Mechanism of copper(I)-catalyzed cyclopropanation: A DFT study calibrated with copper(I) alkene complexes

Article abstract of DOI:10.1016/S0022-328X(03)00520-5

The complete catalytic cycle of copper-catalyzed cyclopropanation is investigated by density functional model calculations. The study focuses on obtaining accurate relative stabilities of the ligand-metal cores of all catalytic intermediates and transition states. The copper iminophosphanamide [H₂P(NH₂) ₂-κ²N]Cu serves as model fragment, since three copper catalyst key intermediates with an iminophosphanamide ligand have been isolated or detected. The electronic structure of the active intermediates, copper(I) carbenes of the d¹⁰-ML ₃ type, and their reactivity towards ethene and vinyl alcohol is investigated. The reliability of the computed data is confirmed by their comparison with experimental data of [t-Bu₂P(NSiMe ₃)₂-κ²N]Cu alkene and alkyne complexes, using relative ligand binding strengths and alkene ligand rotation barriers. The uncharged nature of the model complexes minimizes solvation artifacts and thus renders the evaluation of associative and dissociative ligand exchange pathways possible. The associative alkene ligand exchange with diazoalkane to a κN,κ O-diazoalkane complex, subsequent intramolecular rearrangement to a κ C-complex and N₂ extrusion are identified on the most probable pathway to the electrophilic copper(I) carbene complexes of the Fischer type. They are predicted to react with alkenes to labile cupracyclobutanes with a planar N₂Cu^III C₂ core, which undergo facile reductive elimination of a cyclopropane derivative.

Full text of DOI:10.1016/S0022-328X(03)00520-5

Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
www.elsevier.com/locate/jorganchem
Mechanism of copper(I)-catalyzed cyclopropanation: a DFT study
calibrated with copper(I) alkene complexes
a,b
Bernd F. Straub , Irene Gruber , Frank Rominger , Peter Hofmann *
a
a
a,
a
Organisch-Chemisches Institut der Universit ¨a t Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Department Chemie, Ludwigs-Maximilians-Universit ¨a t M ¨u nchen, Butenandtstr. 5-13 (Haus F), D-81377 Munchen, Germany
Received 26 March 2003; received in revised form 16 May 2003; accepted 16 May 2003
b
Dedicated to Professor Ernst Otto Fischer on the occasion of his 85th birthday
Abstract
The complete catalytic cycle of copper-catalyzed cyclopropanation is investigated by density functional model calculations. The
study focuses on obtaining accurate relative stabilities of the ligandꢀmetal cores of all catalytic intermediates and transition states.
k N]Cu serves as model fragment, since three copper catalyst key intermediates with
/
2
/
The copper iminophosphanamide [H P(NH ) ꢀ
2
2 2
an iminophosphanamide ligand have been isolated or detected. The electronic structure of the active intermediates, copper(I)
1
carbenes of the d -ML
0
3
type, and their reactivity towards ethene and vinyl alcohol is investigated. The reliability of the computed
2
data is confirmed by their comparison with experimental data of [t-Bu P(NSiMe ) ꢀ
/
k N]Cu alkene and alkyne complexes, using
2
3 2
relative ligand binding strengths and alkene ligand rotation barriers. The uncharged nature of the model complexes minimizes
solvation artifacts and thus renders the evaluation of associative and dissociative ligand exchange pathways possible. The associative
alkene ligand exchange with diazoalkane to a kN,kO-diazoalkane complex, subsequent intramolecular rearrangement to a kC-
complex and N extrusion are identified on the most probable pathway to the electrophilic copper(I) carbene complexes of the
2
III
2 2
Fischer type. They are predicted to react with alkenes to labile cupracyclobutanes with a planar N Cu C core, which undergo
facile reductive elimination of a cyclopropane derivative.
#
2003 Elsevier B.V. All rights reserved.
Keywords: Homogeneous catalysis; Carbene ligands; N ligands; Copper; Density functional calculations; Cyclopropanation; Copper carbenes
1
. Introduction
reduced in situ e.g. by addition of hydrazine or by
heating with the diazoalkane substrate [3,4]. The role
and structure of cupracyclobutanes were unclear [3].
Pfaltz rationalized the origin of the excellent enantios-
The mechanism of copper-catalyzed cyclopropana-
tion of alkenes by a-carbonyl diazoalkanes has been of
considerable interest for more than 35 years [1,2].
Nozaki et al. successfully applied the copper-catalyzed
cyclopropanation reaction as the first enantioselective
electivities obtained with C -symmetrically substituted
2
bidentate nitrogen ligands such as semicorrins. The
different steric repulsion of the acyl substituent at the
carbene with bulky groups of the spectator ligand in the
diastereomeric transition states is in accordance with the
observed enantioselectivities [4]. Kinetic investigations
by the group of P e´ rez ascertained the presence of a
homogeneous transition metal catalysis [2]. Initially,
9
copper(II) carbene species of the d -ML type were
5
assumed to be the cyclopropanating species [1]. How-
ever, the importance of copper(I) in the catalytic cycle
was verified by Aratani [3] and the Pfaltz co-workers [4].
Copper(II) complexes are inactive and have to be
preequilibrium before the rate-determining N loss of
2
the diazoalkane [5]. Copper(I) alkene complexes are the
resting states of the catalytic cycle [6,7]. Until 2000, they
were the only isolated or detected intermediates.
The synthesis of copper(I) alkene iminophosphana-
mide precursor complexes 1 and 2 [8] made possible the
isolation of a sterically stabilized copper diazoalkane
*
4-4885.
Corresponding author. Tel.: ꢁ
/
49-6221-54-8502; fax: ꢁ49-6221-
/
5
0
E-mail address: ph@phindigo.oci.uni-heidelberg.de (P. Hofmann).
022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00520-5

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
125
sphanamide fragment towards models of these sub-
strates, which are actually present in cyclopropanation
reactions.
2. Computational methods
For all DFT calculations, the empirically parame-
trized B3LYP method [15] within the GAUSSIAN-98
package was used [16]. The geometry optimizations
and vibrational frequency analyses were performed
using the Stuttgart/Dresden (SDD) multielectron-fit 19
valence electron effective core potential and basis set for
Cu [17] and the 6-31G(d) basis set for C, N, P, O and H
[18]. In addition, geometry optimizations and vibra-
tional frequency analyses were also performed with the
6-31G(d) basis set on all atoms.
All equilibrium structures have been characterized by
the lack of imaginary vibrations, and transition states
have been characterized by the presence of exactly one
imaginary vibration.
Fig. 1. Copper iminophosphanamide complexes 1ꢀ4.
/
complex 3 [9] and the detection of a reactive copper(I)
carbene species 4 (Fig. 1) [10]. This copper carbene
1
0
iminophosphanamide complex of the d -ML type has
3
been identified experimentally as the active species in the
cyclopropanation reaction.
The experimental progress towards unravelling the
mechanism of copper-catalyzed cyclopropanation [5,8ꢀ
0] has been accompanied by quantum-chemical inves-
tigations [11ꢀ14]. Recent theoretical studies by others
/
1
/
on cationic copper model systems are left with the
uncertainty of energetic consequences of solvation and
The SDD ECP and basis set with an additional diffuse
(a ꢂ
/
0.24) and an additional compact polarization f
f
counterion effects [12ꢀ
reliable model energies by the use of simplified models,
such as 5ꢀ8, of experimentally characterized species 1ꢀ
, thus benefiting from minimal solvation artifacts due
/
14]. Here we focus on obtaining
function (a ꢂ3.70) for Cu [17,19] and the 6-311ꢁ
/
/
f
G(2d,p) basis set [20] for C, N, P, O and H were used
for single-point energy calculations and NBO analyses
[21] of structures optimized with the SDD/6-31G(d)
basis set. Single-point energies and NBO analyses were
/
/
4
to the uncharged nature of these iminophosphanamide
complexes (Fig. 2).
also determined with the 6-311ꢁG(2d,p) basis set for
/
Our strategy here is to ensure the reliability of
computed energy barriers within the catalytic cycle by
comparison of experimental alkene rotation barriers in
solution and relative alkene copper binding strengths
with quantum-chemical gas phase data of the analogous
simplified copper iminophosphanamide systems.
As a model for a-carbonyl diazoalkanes such as
methyl-2-phenyl-2-diazoacetate [10], 2-diazoethanal is
used. We are well aware that the steric demand of t-
butyl, trimethylsilyl, phenyl, methoxy and benzo groups
may result in slower intermolecular reactions and
intramolecular rearrangements. Thus, we want to focus
on the inherent reactivity of the copper iminopho-
the structures optimized with the 6-31G(d) basis set. The
energies obtained with the latter method are given in
parentheses unless otherwise noted.
The single-point electronic energies were corrected to
free Gibbs energies at 298.15 K and 1 atm based on the
unscaled harmonic frequencies obtained with the [SDD/
6-31G(d)] or 6-31G(d) basis sets.
The quasirelativistic character of the SDD ECP
should not be of importance in copper chemistry, in
contrast to the chemistry of the heavier congeners silver
and in particular gold [22]. However, the SDD basis set
possesses a valence shell of triple-z quality. Geometries
and energies of interactions of copper with ligand atoms
should significantly benefit from this feature.
3. Discussion and results
First, we want to report new copper(I) iminopho-
sphanamide complexes. Their structures, their intramo-
lecular degenerate rearrangements and their ligand
exchange equilibria are tailor-made for the verification
of the suitability of the level of theory applied here.
Fig. 2. Simplified model complexes 5ꢀ8.
/

