Investigation of ligand steric effects on a highly cis-selective Rh(i) cyclopropanation catalyst

Article abstract of DOI:10.1039/c1nj20343f

Four new Rh(i) complexes bearing chelating imine-functionalized N-heterocyclic carbene ligands have been synthesized and characterized. The catalytic activity of these new Rh(i) complexes has been tested in the cyclopropanation reaction between ethyl diazoacetate and styrene. One of the new complexes, having ethyl groups on the ligand N-aryl ring, exhibited a reactivity and a cis-diastereoselectivity that were comparable to our previously reported Rh(i) cyclopropanation catalyst of this type, and a higher yield and cis-diastereoselectivity were obtained at lower catalyst loadings and higher temperatures. The other new Rh(i) complexes were found to be inferior to the previously reported Rh(i) cyclopropanation catalyst. The catalytic study gave important information about the effect that changing the steric requirements of the substituents at the ligand system has on the efficiency and cis-diastereoselectivity of the complexes as cyclopropanation catalysts.

Full text of DOI:10.1039/c1nj20343f

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PAPER
Investigation of ligand steric effects on a highly cis-selective Rh(I)
cyclopropanation catalystwz
Marianne Lenes Rosenberg,a Eirin Langseth,a Alexander Krivokapic,a
Nalinava Sen Guptaa and Mats Tilset*b
Received (in Montpellier, France) 18th April 2011, Accepted 16th June 2011
DOI: 10.1039/c1nj20343f
Four new Rh(I) complexes bearing chelating imine-functionalized N-heterocyclic carbene ligands
have been synthesized and characterized. The catalytic activity of these new Rh(I) complexes has
been tested in the cyclopropanation reaction between ethyl diazoacetate and styrene. One of the
new complexes, having ethyl groups on the ligand N-aryl ring, exhibited a reactivity and a
cis-diastereoselectivity that were comparable to our previously reported Rh(I) cyclopropanation
catalyst of this type, and a higher yield and cis-diastereoselectivity were obtained at lower catalyst
loadings and higher temperatures. The other new Rh(I) complexes were found to be inferior to
the previously reported Rh(I) cyclopropanation catalyst. The catalytic study gave important
information about the effect that changing the steric requirements of the substituents at the ligand
system has on the efficiency and cis-diastereoselectivity of the complexes as cyclopropanation
catalysts.
Introduction
N-Heterocyclic carbenes (NHC’s) have since Arduengo’s
report on a stable NHC in 19911,2 been extensively used as
ligands for transition metal complexes.3–7 The NHC metal
complexes are excellent catalysts for a wide range of chemical
transformations,3,8,9 for example, for olefin metathesis10 and
C–C cross-coupling reactions,11,12 where the use of NHC
ligands has allowed for major improvements. Chelating
Fig. 1 Rh(I) complex 1, bearing a chelating N-heterocyclic imino-
NHC ligands have also attracted considerable attention, and
carbene ligand.
metal catalysts bearing these ligands have successfully been
used in a range of different reactions.13–15 So far, the NHC
NHC ligand, and the use of this complex in cyclopropanation
reactions.19
ligands have not yet been greatly utilized in the much studied
carbenoid cyclopropanation and insertion reactions. Perez and
This Rh(I) complex, after activation with AgOTf19 or, even
co-workers16,17 have reported the use of some NHC Ag, Au,
better with NaBArf,20 displays a remarkably high reactivity
and Cu complexes in these reactions, showing promising
and cis-diastereoselectivity in the cyclopropanation reaction
results. A Rh(I) complex with a NHC ligand has also
between ethyl diazoacetate (EDA) and styrene (Scheme 1).
previously been tested in cyclopropanation reactions, giving
Excellent results are also obtained with a range of substituted
high yields but low diastereoselectivity.18
We have recently reported the synthesis of a novel Rh(I)
complex 1 (Fig. 1) bearing a chelating imine-functionalized
a Department of Chemistry, University of Oslo,
P.O. Box 1033 Blindern, N-0315, Oslo, Norway
b Centre of Theoretical and Computational Chemistry (CTCC),
Department of Chemistry, University of Oslo,
P.O. Box 1033 Blindern, N-0315, Oslo, Norway.
E-mail: mats.tilset@kjemi.uio.no
w Dedicated to Prof. Didier Astruc on the occasion of his 65th
birthday.
Scheme 1 Highly cis-selective cyclopropanation reaction between
z CCDC reference numbers 800306 & 800307. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1nj20343f
EDA and styrene catalyzed by Rh(I) complex 1.
c
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alkenes.19,20 It can be a challenge in intermolecular cyclo-
propanation reactions to achieve good diastereoselectivity,
especially with simple a-diazoacetates like EDA.21
There are numerous reports on highly efficient and selective
catalysts for the formation of the thermodynamically favored
trans isomer.21–23 On the other hand, efficient and selective
synthesis of the unfavored cis isomer remains more of a
challenge. There are just a few reports on highly cis-selective
catalysts.21,24–37 The reported results using Rh(I) catalyst 1 are
among the highest cis-selectivities reported in these reactions
and, to the best of our knowledge, the highest cis-selectivities
reported thus far using a Rh catalyst. This report was also the
first on the use of a chelating imine-functionalized NHC ligand
system in cyclopropanation reactions.
