Catalytic hydrocarbon functionalization with gold complexes containing n-heterocyclic carbene ligands with pendant donor groups

Article abstract of DOI:10.1002/ejic.201101158

A series of silver and gold complexes bearing N-heterocyclic carbene ligands with a -CH₂CO₂Et pendant group attached to one N atom of the NHC ligand have been prepared. The catalytic properties of the gold complexes toward the decomposition of ethyl diazoacetate (N₂CHCO ₂Et) and the transfer of the carbene CHCO₂Et group to benzene and hexane have been investigated. A somewhat different reaction outcome has been found for this family of gold catalysts compared with the parent IPrAuCl catalyst. Gold-based catalysts containing NHC ligands with potentially coordinating pendant groups have been tested in the functionalization of Caspa2-H and Caspa3-H bonds by carbene insertion from ethyl diazoacetate, showing moderate catalytic activity.

Full text of DOI:10.1002/ejic.201101158

FULL PAPER
DOI: 10.1002/ejic.201101158
Catalytic Hydrocarbon Functionalization with Gold Complexes Containing
N-Heterocyclic Carbene Ligands with Pendant Donor Groups
Manuela Delgado-Rebollo,[a] Álvaro Beltrán,[a] Auxiliadora Prieto,[b]
M. Mar Díaz-Requejo,[a] Antonio M. Echavarren,*[c] and Pedro J. Pérez*[a]
Keywords: Carbenes / Carbene complexes / NHC ligands / Silver / Gold / C–H functionalization / Metalation
A series of silver and gold complexes bearing N-heterocyclic
carbene ligands with a –CH2CO2Et pendant group attached
to one N atom of the NHC ligand have been prepared. The
catalytic properties of the gold complexes toward the decom-
position of ethyl diazoacetate (N2CHCO2Et) and the transfer
of the carbene CHCO2Et group to benzene and hexane have
been investigated. A somewhat different reaction outcome
has been found for this family of gold catalysts compared
with the parent IPrAuCl catalyst.
Introduction
In the last decade, N-heterocyclic carbene complexes of
the coinage metal have been widely employed in numerous
catalytic processes in the homogeneous phase.[1,2] Our
group has participated in this area developing a catalytic
system based on the use of NHC–M (M = Cu, Au) to in-
duce the transfer of carbene units from diazo compounds
to saturated and unsaturated fragments.[3] In particular, the
gold complex IPrAuCl efficiently catalyzed the C–H bond
functionalization of aliphatic hydrocarbons with commer-
cially available ethyl diazoacetate (EDA). Simple, non-acti-
vated alkanes such as n-pentane could be converted into a
mixture of esters by the formal insertion of the :CHCO2Et
group into the different primary or secondary C–H bonds
(Scheme 1, a) using an equimolar mixture of IPrAuCl and
NaBArЈ4 as a halide scavenger [IPr = 1,3-bis(diisopropyl-
phenyl)imidazol-2-ylidene; ArЈ = 3,5-bis(trifluoromethyl)-
phenyl].
Although the copper analogue also promoted the same
transformation, the regioselection was clearly affected by
the metal center, the gold complex being preferred for the
Scheme 1. Functionalization of pentane and benzene with EDA.
activation of primary sites.[3c] The same catalytic system was of the carbene moiety into the Csp2–H bond under very mild
tested for the less common functionalization of the aro- conditions (Scheme 1, b). The chemoselectivity, in this case,
matic C–H bonds of benzene, which was preferentially con- was influenced by the nature of the NHC ligand, and the
verted into the product derived from the formal insertion IPr derivative was the most selective toward the desired tar-
get.[3b,3d]
[a] Laboratorio de Catálisis Homogénea, Departamento de
Química y Ciencia de los Materiales, Centro de Investigación
en Química Sostenible, Universidad de Huelva,
Campus de El Carmen s/n, 21007 Huelva, Spain
Fax: +34-959219942
The ligands employed in our previous work are mono-
dentate, the carbenic atom being the only donor binding
the metal center. We wondered about the effect that ad-
ditional donor atoms, located at the N-substituents, could
induce in the catalytic activity of these compounds in the
aforementioned transformations. Although these (NHC)-
MCl compounds are linear in geometry, the cationic species
generated upon interaction with NaBArЈ4 could transiently
form tricoordinate species, which are known for gold(I)
E-mail: perez@dqcm.uhu.es
[b] Departamento de Ingeniería Química, Química Física y
Química Orgánica, Universidad de Huelva,
Campus de El Carmen s/n, 21007 Huelva, Spain
[c] Institute of Chemical Research of Catalonia (ICIQ),
Av. Països Catalans 16, 43007 Tarragona, Spain
E-mail: aechavarren@iciq.es
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Catalytic Hydrocarbon Functionalization with Gold Complexes
