Discrete bridging and terminal copper carbenes in copper-catalyzed cyclopropanation

Article abstract of DOI:10.1021/ja047935q

The Cu(I)/ β-diketiminate [Me₂NN]Cu(η²- ethylene) (2) catalyzes the cyclopropanation of styrene with N ₂CPh₂ to give 1,1,2-triphenylcyclopropane in 67% yield. Addition of N₂CPh₂ to 2 equiv of 2 allows for the isolation of the dicopper carbene {[Me₂NN]Cu}₂(μ- CPh₂) (3) in which the diphenylcarbene moiety is symmetrically bound between two [Me₂NN]Cu fragments (Cu-C = 1,922(4) and 1.930(4) A) with a Cu-Cu separation of 2.4635(7) A. In toluene-d₈ solution, 3 reversibly dissociates a [Me₂NN]Cu fragment to give [Me₂NN]Cu(toluene) and the terminal carbene [Me₂NN]Cu= CPh₂, Dicopper carbene 3 reacts with 3 equiv of styrene to give 1,1,2-triphenylcyclopropane and 2 equiv of [Me₂NN] Cu(η²-styrene) within minutes, DFT studies with simplified ligands indicate a stronger Cu-C π-back-bonding interaction from two Cu(I) centers to the carbene acceptor orbital in a dicopper carbene than that present in a monocopper carbene. Nonetheless, the terminal carbene [Me ₃NN]Cu=CPh₂ (8) that possesses a p-methyl group on each β-diketiminato N-aryl ring may be isolated and exhibits a shortened Cu-C distance of 1,834(3) A, The stoichiometric cyclopropanation of styrene by 8 in 1,4-dioxane is first-order in both copper carbene 8 and styrene with activation parameters ΔH? = 10,4(3) kcal/mol and ΔS? = -32,3(9) cal/mol·K. In 1,4-dioxane, 8 decomposes to Ph ₂C=CPh₂ via first-order kinetics with activation parameters ΔH? = 21(1) kcal/mol and ΔS? = -8(3) cal/mol·K. Arene solutions of thermally sensitive terminal carbene 8 decompose to [Me₃NN]-Cu(arene), which reacts with 8 still present in solution to give the more thermally stable {[Me₃NN]Cu} ₂-(μ-CPh₂).

Full text of DOI:10.1021/ja047935q

Published on Web 07/21/2004
Discrete Bridging and Terminal Copper Carbenes in
Copper-Catalyzed Cyclopropanation
Xuliang Dai and Timothy H. Warren*
Contribution from the Department of Chemistry, Georgetown UniVersity,
Washington, D.C. 20057
Received April 9, 2004; E-mail: thw@georgetown.edu
Abstract: The Cu(I) â-diketiminate [Me2NN]Cu(η2-ethylene) (2) catalyzes the cyclopropanation of styrene
with N2CPh2 to give 1,1,2-triphenylcyclopropane in 67% yield. Addition of N2CPh2 to 2 equiv of 2 allows for
the isolation of the dicopper carbene {[Me2NN]Cu}2(µ-CPh2) (3) in which the diphenylcarbene moiety is
symmetrically bound between two [Me2NN]Cu fragments (Cu-C ) 1.922(4) and 1.930(4) Å) with a
Cu-Cu separation of 2.4635(7) Å. In toluene-d8 solution, 3 reversibly dissociates a [Me2NN]Cu fragment
to give [Me2NN]Cu(toluene) and the terminal carbene [Me2NN]CudCPh2. Dicopper carbene 3 reacts with
3 equiv of styrene to give 1,1,2-triphenylcyclopropane and 2 equiv of [Me2NN]Cu(η2-styrene) within minutes.
DFT studies with simplified ligands indicate a stronger Cu-C π-back-bonding interaction from two Cu(I)
centers to the carbene acceptor orbital in a dicopper carbene than that present in a monocopper carbene.
Nonetheless, the terminal carbene [Me3NN]CudCPh2 (8) that possesses a p-methyl group on each
â-diketiminato N-aryl ring may be isolated and exhibits a shortened Cu-C distance of 1.834(3) Å. The
stoichiometric cyclopropanation of styrene by 8 in 1,4-dioxane is first-order in both copper carbene 8 and
styrene with activation parameters ∆Hq ) 10.4(3) kcal/mol and ∆Sq ) -32.3(9) cal/mol‚K. In 1,4-dioxane,
8 decomposes to Ph2CdCPh2 via first-order kinetics with activation parameters ∆Hq ) 21(1) kcal/mol and
∆Sq ) -8(3) cal/mol‚K. Arene solutions of thermally sensitive terminal carbene 8 decompose to [Me3NN]-
Cu(arene), which reacts with 8 still present in solution to give the more thermally stable {[Me3NN]Cu}2-
(µ-CPh2).
Introduction
stabilize the intermediate carbene species against competing
carbene dimerization via weak binding to the metal complex
Copper bronze, copper(II) sulfate, and copper(II) oxide are
the longest known catalysts for the decomposition of diazo
compounds N2CR2.1 When performed in the presence of alkenes,
cyclopropanes are one of several organic products that result
from the transient carbene intermediates that are generated.2
Over the last 40 years, soluble copper(I) and copper(II)
complexes have been used to impart greater selectivity for
cyclopropanation and have played an important role in the
development of this valuable group transfer reaction.1 In fact,
the use of chiral copper salicylaldimine complexes marks the
earliest successful use of a chiral, homogeneous metal catalyst
to achieve enantioselectivity. Observation of asymmetric induc-
tion in the cis/trans cyclopropane mixture formed between the
reaction of styrene with N2CHCO2R (R ) ethyl, Ph) demon-
strated participation of the chiral copper complex in the product-
determining step.3
(eq 1).4-6
With the appropriate choice of metal and supporting ligands,
carbene transfer to alkenes can proceed with high levels of
stereoselectivity.1,2,7 While complexes of many transition metals
may be used in catalytic cyclopropanation, copper is attractive
because of its low cost relative to other active metals such as
rhodium and ruthenium.
A variety of copper complexes supported by multidentate
N-donor ligands catalyze the cyclopropanation of olefins with
diazo reagents containing R-carbonyl groups.1,2 While diaste-
reoselectivities are generally modest and may be improved by
increasing the size of the diazo reagent8 and/or supporting
The successful, catalytic cyclopropanation of alkenes with
diazo reagents requires the use of a transition-metal complex
to facilitate loss of N2 from the diazo reagent as well as to
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10.1021/ja047935q CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 10085-10094
10085

A R T I C L E S
Dai and Warren
[But2P(NSiMe3)2-κ2N]CudC(Ph)C(O)OMe (1) in a mixture of
2-phenyl-diazoacetate and [But2P(NSiMe3)2-κ2N]Cu(ethylene)
in toluene-d8.19 Addition of styrene resulted in the rapid
disappearance of 1 and gave a mixture of cyclopropanation
products cis- and trans-methyl-1,2-diphenylcyclopropanecar-
boxylate. Time-resolved FT-IR spectroscopy has also been used
to identify a transient copper carbene upon addition of
N2CHCO2Me to a cationic Cu(I) bis(oxazoline) precatalyst.20
Despite the potential to develop more selective and versatile
catalysts based on a detailed understanding of active species
involved, experimental progress has lagged considerably behind
theoretical studies aimed at characterizing discrete species
involved in copper-catalyzed cyclopropanation. Employing
electron-rich â-diketiminato supporting ligands,21-25 we report
herein detailed structural, spectroscopic, and kinetic studies that
outline the nature of copper carbenes formed in the catalytic
cyclopropanation of styrene with N2CPh2 using a Cu(I) cata-
lyst.26
Figure 1. Supporting ligands in copper cyclopropanation catalysts.