In this contribution, we present the synthesis of several new
Rh(I) complexes that bear chelating imine-functionalized
NHC ligands. These new Rh(I) complexes are analogues of
complex 1 in which substituents at the ligand system have been
varied. The catalytic activity of the new complexes has
been tested in cyclopropanation reactions, giving useful
information about the importance of certain substituents on
the ligand system for obtaining high yields and cis-selectivities.
Scheme 2 Synthesis of complexes 1,19 4, 5 and 6.
Results and discussion
Synthesis
Four new Rh(I) complexes have been synthesized (Fig. 2). In
complexes 4, 5, and 6 the methyl groups at the N-aryl ring in 1
have been replaced with more sterically demanding ethyl (4) and
iso-propyl groups (5) and less sterically demanding hydrogens (6).
In complex 7, the N-methyl group at the NHC in complex 1 has
been replaced by a bulkier tert-butyl group. Complexes 4, 5, and 6
were synthesized in good yields by adaptation of the reported
method for preparation of 1 (Scheme 2).19,38,39
Complex 7 was synthesized by the route shown in Scheme 3.
1-tert-Butyl imidazole is not commercially available and was
prepared by the method of Gridnev and Mihaltseva.40 In the
synthesis of the imidazolium salt 13, the procedure of Green
and co-workers41 was followed. The reaction mixture was
heated at 60 1C for two hours instead of our commonly
employed procedure of stirring the reaction mixture at
ambient temperature (Scheme 2).19,38,39 For the synthesis
of imidazolium salt 13, this procedure proved to be more
efficient. Imidazolium salt 13 was reacted with Rh(acac)CO2
and complex 7 was formed.
Scheme 3 Synthesis of complex 7.
Characterization
13C NMR spectra (d 185.5–183.5) with coupling constants
J(103Rh–13C) = 58–62 Hz, confirming the bonding of the
NHC ligand to Rh.42 Because of the very low solubility of 6, a
13C NMR spectrum could not be obtained for this complex.
As previously reported for complex 1,19 complexes 4, 5 and 7
display characteristic doublets for the carbene carbon in the
1
The H NMR spectrum of complex 4 exhibits two signals for
the ethyl-CH2 protons at the N-aryl ring. Two doublets of
quartets at d 2.74 and 2.32 were observed.
This indicates that rotation around the N(imine)-aryl bond
is severely restricted, rendering the CH2 hydrogens diastereotopic.
A similar feature was also observed in the 1H NMR spectra of
complex 5, where two doublets from the diastereotopic
i-Pr–CH3 groups were found at d 1.28 and 0.90. This has also
been reported for Pd and Pt complexes bearing the same
Fig. 2 The four new Rh(I) complexes.
c
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ligand system.39 There is a rather large separation in chemical
shifts between the two diastereotopic signals in complex 4
(0.42 ppm) and in 5 (0.38 ppm). This agrees with a previously
reported trend, based on a large number of 5- and 6-membered
chelate ring complexes, in that the separation between these
signals is larger in k2(C,N) chelating iminocarbene structures
(typically 0.3–0.6 ppm) than in non-chelating k1(C) imino-
carbene complexes (o0.2 ppm).39,43 The IR nCQN absorption
of the imine was lowered by 48–69 cmꢀ1 in the new Rh(I)
complexes compared to that of the imidazolium salts. This
indicates coordination of the imine nitrogen to the Rh center
with resulting back-bonding into the CQN bond. The
combined data strongly suggest that the iminocarbene ligands
have formed a k2(C,N) chelate at Rh.
Table 1 Selected bond lengths (A) and angles (1) from the X-ray
structures of complexes 1, 5, and 7
Bond lengths and angles
119
5
7
Rh–C1
Rh–CO
Rh–N1
Rh–Cl
C1–Rh–CO
OC–Rh–Cl
Cl–Rh–N
N1–Rh–C1
N2–C1–Rh
N3–C1–Rh
1.953(2)
1.813(3)
2.131(2)
2.367(6)
97.39(1)
90.38(8)
93.40(6)
78.53(9)
113.6(2)
142.4(2)
1.937(2)
1.819(2)
2.130(2)
2.366(5)
96.73(9)
91.39(7)
93.66(5)
78.22(7)
114.9(1)
141.5(2)
1.979(3)
1.808(2)
2.117(2)
2.351(1)
101.6(1)
87.64(2)
91.51(6)
79.14(1)
111.5(2)
145.0(2)
The results from single-crystal X-ray structure determinations
of 5 and 7 are presented in Fig. 3 and 4, respectively. The
X-ray structure analyses confirmed the expected monomeric
structures with chelating NHC iminocarbene ligands and
a cis relationship between the NHC and the CO ligands.