126
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
3.1. Syntheses and structures of copper(I) alkene and
alkyne complexes
The copper(I) maleic anhydride complex 9, the 1,4-
benzoquinone complex 10 and the styrene complex 11
were synthesized in one-pot procedures (Scheme 1). This
synthetic pathway has already been reported by us for
the ethene and norbornene analogue [8].
The maleic anhydride complex 9 has been fully
characterized, including a single crystal X-ray diffrac-
tion study (Fig. 3).
The dimethyl butynedioate ligand easily replaces
ethene in the corresponding complex 1, yielding the
alkyne complex 12 (Scheme 2). This high affinity for
copper(I) will be further discussed in Sections 3.3 and
Fig. 3. Molecular structure of 9 in the solid state. Hydrogens of the
iminophosphanamide ligand are omitted for clarity. Thermal ellipsoids
are drawn at the 50% probability level. Average values of the three
independent molecules with assumed C
(averaged non-crystallographic standard deviation): bond lengths (A)
PꢀN 1.613(5), SiꢀN 1.717(7), CuꢀN 1.966(10), CuꢀC 2.005(25), C1ꢀ
C2 1.435(44), C2ꢀC3 1.430(52), C1ꢀO1 1.409(18), C1ꢀO2 1.193(23).
Bond angles (8) PꢀN1ꢀSi1 146.9(1.6), N1ꢀPꢀN2 102.2(6), Pꢀ
N2 79.4(3). Dihedral angles (8) N1ꢀPꢀN2ꢀ
C2ꢀC3 174.2(6.0).
s
-symmetry in the unit cell
˚
/
/
/
/
/
3
.5.
In the solid state (Fig. 4), the Cꢁ
alkyne ligand in complex 12 remains very short (1.236(4)
/
/
/
/C bond length of the
/
/
/
/
/
Nꢀ
/
Cu
Cu
8
2
9.1(4), N1ꢀ
.1(2.4), N1ꢀCuꢀ
/
Cuꢀ
/
/
/
/
˚
A), indicating weak copper to alkyne backbonding.
/
/
/
1
3
1
In C{ H}-NMR spectra of complex 9 at different
temperatures, both the coalescence of the two doublets
of the diastereotopic quarternary t-Bu carbon atoms to
one doublet and the coalescence of the two singlets of
the t-Bu methyl carbons to one singlet are observed
3
.2. Alkene ligand rotation barriers
We have already reported the intramolecular degen-
erate rotation barrier of the norbornene ligand in
(
Fig. 6).
Comparable coalescence phenomena for the Pt-Bu
complex 6 [8]. Due to the local C2v symmetry of the
iminophosphanamide copper(I) moiety and the lower C
2
s
1
unit in the H-NMR spectra were also observed for the
1
symmetry of the copper alkene fragment, the H- and
%
1
3
1
C{ H}-NMR signals of the t-Bu groups display
1,4-benzoquinone and styrene complexes 10 (DG ꢂ
kJ mol ) and 11 (DG ꢂ
/48.5
ꢃ1
%
ꢃ1
/
37.0 kJ mol ). The C1
temperature-dependent behavior. Determination of the
coalescence temperature gives access to the Gibbs free
activation energy of the responsible molecular rearran-
gement (Scheme 3).
The coalescence of the pseudotriplet/quartet signal of
the t-Bu protons to a doublet in the maleic anhydride
1
complex 9 can be observed by VT- H-NMR spectro-
scopy (Fig. 5).
symmetry of the copper styrene moiety is the origin of
the additional non-equivalence of the trimethylsilyl
groups in the ground state geometry. The coalescence
temperature of the carbon and proton NMR signals of
the trimethylsilyl groups was also measured in this case.
In the 1,4-benzoquinone complex 10, an additional
fluxional process and an intermolecular equilibrium are
active. Kl a¨ ui co-workers has already reported these
processes for a copper(I) benzoquinone complex with
a tridentate ancillary ligand [23]. In their system, a
coalescence temperature of about 305 K corresponds to
ꢃ1
a Gibbs free activation barrier of about 60 kJ mol for
the exchange of the coordinated and the non-coordi-
nated alkene site of the 1,4-benzoquinone ligand. In our
benzoquinone system 10, a Gibbs free activation energy
ꢃ1
1
of 49.7 kJ mol
is derived from the H-NMR spectra
shown in Fig. 7. The higher barrier in the Kl a¨ ui system
can be rationalized by the necessity to break one
coordinative bond of the ancillary ligand in order to
Scheme 1. Syntheses of copper(I) alkene iminophosphanamide com-
plexes.
Scheme 2. Synthesis of alkyne complex 12.

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
127
Scheme 3. Topology of the intramolecular degenerate alkene ligand
rotation in complexes 2, 9, 10 and 11.
Thus, the observed facile alkene site exchange has to
%
2
2
Fig. 4. Molecular structure of [t-Bu
(CO Me) ] 12 in the solid state. Thermal ellipsoids are drawn at
the 50% probability level. Hydrogen atoms are omitted for clarity.
2 3 2
P(NSiMe ) /k N]Cu[h -
ꢀ
proceed intramolecularly via 10b (Scheme 5).
C
2
2
2
The alkene site exchange in complex 10 (Scheme 4 and
Fig. 7), however, will also lead to a coalescence of t-
butyl NMR signals. Since the t-Bu coalescence phenom-
ena and the alkene site exchange coalescence require
similar Gibbs activation barriers, the barrier of the 1,4-
benzoquinone ligand rotation has to be at least 48.5 kJ
˚
Bond lengths (A) Pꢀ
.711(2), CuꢀN 1.979(2) and 1.983(2), Cuꢀ
C21ꢀC24 1.236(4), C21ꢀC22 1.482(4), C24ꢀ
.18(1), C25ꢀO3 1.17(1), C22ꢀO2 1.329(4), C25ꢀ
C23 1.453(4), O4ꢀC26 1.468(5). Bond angles (8) PꢀNꢀ
47.5(1), N1ꢀPꢀN2 102.1(1), PꢀNꢀCu 89.5(1) and 89.8(1), N1ꢀ
N2 78.6(1), C21ꢀC24ꢀC25 149.9(3). Dihedral angle (8) N1ꢀPꢀN2ꢀ
.1.
/
N 1.611(2) and 1.615(2), Siꢀ
C 1.922(3) and 1.929(3),
C25 1.485(4), C22ꢀO1
O4 1.330(6), O2ꢀ
Si 146.3(1) and
Cuꢀ
Cu
/
N 1.710(2) and
1
/
/
/
/
/
/
1
/
/
/
/
/
/
/
1
/
/
/
/
/
/
ꢃ1
1
13
1
/
/
/
/
/
mol , as derived from the H and C{ H}-NMR
coalescence measurements of the t-Bu groups.
0
4
avoid a 20 valence electron copper(I) species in the h -
benzoquinone transition state.
3.3. Relative ligand bond strengths
The presence of a dinuclear m-benzoquinone complex
3 and free benzoquinone accounts for the small signals
1
Relative stabilities of copper(I) alkene iminophospha-
1
namide complexes were derived by integration of H-
1
in the H-NMR spectrum in Fig. 7. Since they do not
participate in the coalescence of the signals of the
mononuclear complex 10, intermolecular alkene site
exchange reactions via dinuclear species such as 13
possess comparably high Gibbs free activation energies.
NMR signals. Solutions of copper(I) alkene complexes
were mixed with a different free alkene. The resulting
equilibria contain four species: two free alkenes and two
copper(I) alkene complexes. The integrals of the alkene
proton signals were normalized with respect to the
number of protons in the alkene. The Gibbs free energy
differences DG were derived from the equilibrium
constants K in Eq. (1).
DGꢂꢃRT ln K
(1)
Scheme 5. Equilibrium of mononuclear complex 10 with dinuclear
complex 13 and 1,4-benzoquinone.
1
3
1
Fig. 6. C{ H}-NMR spectra of 9 in toluene-d
8
at 75.47 MHz:
coalescence phenomena of the signals of the quaternary carbons of t-
butyl groups and of the methyl singlets, respectively. The septet at dꢂ
0.4 is the carbon signal of the CD group of the toluene-d solvent.
1
Fig. 5. H-NMR spectra of 9 in toluene-d
/
8
at 300.13 MHz: coalescence
2
3
8

128
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
nic. Association and dissociation reactions are particu-
larly sensitive towards these solvation effects.
Furthermore, even weakly coordinating counterions
can bind to copper(I) complexes [24]. Cationic copper
complexes with achiral spectator ligands and an en-
antiopure chiral counterion were actually shown to yield
enantioenriched cyclopropanes [25]. Thus, uncharged
models such as the copper(I) iminophosphanamide
system provide the chance to minimize solvation arti-
facts when comparing computed gas phase energies with
experimental chemistry in hydrocarbon solutions. For
neutral systems, a deviation of the computed relative
4
Scheme 4. Alkene site exchange via a h -1,4-benzoquinone transition
state.
The ethene complex 1 plus norbornene is about (49
less stable than norbornene complex 2 plus
ethene. The maleic anhydride complex 9 is (13.69
/2)
ꢃ1
kJ mol
/3) kJ
ꢃ1
mol
less stable than norbornene complex 2. Accord-
ꢃ1
ingly, benzoquinone complex 10 is (12.29
/3) kJ mol
free energies of model systems in the gas phase from
ꢃ1
less stable than complex 2. Dimethyl butynedioate
complex 12 can be quantitatively synthesized from
ethene complex 1 and free alkyne ligand. Ethene is
rapidly evolved even at 1 bar external pressure, indicat-
ing a much higher relative thermodynamic stability of
the dimethyl butynedioate complex. For a comparison
of quantum-chemically derived binding energies and
experimentally observed relative copper alkene bond
strengths, see Section 3.5 and Tables 2 and 3.
solution chemistry data of less than 20 kJ mol
thus
appears to be possible. This is supported by the
comparison of predicted and observed degenerate re-
arrangement barriers for the norbornene complex 2 and
its model 6 and for the diazoalkane complex 3 and its
model 7 [8,9].
3.5. Comparison of theory and experiment: calibration of
computational data
The reliability of computed energies is of major
importance for the quantitative discussion of a catalytic
reaction pathway.
3
.4. Benefit of uncharged model complexes
Computed relative gas phase energies of ionic com-
The accuracy of computed relative energies of model
complexes compared to experimental data generally
suffers from several limitations. In order to ensure
only small energetic deviations between reality and
theory for hypothetical intermediates and transition
states, we compare experimental barriers of degenerate
rearrangements with their computed DFT model coun-
terparts. Furthermore, experimental and quantum-che-
mically derived relative bond strengths of alkene ligands
to copper(I) are compared.
plexes can differ significantly from the experimental
relative energies of the complexes in solution. The
coordination strength of ligands to cationic metal
fragments in the gas phase is larger than in solution,
since the positive charge is stabilized by the larger
available molecular volume. Additionally, donor sol-
vents will be able to coordinate to the metal center itself.
This point is of considerable importance for copper
cyclopropanation catalysts, since they are mostly catio-
Previously, we have confirmed the reliability of the
B3LYP/6-311ꢁG(2d,p)//B3LYP/6-31G(d) level of the-
/
I
III
ory in Cu /Cu chemistry by the good agreement of
structural parameters, relative energies, degenerate re-
arrangement barriers and IR frequencies for the dia-
zoalkane complex 3 and the computed data of its
2
simplified model [H P(NH ) ꢀ
/
k N]Cu[OC H N ꢀ
/
2
2 2
2
2
2
kO,kN] 7 [9].
The degenerate rotation of the norbornene ligand in
copper(I) complex 2 with a free Gibbs energy of
%
ꢃ1
activation of DG 99Kꢂ
/
41.5 kJ mol
[8] provides a
further test of the reliability of the quantum-chemical
1
methodology. The free Gibbs energy of activation of the
ꢃ1
model complex 6 (Fig. 8) is computed to 23.9 kJ mol
ꢃ1
(27.4 kJ mol ).
Essentially, the rotational barrier parallels the differ-
ence of the backbonding strength of the N Cu moiety
1
Fig. 7. H-NMR spectra of 10 in toluene-d
8
at 300.13 MHz:
2
coalescence phenomenon of the singlets of the non-coordinated and
coordinated alkene protons of the 1,4-benzoquinone ligand; additional
traces of free 1,4-benzoquinone at low field and presumably of a
dicopper(I) m-benzoquinone complex 13 at high field.
towards a coplanar or orthogonal alkene. The under-
ꢃ1
rating of the rotation barrier of about 12ꢀ18 kJ mol
/
indicates that the structural simplifications and the level