Results and Discussion
ligands,9 impressive enantioselectivites have been obtained with
chiral supporting ligands such as bis(oxazolines) or semicorrins
(Figure 1).7,10 In addition, Pe´rez and co-workers have shown
that electron-deficient tris(pyrazolyl)borates can shift the chemo-
selectivity of reactions employing N2CHCO2Et to give products
that derive from carbene insertion into C-H bonds.11
Much evidence points to a Cu(I) carbene [Cu]dCRR′ as the
key intermediate in copper-catalyzed cyclopropanation. For
instance, some structurally characterized [M]dCRR′ complexes
of Fe,12 Ru,13 and Os14 isolated from the addition of diazo
compounds to cyclopropanation catalysts stoichiometrically
cyclopropanate alkenes. Several mechanistic15-17 and theoreti-
cal17,18 studies implicate Cu(I) complexes as the catalytically
active species, though copper(I) carbenes reactive toward
cyclopropanation have proven elusive. Hofmann recently de-
scribed the low-temperature NMR detection of the Cu carbene
Catalytic Cyclopropanation. Given its similarity to other
successful copper complexes employed in the cyclopropanation
of alkenes with diazo esters (Figure 1), it is not surprising that
the neutral Cu(I) â-diketiminato complex [Me2NN]Cu(ethylene)22
(2) catalyzes the cyclopropanation of styrene with ethyl diazo-
acetate (EDA). Addition of EDA in one portion to a toluene
solution containing 5 equiv of styrene and 2 mol % 2 gave the
corresponding cyclopropane in 77% yield as a 38:62 cis/trans
mixture (eq 2). We found, however, that the slow addition of
N2CPh2 to a toluene solution containing 10 equiv of styrene
and 5 mol % 2 results in the formation of 1,1,2-triphenyl-
cyclopropane in 67% yield (eq 2).
(8) Doyle, M. P.; Bagheri, V.; Wandless, T. J.; Harn, N. K.; Brinker, D. A.;
Eagle, C. T.; Loh, K.-L. J. Am. Chem. Soc. 1990, 112, 1906.
(9) D´ıaz-Requejo, M. M.; Caballero, A.; Belderra´ın, T. R.; Nicasio, M. C.;
Trofimenko, S.; Pe´rez, P. J. J. Am. Chem. Soc. 2002, 124, 978.
(10) Pfaltz, A. Acc. Chem. Res. 1993, 26, 339.
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Trofimenko, S.; Pe´rez, P. J. Organometallics 2003, 22, 4145. (b) Caballero,
A.; D´ıaz-Requejo, M. M.; Belderra´ın, T. R.; Nicasio, M. C.; Trofimenko,
S.; Pe´rez, P. J. J. Am. Chem. Soc. 2003, 125, 1446. (c) Morilla, M. E.;
Molina, M. J.; D´ıaz-Requejo, M. M.; Belderra´ın, T. R.; Nicasio, M. C.;
Trofimenko, S.; Pe´rez, P. J. Organometallics 2003, 22, 2914. (d) Morilla,
M. E.; Diaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko,
S.; Pe´rez, P. J. Organometallics 2004, 23, 293. (e) D´ıaz-Requejo, M. M.;
Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Pe´rez, P. J. J. Am. Chem.
Soc. 2002, 124, 896.
Notably, the carbene dimerization byproduct Ph2CdCPh2 was
not observed (<1%); the azine Ph2CdN-NdCPh2 accounted
for the remainder of the N2CPh2 consumed. Such use of a non-
R-carbonyl diazo reagent other than N2CH2 for catalytic
cyclopropanation is uncommon. Diazo reagents N2CRR′ that
do not bear heteroatom-containing R-substituents are much more
prone to catalytic dimerization to alkenes R′RCdCRR′ by metal
complexes than the heavily utilized diazo esters.4,27,28 Examples
of selective catalytic group transfer with aryldiazomethanes N2C-
(12) (a) Hamaker, C. G.; Mirafzal, G. A.; Woo, L. K. Organometallics 2001,
20, 5171. (b) Li, Y.; Huang, J.-S.; Zhou, Z.-Y.; Che, C.-M.; You, X.-Z. J.
Am. Chem. Soc. 2002, 124, 13185.
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L.; Teng, P.-F.; Lee, W.-S.; Lo, W.-C.; Peng, S.-M.; Zhou, Z.-Y. J. Am.
Chem. Soc. 2001, 123, 4119. (b) Nishiyama, H.; Aoki, K.; Itoh, H.;
Iwamura, T.; Sakata, N.; Kurihara, O.; Motoyama, Y. Chem. Lett. 1996,
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123, 4843.
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D´ıaz-Requejo, M. M.; Nicasio, M. C.; Pe´rez, P. J. Organometallics 1998,
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(16) D´ıaz-Requejo, M. M.; Pe´rez, P. J.; Brookhart, M.; Templeton, J. L.
Organometallics 1997, 16, 4399.
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P.-O. Chem.-Eur. J. 2002, 8, 177.
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Salvatella, L. J. Am. Chem. Soc. 2001, 123, 7616. (b) Bu¨hl, M.; Terstegen,
F.; Lo¨ffler, F.; Meynhardt, B.; Kierse, S.; Mu¨ller, M.; Na¨ther, C.; Lu¨ning,
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F.; Hofmann, P. J. Organomet. Chem. 2003, 684, 124.
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(20) Ikeno, T.; Iwakura, I.; Yamada, T. J. Am. Chem. Soc. 2002, 124, 15152.
(21) For a general review of â-diketiminate complexes, see: Bourget-Merle,
L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002, 102, 3031.
(22) Cu(I) â-diketiminates have been used to study dioxygen activation: Dai,
X.; Warren, T. H. Chem. Commun. 2001, 1998 and refs 23 and 24.
(23) (a) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.;
Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108. (b) Aboelella, N. W.;
Lewis, E. A.; Reynolds, A. M.; Brennessel, W. W.; Cramer, C. J.; Tolman,
W. B. J. Am. Chem. Soc. 2002, 124, 10660. (c) Spencer, D. J. E.; Reynolds,
A. M.; Holland, P. L.; Jazdzewski, B. A.; Duboc-Toia, C.; Le Pape, L.;
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2003, 42, 7354.
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2001, 40, 5316.
(26) Some of this work has been presented: Dai, X.; Warren, T. H. Abstracts
of Papers, 227th National Meeting of the American Chemical Society,
Anaheim, CA, March 28-April 1, 2004; American Chemical Society:
Washington, DC, 2004; No. INOR 829.
9
10086 J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004

Discrete Copper Carbenes in Cyclopropanation
A R T I C L E S
Scheme 1. Variable Temperature 1H NMR Spectra (300 MHz) of
the â-Diketiminato Backbone C-H Region of 3 in Toluene-d8
Figure 2. ORTEP diagram of the solid-state structure of {[Me2NN]Cu}2-
(µ-CPh2) (3) (all H atoms omitted). Selected bond distances (angstroms),
angles (deg), and twist angles between planes (deg): Cu1-C43 1.922(4),
Cu2-C43 1.930(4), Cu1-Cu2 2.4635(7), Cu1-N1 1.965(3), Cu1-N2
1.955(3), Cu2-N3 1.970(3), Cu2-N4 1.978(3), Cu1-C43-Cu2
79.51(14), N1-Cu1-N2 96.11(13), N3-Cu2-N4 95.85(12), C44-C43-
C50 115.0(3), N1-Cu1-N2/C44-C43-C50 89.1, N3-Cu2-N4/C44-
C43-C50 83.1.
C50 planes). While the spectroscopically characterized di-
phenylcarbene-bridged Rh-Cu complexes Cp(CO)Rh(µ-CPh2)-
(CuCl)2 and Cp(CO)Rh(µ-CPh2)CuCp have been isolated from
the addition of CuCl to Cp(CO)RhdCPh2 followed by subse-
quent treatment with NaCp,31 to our knowledge 3 represents
the first report of a dicopper carbene.
(R)Ar (R ) H, Ar) have only recently been reported, employing
Fe(II) or Os(II) porphyrin precursors whose active species are
the discrete monocarbenes [Fe]dCHAr12 or the unusual bis-
(carbene) [Os](dCPh2)2.14
1
The H NMR spectrum of 3 in toluene-d8 at -70 °C (300
MHz) is consistent with its structure in the solid state, giving
rise to four separate â-diketiminate Ar-Me resonances, two
backbone Me signals, and a solitary backbone C-H singlet at
δ 5.010 ppm (Figure S1). Warming this solution results in the
coalescence of the backbone Me signals, consistent with the
concerted rocking of each [Me2NN]Cu fragment about its
respective Cu-CPh2 vector for which an activation barrier ∆Gq
) 10.1(3) kcal/mol could be established at -32 °C. All four
Ar-Me resonances also coalesce above 25 °C as a result of an
enantiomerization process. While this could involve rapid
dissociation and reassociation of one [Me2NN]Cu fragment from
3, more likely it is a twisting of the bridging diphenylcarbene
ligand about the vector containing C43 that bisects the Cu1-
Cu2 axis. Room temperature 13C{1H} NMR spectra of 3 reveal
the carbene CPh2 signal at δ 189.4 ppm, which is downfield of
all other resonances, but typical for bridging carbenes that appear
in the range 100-210 ppm if a metal-metal bond is present.32
For instance, the bridging CPh2 moiety in Cp(CO)Rh(µ-CPh2)-
CuCp appears at δ 206.3 ppm.31
Variable temperature NMR spectra also reveal that 3 is in a
reversible equilibrium with two new C2V symmetric copper
â-diketiminate species 4 and 5 (Scheme 1). Two backbone C-H
signals at δ 4.958 and 4.801 ppm with corresponding Ar-Me
resonances at δ 2.03 and 2.40 ppm are observed for 4 and 5,
respectively. These signals increase in intensity from ca. 2 to
30% relative to 3 over the temperature range -70 to 60 °C.