The complexes assume a slight distortion from the square-planar
geometry, as expected for unsymmetrically substituted four-
coordinate d8 species. Some selected bond lengths and angles
for 5 and 7 are shown in Table 1. Bond lengths and angles for 1
from the previously reported X-ray structure19 are included
for comparison. As can be seen in Table 1, the carbene C1–Rh
bond is shortest in complex 5 and longest in complex 7. For all
complexes 1, 5, and 7, there are deviations from the ideal 901
L–Rh–L0 angles, ranging from 78–791 for the N1–Rh–C1
chelate bite angle to 97–1021 for the C1–Rh–CO angle. These
observations are consistent with the reported structures of Pd
and Pt complexes bearing this type of ligand systems.38,39 The
C1–Rh–CO angle is greatest in complex 7, presumably a result
of the larger steric pressure from the adjacent N-tert-butyl group
in 7 compared to the N-methyl group in 1. This also results in
smaller OC–Rh–Cl and Cl–Rh–N1 angles in 7 than in 1 and 5.
Significant ring strain in the chelate in complexes 1, 5 and 7 is
evident from the 27–341 difference between the N3–C1–Rh and
N2–C1–Rh angles.
Catalysis
We have reported that in order for the cyclopropanation
catalyst 1 to be active, it has to be activated prior to the
reactions, presumably by generation of a vacant coordination
site through removal of the chloride ligand. Both AgOTf19 and
NaBArf20 have been found to be efficient activating agents.
Some of the previously reported results are summarized in
Table 2. As can be seen, excellent results were obtained with
both activating agents in the cyclopropanation reaction of
styrene with EDA when a catalyst loading of 5 mol% was used
(Table 2, entries 1 and 5). However, NaBArf (which has a
Fig. 3 ORTEP drawing of complex 5 at 105 K (H atoms and CH2Cl2
omitted for clarity, ellipsoids at 50% probability).
larger and more loosely coordinating anion than AgOTf44,45
)
proved to be superior at lower catalyst loadings (entries 2, 3, 6,
and 7). The best conditions found so far for cyclopropanation
with EDA catalyzed by 1 are in CH2Cl2 at 0 1C. Increasing the
reaction temperature to ambient results in lower diastereo-
selectivity (entry 4). Formal carbene dimerization is a common
side reaction in carbenoid reactions. In order to circumvent
this problem, the diazo compound is often added very slowly
to the reaction mixture, resulting in rather time consuming
cyclopropanation procedures. Using catalyst 1, EDA can
be added in one portion without any sign of dimerization.
However, when the catalyst loading is reduced to 1 mol%, the
yield of the cyclopropanes and the diastereoselectivity are reduced
(entries 3 and 7) and formal carbene dimerization is observed.
Fig. 4 ORTEP drawing of complex 7 at 293 K (H atoms omitted for
clarity, ellipsoids at 50% probability).
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Table 2 Previously reported results from cyclopropanation reactions with EDA and styrene using 1 as catalyst and AgOTf or NaBArf as
activating agenta
Entry
Mol% 1
Equiv. styrene
Mol% activator
T/1C
Time/h
Yield %f (cis : trans)g
1b
2b
3b
4b
5c
6c
7c
5
2.5
1
5
5
5
10
10
10
5
5d
0
0
0
20
0
0
2
3
48
3
1
2
24
98 (499 : 1)
89 (94 : 6)
50 (92 : 8)
2.5d
1d
5d
96 (92 : 8)
5e
98 (499 : 1)
98 (499 : 1)
49 (96 : 4)
2.5
1
5
5
2.5e
1e
0
a
b
Reaction conditions: 1.00 mmol of EDA and given quantities of 1, activating agent and styrene in 20 mL CH2Cl2. From ref. 19. From ref. 20.
c
d
e
AgOTf. NaBArf. Isolated yield based on EDA limiting reagent. cis : trans ratio determined by H NMR and GC.
f
g
1
Catalysts 4–7 were also tested in the cyclopropanation
was observed. It appears that the iso-propyl groups impose too
reaction between EDA and styrene, following the previously
reported procedure. These results are presented in Table 3.
With 2.5 mol% of catalyst 4 and NaBArf as the activating
agent (entry 1) it was found that 4, which is 2,6-diethyl
substituted at the N-aryl ring, is as efficient and selective as
catalyst 1 which is 2,6-dimethyl substituted at the N-aryl
ring. Reducing the catalyst loading to 1 mol% of 4 at 0 1C
(entry 2) led to lower yield and diastereoselectivity, as was
also reported for 1.19,20 A notable difference between the
two catalysts 1 and 4 was that the reaction was slower with
catalyst 4 than with 1, and after 24 hours there was still
unreacted EDA left in the reaction mixture. The reaction
temperature was raised to ambient in the hope that this would
improve the yield and diastereoselectivity. Indeed, it turned
out that by increasing the temperature, the yield increased to
99% and a quite satisfactory cis :trans selectivity of 98 : 2 was
still obtained (entry 3). As mentioned earlier,19,20 increasing the
reaction temperature using catalyst 1 resulted in lower yield and
diastereoselectivity.
much of a steric hindrance, resulting in a slower and less
diastereoselective cyclopropanation catalyst.