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
129
%
˚
2.053 A;
Fig. 8. Ball-and-stick models of norbornene complex 6 and transition state 6 . Relevant bond lengths for 6: d(Cuꢀ
/
N1)ꢂ
/
d(Cuꢀ
/
N2)ꢂ
/
%
˚
˚
˚
˚
d(Cuꢀ
/
C2)ꢂ
/
d(Cuꢀ
/
C3)ꢂ
/
2.036 A; d(C2ꢀ
2.082 A; d(C2ꢀ
/
C3)ꢂ
/
1.398 A. Relevant bond distances for 6 : d(Cuꢀ
/
N1)ꢂ
/
1.931 A; d(Cuꢀ
/N2)ꢂ
/
2.396 A; d(Cuꢀ
/
C2)ꢂ
/
˚
˚
˚
2.091 A; d(CuꢀC3)ꢂ
/
/
/
C3)ꢂ1.383 A.
/
of theory lead to an underestimation of the backbonding
capability of the copper fragment.
In the model system, the iminophosphanamide ligand
The backbonding capability of copper(I) computed for
our model systems appears to be too low, which might
also result in somewhat destabilized copper(III) model
species at this level of theory.
displays an interesting feature: the nitrogens of the
2
[
H P(NH) ꢀ
/
k N]Cu model systems are pyramidalized
The experimental Gibbs free energy difference of (49
/
2
2
ꢃ1
(
planar nitrogen substituent environment in the real [t-
1308B
/
d(NPNH)B
/
1408), in contrast to the nearly
2) kJ mol between ethene complex 1 plus norbornene
and norbornene complex 2 plus ethene is perfectly
reflected in the computed energy difference of the ethene
model complex 5 and the norbornene model complex 6
at triple-z basis set levels of theory (Tables 2 and 3). The
complexes 14 and 15 are computed to be the least stable
model complexes, again in accordance with experiment.
Therefore, the qualitative ligand to metal strength
ordering is correctly reproduced by the used level of
theory. The quantitative comparison between experi-
2
Bu P(NSiMe ) ꢀk N]Cu complexes. As a consequence,
/
2
3 2
two diastereomers e.g. for carbene model complex 8
ꢃ1
with a free Gibbs energy difference of 0.4 kJ mol (0.8
ꢃ1
kJ mol ) were found. No efforts were made to
calculate the diastereomers of all models relevant in
the computed reactions, since the energy differences of
the diastereomers should also be very small and
irrelevant for the experimental system with its trimethyl-
silyl groups and planarized nitrogen binding environ-
ment.
ꢃ1
ment and theory is accurate within 10 kJ mol , which
is very satisfactory for an isodesmic reaction involving a
transition metal.
We performed further calibrations using the alkene
rotation in the copper maleic anhydride complex 9 and
All comparisons of experimental and computed data
emphasize that systematic errors at the present level of
theory, solvation and electronic effects of the t-butyl
and trimethylsilyl groups result in deviations of relative
energies of computed models from experimental data of
1
,4-benzoquinone iminophosphanamide complex 10
and the intramolecular alkene site exchange in the latter
11a]. Furthermore, we compared the alkene and alkyne
[
copper(I) binding energies of the complexes 5, 6, 14, 15
and 16 (Fig. 9) with the relative binding capabilities of
their experimental counterparts 1, 2, 9, 10 and 12.
The data of both the experimental NMR investiga-
tions as well as the quantum-chemical data for the
simplified model complexes is depicted in Table 1.
The quantum-chemical model prediction of alkene
ligand rotation barriers is consistently underestimated.
ꢃ1
less than 20 kJ mol . Therefore, the following model
calculations for catalytic cyclopropanation should re-
flect reality in a reasonable fashion.
3.6. Mechanism of copper-catalyzed cyclopropanation
A complete cycle of copper-catalyzed cyclopropana-
tion is described. Various reaction pathways of a
copper(I) ethene iminophosphanamide model complex
with 2-diazoethanal yielding a copper(I) carbene species
are evaluated. The electronic structure of copper car-
1
0
benes of the d -ML type is characterized. Their
3
intrinsic reactivity towards alkenes and the stability of
cupracyclobutanes are of major interest for the under-
standing of the origin of enantioselectivities and dia-
stereoselectivities with enantiopure chiral chelating N-
ligands.
Fig. 9. Computed copper(I) alkene model complexes.

130
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
Table 1
Comparison of experimental Gibbs free activation energies of 11, 2, 10 and 9 with calculated ligand rotation barriers of model complexes 6, 15 and 14
Gibbs free rotation energy barriers for [Cu]ꢀ
/
L in (kJ molꢃ1
)
Experimental
Quantum-chemical model prediction
Difference
Lꢂ/styrene
37.09
/
2.0 (11)
ꢀ/
ꢀ/
Lꢂ
/
norbornene
41.59
/
2.0 (2)
23.9 (26.0) (6)
17.6 (15.5)
12.5(ꢀ12.2)
0.7 (2.3)
13.2 (12.4)
Lꢂ
/
1,4-benzoquinone (intramolecular alkene site exchange)
ꢀ/48.5 (10)
36.0 (36.3) (15)
49.0 (47.4)
ꢀ/
/
4
9.79
/
2.0
Lꢂ
/
maleic anhydride
53.79
/
2.0 (9)
40.5 (41.3) (14)
3
.6.1. Preequilibrium
Copper(I) alkene complexes are the resting states of
the catalytic cycle of copper-catalyzed cyclopropanation
6,7]. In the computed carbene complex formation
tive alkene diazoalkane ligand exchange mechanisms
were investigated in order to find the lowest-energy
pathway (Fig. 10).
[
In the first associative ligand replacement reaction,
the diazoalkane complex 7 can be formed from ethene
complex 5 and N CHCHO via transition state 17. The
pathway (Fig. 10), the ethene model complex 5 is indeed
the most stable species before the rate-determining N2
loss via transition state 23. Copper diazoalkane model
complex 7 is more stable than its isomers 20 and 22
2
second associative ethene diazoalkane exchange via
1
transition state 18 leads directly to the k C-diazoalkane
complex 22. In a dissociative pathway, the liberated 14
valence electron copper fragment coordinates to the
diazoalkane, yielding either one of the diazoalkane
complex isomers 7, 20 or 22.
(
Figs. 10ꢀ
/
12). Thus, the kN,kO coordination mode of
a-carbonyl diazoalkanes is the most favored for copper
iminophosphanamides. Indeed, the diazoalkane com-
plex 3 has been isolated and structurally characterized
[
9]. The steric inhibition of the N extrusion pathway
2
The formation of diazoalkane complex 7 can only be
of relevance for cyclopropanation, if a facile intramole-
cular rearrangement pathway to diazoalkane complex
and the thermodynamic stabilization due to the pro-
nounced diazoniophenolate character of the diazophe-
nanthrone ligand are responsible for the unusual
stability of 3 [9]. The existence of a preequilibrium
involving copper alkene complexes is reflected in the
rate-retarding effect of high alkene concentrations in
cyclopropanation catalysis [5,9]. Experimentally, an
equilibrium of ethene complex 1, diazoalkane, free
ethene and diazoalkane complex 3 has been confirmed
2
2 is accessible, from which N2 is liberated. The
2
rearrangement of diazoalkane complex 7 to the h -
carbonyl model complex 20 and subsequently to the
1
copper(I) k C diazoalkane complex 22 is indeed pre-
dicted to be facile.
The Gibbs ethene ligand dissociation energy in 5 is
ꢃ1
computed to be 86.1 (75.1) kJ mol , which is very
similar to the barrier for the associative exchange via 17
[
9].
(
underestimation of the backbonding capability of cop-
Figs. 10 and 11 and Table 4). However, the theoretical
3
.6.2. Associative versus dissociative ligand exchange
The reactivity of a-carbonyl diazoalkanes could be at
ꢃ1
per(I) of 12ꢀ18 kJ mol derived in Section 3.5 from the
/
least in part due to the role of the oxo functionality as
ligand atom in the copper carbene formation pathway.
The question whether the alkene diazoalkane ligand
exchange is associative or dissociative, is not relevant for
the explanation of the existence of a preequilibrium.
Nonetheless, the comparison of alternative pathways
would increase the general understanding of copper(I)
reaction mechanisms. Two associative and the dissocia-
rotation barriers in copper(I) alkene complexes indicates
that real copper alkene bond strengths could well be
somewhat higher than computed. Since the Gibbs free
energy of transition state 18 is predicted to be higher
than that of 17, the kN,kO exchange via 17 with a
subsequent rearrangement pathway appears to be the
most probable route for copper carbene complex
formation. The possibility of an intramolecular rearran-
Table 2
Computed copper alkene dissociation energies of model complexes 5, 6, 14, 15 and 16
5
6
14
15
16
6-31G(d)
Zero point correction
141.1 (250.2)
8.7 (10.8)
131.1 (264.5)
5.2 (7.5)
123.0 (264.7)
6.6 (7.9)
123.8 (260.8)
6.2 (7.4)
136.3 (276.8)
5.8 (6.8)
6
-311ꢁ
/
G(2d,p)
140.2 (133.5)
131.5 (122.7)
134.0 (125.6)
138.4 (132.5)
133.2 (125.0)
132.9 (125.4)
131.6 (128.4)
125.0 (120.5)
125.5 (121.1)
132.9 (128.8)
126.7 (121.4)
126.9 (121.7)
143.2 (139.6)
137.4 (132.8)
137.5 (133.1)
6-311ꢁ
/G(2d,p) zero point corrected
BDE (298.15 K, 1 atm)
Energies in (kJ molꢃ1).