Since 5 can be identified as [Me2NN]Cu(toluene) by addition
of an authentic sample to the mixture at room temperature
(prepared by addition of Tl[Me2NN] to CuBr in toluene), we
suggest that 3 reversibly dissociates one [Me2NN]Cu fragment
Synthesis and Characterization of Dicopper Carbene
{[Me2NN]Cu}2(µ-CPh2). To investigate the origin of selectivity
toward cyclopropanation of styrene by N2CPh2 catalyzed by 2,
we slowly added N2CPh2 to 2 equiv of [Me2NN]Cu(ethylene)
(2) in toluene at -35 °C. Immediate effervescence occurred
along with the formation of a purple solution from which the
unanticipated dicopper carbene {[Me2NN]Cu}2(µ-CPh2) (3) was
isolated in 40% yield as purple crystals from ether (eq 3).
The X-ray structure of 3 • 0.75 hexane shows the diphenylcar-
bene unit nearly symmetrically bound between two [Me2NN]-
Cu fragments separated by 2.4635(7) Å with Cu1-C43 and
Cu2-C43 distances of 1.922(4) and 1.930(4) Å (Figure 2). The
Cu-C distances in 3 are at the low end of the range of 1.911-
(2)-2.06(2) Å established for bridging sp2-hybridized aryl
groups in multinuclear Cu(I) aryls such as [CuMes]x (Mes )
2,4,6-trimethylphenyl; x ) 4 or 5),29 which generally exhibit
Cu-Cu distances in the range 2.4 to 2.5 Å.30 The two [Me2-
NN]Cu fragments are related by a noncrystallographic C2 axis
through C43 that bisects the Cu-Cu vector, and each [Me2-
NN]Cu fragment is nearly orthogonal to the CPh2 moiety (83.1
and 89.1° twist angles between the N-Cu-N and C44-C43-
(27) (a) Tomilov, Y. V.; Dokichev, V. A.; Dzhemilev, U. M.; Nefedov, O. M.
Russ. Chem. ReV. 1993, 62, 799. (b) Oshina, T.; Nagai, T. Tetrahedron
Lett. 1980, 21, 1251.
(28) Shankar, B. K. R.; Shechter, H. Tetrahedron Lett. 1982, 23, 2277.
(29) (a) Meyer, E. M.; Gambarotta, S.; Floriani, C.; Chiesi-Vila, A.; Guastini,
C. Organometallics 1989, 8, 1067. (b) Eriksson, H.; Håkansson, M.
Organometallics 1997, 16, 4243. (c) Gambarotta, S.; Floriani, C.; Chiesi-
Vila, A.; Guastini, C. Chem. Commun. 1983, 1156.
(31) Werner, H.; Schwab, P.; Bleuel, E.; Mahr, N.; Windmu¨ller, B.; Wolf, J.
Chem.-Eur. J. 2000, 6, 4461.
(32) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
Wiley-Interscience: New York, 2001.
(30) van Koten, G.; James, S. L.; Jastrebeski, J. T. B. H. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: New York, 1995; Vol. 3, p 57.
9
J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004 10087

A R T I C L E S
Dai and Warren
Table 1. Calculated Bond Distances (angstroms) and Angles
(deg) for Models 6, 7-C2v, and 7-C2
parameter 7-C
6
7-C
2
2v
Cu-C
1.785 1.885 1.892
Cu-N
1.917 1.932 1.904 (Cu-N1) 1.924 (Cu-N2)
Cu-Cu
N-Cu-N
Cu-C-Cu
n/a
2.530 2.321a
92.5
n/a
95.5
84.5
90
95.3
75.7
89.2
N-Cu-N/H-C-H 90
a
In sterically unencumbered 7-C2, the Cu centers can approach each
other much closer than possible in 3 (Cu-Cu ) 2.4635(7) Å). Constraining
the Cu-Cu distance to 2.46 Å did not result in calculated energies or
metrical parameters markedly different from those of unconstrained 7-C2.
Direct reaction of N2CPh2 with dicopper carbene 3 gives the
azine Ph2CdN-NdCPh2 isolated in 37% yield as colorless
crystals from pentane (Scheme 2). Benzophenone azine forms
quantitatively upon addition of N2CPh2 to the discrete rhodium
carbene Cp(SbiPr3)RhdCPh2.31 This azine is also the primary
product when N2CPh2 is added to Rh2(OAc)4,28 the prototypical
member of an efficient family of dirhodium(II) carboxylate and
carboxamide catalysts for alkene cyclopropanation with R-car-
bonyl diazo reagents.5,7,33 This byproduct identified in the
cyclopropanation of styrene with N2CPh2 catalyzed by 2 thus
results from competition between styrene and the diazo reagent
for the copper carbene, underscoring the requirement for slow
addition of the diazo reagent to minimize the concentration of
N2CPh2 during the course of catalytic cyclopropanation.
Given the ready reactivity of 3 toward styrene, its thermal
stability in arene solutions is surprising. Fifty-seven percent of
3 in an initially 20 mM benzene-d6 solution is still present after
56 h at room temperature. This paradoxical behavior may be
explained by the equilibrium between the dicopper carbene 3
and the presumably less thermally stable terminal carbene [Me2-
NN]CudCPh2 (4) along with a [Me2NN]Cu(arene) species
(Scheme 2). As 3 decomposes, Ph2CdCPh2 and additional [Me2-
NN]Cu(arene) are formed, thereby shifting the equilibrium
toward more thermally stable 3. We see no evidence of [Me2-
NN]Cu(Ph2CdCPh2) formation; presumably, the tetrasubstituted
alkene is too sterically encumbered to bind to the [Me2NN]Cu
fragment.
DFT Calculations Employing Simplified Models. DFT
calculations employing simplified models of the mono- and
dicopper carbenes (Table 1) were carried out to identify
electronic factors that could account for the stability of the
bridged carbene 3 relative to the terminal carbene 4. In accord
with previous theoretical studies on Cu(I) carbenes supported
by bidentate N-donor ligands,17,18 the hypothetical monomeric
carbene [H5C3N2]CudCH2 (6) favors an orientation in which
the methylene moiety is orthogonal to the backbone. This
disposition maximizes back-bonding to the empty carbene p
orbital from the filled d orbital of the d10 Cu(I) center that is
most destabilized by the â-diketiminate supporting ligand
(Figures 4 and 5). Bearing this geometric relationship in mind,
inspection of the X-ray structure of 3 shows that the CPh2 moiety
is essentially orthogonal to the backbone of each unique [Me2-
NN]Cu fragment (83.1 and 89.1° twist angles). Thus, the
diphenylcarbene moiety in 3 adopts an orientation that would
appear to allow back-bonding from each [Me2NN]Cu fragment
the carbene is bound to.
Figure 3. van’t Hoff plot for the dissociation of a [Me2NN]Cu fragment
from 3 in toluene-d8.
Scheme 2. Reactivity of Dicopper Carbene 3
to give an equilibrium concentration of the monomeric carbene
[Me2NN]CudCPh2 (4). A related Cu(I) benzene adduct sup-
ported by a fluorinated â-diketiminate ligand has been structur-
ally characterized by Sadighi.24 The temperature dependence
of the equilibrium constant over the temperature range -20 to
60 °C allows the determination of the thermodynamic param-
eters ∆H ) 9.1(3) kcal/mol and ∆S ) 24(1) cal/mol‚K for this
process (Figure 3). Notably, no significant broadening of the
closely spaced backbone C-H 1H NMR resonances of 3 and 4
occurs until the temperature is raised above 60 °C, indicating
that 3 and 4 do not interconvert quickly on the NMR time scale
over this temperature range.
Reactivity of Dicopper Carbene {[Me2NN]Cu}2(µ-CPh2).
