13] D.A. Evans, J.A. Murry, P. von Matt, R.D. Norcross, S. Miller, J.
Angew. Chem. Int. Ed. Engl. 34 (1995) 798.
[14] B.F. Straub, F. Rominger, P. Hofmann, Organometalics 19 (2000)
4305.
[15] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[16] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1988) 785.
[17] (a) P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270;
(b) P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 284;
(c) P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299;
(d) T.H. Dunning, P.J. Hay, in: H.F. SchaeferIII (Ed.), Modern
Theoretical Chemistry, vol. 3, Plenum, New York, 1976, p. 1.
Tables of total energy and cartesian coordinates for the
DFT-optimized geometries of the complexes discussed in
the text. Spectra and information related to the uv-vis
spectroscopic study of 10a0–Cu(CH3CN)6PF6 and for the
kinetics of EDA decomposition using this complex.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2007.06.068.
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