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
131
Table 3
Observed relative copper alkene bond strengths of complexes 1, 2, 9, 10 and 12
1
2
9
10 12
Relative Gibbs free energies at ambient conditions, accuracy 2ꢀ
/
3 kJ molꢃ1
11.6
13.6
1.4
0
ꢁ/11.6
Energies in (kJ molꢃ1).
1
gement pathway to k C-diazoalkane complexes has
been previously analyzed by us [9]. In a computational
study on cationic copper species, kN,kO-diazoalkane
complexes such as 3 were assumed to be only inter-
mediates in an unproductive side-equilibrium [13].
However, the competing pathways were not considered
in this study.
copper center. In the gas phase, a copper ethene
diazoalkane complex analogous to structure 18 has
ꢃ1
been computed to be only 27.2 kJ mol
higher in
Gibbs free energy than free diazoalkane and a copper(I)
ethene N,N?-dimethyl malonaldiimine cation [13].
Long-range interactions of the copper center with the
diazo or carbonyl group appear to be only reproducible
with diffuse basis functions on copper. For transition
state 19 and diazoalkane complex 22 (Figs. 10 and 12),
the SDD ECP and basis set on copper have a distinct
stabilizing effect. For 22, a weak coordination of the
carbonyl group was obtained in the B3LYP/6-31G(d)
In the present work, the dissociative and one of the
two associative pathways come out with too similar free
activation barriers to make reliable general predictions
for copper(I) systems, in particular for cationic catalysts.
Increased steric demand of substituents in real systems
will inevitably lead to a destabilization of transition
states for associative ligand exchange reactions, while
the positive charge in cationic copper complexes should
favor associative pathways with a tetracoordinated
geometry optimization. With the Stuttgartꢀ
/
Dresden
basis set on copper, only the coordination of the diazo
1
0
carbon was found. Complex 22 is classified as a d -ML
3
type complex with an approximate Y-shape coordina-
Fig. 10. Computed formation pathway of copper(I) carbene model complex 8 from ethene complex 5. Gibbs free energies refer to B3LYP/SSD*ꢁ
/
,6-
3
11ꢁG(2d,p)//B3LYP/SDD,6-31G(d) (in parentheses B3LYP/6-311ꢁG(2d,p)//B3LYP/6-31G(d)).
/
/

132
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
Fig. 11. Ball-and-stick models of diazoalkane complex 7 and the transition states 17 and 18 for the computed associative ethene diazoalkane ligand
exchange.
2
3
1
Fig. 12. Ball-and-stick models of the transition state 19 for the h -carbonyl model complex 20, h -transition state 21 and the k C diazoalkane
complex 22.

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
133
Table 4
Relevant bond lengths (A) of the copper iminophosphanamide complexes 7, 8 and 17ꢀ
˚
/
23 [B3LYP/{SDD/6-31G(d)}]
7
8
17
18
19
20
21
22
23
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Oꢀ
d(C1ꢀ
d(C2ꢀ
d(N3ꢀ
d(C3ꢀ
/
N1)
N2)
N3)
C1)
C2)
C3)
C4)
O)
2.134
1.973
1.987
2.850
3.256
2.012
2.023
2.264
2.036
2.373
3.112
3.488
2.131
2.125
2.209
1.240
1.430
1.328
1.152
1.367
2.115
2.079
3.586
2.725
2.069
2.790
2.759
3.455
1.215
1.488
1.353
1.126
1.340
2.248
1.962
3.766
2.753
3.489
2.095
1.991
4.208
2.162
3.081
2.014
2.094
3.791
2.079
2.290
2.038
2.096
3.598
2.723
2.010
2.070
2.037
3.696
2.831
1.866
/
/
ꢀ/
2.923
/
/
1.794
/
ꢀ/
ꢀ/
1.914
1.266
1.394
1.358
1.187
ꢀ/
ꢀ/
3.582
1.218
1.464
ꢀ/
ꢀ/
1.935
1.251
1.428
1.322
1.132
ꢀ/
ꢀ/
1.920
1.277
1.455
1.313
1.137
ꢀ/
ꢀ/
2.391
1.245
1.471
1.325
1.133
ꢀ/
ꢀ/
3.478
1.213
1.494
1.360
1.126
ꢀ/
ꢀ/
3.643
1.214
1.475
1.778
1.117
/
/
/
C1)
/C2)
/N4)
ꢀ/
ꢀ/
ꢀ/
/
N4)
/C4)
ꢀ/
ꢀ/
ꢀ/
ꢀ/
ꢀ/
ꢀ/
tion geometry of the copper center (a(Nꢀ
/
Cuꢀ
/
C)ꢂ
/
of catalytically active copper(I) species. It is thus
understandable that experimentally hard bidentate or
tridentate s- and p-donor nitrogen ligands are best
suited to yield electron-rich copper(I) fragments with a
high backbonding capability and stability towards
disproportionation to metallic copper and copper(II).
The most important structural data of the model
complexes of the cyclopropanation cycle are shown in
Table 4.
1
1
37.08 and 146.58). The dative bond of the k C
ꢁ
diazoalkane ligand leads to a diazonio type RN ꢁ
/
N
˚
group (d(Nꢀ
/
N)ꢂ1.126 A). Previous classifications of
/
1
k C diazoalkane species as copper(III) complexes (T
8
shape, d -ML type) [14] are difficult to rationalize in
this context.
3
3
.6.3. Carbene complex formation
The N2 extrusion via transition state 23 has the
highest overall free Gibbs energy of activation of the
computed catalytic cycle (Figs. 10 and 13). The diazo
carbon in 23 needs a high backbonding capability of the
copper fragment in order to electronically stabilize the
empty p orbital of the carbene carbon. Accordingly,
copper complexes with p-acids such as phosphanes are
only suited as precatalysts for a-carbonyl diazoalkane
decomposition and cyclopropanation reactions. They
have to lose phosphane in order to yield catalytically
active species. The effective depopulation of the copper
3.6.4. Structure of copper carbenes
The copper carbon bond length in 8 is computed to be
˚
1.794 A, which is significantly shorter than in thiazole
derived carbene copper complexes in the solid state
˚
(1.868(6) and 1.888(6) A) [26] and in MP2 calculations
of WanzlickꢀArduengo carbene copper complexes
/
˚
(1.84ꢀ
/
1.87 A) [27]. The Cuꢀ
/
C bond is comparable to
˚
the previously computed 1.782 A in a cationic copper
carbene model complex [14]. The orthogonality of the
carbene moiety and the PN Cu fragment in the cop-
2
1
0
d
shell by p-acidic ligands reduces the backbonding
per(I) carbene complex 4 has been unambiguously
ascertained in a previous report, since the t-butyl
substituents at the phosphorus are diastereotopic at
the NMR time scale at room temperature [10]. In
accordance with this observation, the carbene model
strength to other ligands and thus hampers the forma-
tion of the strongly p-acidic carbene ligand from the s-
donor kC-diazoalkane ligand. Induction periods are
required for phosphane dissociation and the formation
2
Fig. 13. Ball-and-stick models of the transition state for N extrusion 23 and carbene complex 8.

134
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
complex 8 has nearly orthogonal carbene and PN Cu
2
3.6.5. Isomers of carbene complexes
fragments. Carbene complex 8, N and C H have a
2
Due to the capability of the carbonyl group to
coordinate to the copper center, isomers of the carbene
complex 8 are possible. Indeed, a copper(III) model
2
4
slightly negative free Gibbs energy of formation from
ethene complex 5 and 2-diazoethanal. From a purely
electronic point of view, the carbene complex formation
complex 24 with nearly coplanar PN
moieties is a local minimum (Figs. 14 and 15). Model 24
Cu and CuC O
2
2
ꢃ1
ꢃ1
is disfavored by 54.5 kJ mol
(49.9 kJ mol ). The
ꢃ1
ꢃ1
large difference between electronic and free Gibbs
energies is mainly due to the entropically favored N2
formation. Nevertheless, copper(I) fragments are excel-
lent leaving groups, both from a kinetic and from a
thermodynamic point of view. In accordance with the
exergonic carbene model complex formation, the experi-
mental preparation of the parent complex 4 in a steady-
state concentration from ethene complex 1 and methyl-
is less stable by DGꢂ/16.3 kJ mol
(17.1 kJ mol
)
than the carbene complex 8. A phenyl substituent at the
carbene as in the experimental complex 4 should further
mesomerically stabilize the carbene complex. Addition-
ally, a cupraoxacyclobutene analogous to 24 would be
destabilized by the steric repulsion of the trimethylsilyl
groups with the phenyl substituent compared to the
experimental carbene complex 4. This is in accordance
%
with the free Gibbs energy of activation of DG ꢀ
/
60 kJ
2
-phenyl-2-diazoacetate has been successful [10]. In the
ꢃ1
mol
10].
Interestingly, the free Gibbs energy difference at the
for carbene rotation in the carbene complex 4
real system 4, the mesomeric stabilization of the
electrophilic carbene by a phenyl substituent further
favors the formation of the carbene complex.
[
B3LYP/6-31G(d) level of theory favors copper(III)
model 24 with its higher copper coordination number
An NBO analysis of 8 reveals that copper(I) carbene
10
complexes of the d -ML type possess a true copper
3
ꢃ1
by 10.6 kJ mol , while the B3LYP/[SDD/6-31G(d)]
methodology predicts carbene complex 8 to be more
carbon double bond. The p-bond is mainly localized at
the copper fragment (74.6%) with 9.5% s and 90.4% d
character. The carbene carbon contributes mainly by its
p orbital (95.7%). The s-bond is a dative bond of the
lone pair of the singlet carbene fragment (82.6% carbon
character with 30.9% s and 69.1% p contribution) to the
copper fragment (17.4% copper character with 12.7% d
ꢃ1
stable by 20.1 kJ mol
*in accordance with the
/
relative single-point energies obtained with the more
flexible basis sets. Thus, the accurate prediction of
energies of some copper species and even the shape of
energy surfaces may significantly depend on the quality
of the basis set on copper. A triple-z quality basis set on
copper seems to be mandatory for a reasonable compu-
tational treatment of some copper species. Similar basis
set effects may also be relevant for the quantum-
chemical study of similar species in other copper-
catalyzed reactions such as the aziridination of alkenes
2
and 87.0% s contribution). The carbene carbon is sp
hybridized (CH bond 32.3% s and 67.6% p; CC bond
3
2.7% s and 67.0% p).
The asymmetry of the carbene fragment induces a
1
0
distortion of the Y-shape of the ideal d -ML geometry
3
and subsequently leads to sd mixing at the copper
center. This intrafragment polarisation is also the origin
of the significant copper s character in the p-bond. The
facile sd mixing of copper and the facile sp mixing at the
carbene carbon rationalize the fact of symmetry-allowed
by PhIꢂ
/
NTs [30,31].
3.6.6. Analogies to aziridination
The isolobality of acyl carbene and sulfonyl nitrene
ligands has led to the assumption that analogous nitrene
complexes could be the active species in copper-cata-
lyzed aziridination of alkenes. In a DFT study with
triple-z plus polarization quality basis set on copper,
Gillespie et al. have only found copper complexes A with
a monodentate nitrene ligand (Fig. 16) [30]. In contrast,
Brandt et al. found bidentate kN, kO sulfonyl nitrene
structures B throughout [31]. For geometry optimiza-
tion, they used a double-z quality basis set without
polarization or diffuse functions on copper. Even
though different spectator ligands were used in these
[
benes of the d -ML type.
2ꢁ2] addition reactions of alkenes with copper car-
/
1
0
3
An ab initio study of B o¨ hme and Frenking has
confirmed that the copper carbon bond in copper
complexes of WanzlickꢀArduengo carbenes, which
/
have weak or no p acceptor character has only little
backbonding character [27]. The low p-acidity of
¨
Ofele carbene ligands has been
Wanzlickꢀ
/
Arduengoꢀ
/
1
shown in Ref. [28]. However, copper carbenes of the d -
0
ML type such as 4 or model 8 possess a much stronger
3
copper carbon p-interaction.
In summary, copper(I) carbene complexes such as 4
or 8 should be classified as Fischer [29] carbenes. The
designation of the active species in copper-catalyzed
cyclopropanation as copper carbenoids, however, is
misleading. Copper ‘carbenoids’ do not only react as if
they were carbene complexes; they are carbene com-
plexes.
Fig. 14. Copper(I) carbene model complex 8 and isomeric copper(III)
vinyl alcoholate derivative 24.