Interestingly, for catalyst 6, with no ortho substituents at the
N-aryl ring, the results were strikingly different from those
observed for catalyst 1. The cyclopropanes 2 and 3 were
formed in only 29% yield using AgOTf as an activating agent
and the cis-selectivity was completely lost, as a 50 : 50 mixture
of the cis and trans isomers was obtained (entry 7). Changing
the activating agent to NaBArf had little effect on the yield
and selectivity (entry 8), but did result in a more reactive
catalyst. With AgOTf there was still unreacted EDA left in the
reaction mixture after 24 hours, while with NaBArf complete
conversion of EDA was achieved within 1 hour. Products
from formal carbene dimerization were observed as the only
byproducts with both activating agents. There were no signs of
homologated products.
Complex 7, in which the N-methyl group at the NHC has
been replaced with a bulkier tert-butyl group, also proved to
be a less efficient and less diastereoselective cyclopropanation
catalyst than catalyst 1 (entry 9). With a catalyst loading of
5 mol% of 7 and NaBArf as the activating agent, 74% of the
cyclopropanes 2 and 3 were formed, and the diastereomeric
ratio was 93 : 7 in favor of the cis isomer. Reducing the catalyst
loading did not reduce the reactivity or selectivity (entry 10).
Rh(I) catalyst 1 is a highly efficient and cis-diastereoselective
cyclopropanation catalyst in reactions between EDA and
sterically unhindered, electron rich alkenes and cyclic alkenes.19,20
On the other hand, with the acyclic aliphatic alkene 1-octene,
the reactivity is good, but the cis-selectivity is only moderate
(Table 4, entry 1).
For complex 5, with the even bulkier iso-propyl groups at
the 2,6-positions of the N-aryl ring, both yield and selectivity
were considerably lower than for 1 when AgOTf was used as
the activating agent (entry 4). An increase of the temperature
(entry 5) did not have the same positive effect as for catalyst 4;
the yield was improved but, as observed for catalyst 1, the
selectivity was lowered. By changing the activating agent to
NaBArf, both yield and selectivity were improved (entry 6),
but the results were still significantly poorer than for 1 and 4.
Complete conversion of EDA was achieved in five hours using
NaBArf as the activating agent, but formal carbene dimerization
Table 3 Catalytic testing of complexes 4–7 in the cyclopropanation reaction between EDA and styrenea
Entry
Cat.
Mol% cat.
Mol% activator
T/1C
Time/h
Yield %d (cis : trans)e
1
2
3
4
5
6
7
8
9
10
4
4
4
5
5
5
6
6
7
7
2.5
1
1
5
5
5
5
5
5
2.5
2.5c
2.5c
1c
0
0
r.t.
0
r.t.
0
0
0
0
0
2
24
2
24
24
5
24
1
3
98 (499 : 1)
55 (96 : 4)
99 (98 : 2)
53 (92 : 8)
73 (89 : 11)
65 (95 : 5)
29 (50 : 50)
30 (53 : 47)
74 (93 : 7)
71 (95 : 5)
5b
5b
5b
5b
5c
5c
2.5c
5
a
Reaction conditions: 1.00 mmol of EDA, 5 equiv. of styrene and given quantities of catalysts 4–7 and activating agent in 20 mL CH2Cl2.
b
c
AgOTf. NaBArf. Isolated yield based on EDA limiting reagent. cis : trans ratio determined by H NMR and GC.
d
e
1
c
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Table 4 Catalytic testing of complexes 1, 4 and 7 in the cyclopropanation reaction between EDA and 1-octenea
Entry
Cat.
Mol% cat.
Mol% NaBArf
T/1C
Time/h
Yield %b (cis : trans)c
1
2
3
1
4
7
2.5
2.5
5
2.5
2.5
5
0
0
0
5
5
24
95 (75 : 25)20
85 (77 : 23)
55 (68 : 32)
a
Reaction conditions: 1.00 mmol of EDA, 5 equiv. of 1-octene and given quantities of catalysts 1, 4, and 7 and NaBArf in 20 mL CH2Cl2.
b
c
Isolated yield based on EDA limiting reagent. cis : trans ratio determined by H NMR and GC.
1
Complexes 4 and 7 were also tested as catalysts for the
Conclusion
cyclopropanation reaction between EDA and 1-octene, to
see whether larger substituents at the NHC nitrogen or at
the N-aryl ring would have a positive effect on the diastereo-
selectivity. Catalyst 4 gave the same selectivity as obtained
with catalyst 1 (entry 2), and with catalyst 7 both yield and
diastereoselectivity were lower than for 1 (entry 3).
Four new Rh(I) complexes 4–7 which bear chelating
imine-functionalized NHC ligands have been synthesized
and characterized. The testing of these complexes in cyclo-
propanation reactions showed that the substituents at the
ortho positions of the N-aryl ring are very important for
achieving high cis-selectivity in these Rh(I) catalyzed cyclo-
propanations. The results also show that having sterically too
demanding substituents at the N-aryl ring or at the NHC
nitrogen has a detrimental effect on the efficiency and diastereo-
selectivity of the catalysts. Complex 4 with ethyl groups at the
ortho positions on the N-aryl ring was shown to be an even
more efficient and cis-selective cyclopropanation catalyst than
the original catalyst 1 at lower catalyst loadings and higher
temperatures. These interesting results broaden the scope,
selectivity and efficiency of our recently discovered, highly
cis-selective Rh(I) catalyzed cyclopropanation reactions.