Dicopper carbene 3 is reactive toward cyclopropanation. As
monitored by 1H NMR spectroscopy, a benzene-d6 solution of
3 in the presence of 3 equiv of styrene quantitatively gives 1,1,2-
triphenylcyclopropane and 2 equiv of Cu-styrene complex
[Me2NN]Cu(styrene)22 within minutes (Scheme 2). In light of
the sterically crowded nature of dicopper carbene 3, we assume
that the reactive species is the terminal carbene 4 formed by
the reversible dissociation of one [Me2NN]Cu fragment from
3. Scouting experiments indicate that the facility of carbene
transfer is sensitive to the degree and pattern of alkene
substitution. While reaction of 3 with a 50-fold excess of
R-methylstyrene in toluene at room temperature gives clean
conversion to the corresponding cyclopropane, the 1,2-disub-
stituted trans-â-methylstyrene requires heating at 45 °C for 2 h
under similar conditions to completely consume the copper
carbene 3. In the latter case, carbene dimerization competes with
cyclopropanation to give a 73:27 ratio of Ph2CdCPh2 to
cyclopropane. The aliphatic olefins cyclooctene and 1-hexene
(50 equiv) also sluggishly react with 3 at 45 °C to predominately
give Ph2CdCPh2 with a low (ca. 20%) conversion to the
corresponding cyclopropanes.
(33) (a) Doyle, M. P. Chem. ReV. 1986, 86, 919. (b) Burke, S. D.; Grieco, P. A.
Org. React. 1979, 26, 361.
9
10088 J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004

Discrete Copper Carbenes in Cyclopropanation
A R T I C L E S
with Cu(I) complexes, two Cu(I) centers can be more effective
than one at stabilizing a strongly π-accepting carbene.
Synthesis and Characterization of Terminal Carbene
[Me3NN]CudCPh2. Nonetheless, steric factors undoubtedly
play a role in the relative stability of mono- and dicopper
carbenes. Given the small enthalpy of dissociation from 3 to
generate the terminal carbene [Me2NN]CudCPh2 (4), we felt
that a slight modification to the â-diketiminate ligand might
sufficiently destabilize this singly bridged dicopper species to
isolate a terminal carbene.34 Low-temperature addition of N2-
CPh2 to an ether solution of [Me3NN]Cu(toluene) that possesses
an additional methyl group in the 4-position of each â-diketimi-
nato N-aryl ring results in the isolation of the terminal carbene
[Me3NN]CudCPh2 (8) as purple crystals in 20% recrystallized
Figure 4. π-Back-bonding interactions in C2V symmetric copper carbenes.
To qualitatively assess differences in back-bonding interac-
tions between mono- and dicopper carbenes, a dicopper carbene
was modeled with the C2V-species {[H5C3N2]Cu}2(µ-CH2) (7-
C2v) (Table 1, Figure 5). In 7-C2v, the difference in energy
between the orbitals ascribed to Cu-C π-bonding and π*-
antibonding (2.92 eV) is greater than the corresponding differ-
ence in 6 (2.39 eV), consistent with a stronger π-back-bonding
interaction in the dicopper carbene. Modeling the C2 symmetric
structure that 3 possesses in solution, we further reduced the
symmetry of the simplified dicopper carbene to C2 (7-C2) (Table
1). The mutual “twisting” of each [â-diketiminato]Cu fragment
along the Cu-C vectors allows more direct overlap between
the corresponding orbitals of predominately d heritage on Cu
and the carbene acceptor orbital in 7-C2 (Figure 6) as compared
to those in 7-C2v. This results in an even larger energy separation
between the Cu-C π and π* orbitals (3.54 eV), suggesting a
further enhanced interaction. These calculations on simple
models indicate stronger π-back-bonding in dicopper carbenes
via a three-center, four-electron interaction in comparison to
the simple π-back-bonding present in monocopper carbenes.
Considering the weak back-bonding ability typically associated
1
yield (eq 4). H NMR analysis of an aliquot of the reaction
mixture indicates >90% conversion to the terminal carbene 8,
its isolation in pure form hampered by its high solubility and
thermal sensitivity.
The X-ray structure of 8 reveals a contracted Cu-C bond
distance of 1.834(3) Å relative to that of the dicopper carbene
3 (Figure 7). This Cu-C distance is consistent with multiple-
bond character, and 8 exhibits structural parameters similar to
those of the related d10 Ni(0) carbene (tBu2PCH2CH2PBut2)-
NidCPh2, which has a Ni-C distance of 1.836(2) Å.35 The
Cu-C bond in 8 is shorter than that (1.882(3) Å) found in a
structurally characterized three-coordinate cationic copper(I)
Figure 5. Correlation diagram illustrating Cu-C π-interactions in C2V symmetric monocarbene 6 (left) and dicopper carbene 7-C2v (right).
9
J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004 10089

A R T I C L E S
Dai and Warren
Figure 6. Cu-carbene orbital interactions in C2 symmetric {[H5C3N2]Cu}2-
(µ-CH2) (7-C2).
Figure 8. Representative UV-vis spectrum for the cyclopropanation of
styrene (100 equiv) by [Me2NN]CudCPh2 (8) (dioxane, 30.2 °C).
carbene π-back-bonding is expected to be less important.39
Maximizing π-back-bonding with the [Me3NN]Cu fragment, the
carbene ligand in 8 is essentially orthogonal to the â-diketimi-
nate backbone (89.0° twist angle). The 1H NMR spectrum of 8
in benzene-d6 exhibits C2V symmetry, and the backbone C-H
resonance at δ 4.989 ppm has a similar chemical shift as that
ascribed to [Me2NN]CudCPh2 (4) at δ 4.958 ppm. The carbene
carbon resonates at δ 253.1 ppm in the 13C{H} NMR spectrum
of 8, downfield of the carbene resonance in the bridged species
3 as well as Hofmann’s terminal But2P(NSiMe3)2-κ2N]-
CudC(Ph)C(O)Me (1) observed at δ 229.9 ppm,19 but within
the approximate range 200-400 ppm established for terminal
carbenes.32
Kinetic Studies for Cyclopropanation of Styrene Deriva-
tives by [Me3NN]CudCPh2. Terminal carbene 8 quickly reacts
with styrene at room temperature to give 1,1,2-triphenylcyclo-
propane and exhibits clean pseudo-first-order kinetics in 1,4-
dioxane in the presence of excess styrene (eq 5).
Figure 7. ORTEP diagram of the solid-state structure of [Me3NN]Cud
CPh2 (8) (all H atoms omitted). Selected bond distances (angstroms), angles
(deg), and twist angles between planes (deg): Cu-C24 1.834(3), Cu-N1
1.906(3), Cu-N2 1.922(3), N1-Cu-N2 96.10(12), C25-C24-C31
117.3(3), N1-Cu-N2/C25-C24-C31 89.0.
Fischer carbene [Cu{)CR1(OR2)}(MeCN)(OEt2)]+ (R1 ) (E)-
CHdCH-2-furyl; R2 ) menthyl) isolated by carbene transfer
from a Cr complex.36 In addition, the Cu-C distance in 8 is
shorter than that found in three-coordinate (1.914(4)-
1.9938(14) Å)37 and two-coordinate (1.850(4)-1.9124(16)
Å)38,39 N-heterocyclic copper(I) carbenes for which copper
Monitoring the decrease of the UV-vis signal of 8 at λ ) 566
nm (Figure 8), the observed first-order rate constant at 40 °C
doubles from 1.5(1) × 10-4 s-1 to 2.8(1) × 10-4 s-1 upon
increasing the equivalents of styrene present from 50 to 100,
indicating that the reaction is also first-order in styrene. Rate
constants obtained over the temperature range 17.5-60.2 °C
(Table 2) under pseudo-first-order conditions employing 100
equiv of styrene allow the construction of an Eyring plot that
affords the activation parameters ∆Hq ) 10.4(3) kcal/mol and
∆Sq ) -32.3(9) cal/mol‚K (Figure 9). These values are clearly
consistent with a facile associative mechanism and may be
compared to those reported for the stoichiometric cyclopropa-
nation of vinyl acetate with (CO)5WdCH(p-OMeC6H4) (∆Hq
(34) While the two [â-diketiminato]Ni fragments in {[Me2NN]Ni}2(µ-NAd) are
bridged by a single adamantylimido ligand, the corresponding terminal Ni-
imide [Me3NN]NidNAd may be isolated provided that additional Me
groups in the p-positions of each â-diketiminato N-aryl ring are present:
Kogut, E.; Wiencko, H. L.; Zhang, L.; Warren, T. H., submitted for
publication to J. Am. Chem. Soc.
(35) Mindiola, D.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 9976.
(36) Barluenga, J.; Lo´pez, L. A.; Lo¨ber, O.; Toma´s, M.; Garc´ıa-Granda, S.;
Alvarez-Ru´a, C.; Borge, J. Angew. Chem., Int. Ed. 2001, 40, 3392.