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
135
carbene coordination mode in the real carbene complex
4
.
The small Nꢀ
/
Cuꢀ/N bite angle in copper iminopho-
sphanamides results in a particularly high backbonding
capability of the copper center [8], which strengthens the
p-bond to the methylene group. Upon rotation of the
methylene ligand, the capto-dative stabilization of the
four-membered PN Cu cycle disappears, and the ring
2
strain results in the formation of a carbene complex 26
1
of the d -ML type comparable to the copper(I)
0
2
complexes previously reported with Wanzlickꢀ
engo carbene ligands [26]. The CuC bond length of 1.788
/
Ardu-
Fig. 15. Ball-and-stick models of the copper(III) complex 24. Relevant
˚
computed bond lengths and angles for 24: d(Cuꢀ
/
N1)ꢂ
C2)ꢂ1.931 A;
CuꢀN2)ꢂ77.58;
/
2.058 A;
˚
˚
˚
˚
˚
A in 25 increases to 1.795 A in 26, although the
d(Cuꢀ
d(C1ꢀ
a(C2ꢀ
/
N2)ꢂ
C2)ꢂ1.345 A; d(C1ꢀ
CuꢀO)ꢂ70.48.
/
1.929 A; d(Cuꢀ
/
O)ꢂ
/
1.892 A; d(Cuꢀ
/
/
˚
˚
/
/
/
O)ꢂ1.341 A; a(N1ꢀ
/
/
/
/
coordination number of copper decreases. In 27, the
/
/
/
˚
copper carbon distance of 1.806 A is again slightly
larger.
Due to the flat energy surface, sterically demanding
substituents at the iminophosphanamide ancillary li-
gand may well lead to a ground state with a geometry
such as in stationary point 27 (Fig. 18 and Table 5).
studies, the contrasting findings indicate that analogous
basis set effects could also be present in copper nitrene
chemistry.
3
.6.7. Carbene ligand rotation
The rotation barriers of carbene and alkene ligands
3.6.8. Cyclopropanation
around CuN2 fragments reflect the strength of the
backbonding character. Such data is of relevance for
the detailed understanding of the copper carbon double
bond.
The CꢀC bond formation reaction from carbene
/
complex 8 to the copper complex 29 is highly exergonic,
reflecting the weakness of the copper carbene double
bond compared to carbonꢀ
In complex 29, one of the two carbonꢀ
has already been formed. The highly delocalized species
9 can either be viewed as a copper(I) complex with
strong back-donation into a cyclopropane CꢀC s*
/
carbon bonds (Fig. 19).
The intrinsic barrier for carbene rotation at an N Cu
2
/carbon bonds
fragment is difficult to derive from data of an acyl
substituted carbene model complex such as 8. The
possibility of oxygen coordination to the copper center
and formation of copper(III) complexes such as 24
interferes with the rotation of the carbene. Thus, the
rotation of a methylene ligand in model complex 25 was
investigated (Figs. 17 and 18).
2
/
molecular orbital and the p* orbital of the carbonyl
moiety, or as an ylidic 4-cupronio(III)-1-buten-1-olate
with an intramolecular coordination of the partial
carbonꢀcarbon double bond. The Gibbs free energy of
/
ꢃ1
A transition state 26 for the rotation of the methylene
activation via transition state 28 of 54.8 kJ mol (59.0
ꢃ1
ꢃ1
ꢃ1
group is found to lie 35.9 kJ mol
higher in energy than equilibrium structure 25 (Fig. 17).
The C -symmetric structure 27 is only a third-order
(37.1 kJ mol
)
kJ mol ) has no inherent electronic origin, since the
ꢃ1
electronic barrier only amounts to 3.8 kJ mol
ꢃ1
(4.8 kJ
mol ). Zero point corrections and entropic contribu-
tions result in this significant difference between
2
[
3
SDD/6-31G(d)] or a second-order stationary point [6-
ꢃ1
1G(d)] with a 48.5 kJ molꢃ1 (47.7 kJ mol ) higher
free Gibbs energy. The methylene rotation barrier is
%
tot
%
DE and DG .
An alkene with an electron-donating substituent such
as vinyl alcohol as a model of enol ethers has an even
significantly lower than the experimental lower limit of
ꢃ1
6
0 kJ mol
10]. However, an analogous underestimation of the
for carbene ligand rotation in complex 4
lower Gibbs free activation energy for the Cꢀ
/
C bond
formation reaction. Transition state 33 has a 43.4 kJ
[
electronic ligand rotation barrier as in the norbornene
ꢃ1
ꢃ1
mol
(43.6 kJ mol ) higher Gibbs free energy than
model complex 6 is probable. Furthermore, the steric
2
carbene complex 8 and vinyl alcohol (Fig. 20). The high
reactivity of electron-rich alkenes in copper-catalyzed
ꢃ
demand of the [t-Bu P(NSiMe ) ꢀk N] ligand system
/
2
3 2
should contribute to the destabilization of the coplanar
Fig. 16. Proposed monodentate and kN, kO sulfonyl nitrene inter-
mediates in copper-catalyzed aziridination of alkenes [30,31].
Fig. 17. Copper(I) carbene model complex 25, transition state 26 for
carbene rotation and C -symmetrical higher-order stationary point 27.
2

136
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
2
Fig. 18. Ball-and-stick models of the carbene complex 25, the transition state for carbene rotation 26 and the C symmetric stationary point 27.
cyclopropanation is in accordance with the lowering of
the activation barrier for vinyl alcohol compared to
ethene (Table 6).
The electrophilic character of the carbene complex in
the transition state 28 is reflected in the NBO net charge
ligand (Fig. 21). Cupracyclobutane model 31 undergoes
extremely facile reductive elimination to formyl cyclo-
propane and the copper(I) fragment 32. Thus, it is
probable that cyclopropanation reactions with sterically
demanding semicorrin, [4] bisoxazolylmethane [32] or
iminophosphanamide [8,10] ligands result in the direct
formation of cyclopropane derivatives without cupracy-
clobutane intermediates. Less electron-donating specta-
tor ligands destabilize the oxidation state III of copper.
Accordingly, a cationic copper carbene model complex
was reported to prefer direct cyclopropanation in the
gas phase, [14] avoiding the high oxidation state III on
copper.
Steric shielding of the copper center will further
increase the activation barrier for the reaction of copper
carbenes with alkenes. The successful detection of
carbene complex 4 in the presence of ethene underlines
the role of sterically demanding spectator ligands. Even
addition of an excess of styrene to a solution with a
steady-state concentration of complex 4 does not result
in an immediate disappearance of the violet color of the
carbene complex, but in a slow fading of the color
within few minutes [10,11a].
transfer of ꢃ0.14 of the ethene nucleophile to the
/
carbene. In accordance with the formal oxidation of
the copper(I) carbene 8 to the copper(III) complex 31,
the direction of the charge transfer changes in the course
of the reaction. As expected, the charge transfer is even
more pronounced in transition state 31 with its electron-
rich vinyl alcohol as a nucleophile (Table 7).
Model complex 29 and transition state 30 could only
be localized at the B3LYP/[SDD/6-31G(d)] level of
theory. Upon B3LYP/6-31G(d) geometry-optimization
of structure 29, cupracyclobutane 31 is directly formed.
Again, the importance of the basis set quality on copper
is apparent.
The shallow nature of the reaction energy surface
around the structures 29ꢀ32 makes these complexes
/
very sensitive towards steric repulsion of the spectator
Table 5
˚
Relevant bond lengths (A) and bond angles (8) of the copper
iminophosphanamide complexes 25, 26 and 27 [B3LYP/{SDD/6-
3.6.9. Comparison with the Pfaltz model
The transition state 30 has essentially the same
31G(d)}]
2
5
26
27
structure of the copper carbene alkene backbone as in
the model put forward by the Pfaltz co-workers in order
to rationalize the origin of the enantioselectivities
Bond length
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
/
N1)
N2)
C)
2.031
2.031
1.788
2.570
1.875
1.795
2.090
2.090
1.806
obtained with their C -symmetric semicorrin system
2
/
/
[4]. In their model, the orthogonal carbene and N Cu
2
moieties of the disfavored transition state lead to a
repulsive interaction of the alkoxy carbonyl substituent
of the carbene with a sterically demanding substituent of
Bond angles
a(N2ꢀCuꢀC)
/
/
141.9
169.2
143.2

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
137
Fig. 19. Computed pathway of alkene cyclopropanation by copper(I) carbene model complex 8. Gibbs free energies refer to B3LYP/SSD*ꢁ
/
,6-311ꢁ
/
G(2d,p)//B3LYP/SDD,6-31G(d) (in parentheses B3LYP/6-311ꢁG(2d,p)//B3LYP/6-31G(d)).
/
the spectator ligand in one quadrant. In the model
structures 29ꢀ31 (Fig. 21), the acyl group is indeed
strongly bent towards the spectator ligand. Thus, a large
energy difference of the diastereomeric transition states
with chiral spectator ligands can be caused by the
increasing steric repulsion of acyl and spectator ligand
The copper carbon double bond character in the
copper carbenes 8 and 25 is reflected in high carbene
bond strengths. A Gibbs free energy for carbene
/
ꢃ1
ꢃ1
dissociation of 195.4 kJ mol
(187.9 kJ mol ) in
model 8 clearly demonstrates that the generation of free
carbenes from copper carbene complexes is no alter-
native mechanistic pathway in copper-catalyzed cyclo-
propanation. MP2 and CCSD(T) computations of CuCl
in the course of the Cꢀ/C bond formation process.
3
.6.10. Absolute carbene dissociation energies
The bond strengths of ethene, singlet formyl carbene
Wanzlickꢀ/Arduengo carbene complexes revealed dis-
and singlet methylene towards the copper iminopho-
sphanamide fragment 32 in the model complexes 5, 8
and 25 were computed (Table 8).
Table 6
˚
Relevant bond lengths (A) and bond angles (8) of the copper
iminophosphanamide complexes 28ꢀ
/
31 and 33 [B3LYP/{SDD/6-
3
1G(d)}]
The ethene bond strengths to the uncharged copper
model fragment are weaker than the previously com-
28
29
30
31
33
ꢁ
puted bond strengths in [Cu(C H )] of 184ꢀ238 kJ
/
2
4
ꢃ1
2
ꢁ
Bond length
mol
an ethene free dissociation energy of 143 kJ mol
[33]. In [{H C(CHꢂ
/
NMe)2ꢀk N}Cu(C H )] ,
/
2
2
4
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(Cuꢀ
d(C1ꢀ
d(C1ꢀ
d(C2ꢀ
d(C2ꢀ
d(C3ꢀ
/
N1)
N2)
C1)
C2)
C3)
2.030
1.976
2.852
1.796
2.796
1.220
1.471
2.226
2.505
1.359
2.163
1.992
2.618
2.217
2.201
1.214
1.494
1.635
1.515
1.488
2.106
2.028
2.624
2.105
2.073
1.214
1.491
1.777
1.510
1.489
2.027
2.023
2.690
1.984
1.951
1.219
1.476
2.196
1.525
1.512
2.094
2.034
2.894
1.841
2.966
1.223
1.471
2.269
2.664
1.361
ꢃ1
has
/
been computed [13]. The positive charge in these
complexes significantly strengthens the ethene coordina-
tion to copper(I) fragments in the gas phase.
/
/
/
/O)
/
C2)
C3)
C4)
C4)
/
/
/
Bond angles
a(N2ꢀ
a(N1ꢀ
a(Cuꢀ
/
Cuꢀ
Cuꢀ
C2ꢀ
/
C2)
143.5
87.6
121.3
133.6
107.4
87.4
122.2
111.6
92.2
108.5
107.4
101.0
149.0
86.1
121.4
/
/C3)
Fig. 20. Model transition states 28 and 33 for the reaction of the
electrophilic copper(I) carbene complex 8 with ethene and vinyl
alcohol.
/
/C1)