The above results show that the steric bulk of the substituents at
the imine-functionalized NHC ligand plays a major role
for both the efficiency and the diastereoselectivity of the
Rh(I) catalyzed cyclopropanations. By exchanging the methyl
groups at the N-aryl ring in catalyst 1 with ethyl groups
(catalyst 4), the high yields and diastereoselectivities observed
with catalyst 1 were maintained. In addition, catalyst 4 is a
more efficient and diastereoselective catalyst at lower catalyst
loadings and higher temperatures. Increasing the steric bulk
even further with iso-propyl groups at the ortho-positions
(catalyst 5), on the other hand, gives lower yields and selectivity.
The catalytic results obtained with catalyst 6 further confirm
the importance of the ortho substituents at the N-aryl ring.
Removing the ortho substituents resulted in low yields of the
cyclopropanes (2 and 3) and the cis-selectivity was lost
completely. The catalytic results obtained using catalyst 7
show that increasing the steric bulk of the substituent at the
NHC nitrogen has a detrimental effect on the reactivity and
selectivity.
Experimental
General procedures
All reactions involving organometallic compounds were carried
out with the use of dry box and inert-atmosphere techniques
unless otherwise noted. Solvents for reactions were dried
according to standard procedures. THF, CH2Cl2, Et2O, and
CH3CN were dried using a MB SPS-800 solvent purifying
system from MBraun. NMR spectra were recorded on Bruker
Avance DPX 200, DPX 300, and DRX 500 instruments. The
NMR spectra were recorded at 25 1C. The assignment of
1H and 13C signals was aided by COSY 45, HMQC, NOESY,
and HMBC spectroscopy. For brevity, the following
abbreviations are used for the assignment: Ph = phenyl,
Dmp = 2,6-dimethylphenyl, Dipp = 2,6-di-iso-propylphenyl,
Dep = 2,6-diethylphenyl: Ho/m/p and Ci/o/m/p denote the ipso/
ortho/meta/para atoms relative to the point of attachment
to the imine nitrogen. The imidazolium salts 10–12 were
synthesized according to previously reported procedures.19,38
Mass spectra were obtained on a Micromass QTOF II spectro-
meter (ESI) and on a Fisons VG Prospec sector instrument
at 70 eV (EI). Elemental analyses were performed by Mikro
Kemi AB, Uppsala, Sweden, and Mikroanalytisches
Laboratorium Kolbe, Germany. When appropriate, the presence
of CH2Cl2 in the stated proportions was assumed on the basis
of the fit with the analyses for C, H, and N and confirmed by
1H NMR spectroscopy.
An ongoing computational mechanistic study indicates that
the origin of the high cis-selectivities lies in an unfavorable
steric interaction in the transition state between the alkene
substituent and the ortho substituents of the N-aryl ring in
the path leading to the trans isomer (Scheme 4). This explains
why the cis-selectivity is affected by changing the steric bulk
of the ortho substituents and why the cis-selectivity is lost
when the ortho substituents are removed. Increasing the steric
bulk of the NHC nitrogen as in catalyst 7 affects the transition
state leading to the cis isomer and thereby lowers the cis-
selectivity.
Imidazolium salt 10. 1-Methyl imidazole (0.32 mL,
4.05 mmol, 1.1 equiv.) was added dropwise to the iminochloride
9 (1.00 g, 3.68 mmol, 1.0 equiv.) in dry THF (19 mL). The reaction
Scheme 4 Depiction of transition states leading to cis and trans
products.
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mixture was stirred at ambient temperature for 4 days. The
THF was removed under vacuum. The crude product was
recrystallized from CH2Cl2/THF. This yielded 10 (0.911 g,
70%) as a yellow solid. 1H NMR (CD2Cl2, 300 MHz): d 10.63
(s, 1H, NCHN), 8.01–7.97 (m, 1H, NCHCHN), 7.88 (br s, 1H,
NCHCHN), 7.55–7.30 (m, 5H, Ph–H), 7.01 (s, 3H, Dep-H),
4.30 (s, 3H, NCH3), 2.50–2.38 (m, 2H, CH2CH3), 2.35–2.20
(m, 2H, CH2CH3), 1.09 (t, 6H, J = 7.5 Hz, CH2CH3).
13C{1H} NMR (CD2Cl2, 75 MHz): d 147.8, 142.4, 138.9
(NCHN), 132.6, 132.1 (Ph-CH), 129.5 (Ph-CH), 129.2
(Ph-CH), 126.3 (Dep-CH), 125.3 (Dep-CH), 125.0 (NCHCHN),
119.6 (NCHCHN), 37.6 (NCH3), 24.8 (CH2CH3), 13.6
(CH2CH3). One quaternary carbon was not observed. ESI-MS
(Dipp-Ci), 132.3 (Ph-CH), 129.5 (Ph-Co), 129.1 (Ph-Ci),
127.4 (Ph-CH), 127.0 (Dipp-Cp), 123.9 (NCHCHCH3), 123.5
(Dipp-Cm), 117.3 (NCHCHNCH3), 39.0 (NCHCHNCH3),
28.9 (CH(CH3)2), 24.7 (CH(CH3)2), 23.2 (CH(CH3)2).