(37) (a) Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M. E.;
Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027. (b) Hu,
X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003, 22, 3016. (c)
Arnold, P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem. Commun.
2001, 2340.
) 10.0(3) kcal/mol and ∆Sq ) -34.2(7) cal/mol‚K).40
A
Hammett plot employing p-substituted styrenes CH2dCHAr
(38) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2003, 125,
12237. (b) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P.
Organometallics 2004, 23, 1191.
(39) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2004, 23, 755.
(40) Fischer, H.; Mauz, E.; Jaeger, M. J. Organomet. Chem. 1992, 427, 63.
9
10090 J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004

Discrete Copper Carbenes in Cyclopropanation
A R T I C L E S
Table 3. Observed First-Order Rate Constants for Thermal
Table 2. Observed First-Order and Actual Second-Order Rate
Constants for Styrene Cyclopropanation by 8 under
Decomposition of 8
Pseudo-First-Order Conditions with [Styrene] ) 0.0839(2) M
T (°C)
k (s-1
)
T (°C)
k
obs (s-1
)
k (M-1 s-1
)
32.2
40.0
49.0
60.0
1.92(5) × 10-5
4.0(1) × 10-5
1.26(5) × 10-4
3.4(1) × 10-4
17.5
30.2
40.5
50.0
60.2
7.79(5) × 10-5
1.69(5) × 10-4
2.83(5) × 10-4
5.31(5) × 10-4
9.0(1) × 10-3
9.29(6) × 10-4
2.02(6) × 10-3
3.42(6) × 10-3
6.38(6) × 10-3
1.08(2) × 10-2
Scheme 3. Formation of Dicopper Carbene 9 via Decomposition
of 8 in Arene Solvents
copy (Table 3) are only 30-100 times lower than the actual
second-order rate constants determined in the cyclopropanation
of styrene at these temperatures. This comparison underscores
the importance of excess alkene present under typical catalytic
conditions dilute in copper catalyst to achieve clean cyclopro-
panation. Activation parameters ∆Hq ) 21(1) kcal/mol and ∆Sq
) -8(3) cal/mol‚K reveal a significantly larger ∆Hq for carbene
loss relative to transfer to styrene. This is to be expected as the
breaking Cu-carbene bond is not compensated by C-C bond
formation in the transition state and suggests a lower limit to
the Cu-carbene bond strength.
Figure 9. Eyring plot for the cyclopropanation of styrene by 8 in dioxane
over the temperature range 17.5-60.2 °C.
In more concentrated arene solutions (40 mM), [Me3NN]-
CudCPh2 (8) decomposes to [Me3NN]Cu(arene), which com-
bines with 8 still present to form an equilibrium concentration
of the dicopper carbene {[Me3NN]Cu}2(µ-CPh2) (9) (Scheme
3). In benzene-d6, this new species exhibits a backbone C-H
1H NMR resonance at δ 5.018 ppm, and its 13C{1H} NMR
spectrum reveals a carbene signal at δ 189.0 ppm, chemical
shifts nearly identical to those observed for 3. Arene solutions
of 8 still retain cyclopropanation activity after 2 days at room
temperature as the more thermally stable dicopper carbene 9
becomes the predominant Cu-containing species after a few
hours in solution.
Conclusions
The unprecedented dicopper carbene bonding motif in {[â-
diketiminato]Cu}2(µ-CPh2) (3 and 9) results in an extraordinary
solution lifetime for metal carbene species that readily cyclo-
propanate alkenes at room temperature. DFT calculations
indicate that the additional Cu(I) fragment present in dicopper
carbenes enhances π-back-donation via a three-center, four-
electron interaction with the carbene acceptor orbital as com-
pared to the simple back-bonding present in a monocopper
carbene. The active intermediate in cyclopropanation is the
terminal carbene [Me2NN]CudCPh2 (4) formed by reversible
dissociation of one [Me2NN]Cu fragment from 3 or the
structurally characterized [Me3NN]CudCPh2 (8), which exhibits
clean, pseudo-first-order kinetics in the cyclopropanation of
styrene derivatives. To our knowledge, the X-ray structures of
{[Me2NN]Cu}2(µ-CPh2) (3) and [Me3NN]CudCPh2 (8) are the
first of copper carbenes reactive toward cyclopropanation.
Comparison of the activation parameters for styrene cyclopro-
panation and thermal loss of diphenylcarbene from 8 indicates
only a small difference in ∆Gq at room temperature that
narrowly favors cyclopropanation. As the rate of styrene
cyclopropanation is also first-order in styrene, the use of excess
Figure 10. Hammett plot of cyclopropanation of p-substituted styrenes.
(* denotes estimated value due to overlapping GC peaks in competition
experiment.)
(Ar ) p-XC6H5; X ) OMe, Me, H, CF3) indicates the
electrophilic nature of copper carbene 8 as electron-rich sub-
stituentents (X ) OMe, Me) accelerate cyclopropanation,16,17
though the electron-poor styrene derivative (X ) CF3) has little
influence on the rate compared to styrene measured directly by
kinetics or via competition experiments (Figure 10).
Decomposition of [Me3NN]CudCPh2. Dilute (ca. 0.8 mM)
solutions of the terminal carbene 8 in 1,4-dioxane cleanly
decompose in a first-order fashion to the carbene coupling
product Ph2CdCPh2 and the solvated [Me3NN]Cu fragment
(eq 6).
The first-order rate constants for decomposition obtained over
a temperature range 30-60 °C measured by UV-vis spectros-
9
J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004 10091

A R T I C L E S
Dai and Warren
Employing typical bond distances and angles for the â-diketiminate
ligand, coordinates for the simplified molecules [C3H2N2]CudCH2 (6)
and {[C3H2N2]Cu}2(µ-CH2) in C2V symmetry (7-C2v) as well as C2
symmetry (7-C2) (z-axis unique) were developed, optimized, and
converged. In the absence of symmetry constraints, optimization of
[C3H2N2]CudCH2 starting with a twist angle of 45° between the
â-diketiminate backbone and carbene planes gave a converged structure
nearly identical to the C2V model 6, indicating an electronic preference
for the carbene to be perpendicular to the â-diketiminate backbone.
An alternative C2V structure for 6 in which the â-diketiminate backbone
and carbene moieties are coplanar was significantly higher in electronic
energy. Despite the dramatic simplification of models 6 and 7 relative
to {[Me2NN]Cu}2(µ-CPh)2 (3) and [Me3NN]CudCPh2 (8), bond
distances in the models were in reasonable agreement to those found
in the X-ray structures of 3 and 8, but tended toward shorter Cu-
ligand bond distances because of the absence of steric effects present
in the full systems 3 and 8.
styrene minimizes thermal decomposition of the intermediate
metal carbene, an important consideration in most catalytic
cyclopropanation protocols.
Future reports will explore the ability of this Cu(I) system to
stabilize carbenes derived from a wider range of diazo reagents
and examine their group transfer reactivity to other unsaturated
substrates. Furthermore, the unique bonding mode observed in
the dicopper carbenes 3 and 9 suggests the use of two
[â-diketiminato]Cu fragments in concert to stabilize other highly
electrophilic functional groups such as nitrenes (NR) for related
copper-catalyzed group transfer processes.41
Experimental Section
General Considerations. All experiments were carried out in a dry
nitrogen atmosphere using glovebox and standard Schlenk line tech-
niques when required. 4A molecular sieves were activated at 180 °C
in vacuo for 24 h. Anhydrous toluene was purchased from Aldrich
and was stored over 4A molecular sieves prior to use. Diethyl ether,
pentane, and hexane were distilled before use from sodium/benzophe-
none. All deuterated solvents were sparged with nitrogen, dried over
4A molecular sieves, and stored under nitrogen. 1H and 13C{1H} NMR
spectra were recorded on Mercury Varian 300 MHz spectrometer at
300 and 75.4 MHz, respectively, at 25 °C unless otherwise noted. The
spectra were indirectly referenced to TMS using residual solvent signals
as internal standards. GC-MS spectra were recorded on a Fisions
Instruments MD800, UV-vis spectra were taken with an Agilent 8453
diode array spectrometer, and elemental analyses were performed on a
Perkin-Elmer PE2400 microanalyzer in our laboratory.
Anhydrous CuBr was obtained from Strem and used as received.