138
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
Fig. 21. Ball-and-stick models of the transition state for ethene cyclopropanation 28, cyclopropanation intermediate 29, transition state 30 and
cupracyclobutane 31.
sociation energies of 282 and 319 kJ molꢃ1, [27] which
are very similar to our D values of 257.9ꢀ312.4 kJ
calculations [12ꢀ14,34]. Several similarities exist for
/
/
the mechanism of the copper(I) and palladium(0)
catalytic cycle. For both metals, the catalyst resting
e
ꢃ1
mol
.
1
0
state is a d alkene complex and the reactive inter-
In the cyclopropanation reaction of alkenes with
copper carbenes, the alkene and the copper carbene
1
0
mediate is a d carbene complex. The rate-determining
step is the N2 elimination from a kC-diazoalkane
complex, respectively (Scheme 6).
double bond disappear and three strained carbonꢀ
/
carbon single bonds are formed. Though one bonding
interaction is formally lost, the cyclopropanation reac-
tion is exergonic due to the relative weakness of the
Whereas copper(I) fragments bind stronger to elec-
tron-rich donor alkenes than to electron-deficient al-
kenes, the opposite is true for palladium(0) fragments.
Thus, the palladium(0) cyclopropanation catalyst can be
inhibited by maleic esters, [35] while the activity of
copper(I) catalysts is insensitive towards such alkenes.
In contrast to copper(I), palladium(0) binds stronger to
copperꢀ
/
carbon double bond compared to carbonꢀ
/
carbon single bonds.
3
d
.6.11. Comparison of cyclopropanation mechanisms via
0
1
transition metal carbene intermediates
Both for copper(I) and palladium(0) cyclopropana-
tion catalysis, mechanistic insight is based upon experi-
Table 8
Electronic bond strengths corrected by zero point energies and free
mental investigations and quantum-chemical model
1 1
CHCHO and CH
Gibbs energies of dissociation of C
ꢃ1
2
H
4
,
2
in kJ
mol
Table 7
NBO charges in ethene, 8, 28, 29, 30, 31, vinyl alcohol and 33.
a
b
c
D
e
D
0
BDE
DG298.15K
Cu
Carbene C
Alkene fragment
50
/
32ꢁ
/
C
2
H
4
140.2
133.5)
261.4
131.8
(122.7)
251.5
134.0
(125.6)
252.8
86.1 (75.1)
(
C
2
H
4
ꢀ/
ꢀ/
0
80
/
195.4
1
8
0.99
0.94
0.85
0.90
1.05
ꢀ
/
0.45
0.47
0.39
0.39
0.45
ꢀ/
32ꢁ
/
CHCHO
1
(257.9)
312.4
(310.4)
(246.6)
291.9
(288.1)
(248.0)
296.9
(293.4)
(187.9)
251.4
(245.4)
28
29
30
31
ꢃ
/
0.14
0.08
0.02
0.14
0
250
/32ꢁ CH
2
/
ꢃ
/
ꢃ
/
ꢃ/
ꢃ/
All electron basis set energies in parentheses.
a
C
2
H
3
OH
ꢀ/
0.95
ꢀ/
Difference of total electronic energies.
Including zero point vibrational energy correction.
b
33
ꢃ
/
0.49
0.16
c
BDEꢂDH (298.15 K, 1 atm).
/

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
139
Cartesian coordinates for structures in Tables 9ꢀ
/11 can
be obtained from authors.
The N2 substitution in diazoalkanes by copper(I)
fragments can be viewed as umpolung reaction. The
nucleophilic diazo carbon is converted into an electro-
philic center of a Fischer-type carbene complex.
5. Experimental
Scheme 6. Initializing mechanistic steps of alkene cyclopropanation by
metal fragments.
10
d
5.1. General
an acceptor-substituted alkene substrate than to chelat-
ing N-ligands [34,36]. As a consequence, chiral N-
ligands induce high enantioselectivities in copper-cata-
lyzed alkene cyclopropanation. Analogous ligands in
palladium-catalyzed cyclopropanation lead to 0% ee,
since alkene substrate ligands replace the nitrogen
ligands in catalytically active palladium(0) species.
The most important difference between the copper(I)
and the palladium(0) mechanism is the actual cyclopro-
panation sequence itself. The reactive copper(I)carbene
intermediate undergoes a one-step, intermolecular, di-
rect carbene fragment transfer reaction with the elec-
tron-rich alkene substrate to yield the cyclopropane
product. The analogous palladium dialkene carbene
intermediate, however, needs two steps: first, an intra-
molecular ‘cis-rearrangement’ leads to the formation of
a monoalkene pallada(II)cyclobutane. In the second
step, a reductive elimination produces the cyclopropane
product. The larger palladium atom and the lack of
sterically demanding ancillary ligands enable the forma-
tion of transient pallada(II)cyclobutanes in this reaction.
The starting material t-Bu P[N(SiMe )]NH(SiMe )
2
3
3
was synthesized according to a literature procedure
37]. Benzoquinone was sublimed prior to use. Deuter-
ated solvents were dried and distilled prior to use.
[
5
.2. Synthesis of maleic anhydride complex 9
Under an argon atmosphere, ca. 2.5ꢀ3 ml of 1.6 M n-
/
BuLi in hexane (4.5 mmol) was added to 1.2 g (3.75
mmol) t-Bu P[N(SiMe )]NH(SiMe ) in 10 ml of hexane.
The solution was heated to reflux temperature for 1 min,
2
3
3
then added to a suspension of 4.97 g (24 mmol) CuBr×
/
SMe in 10 ml of hexane at 195 K and stirred while
2
warming to ambient temperature. A copper mirror on
the wall of the glass can sometimes be formed. 0.4 g (4.1
mmol) maleic anhydride was added. After stirring for 90
min at ambient temperature, the suspension was filtered
through Celite. The Celite was washed once with
hexane. The solution was concentrated in vacuo. Yellow
crystals precipitated. From the orangeꢀ
crude 9 was crystallized at ꢃ788C. Recrystallization
yielded 613 mg (68%), m.p.: 84 8C, 89ꢃ
/yellow solution
/
1
/
90 8C dec. H-
NMR (298 K, toluene-d , 300.13 MHz): d 4.24 (s,
8
1
3
CH; 2H), 1.03 (d, JPH
JCH
JCH
ꢂ
/
181 Hz, ꢂ
/
ꢂ14 Hz,
/
1
1
4
. Conclusions and summary
ꢂ
/
129 Hz, C(CH ) ; 18H), 0.21 (sꢁ
/
sat., JCHꢂ
/
3 3
1
3
1
1
20 Hz, Si(CH ) ; 18H). C{ H}-NMR (298 K, tolu-
3 3
The generally observed preequilibrium in copper-
ene-d , 75.47 MHz): d 165.3 (s, CO), 70.9 (s, ꢂ
/
CH), 37.7
3
8
1
catalyzed cyclopropanation of alkenes is due to an
equilibrium of copper diazoalkane complexes, copper
alkene complex and free diazoalkane and alkene ligand.
Diazoalkane alkene ligand exchange pathways, which
are faster than the N2 loss and carbene complex
formation, have been found computationally.
All computed intermediates and transition states of
the catalytic cycle are closed-shell diamagnetic copper(I)
or copper(III) complexes, in accordance with experi-
mental data of observed or isolated intermediates. A
high backbonding capability of the copper fragment is
mandatory to stabilize the active copper carbene species.
Electron-rich chelating nitrogen ligands indeed lead to a
high backbonding capability of copper(I) and thus result
in thermodynamically stable copper alkene complex
resting states. Despite their thermodynamic stability,
these alkene complexes are kinetically highly active.
(d, JCPꢂ
JPC
300.13 MHz): C(CH ) , T ꢂ
/
60 Hz, C(CH ) ), 27.7 (s, C(CH ) ), 5.63 (d,
3 3 3 3
1
ꢂ
/
3 Hz, Si(CH ) ). VT { H}-NMR (toluene-d ,
3 3
8
/
(2519
/
4.0) K, k ꢂ
/13.5
3
3
c
c
1
3
1
Hz. VT
C{ H}-NMR (toluene-d , 75.47 MHz):
8
C(CH ) , T ꢂ
/
(2599
/
4.0) K, k ꢂ
/
39 Hz; C(CH3)3,
79 Hz; temperature calibration
3 3
c
c
T ꢂ
/
(267.59
/
4.0) K, k ꢂ
/
c
c
2
by external MeOH. Si{ H}-NMR DEPT (298 K,
9
1
2
sat., JPsi
toluene-d , 59.63 MHz): d ꢃ
/
6.5 (dꢁ
/
ꢂ11
/
8
1
31
43 Hz). P{ H}-NMR (298 K, toluene-d ,
1
Hz, JCSiꢂ
/
8
ꢃ1
121.50 MHz): d 75.9 (s). IR (KBr pellet, cm ): 2951
(s), 2900 (m), 1848 (w), 1834 (m), 1775 (s), 1570 (s), 1478
(m), 1395 (m), 1348 (s), 1250 (d), 1227 (m), 1105 (s),
1050 (w), 922 (w), 892 (m), 838 (vs), 759 (m), 699 (m),
670 (m), 643 (m), 625 (m), 481 (m). UV (pentane, nm):
207, 295. Calc. for C H N CuO PSi : C, 44.93; H,
1
8
38
2
3
2
7.96; N, 5.82; P, 6.44. Found: C, 44.93; H, 8.06; N, 6.05;
P, 6.40%.