ESI-MS (CH3CN): m/z 517 (M+–Cl
+ CH3CN). IR
(CH2Cl2): nCQN 1602 cmꢀ1, nCQO 1978 cmꢀ1. Anal. calc. for
C24H27N3RhOCl ꢁ 1/7 CH2Cl2: C, 55.4; H, 5.32; N, 8.21%.
Found: C, 55.5; H, 5.2; N, 8.1%.
Complex 6. The imidazolium salt 12 (0.100 g, 0.321 mmol,
1.0 equiv.) and Rh(acac)(CO)2 (0.0828 g, 0.321 mmol,
1.0 equiv.) were mixed in a round-bottom flask in the dry
box. Dry degassed THF (21 mL) was added and the reaction
mixture was stirred at ambient temperature under an argon
atmosphere for 4 h. THF was removed under reduced pressure
and the crude product was recrystallized from dry CH2Cl2/
pentane. This yielded 6 (0.101 g, 71%) as a green crystalline
solid. 1H NMR (CD2Cl2, 500 MHz): d 7.50–7.34 (m, 3H),
7.30–7.24 (m, 2H), 7.01 (d, 2H, J = 8.5 Hz), 6.84–6.75
(m, 4H), 3.70 (s, 3H, NCH3), 2.24 (s, 3H, tolyl-CH3). Solubility
problems precluded 13C NMR and further assignments.
(CH3CN): m/z 346 (M+–Clꢀ). IR (CH2Cl2): nCQN 1673 cmꢀ1
.
Complex 4. The imidazolium salt 10 (0.200 g, 0.566 mmol,
1.0 equiv.) and Rh(acac)(CO)2 (0.146 g, 0.566 mmol, 1.0 equiv.)
were mixed in a round-bottom flask in the dry box. Dry
degassed THF (30 mL) was added and the reaction mixture
was stirred at ambient temperature under an argon atmosphere
for 4 h. THF was removed under reduced pressure and the
crude product was recrystallized from dry CH2Cl2/pentane.
This yielded 4 (0.260 g, 97%) as a red/orange crystalline solid.
1H NMR (CD2Cl2, 600 MHz): d 7.47 (t, 1H, J = 7.6 Hz,
Ph-Hp), 7.36 (t, 2H, J = 7.9 Hz, Ph-Hm), 7.23 (d, 2H, J =
7.6 Hz, Ph-Ho), 7.09 (t, 1H, J = 7.6 Hz, Dep-Hp), 6.99 (d, 2H,
ESI-MS (CH3CN): m/z 447 (M+–Cl
+ CH3CN). IR
(CH2Cl2): nCQN 1605 cmꢀ1, nCQO 1962 cmꢀ1. Anal. calcd.
for C19H17N3RhOCl ꢁ 1/7 CH2Cl2: C, 50.7; H, 3.8; N, 9.3%.
Found: C, 51.0; H, 3.9; N, 9.1%.
Imidazolium salt 13. Iminochloride 8 (1.21 g, 4.97 mmol,
1.0 equiv.) in dry THF (25.4 mL) was added to 1-tert-butyl-
imidazole (0.679 g, 5.47 mmol, 1.1 equiv.). The reaction
mixture was heated at 60 1C for 2 h and then cooled to
ambient temperature. The volatiles were removed under
vacuum. It was then attempted to recrystallize the crude
product from CH2Cl2/pentane, but without success. The
solvents were removed and this yielded 13 as a light yellow
foam (1.68 g, 92%) that was used without further purification.
1H NMR (CD2Cl2, 300 MHz): d 8.95 (s, 1H, NCHN),
7.53–7.22 (m, 5H, Ph-H), 7.22 (br s, 1H, NCHCHN),
7.09–7.00 (m, 3H, Dmp-H), 6.95–6.90 (m, 1H, NCHCHN),
2.21 (s, 6H, Dmp-CH3), 1.60 (s, 9H, t-Bu). 13C{1H} NMR
(CD2Cl2, 75 MHz): d 166.2 (CQN), 136.5, 135.6, 134.6, 132.5
(NCHN), 131.5, 128.4, 128.2, 128.0, 127.0, 119.9 (NCHCHN),
119.1 (NCHCHN), 59.5 (C(CH3)3), 29.9 (C(CH3)3), 18.5
(Dmp-CH3). ESI-MS (CH3CN): m/z 208 (M+–C7H12N2).
HR-MS: m/z 208.1120, calcd. for C15H14N 208.1126 (ꢀ3.0 ppm).