Styrene and para-substituted styrenes were obtained from Aldrich and
passed through activated Al2O3 before use. Ethyl diazoacetate containing
up to 10% dichloromethane was obtained from Aldrich and used as
received. Tetraphenylethylene and naphthalene for GC-MS standards
were obtained from Aldrich and Acros, respectively, and used as
received. Tl[Me2NN],22 Tl[Me3NN],34 [Me2NN]Cu(ethylene),22 di-
phenyldiazomethane,42 and para-trifluoromethylstyrene43 were synthe-
sized according to literature procedures.
Computational Details. The DFT calculations employed the
Becke-Perdew exchange correlation functional44 using the Amsterdam
Density Functional suite of programs (ADF 2002.03).45 Slater-type
orbital (STO) basis sets employed for H, C, and N atoms were of triple-ú
quality augmented with two polarization functions (ZORA/TZP2-ADF
basis V), while an improved triple-ú basis set with two polarization
functions (ZORA/TZP2+) was employed for the Cu atom. Scalar
relativistic effects were included by virtue of the zero order regular
approximation (ZORA).46 The 1s electrons of C and N as well as the
1s-2p electrons of Cu were treated as frozen core. The Vosko, Wilk,
and Nusair (VWN) functional was used for local density approximation
(LDA).47 The contour plots in Figures 5 and 6 were rendered with the
MOLEKEL molecular graphics package.48
Catalytic Cyclopropanation of Styrene and EDA by [Me2NN]-
Cu(ethylene) (2). EDA (0.28 mg, 2.211 mmol) (commercial sample
contained ca. 10% CH2Cl2) was added to a solution of [Me2NN]Cu-
(ethylene) (0.017 g, 0.044 mmol) and styrene (1.150 g, 11.06 mmol)
in 5 mL of toluene. The mixture was stirred for 2h, naphthalene (0.195
g) was added as an internal standard, and an aliquot of the resulting
solution was diluted and analyzed by GC-MS to give a 76.9% yield
of 1-ethoxycarbonyl-2-phenyl-cyclopropane with diastereoselectivity
of cis/trans ≈ 38/62. The trans isomer was isolated and characterized
1
by H NMR, which was consistent with literature data.49
Catalytic Cyclopropanation of Styrene and N2CPh2 by [Me2NN]-
Cu(ethylene) (2). To a solution of [Me2NN]Cu(ethylene) (0.015 g,
0.038 mmol) and styrene (0.780 g, 7.50 mmol) in 10 mL of toluene, a
solution of N2CPh2 (0.165 g, 0.757 mmol) in 9 mL of toluene was
added by syringe pump at room temperature over a period of 20 h.
After stirring at RT for another 6 h, we added naphthalene (0.112 g)
as an internal standard, and an aliquot of the resulting solution was
passed through silica gel and analyzed by GC-MS to give a 68% yield
of 1,1,2-triphenylcyclopropane14 along with the azine Ph2CdN-Nd
50
CPh2 in ca. 30% yield.
Preparation of {[Me2NN]Cu}2(µ-CPh2) (3). A cooled (-35 °C)
solution of N2CPh2 (0.074 g, 0.34 mmol) in 3 mL of toluene was added
with stirring to a cooled (-35 °C) solution of [Me2NN]Cu(ethylene)
(0.268 g, 0.675 mmol) in 5 mL of toluene. The color of solution turned
dark purple immediately, and effervescence was observed. After being
stirred at room temperature for 3 min, the reaction mixture was placed
into the freezer and allowed to stand overnight. The volatiles were
removed in vacuo, and the residue was extracted with ether (10 mL)
and filtered through Celite. The filtrate was concentrated and allowed
to stand at -35 °C. Dark purple crystals which had formed were
collected on a frit, washed with cold ether, and dried in vacuo to afford
0.122 g (40.3%) of product as a 1:1 ether solvate. Recrystallization
from hexane afforded crystals of 3 • 0.75 hexane suitable for X-ray
1
diffraction. H NMR (toluene-d8, -70 °C): δ 7.3-6.4 (m, 20, Ar),
6.16 (d, 2, o-CPh2), 5.010 (s, 2, backbone-CH), 2.951 (s, 6, Ar-CH3),
1.953 (s, 6, Ar-CH3), 1.920 (s, 6, Ar-CH3), 1.639 (s, 6, backbone-
CH3), 1.565 (s, 6, backbone-CH3), 1.213 (s, 6, Ar-CH3). 13C{1H} NMR
(toluene-d8, -70 °C): δ 189.41 (CPh2), 162.75 (imine), 161.18 (imine),
152.98, 148.46, 147,50, 138.14, 137.04, 133.12, 132.91, 129.36, 128.84,
128.21, 127.32, 125.13, 124.82, 124.49 (four aromatic resonances
obscured or coincident), 96.81 (backbone-CH), 21.46, 21.21, 20.95,
20.70, 20.45. 20.19 (one backbone Me obscured or coincident). Anal.
Calcd for C55H60N4Cu2: C, 73.06; H, 6.69; N, 6.20. Found: C, 73.19;
H, 6.65; N, 6.09.
(41) A disilver(I) complex has been recently reported to efficiently catalyze the
aziridination of alkenes with PhIdNTs: Cui, Y.; He, C. J. Am. Chem. Soc.
2003, 125, 16202.
(42) Smith, L. I.; Howard, K. L. Organic Syntheses; Wiley & Sons: New York,
1955; Vol. 3, p 351.
(43) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H. J. Am.
Chem. Soc. 1994, 116, 9869.
(44) (a) Becke, A. Phys. ReV. A 1988, 38, 3098. (b) Perdew, J. P. Phys. ReV. B
1986, 34, 7406. (c) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(45) (a) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.;
Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931. (b) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E.
J.; Acc., T. C. Theor. Chem. Acc. 1998, 99, 391. (c) ADF2002.03, SCM;
Theoretical Chemistry, Vrije Universieit: Amsterdam, The Netherlands.
http://www.scm.com.
(48) (a) Flu¨kiger, P. F. Ph.D. Thesis, University of Geneva, Geneva, Switzerland,
1992. (b) Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766. (c) Swiss
National Supercomputing Centre Home Page. http://www.cscs.ch/molekel.
(49) Solladie-Cavallo, A.; Diep-Vohuule, A.; Isarno, T. Angew. Chem., Int. Ed.
1998, 37, 1689.
(46) (a) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909. (b)
Ziegler, T.; Tschinke, V.; Baerends, E. J.; Snijders, J. G.; Ravenek, W. K.
J. Phys. Chem. 1989, 93, 3050. (c) van Lenthe, E.; Baerends, E. J.; Snijders,
J. G. J. Chem. Phys. 1993, 99, 4597.
(47) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(50) Barluenga, J.; Fustero, S.; Gomez, N.; Gotor, V. Synthesis 1982, 966.
9
10092 J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004

Discrete Copper Carbenes in Cyclopropanation
A R T I C L E S
Table 4. Cyclopropanation of Alkenes by {[Me2NN]Cu}2(µ-CPh2)
(3)
At -70 °C, [Me2NN]CudCPh2 (4) and [Me2NN]Cu(toluene) (5)
were also present in ca. 2% of the concentration {[Me2NN]Cu}2(µ-
CPh2) (3) as determined by H NMR. Selected H NMR resonances
1
1
yield of
cyclopropanea
reaction
timeb
olefin
styrene
tetraphenylethylenea
for [Me2NN]CudCPh2 (4): δ 7.400 (d, 2, o-Ph), 4.958 (s, 1, backbone-
1
CH), 2.026 (s, 6, Ar-CH3). Selected H NMR resonances for [Me2-
100
0
1/2
NN]Cu(toluene) (5): δ 4.801 (s, 1, backbone-CH), 2.384 (s, 6, Ar-
CH3). With increasing temperature, 4 and 5 increased in intensity at
the expense of 3.
R-methylstyrene
100
0
73.2
2
trans-â-
26.8 (32.0c)
2 (45 °C)d
methylstyrene
cycloocetene
1-hexene
19.7
21.0
0
80.3
79.0
100
2 (45 °C)d
2 (45 °C)d
2 (45 °C)d
The thermal decomposition of compound 2 in benzene-d6 (ca. 20
1
mM) was monitored by H NMR spectroscopy. After 56 h at room
No. olefin
temperature, 57% of the initial concentration of 2 was still present.