140
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
Table 9
Computed energies of the model compounds 5ꢀ
/
22 at the B3LYP/[SDDꢁ
/
*/6-311ꢁ
/
G(2d,p)]//B3LYP/[SDD/6-31G(d)] level of theory (a.u.)
E[SDD/6-
1G(d)]
Single-point electronic energy E[SDD/6-311ꢁ
G(2d,p)]
/
Gibbs free energy correc- G[SDD/6-
tion
G[SDD/6-311ꢁ
G(2d,p)]
/
3
31G(d)]
5
6
6
7
8
8
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
ꢃ
/
729.220085
923.356235
923.344701
912.700690
803.154874
803.154152
ꢃ
/
729.345852
923.532889
923.523023
912.882129
803.304120
803.303643
0.070664
0.166170
0.165398
0.058688
0.046659
0.046318
0.068792
0.067224
0.096703
0.094915
0.096078
0.057047
0.104744
0.097972
0.052939
0.055089
0.055037
0.052302
0.051591
0.050504
0.042476
0.040048
0.040708
0.095720
0.100561
0.101803
0.103322
0.020366
0.099159
ꢃ
/
729.149421
923.190065
923.177691
912.642002
803.108216
803.107834
ꢃ
/
729.275188
923.366719
923.357625
912.823441
803.257461
803.257325
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
%
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
a
b
4
4
5
5
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
1029.915298
1029.897253
1032.077722
1032.060899
1032.057957
ꢃ
/
1030.139189
1030.122181
1032.297776
1032.282258
1032.278494
ꢃ
/
1029.846505
1029.830029
1031.981019
1031.965984
1031.961879
ꢃ
/
1030.070397
1030.054957
1032.201073
1032.187343
1032.182416
%
%
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
%
5b
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
ꢃ/
ꢃ/
ꢃ/
ꢃ/
ꢃ
/
954.594858
991.280046
991.265252
912.678412
912.683723
912.668940
912.676743
912.660036
803.151088
689.832180
689.815927
689.811591
881.742405
881.807990
881.807290
881.814054
650.578891
956.962433
ꢃ
/
954.795679
991.489861
991.477668
912.861212
912.868054
912.853787
912.863092
912.846088
803.301750
689.942308
689.926191
689.922081
881.920198
881.982177
881.981713
881.988790
650.674917
957.175485
ꢃ
/
954.537811
991.175302
991.167280
912.625473
912.628634
912.613903
912.624441
912.608446
803.100583
689.789704
689.775879
689.770883
881.646685
881.707430
881.705487
881.710732
650.558525
956.863274
ꢃ
/
954.738632
991.385117
991.378708
912.808273
912.812965
912.798750
912.810790
912.794497
803.251246
689.899832
689.886143
689.881373
881.824478
881.881616
881.879910
881.885468
650.654551
957.076326
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ/
ꢃ/
ꢃ/
ꢃ/
1
5
.3. Synthesis of benzoquinone complex 10
Under an argon atmosphere ca. 2.5ꢀ3 ml of 1.6 M n-
BuLi in hexane (ca. 4.5 mmol) was added to 1.2 g (18.0
138.0 (s, ꢂ
/
CH), 78.4 (s, CuCH), 38.0 (d, JCPꢂ
/
59 Hz,
62 Hz, C(CH ) ), 28.1 (s,
1
C(CH ) ), 37.0 (d, JCP
ꢂ
/
3
3
3 3
3
C(CH ) ), 26.8 (s, C(CH ) ), 5.5 (d, JPC
/
ꢂ2 Hz,
/
3
3
3 3
29
1
1
JCH
ꢂ57 Hz, Si(CH3)3). Si{ H}-NMR DEPT (298
/
2
mmol) t-Bu P[N(SiMe )]NH(SiMe ) in 10 ml of hexane.
2
K, toluene-d , 59.63 MHz): d ꢃ
/
8.2 (dꢁ
/
sat., JPsiꢂ12
/
3
3
8
1
31
1
57 Hz). P{ H}-NMR (298 K, toluene-d ,
The solution was heated to reflux temperature for 1 min,
then added to a suspension of 1.0 g (4.9 mmol) CuBr×
SMe in 10 ml of hexane at 195 K and stirred while
warming to ambient temperature. 0.44 g (4.1 mmol) 1,4-
benzoquinone was added. The suspension was filtered
Hz, JCSiꢂ
/
8
1
121.50 MHz): d 73.6 (s). VT { H}-NMR (toluene-d ,
/
8
300.13 MHz): CH, T ꢂ
/
(2679
/
4.0) K, k ꢂ
/
466 Hz. VT
C{ H}-NMR (toluene-d , 75.47 MHz): C(CH ) ,
2
c
c
1
3
1
8
3 3
T ꢂ
/
(2419
/
4.0) K, k ꢂ
/
71 Hz; C(CH ) , T ꢂ
/
(2449
/
c
c
3 3
c
through Celite. From the redꢀ
was isolated by removing the solvent in vacuo. Excess of
,4-benzoquinone was removed at 10ꢃ3 mbar. After
/
orange solution crude 10
4.0) K, k ꢂ
MeOH.
/
102 Hz; temperature calibration by external
c
1
dissolving 10 in hexane and filtration, attempts to
crystallize 10 failed. The metallic golden material thus
5.4. Synthesis of styrene complex 11
1
has not yet been purified. H-NMR (298 K, toluene-d ,
Under an argon atmosphere, 8 ml of 1.6 M n-BuLi in
hexane (15 mmol) was added to 3.2 g (10 mmol) t-
Bu P[N(SiMe )]NH(SiMe ) in ca. 15 ml of hexane. The
8
3
3
Hz,
JCH
00.13 MHz): d 5.34 (br s, ꢂ
/
CH; 4H), 1.07 (d, JPHꢂ
/
14
sat.,
118 Hz, Si(CH ) ; 18H). H-NMR (213 K,
1
J
CHꢂ
/
126 Hz, C(CH ) ; 18H), 0.23 (sꢁ
/
3 3
2
3
3
1
1
ꢂ
/
solution was heated to reflux temperature for 1 min,
3 3
toluene-d , 300.13 MHz): d 5.85 (s, ꢂ
/
CH; 2H), 4.29 (s,
then added to a suspension of 2.6 g (13 mmol) CuBr×
/
8
1
J
CHꢂ
/
181 Hz, CuCH; 2H), 1.05 (d, C(CH ) ; 18H),
3 3
SMe in 20 ml of hexane at 195 K and stirred while
2
1
.29 (s, Si(CH ) ; 18H). C{ H}-NMR (298 K, toluene-
3
1
0
d , 75.47 MHz): d 186.3 (s, CO), 37.9 (d, J
warming to ambient temperature. Twelve milliliter of
styrene was added to the black suspension. The suspen-
sion was filtered through Celite. Only a part of the black
colloidal copper, presumably copper hydrides or copper
3
3
1
ꢂ60 Hz,
/
8
CP
1
C(CH ) ), 28.0 (s, C(CH ) ), 5.7 (s, Si(CH ) ). C{ H}-
3
1
3
3
3 3
3 3
NMR (213 K, toluene-d , 75.47 MHz): d 186.5 (s, CO),
8