J
= 7.6 Hz, Dep-Hm), 6.85 (d, 1H, J = 2.3 Hz,
NCHCHNCH3), 6.81 (d, 1H, J = 2.3 Hz, NCHCHNCH3),
3.72 (s, 3H, NCH3), 2.74 (dq, 2H, J = 15.0, 7.5 Hz, CH2CH3),
2.32 (dq, 2H, J = 15.0, 7.5 Hz, CH2CH3), 1.21 (t, 6H, J = 7.5
Hz, CH2CH3). 13C{1H} NMR (CD2Cl2, 151 MHz): d 191.5 (d,
1J(103Rh–13C) = 80 Hz, CO), 185.5 (d, 1J(103Rh–13C) =
58 Hz, C(carbene)), 159.4, 140.4, 136.3, 131.8 (Ph-CH), 128.7
(Ph-CH), 128.1 (Ph-CH), 126.8, 126.4 (Dep-CHp), 124.5
(Dep-CHm), 123.5 (NCHCHNCH3), 116.7 (NCHCHNCH3),
38.6 (NCH3), 24.5 (CH2CH3), 13.0 (CH2CH3). IR (CH2Cl2):
nCQN 1612 cmꢀ1, nCQO 1986 cmꢀ1. ESI-MS (CH3CN): m/z
489 (M+–Cl + CH3CN). Anal. calcd. for C22H23N3RhOCl:
C, 54.61; H, 4.79; N, 8.69%. Found: C, 54.26; H, 4.46;
N, 8.69%.
Complex 5. The imidazolium salt 11 (0.100 g, 0.262 mmol,
1.0 equiv.) and Rh(acac)(CO)2 (0.0676 g, 0.262 mmol,
1.0 equiv.) were mixed in a round-bottom flask in the dry
box. Dry degassed THF (17 mL) was added and the reaction
mixture was stirred at ambient temperature under an argon
atmosphere for 4 h. THF was removed under reduced pressure
and the crude product was recrystallized from dry CH2Cl2/
pentane. This yielded 5 (0.0794 g, 60%) as a red crystalline
solid. 1H NMR (CD2Cl2, 500 MHz): d 7.47 (t, 1H, J = 7.6 Hz,
Ph-Hp), 7.36 (t, 2H, J = 7.9 Hz, Ph-Hm), 7.25 (d, 2H, J =
7.9 Hz, Ph-Ho), 7.16 (t, 1H, J = 7.7 Hz, Dipp-Hp), 7.01 (d, 2H,
IR (CH2Cl2): nCQN 1673 cmꢀ1
.
Complex 7. The imidazolium salt 13 (0.226 g, 0.615 mmol,
1.0 equiv.) and Rh(acac)(CO)2 (0.159 g, 0.615 mmol, 1.0 equiv.)
were mixed in a round-bottom flask in the dry box. Dry
degassed THF (40 mL) was added and the reaction mixture
was stirred at ambient temperature under an argon atmosphere for
4 h. The solvent was removed under reduced pressure and the
crude product was recrystallized from dry CH2Cl2/pentane.
This yielded 7 (0.152 g, 50%) as a red crystalline solid.
1H NMR (CD2Cl2, 300 MHz): d 7.24 (tt, 1H, J = 7.4,
1.6 Hz, Ph-Hp), 7.39 (tt, 2H, J = 7.4, 1.2 Hz, Ph-Hm), 7.26
(dt, 2H, J = 7.4, 1.6 Hz, Ph-Ho), 7.08 (d, 1H, J = 2.5 Hz,
NCHCHN-t-Bu), 6.97–6.89 (m, 3H, Dmp-H), 6.78 (d, 1H,
J = 2.5 Hz, NCHCHN-t-Bu), 2.20 (s, 6H, Dmp-CH3), 1.89
(s, 9H, t-Bu). 13C{1H} NMR (CD2Cl2, 75 MHz): d 193.2
J
= 7.7 Hz, Dipp-Hm), 6.94 (d, 1H, J = 2.3 Hz,
NCHCHNCH3), 6.85 (d, 1H, J = 2.3 Hz, NCHCHNCH3),
3.73 (s, 3H, NCH3), 2.99 (sept., 2H, J = 6.8 Hz, CHMe2), 1.28
(d, 6H, J = 6.8 Hz, CH(CH3)2, 0.90 (d, 6H, J = 6.9 Hz,
CH(CH3)2. 13C{1H} NMR (CD2Cl2, 135 MHz): d 191.8
(d, J(103Rh–13C) = 82 Hz, CO), 185.9 (d, J(103Rh–13C) =
1
1
58 Hz, C(carbene)), 159.5 (CQN), 141.6 (Dipp-Co), 139.1
c
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New J. Chem., 2011, 35, 2306–2313 2311

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(d, 1J(103Rh-13C) = 79 Hz, CQO), 183.5 (d, 1J(103Rh-13C) =
62 Hz, C(carbene), 160.3 (CQN), 142.1, 132.1, 131.6, 129.0,
128.2, 127.6, 127.4, 126.3, 120.3 (NCHCHN-t-Bu), 116.4
(NCHCHN-t-Bu), 59.4 (C(CH3)3), 31.2 (C(CH3)3), 19.3
(Dmp-CH3). ESI-MS (CH3CN): m/z 503.2 (M+-Cl + CH3CN).
HR-MS: m/z 503.1304, calcd. for C25H28ON4Rh 503.1318
(ꢀ2.8 ppm). IR (CH2Cl2): nCQN 1625 cmꢀ1, nCQO 1984
cmꢀ1. Anal. calcd. for C23H25N3RhOCl ꢁ 0.35 CH2Cl2: C,
53.2; H, 4.8; N, 8.9%. Found: C, 53.0; H, 4.9; N, 8.4%.