The other â-diketiminate containing product observed was the solvento
species [Me2NN]Cu(benzene-d6).
a
b
c
In percent. In hours. This yield was determined directly from the
cyclopropane peak area using the same response factor determined for 1,1,2-
triphenylcyclopropane. No change after stirring for 2 h at room temper-
ature and then heating to 45 °C for 2 h.
d
Preparation of [Me2NN]Cu(toluene) (5). A solution of [Me2NN]-
Tl (0.450 g, 0.883 mmol) in 5 mL of toluene was stirred with powdered
CuBr (0.325 g, 2.262 mmol) for 1 day. The volatiles were removed in
vacuo, the residue was extracted with 10 mL of pentane, and the mixture
was filtered through Celite. The filtrate was concentrated, a few drops
of toluene were added, and the solution was allowed to stand at -35
Survey of Reactivity of {[Me2NN]Cu}2(µ-CPh2) (3) with Different
Olefins. A 10.0-mL solution of {[Me2NN]Cu}2(µ-CPh2) (0.101 g, 0.112
mmol, 0.012 mM) in toluene was divided into five 2.0-mL portions.
Fifty equivalents of each olefin (1.117 mmol) were added individually
to separate 2.0-mL portions of dicopper carbene. In the case of styrene
and R-methylstyrene, the corresponding solutions turned yellow within
2 h at room temperature. For the other alkenes, the resulting solutions
were heated at 45 °C for 2 h to completely discharge the intense purple
color of the copper carbene species. A solution of naphthalene (0.052
g) in 10.0 mL of toluene was divided to five 2-mL portions and added
individually as an internal standard to each of the five solutions above.
The resulting solutions were analyzed by GC-MS (Table 4). In all
cases the yield of Ph2CdCPh2 was determined directly against the
internal naphthalene standard. The yield of cyclopropane was obtained
by mass balance since no other Ph2C-containing products were
observed. In the case of styrene, the quantitative conversion to 1,1,2-
triphenylcyclopropane was verified by using an authentic standard. A
blank solution without olefin was decomposed under the same
conditions to give a quantitative yield of Ph2CdCPh2.
Preparation of [Me3NN]Cu(toluene). An analogous procedure as
that for [Me2NN]Cu(toluene) was followed employing [Me3NN]Tl
(0.491 g, 0.913 mmol) and CuBr (0.386 g, 2.690 mmol) to give 0.148
g (33.2%) dark brown crystals. 1H NMR (benzene-d6): δ 6.8-7.1 (m,
11, Ar-H), 4.797 (s, 1, backbone-CH), 2.269 (s, 6, Ar-CH3), 2.106
(s, 3, toluene-CH3), 2.032 (s, 12, Ar-CH3), 1.672 (s, 6, backbone-
CH3). 13C{1H} NMR (benzene- d6, partial data): 162.72, 160.96, 148.19,
141.80, 137.80, 131.97, 130.30, 129.26, 94.46 (backbone-CH), 23.27,
21.15, 18.98, 18.65. Anal. Calcd for C30H37N2Cu: C, 73.66; H, 7.62;
N, 5.73. Found: C, 73.34; H, 7.31; N, 5.31.
1
°C to give 0.118 g (28.9%) dark brown crystals. H NMR (benzene-
d6): δ 6.9-7.1 (m, 11, Ar-H), 4.769 (s, 1, backbone-CH), 2.128 (s, 3,
toluene-CH3), 2.011 (s, 12, Ar-CH3), 1.625 (s, 6, backbone-CH3). 13C-
{1H} NMR (benzene-d6, partial data): 162.42, 150.55, 132.33, 130.63,
129.28, 94.42 (backbone-CH), 23.28, 19.03, 18.69. Anal. Calcd for
C28H33N2Cu: C, 72.93; H, 7.21; N, 6.07. Found: C, 72.66; H, 7.09;
N, 5.94.
Determination of Thermodynamic Parameters ∆H and ∆S for
the Dissociation of One [Me2NN]Cu Fragment from {[Me2NN]Cu}2-
(µ-CPh2) (3) in Toluene. A sample of {[Me2NN]Cu}2(µ-CPh2) (0.020
g, 0.024 mmol) was dissolved in 1 mL of toluene-d8 in a volumetric
flask and transferred to an NMR tube. 1H NMR spectra were acquired
every 10° over the range of temperatures -20 to 60 °C. The use of
relative integrals for the â-diketiminato backbone C-H resonances
corresponding to {[Me2NN]Cu}2(µ-CPh2) (3) (δ 5.010 ppm), [Me2-
NN]CudCPh2 (4) (δ 4.958 ppm), and [Me2NN]Cu(toluene) (5) (δ 4.801
ppm) against an internal standard allowed the calculation of the
equilibrium constants at different temperatures using the following
dependence with an initial {[Me2NN]Cu}2(µ-CPh2)} (3) concentration
of 0.024 M:
Keq
)
[initial concn (M) of 3] × {([Me2NN]CudCPh2) × ([Me2NN]Cu(toluene))}
{[Me2NN]Cu}2(µ-CPh2)
Preparation of [Me3NN]CudCPh2 (8). A cooled (-35 °C) solution
of N2CPh2 (0.061 g, 0.280 mmol) in 3 mL of ether was added with
stirring to a cooled (-35 °C) solution of [Me3NN]Cu(toluene) (0.135
g, 0.276 mmol) in 5 mL of ether. The color of solution turned metallic
purple immediately. After being stirred at room temperature for 3 min,
the reaction mixture was filtered through Celite, and the filtrate was
concentrated and allowed to stand at -35 °C to give dark purple
crystals. The product was isolated and recrystallized from pentane at
-35 °C to afford 0.031 g (20%) of product. 1H NMR (benzene-d6): δ
7.376 (dd, 4, m-CPh), 7.138 (m, 2, p-CPh), 6.866 (t, 4, o-CPh), 6.643
(s, 4, Ar-H), 4.941 (s, 1, backbone-CH), 2.331 (s, 12, Ar-o-CH3), 1.996
(s, 6, Ar-p-CH3), 1.593 (s, 6, backbone-CH3). 13C{1H} NMR (benzene-
d6): δ 253.10 (CPh2), 162.23, 155.86, 148.83, 132.54, 130.89, 129.23,
128.92, 128.88, 128.82, 96.44 (backbone-CH), 23.18 (Ar-o-CH3), 21.27
(Ar-p-CH3), 19.31 (backbone-CH3). Anal. Calcd for C36H39N2Cu: C,
76.77; H, 6.98; N, 4.97. Found: C, 76.89; H, 6.73; N, 4.63.
A van’t Hoff plot of ln Keq vs 1/T allowed in calculation of ∆H ) 9.1
(3) kcal/mol (from slope) and ∆S ) 24(1) cal/mol‚K (from intercept)
(Figure 3).
Reaction of {[Me2NN]Cu}2(µ-CPh2) (3) with Styrene. Styrene
(0.010 g, 0.009 mmol) and {[Me2NN]Cu}2(µ-CPh2) (0.026 g, 0.003
mmol) were dissolved in 0.75 mL of benzene-d6 and stirred until the
purple color of 2 completely disappeared (ca. 45 min). 1H NMR analysis
showed that [Me2NN]Cu(styrene)22 at δ 4.858 ppm (1H, s, backbone
C-H), 2.215 ppm (6H, br. s, Ar-CH3), 1.724 ppm (6H, br. s, Ar-
CH3), 1.585 ppm (6H, s, backbone-CH3), and 1,1,2-triphenylcyclo-
propane at δ 2.870 ppm (1H, m, CHPh) were formed in a 2:1 ratio.
1,1,2-Triphenylcyclopropane14 was also identified by GC-MS.
Reaction of {[Me2NN]Cu}2(µ-CPh2) (3) with N2CPh2. A solution
of N2CPh2 (0.028 g, 0.128 mmol) in 2 mL of toluene was added with
stirring to a solution of {[Me2NN]Cu}2(µ-CPh2) (0.114 g, 0.126 mmol)
in 5 mL of ether. After being stirred at room temperature for 15 min,
the reaction mixture was placed into the freezer and allowed to stand
overnight. Colorless crystals were isolated by decantation of the
solution, rinsed with chilled pentane, and dried in vacuo to give 0.019
g (37%) of the azine Ph2CdN-NdCPh2,50 whose identity was
confirmed by GC-MS.