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
141
Table 10
Computed energies of the model compounds 5ꢀ
/
23 at the B3LYP/6-311ꢁ
/
G(2d,p)//B3LYP/6-31G(d) level of theory (a.u.)
E[6-31G(d)]
Single-point electronic energy E[6-311ꢁ
/
G(2d,p)] Gibbs free energy correction G[6-31G(d)]
G[6-311ꢁ
/
G(2d,p)]
5
6
6
7
8
8
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
3
3
3
ꢃ
/
2172.168525
2366.313941
2366.299621
2355.665586
2246.108291
2246.109087
2472.876137
2472.856022
2475.036785
2475.017998
2475.019872
2397.555270
2434.238881
2434.216553
2355.616332
2355.636412
2355.622951
2355.626014
2355.607988
2246.116629
2132.781499
2132.757744
2132.754748
2324.691782
2324.779338
2093.485775
2399.912595
ꢃ
/
2172.492752
2366.680111
2366.669244
2356.029340
2246.452242
2246.452271
2473.287436
2473.270311
2475.445678
2475.430466
2475.426039
2397.943746
2434.639495
2434.620870
2355.995740
2356.015009
2356.000990
2356.004593
2355.993496
2246.449552
2133.090981
2133.073957
2133.071474
2325.067996
2325.135827
2093.824366
2400.323839
0.071831
0.167350
0.166903
0.060435
0.047645
0.047962
0.069231
0.067828
0.096892
0.095472
0.096787
0.057046
0.104293
0.102687
0.056515
0.056611
0.056992
0.055917
0.053069
0.051474
0.043516
0.040529
0.042178
0.098039
0.105111
0.019871
0.100309
0.031538
0.012851
0.029696
0.015421
0.063504
0.005923
0.001123
0.026816
0.125138
0.056158
0.016626
ꢃ
/
2172.096694
2366.146591
2366.132717
2355.605151
2246.060645
2246.061125
2472.806906
2472.788194
2474.939892
2474.922526
2474.923085
2397.498223
2434.134587
2434.113866
2355.559818
2355.579801
2355.565959
2355.570097
2355.554919
2246.065155
2132.737983
2132.717214
2132.712570
2324.593742
2324.674227
2093.465903
2399.812287
ꢃ2172.420921
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
2366.512761
2366.502341
2355.968905
2246.404597
2246.404309
2473.218205
2473.202483
2475.348786
2475.334994
2475.329252
2397.886700
2434.535202
2434.518183
2355.939225
2355.958398
2355.943998
2355.948676
2355.940427
2246.398078
2133.047465
2133.033332
2133.029296
2324.969957
2325.030716
2093.804495
2400.223530
%
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
a
b
4
4
5
5
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
%
%
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
%
5b
6
7
8
9
0
1
2
3
4
5
6
7
8
1
2
3
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ/
ꢃ/
ꢃ/
ꢃ/
C
2
H
3
OH
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
/
153.802228
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
/
153.867075
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
/
153.770690
ꢃ
/
153.835537
N
2
/109.524129
/109.562947
ꢃ
/
/
109.536980
ꢃ
/
109.575798
C
2
H
4
/78.587458
/78.617536
/
78.557762
ꢃ
/
78.587840
N
2
CHCHO
CHO
CHCHO
CH
/262.071651
/262.159010
/
262.056230
ꢃ
/
262.143589
C
3
H
5
/231.218147
/231.292916
/
231.154644
ꢃ
/
231.229412
/152.474031
/152.529641
/
152.468108
ꢃ
/
152.528518
2
/39.128249
/39.148393
ꢃ/
/
39.129372
ꢃ
/
39.149516
C
C
C
C
4
7
6
2
H
H
H
O
2 3
/379.289544
/379.414165
/
379.262729
ꢃ
/
379.387349
10
ꢃ
ꢃ
ꢃ
/
272.727426
381.451687
303.964053
ꢃ
ꢃ
ꢃ
/
272.805262
381.572242
304.066219
ꢃ
ꢃ
ꢃ
/
272.602289
381.395529
303.947427
ꢃ
/
272.680124
381.516084
304.049593
O
4 2
/
/
/
ꢃ
/
(CHO)
2
/
/
/
ꢃ/
1
3
29 1
2 Hz, Si(CH3)3). Si{ H}-NMR
clusters, were removed. The black solution was concen-
ꢃ3
JSiC
ꢂ
/
57 Hz, JPCꢂ
/
trated in vacuo at 10
removed within 12 h at 10
mbar. Excess of styrene was
ꢃ5
DEPT (298 K, toluene-d
2
8
, 59.63 MHz): d ꢃ
15.5 Hz). P{ H}-NMR (298 K, toluene-d ,
8
/14.5 (d,
31
1
mbar. The black oil
J
Psi
ꢂ
/
1
2
crystallized within several days at ambient temperatures.
Small colorless crystals embedded in a black matrix were
isolated. Further attempts of filtration and crystalliza-
121.50 MHz): d 61.7 (sꢁ
/
sat., JCP
C{ H}-NMR (toluene-d
(181.594.0) K, k 42 Hz; Si(CH
4.0) K, k 61 Hz; temperature calibration
by external MeOH.
ꢂ
/
64 Hz, JPsi
ꢂ14
/
1
3
1
Hz). VT
C(CH , T
, 75.47 MHz):
8
)
ꢂ
/
/
ꢂ
/
) ,
3 3
3
3
c
c
tion failed. Complex 11 is not available in analytically
1
pure form. H-NMR (298 K, toluene-d , 300.13 MHz):
T
c
ꢂ
/
(185.59
/
c
ꢂ
/
8
3
3
d 7.25 (d, JHHꢂ
/
8 Hz, o-CH; 2H), 7.02 (‘t’, JHHꢂ
/
7.5
7 Hz, p-CH; 1H), 5.02
CH gem; 1H), 3.92 (d,
3
Hz, m-CH; 2H), 6.96 (d, JHHꢂ
/
5.5. Synthesis of dimethyl butynedioate complex 12
3
(
3
dd, JHHꢂ
/
9.2 Hz and 14.6, ꢂ
14.6, ꢂCH cis; 1H), 3.59 (d, J
CH trans; 1H), 1.17 (d, J
/
3
JHH
ꢂ
/
/
ꢂ
/
9.2 Hz, ꢂ
14.5 Hz, JCHꢂ128 Hz,
110 Hz, Si(CH3)3;
/
Under an argon atmosphere, 262 mg dimethyl buty-
nedioate (1.79 mmol) and 752 mg (1.82 mg) ethene
complex 1 were dissolved in 5 ml Et O. Gas evolution
occurred. Upon standing overnight at 223 K, yellow
crystals precipitated. The crystals were dissolved in
Et O/THF and filtered through Celite. The solution
HH
3
1
ꢂ
/
/
PH
1
C(CH ) ; 18H), 0.11 (sꢁ
/
sat., JCHꢂ
/
3
1
3
3
2
1
1
1
9
8H). C{ H}-NMR (298 K, toluene-d , 75.47 MHz): d
8
39.7 (s, ipso-C), 128.8 (s), 127.5 (s, p-CH), 12 6.3 (s),
1
1.0 (s, PhC), 68.2 (s, CH ), 37.4 (d, JCP
ꢂ
/
64 Hz,
2
2
2
C(CH ) ), 28.1 (d, JCPꢂ
/
2 Hz, C(CH ) ), 5.5 (dꢁ
/
sat.,
was concentrated in vacuo and cooled to 223 K. Yellow
3
3
3 3

142
B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ143
/
Table 11
Zero point energies and H corrections (a.u.)
Acknowledgements
A doctoral fellowship for B.F.S. from the Fonds der
Chemischen Industrie is gratefully acknowledged.
B3LYP/[SDD/6-31G(d)]
ZPE H correction
B3LYP/6-31G(d)
ZPE H correction
CH
CHCHO
2
0.016586 0.020368
0.030043 0.034523
0.051126 0.055212
0.153656 0.159801
0.055890 0.062019
0.085366 0.092543
0.047306 0.054611
0.103616 0.113294
0.204834 0.217622
0.082973 0.093535
0.107585 0.120081
0.136918 0.150576
0.098683 0.112502
0.073581 0.082021
0.049184 0.055731
0.016586 0.020368
0.030039 0.034523
0.051126 0.055212
0.153656 0.159801
0.055890 0.062019
0.085366 0.092543
0.047306 0.054611
0.104684 0.114029
0.205844 0.218313
0.083702 0.094098
0.108255 0.120623
0.137529 0.151081
0.099230 0.112918
0.074429 0.082667
0.049342 0.055817
C
C
C
C
C
5
2
7
4
6
2
H
H
H
H
4
References
10
2
O
3
[1] (a) W. Kirmse, Angew. Chem. 115 (2003) 1120 (Angew. Chem.
Int. Ed. 42 (2003) 1088);
4
O
2
(CHO)
2
(
b) M.P. Doyle, M.A. McKervey, T. Ye, Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds, Wiley,
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6
8
[2] (a) H. Nozaki, S. Moriuti, H. Takaya, R. Noyori, Tetrahedron
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1
1
1
2
3
4
5
6
5
2
(
(
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[
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[
(
Prieto, P.J. P e´ rez, Organometallics 18 (1999) 2601.
b) M. Mar D ´ı az-Requejo, T.R. Belderrain, M.C. Nicasio, F.
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1
1
01ꢀ
/
102 8C (dec., turns red). H-NMR (298 K, C D ,
[6] P.J. P e´ rez, M. Brookhart, J.L. Templeton, Organometallics 12
(1993) 261.
[7] (a) R.W. Quan, Z. Li, E.N. Jacobsen, J. Am. Chem. Soc. 11
6
6
3
3
Hz, JCH
00.13 MHz): d 3.40 (s, OMe), 1.15 (dꢁ
/
sat., JPHꢂ
/
14
sat.,
1
ꢂ
/
128 Hz, C(CH ) ; 18H), 0.17 (sꢁ
/
3 3
(
1996) 8156;
1
13
1
JCH
ꢂ
/
117 Hz, Si(CH ) ; 18H). C{ H}-NMR (298
3 3
(b) M.M.-C. Lo, G.C. Fu, J. Am. Chem. Soc. 120 (1998) 10270;
(c) L. Cavallo, M.E. Cucciolito, A. De Martino, F. Giordano, I.
Orabona, A. Vitagliano, Chem. Eur. J. 6 (2000) 1127.
K, C D , 75.47 MHz): d 157.2 (s, CO), 100.3 (s, ꢁ
/
C),
61 Hz, C(CH ) ), 27.7 (d,
6
6
1
5
2.6 (s, OMe), 37.8 (d, JCPꢂ
/
3 3
2
1
sat., JSiC
[8] B.F. Straub, F. Eisentr a¨ ger, P. Hofmann, Chem. Commun. (1999)
2
JCP
3
ꢂ
/
2
Hz, C(CH3)3), 4.9 (dꢁ
/
ꢂ
/58
507.
[9] B.F. Straub, F. Rominger, P. Hofmann, Organometallics 19
(2000) 4305.
29
1
Si{ H}-NMR DEPT
Hz, JPCꢂ
/
2 Hz, Si(CH3)3).
2
( sat., JPsiꢂ10
Hz). P{ H}-NMR (298 K, C D , 121.50 MHz): d 73.5
298 K, C D , 59.63 MHz): d ꢃ
/
7.9 (dꢁ
/
/
6
6
31
1
[
[
10] B.F. Straub, P. Hofmann, Angew. Chem. 113 (2001) 1328
(Angew. Chem. Int. Ed. 40 (2001) 1288).
6
6
1
ꢃ1
(
(
(
s, JCPꢂ64 Hz). IR (KBr pellet, cm ): 3000 (w), 2951
s), 2900 (m), 1949 (m), 1920 (m), 1732 (s), 1714 (s), 1478
/
11] (a) Bernd F. Straub, Dissertation, Ruprecht-Karls Universit a¨ t,
Heidelberg, (2000);
w), 1428 (m), 1397 (w), 1347 (m), 1248 (vs), 1159 (m),
(b) B.F. Straub, F. Rominger, P. Hofmann, Unraveling the
Mechanism of Copper-Catalyzed Cyclopropanation, IV Sympo-
sium ‘Selective Reactions of Metal-Activated Molecules’, W u¨ rz-
burg, September 2000.
1
123 (s), 1037 (m), 923 (w), 836 (s), 759 (m), 701 (m), 667
w), 644 (m), 625 (w), 521 (w), 480 (w). UV (pentane):
(
2
07, (241 sh), 296. FABMS (m-nitrobenzyl alcohol): m/
ꢁ
[
[
[
[
12] M. B u¨ hl, F. Terstegen, F. L o¨ ffler, B. Meynhardt, S. Kierse, M.
M u¨ ller, C. N a¨ ther, U. L u¨ ning, Eur. J. Org. Chem. (2001) 2151.
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Salvatella, J. Am. Chem. Soc. 123 (2001) 7616.
z Calc.
C H N CuO PSi : C, 45.73; H, 8.06; N, 5.33; P,
(%)ꢂ
/524.2
(21.7)
[M ].
for
2
0
42
2
4
2
5
.90. Found: C, 45.99; H, 8.17; N, 5.62; P, 5.74%.
14] T. Rasmussen, J.F. Jensen, N. Ostergaard, D. Tanner, T. Ziegler,
P.-O. Norrby, Chem. Eur. J. 8 (2002) 177.
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(
(
b) S.H. Volko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 1200;
c) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
6
. Supplementary material
[16] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr.,
R.E. Stratman, J.C. Burant, S. Dapprich, J.M. Millam, A.D.
Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q.
Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghava-
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Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L.
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Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.
Johnson, W. Cheng, M.W. Wong, J.L. Andres, C. Gonzalez, M.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
data Centre, CCDC no. 214068 for compound 9 and no.
2
14069 for compound 12. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: ꢁ
c.uk or www: http://ccdc.cam.ac.uk.
/
44-1223-336033; e-mail deposit@ccdc.cam.a-

B.F. Straub et al. / Journal of Organometallic Chemistry 684 (2003) 124ꢀ
/143
143
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J.G. Toerien, Angew. Chem. 106 (1994) 687 (Angew. Chem. Int.
Ed. Engl. 33 (1994) 672).
[
[
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