Table 5 Crystallographic data for complexes 5 and 7
Compound
Formula
5
7
C24H27ClN3ORhꢂ
CH2Cl2
596.79
Monoclinic
Red
P21/n
C25H25ClN3ORh
Formula weight
Crystal system
Colour
Space group
497.82
Orthorhombic
Red
Fdd2
a/A
b/A
c/A
a/1
11.1361(8)
14.0517(11)
17.3518(13)
90
19.6410(7)
47.2800(14)
9.725(3)
90
General procedure for cyclopropanation reactions
b/1
100.9510(10)
90
2665.8(3)
4
105
90
90
9031(3)
16
293
The selected Rh(I) complex and the activating agent were
stirred in dry CH2Cl2 (13.0 mL) at ambient temperature for
1 h under an argon atmosphere. The reaction mixture was
cooled to 0 1C and the alkene was added. Then EDA (0.12 mL,
1.00 mmol, 1.0 equiv.) in dry CH2Cl2 (7.0 mL) was added in
one portion. The reaction mixture was stirred at 0 1C. CH2Cl2
was removed in vacuo and the crude product was purified by
flash chromatography on silica gel (ethyl acetate:hexane) to
afford the cyclopropanes. The cis : trans ratio and character-
ization of the cyclopropanes were determined with GC analysis
and 1H NMR spectroscopy by comparison with literature
data.
g/1
V/A3
Z
T/K
F(000)
Radiation
1216
4064
0.17073
1.88–28.2
23 563
6344
4788/0/298
0.71073
1.72–28.62
19 450
5446
y range/1
Reflections measured
Unique reflections
No. of data/restraint/
param.
Goodness of fit, F
R1, wR2 [I 4 3s(I)]
R1, wR2 (all data)
Large diff. peak/e Aꢀ3
5446/1/262
1.1229
1.025
0.0255, 0.0262
0.0359, 0.0374
ꢀ0.42 to 0.47
0.0226, 0.0253
0.0515, 0.0530
ꢀ0.198 to 0.290
Ethyl cis-2-phenylcyclopropane-1-carboxylate (2)46
initially refined with soft restraints on the bond lengths and
angles to regularize their geometry (C–H in the range
0.93–98, A) and isotropic ADPs (U(H) in the range 1.2–1.5 ꢁ
1H NMR (CDCl3, 300 MHz): d 7.3 (m, 4H, Ph-H), 7.18
(m, 1H, Ph-H), 3.87 (q, 2H, J = 7.1 Hz, OCH2CH3),
2.67–2.52 (m, 1H, cyclopropane-H), 2.06 (ddd, 1H, J = 9.3,
7.9, 5.7 Hz, cyclopropane-H), 1.70 (ddd, 1H, J = 9.3, 7.4,
5.3 Hz, cyclopropane-H), 1.40–1.23 (m, 1H, cyclopropane-H),
0.95 (t, 3H, J = 7.1 Hz, OCH2CH3). 13C{1H} NMR (CDCl3,
75 MHz): d 170.8 (CO), 136.5 (Ph-C), 129.2 (Ph-CH), 127.7
(Ph-CH), 126.5 (Ph-CH), 60.0 (OCH2CH3), 25.3 (cyclo-
propane-CH), 21.7 (cyclopropane-CH), 13.9 (OCH2CH2),
11.0 (cyclopropanes-CH2). EI-MS: m/z (%) 190 (M+, 42),
162 (7), 145 (20), 117 (100), 91 (24).
U
equiv of the adjacent atom), after which they were refined with
riding constraints. Table 5 lists the experimental and crystallo-
graphic data. Crystallographic data for both complexes may
be obtained as individual CIF files in the ESI.w
Acknowledgements
We acknowledge generous financial support from the Norwegian
Research Council (NFR) under grant no. 177325/V30 (stipend
to MLR). We thank Senior Engineer Dirk Petersen for assistance
with obtaining NMR data.
X-Ray crystallography
Crystals of complexes 5 and 7 were grown from dichloro-
methane/pentane. The crystal of complex 5 was mounted on a
glass fiber with perfluoropolyether, and the data were collected
at 105 K. The crystal of complex 7 was mounted on thin glass
fiber clamped on a brass pin and the data were collected at
293 K. Data for both complexes were collected on a Bruker D8
diffractometer with an Apex II detector and an Oxford
Cryosystems Cryostream Plus device, using graphite-mono-
chromated Mo Ka radiation. Data collection method: j and
o-scans, range 0.51. The data were integrated with SAINT47
and semiempirical absorption correction was performed with
SADABS.48 The structure of complex 5 was solved with
Sir92,49 and refined on F with the program Crystals.50 The
structure of complex 7 was solved by direct methods with
Sir9249 and refined using full matrix least squares against |F|2
with SHELXL as implemented in Farrugia’s WinGX suite.51
For both complexes the non-hydrogen atoms were refined
with anisotropic thermal parameters; the H atoms were all
located in a difference map, but those attached to carbon
atoms were repositioned geometrically. The H atoms were
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