Kinetic Studies for the Cyclopropanation of Styrene with
[Me3NN]CudCPh2 (8) in 1,4-Dioxane. A 25.0-mL stock solution of
[Me3NN]CudCPh2 (11.8 mg, 0.021 mmol) and styrene (218 mg, 2.10
mmol) in 1,4-dioxane was prepared using a volumetric flask. This stock
solution was divided to five 5.0-mL portions, and each was frozen in
dry ice until used. In separate experiments, the decreasing concentration
9
J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004 10093

A R T I C L E S
Dai and Warren
Table 5. Relative Rates of Cyclopropanation of Styrene
Derivatives by [Me3NN]CudCPh2 (8)
A∞] versus time gave straight lines with observed rate constants that
appear in Table 3. Uncertainties in reported rate constants were
estimated on the basis of inspection of the sensitivity of the fits of the
ln[At - A∞] versus time plots.
styrene
derivative
rate constant for
cyclopropanation
(UV−vis, kobs (s-1))
yield of (X)H)
cyclopropane
(%)
kX/kH
kX/kH
(p-X−C H −CHdCH )
(competition) (UV−vis)
6
4
2
Thermal Decomposition of [Me3NN]CudCPh2 (8) in Benzene-
d6. An approximate 40 mM solution of [Me3NN]CudCPh2 (0.015 g)
X ) OMe
X ) Me
X ) H
7.27(9) × 10-4
3.65(5) × 10-4
2.47(5) × 10-4
1.35(5) × 10-4
30.3a
41.5
100
2.3a
1.41
1.00
1.02
2.95
1.48
1.00
1.28
1
in benzene-d6 (0.7 mL) was prepared and monitored by H and 13C-
{1H} NMR spectroscopy at room temperature. Within 2.5 h, the 13C
NMR signal of CPh2 at δ 253.10 ppm decayed, and a new resonance
at δ 189.0 ppm grew in. After 12 h, there was a 1.55:1.00:0.19 molar
ratio of {[Me3NN]Cu}2(µ-CPh2)/[Me3NN]CudCPh2/[Me3NN]Cu-
(benzene-d6) in the solution. Partial NMR data for {[Me3NN]Cu}2(µ-
CPh2) (9): 1H NMR (benzene-d6): δ 5.018 (s, backbone-CH), 1.624
(s, backbone-CH3), 1.8-2.3 (br, Ar-CH3). 13C{1H} NMR (benzene-
d6): δ 189.0 (CPh2), 97.11 (backbone-CH), 23.92 (Ar-o-CH3), 21.28
(Ar-p-CH3), 19.99 (backbone-CH3). Partial data for [Me3NN]Cu-
(benzene-d6): 1H NMR (benzene-d6): δ 4.810 (s, backbone-CH), 2.033
(s, 12, Ar-CH3).
X ) CF3
49.5
b
(1.05(5) × 10-4
)
a
The yield of 1,1,2-triphenylcyclopropane could not be accurately
obtained because of the overlapping of p-methoxystyrene and the naph-
thalene standard in the gas chromatogram. The ratio of kX/kH comes from
b
the ration of peak area in GC-MS. The relative rate for cyclopropanation
of para-trifluoromethylstyrene vs styrene was determined in a separate run;
the number in parentheses is the corresponding rate constant for styrene
cyclopropanation under the slightly different conditions.
of [Me3NN]CudCPh2 over time was quantified by UV-vis spectros-
copy by monitoring the decrease in intensity of the band due to 8 at
λmax ) 566 nm at the temperatures 17.5, 30.2, 40.5, 50.0, and 60.2 °C.
Data were generally taken for ca. 3 half-lives. Temperatures can be
deemed accurate (0.5 °C. Plots of ln[At - A∞] versus time gave straight
lines with observed rate constants that appear in Table 2. Uncertainties
in reported rate constants were estimated on the basis of inspection of
the sensitivity of the fits of the ln[At - A∞] versus time plots. The
X-ray Structure Refinement Details. Single crystals of each
compound were mounted under mineral oil on glass fibers and
immediately placed in a cold nitrogen stream at -90(2) °C on a Bruker
SMART CCD system. Hemispheres of data were collected (0.3°
ω-scans; 2θmax ) 56°; monochromatic Mo KR radiation, λ ) 0.7107
Å) and integrated with the Bruker SAINT program. Structure solutions
were performed using the SHELXTL/PC suite51 and XSEED.52 Intensi-
ties were corrected for Lorentz and polarization effects, and an empirical
absorption correction was applied using Blessing’s method as incor-
porated into the program SADABS.53 Non-hydrogen atoms were refined
with aniostropic thermal parameters, and hydrogen atoms were included
in idealized positions. Because disordered hexane molecules of solvation
were found in the initial refinement of {[Me2NN]Cu}2(µ-CPh2) (3) that
could not be satisfactorily modeled, the SQUEEZE subroutine of
PLATON54 was used. A total of 314 solvent electrons were identified
corresponding to ca. 0.75 molecules of hexane per {[Me2NN]Cu}2(µ-
CPh2) (six hexanes per unit cell), and the reflection data were refined
excluding the solvent to give refinement details collected in Tables
S1-S5.
concentration of styrene was 0.08385 M in each case, leading to kact
obs/0.0839 M.
Relative Rates for Cyclopropanation of para-Substituted Sty-
)
k
renes (p-X-C6H4-CHdCH2) with [Me3NN]CudCPh2 (8). Two
separate methods were used to determine the relative rates of cyclo-
propanation kX/kH for para-substituted styrenes (X ) OMe, M, H, CF3)
compared to that of styrene (X ) H) collected in Table 5 used to prepare
the Hammet plot in Figure 10.
(a) Competition Experiments. A solution of [Me3NN]CudCPh2
(10.9 mg, 0.019 mmol) with an internal standard of naphthalene in
1,4-dioxane was diluted to 10.0 mL using a volumetric flask. The
solution was divided to five 2-mL portions to which styrene and a
substituted styrene were added. The solutions were stirred for 1 h and
analyzed by GC-MS. The yield of 1,1,2-triphenylcyclopropane (YH)
was determined against the naphthalene standard. As <1% Ph2CdCPh2
was observed in each of these competition experiments, the yield of
the p-substituted 1,1,2-triphenylcyclopropane (YX) was determined by
mass balance. The relative rate kX/kH was thus determined by the
relationship: kX/kH ) YX/YH ) (100 - YH)/YH (Table 5).
(b) Direct Kinetic Measurements. For comparison, kinetic studies
were carried out with para-substituted styrenes (X ) OMe, Me, H,
CF3) by the method described for the cyclopropanation of styrene by
8 using 100 equiv of the substituted styrene. The temperature of the
UV-vis cell was thermostated at 40.0 °C during the kinetic measure-
ments for each member of the series. The rate constants kX and kH thus
were independently measured under identical conditions, allowing for
the tabulation of kX/kH for each styrene derivative (Table 5).
Kinetic Studies for Decomposition of [Me3NN]CudCPh2 (8) in
1,4-Dioxane at Various Temperatures. A 25.0-mL stock solution of
[Me3NN]CudCPh2 (11.8 mg, 0.021 mmol) in 1,4-dioxane was prepared
using a volumetric flask. This stock solution was divided to five 5.0-
mL portions and frozen in dry ice until used. In separate experiments,
the decreasing concentration of [Me3NN]CudCPh2 over time was
quantified by UV-vis spectroscopy by monitoring the decrease in
intensity of the band due to 8 at λmax ) 566 nm at the temperatures
32.2, 40.0, 49.0, and 60.0 °C. Data were generally taken for ca. 3 half-
lives. Temperatures can be deemed accurate (0.5 °C. Plots of ln[At -
Acknowledgment. T.H.W. thanks the NSF CAREER pro-
gram (No. 0135057) for support of this work.
Supporting Information Available: VT 1H NMR spectra for
3, additional calculational details for calculated structures 6,
7-C2v, and 7-C2, plots of ln[(At - A∞)/(A0 - A∞)] versus time
for cyclopropanation of styrene by 8 from 17.5 to 60.2 °C,
representative UV-vis plot for thermal decomposition of 8,
plots of ln[(At - A∞)/(A0 - A∞)] versus time for thermal
decomposition of 8 from 30.2 to 60.0 °C, tables of X-ray
refinement data, bond distances, and bond angles along with
fully labeled ORTEP plots for 3 and 8 (also provided in CIF
format). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA047935Q
(51) (a) SHELXTL-PC, version 5.10; Bruker-Analytical X-ray Services: Madi-
son, WI, 1998. (b) Shelrick, G. M. SHELX-97, Universita¨t Go¨ttingen,
Go¨ttingen: Germany, 1997.
(52) (a) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189. (b) XSEED Home
Page, http://xseed.net.
(53) (a) Sheldrick, G. M. SADABS, version 2.01; Bruker-Analytical X-ray
Services: Madison, WI, 1999. (b) Based on the method described in:
Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
(54) Spek, A. L. Acta Crystallogr. 1990, A46, C43.
9
10094 J. AM. CHEM. SOC. VOL. 126, NO. 32, 2004