complexes such as dppomf(AuCl) [dppomf = 1,1Ј-bis(di- eral bases was unsuccessful, with extensive decomposition
phenylphosphanyl)octamethylferrocene][4]
and
(phos- and appearance of colloidal gold being observed. Therefore,
phane)gold(I)-azadipyrromethene.[5] In our commitment to we decided to move to the well-known strategy of ligand
developing more active and selective catalysts for these transmetallation[8] from silver derivatives of the general for-
transformations, we wondered if combinations of a strong mula (NHC)AgX (X = Cl, Br).
donor such as the N-heterocyclic carbene ligand, with a
The preparation of the silver complexes was first at-
more weakly coordinating donor, would allow us to gain tempted following the described procedures for the synthe-
access to a class of catalysts that benefit from the hemilabile sis of related NHC–sulfonate silver complexes.[9] The pro-
behavior of one of its ligands (Scheme 2). Thus, a tricoord- cedure consisted of refluxing the imidazolium salt in CHCl3
inate geometry involving the NHC ligand, the hemilabile in the presence of 3 equiv. of Ag2O. However, in our case
arm, and the carbene group could induce a distinct catalytic intractable mixtures were obtained. Moving to a less acidic
behavior in the targeted C–H bond functionalization reac- medium and softer conditions such as CH2Cl2 as the sol-
tions. With this objective in mind we decided to synthesize vent and only 1.2 equiv. of Ag2O at room temperature af-
new gold complexes with N-heterocyclic carbene ligands forded the silver complexes 5 and 7, from the imidazolium
functionalized with ester groups to check their catalytic ac- salts 1 and 3, respectively, in 87% and 60% yields (Table 1,
tivity in C–H bond functionalization of aliphatic and aro- Entries 1 and 3). Although low conversions were obtained
matic hydrocarbons with diazo compounds.
with the imidazolium salts 2 and 4 at room temperature,
the same protocol at 40 °C proved suitable to prepare the
corresponding silver complexes 6 and 8 in 60 and 90%
yields, respectively (Entries 2 and 4). While complexes 5, 6,
and 8 display a high stability under an inert atmosphere,
complex 7 decomposes rather quickly in both the solid state
and in solution.
Scheme 2. Plausible coordination modes of NHC ligands with a
weak donor in a pendant arm.
Table 1. Synthesis of the silver complexes (NHC)AgX 5–8.
Results and Discussion
Syntheses of the Imidazolium Salts and Silver Complexes
The imidazolium salts 1–2 were readily prepared in
quantitative yields according to literature procedures,[6]
while compounds 3–4 (Scheme 3) were obtained by direct
alkylation of N-mesityl- and N-(2,3-diisopropylphenyl)-
imidazole with slight modifications of the procedure re-
ported for the synthesis of the mesityl derivative.[7] These
hygroscopic imidazolium salts were characterized by spec-
Entry
R
X
Silver
complexes [°C]
Temp. Time
Yield
[%]
[h]
1
2
3
4
CH2CO2Et
CH3
mesityl
Cl
Cl
Br
Br
5
6
7
8
r. t.
40
r. t.
40
14
7
24
12
87
60
60
90
2,6-Pr2C6H3
1
troscopic and analytical methods. In their H NMR spec-
tra, in addition to the ring and N-substituents, all of them
showed a typical resonance in the downfield region within
the 9.0–10.5 ppm range assigned to 2-H of the heterocyclic
ring.
1
The H NMR spectra of the C2-symmetric complex 5
show single resonances centered at δ = 7.10 and 4.92 ppm
for the unsaturated backbone of the imidazole ring and the
methylene protons of the N-alkyl chain, respectively. Com-
plexes 6, 7 and 8 display single resonance signals between
4.90 and 5.03 ppm for the methylene protons of the arm,
the imidazole ring protons appearing between 6.85 and
7.27 ppm as doublets or broad singlets from the loss of the
C2 symmetry. The 13C NMR spectra of complexes 5–8
show two low-field signals attributed to the carbonyl car-
bon (167–169 ppm) and to the carbenic carbon (183–
184 ppm). The similar values in chemical shift of the carb-
enic carbon clearly evidenced the lack of a significant effect
in the electronic-donor properties of the NHC ligands in
Scheme 3. Imidazolium salts employed in this work.
Once we obtained the above NHC precursors, we tried complexes 5–8. Additionally, these values of chemical shift
the direct synthesis of the gold complexes (NHC)AuCl. are similar to those described for the unsaturated NHC-Ag
However, the direct reaction of the imidazolium salts 1–4 complexes, such as (IMes)AgCl (δ = 185.0 ppm) or (IPr)-
with AuCl(SMe2) as the AuI source in the presence of sev- AgCl (δ = 184.6 ppm).[10] On the other hand, complexes 5
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A. M. Echavarren, P. J. Pérez et al.
FULL PAPER
and 6 show sharp singlets for the carbenic carbon charac- Table 2. Selected chemical shifts (δ values, ppm) for the complexes
(NHC)MX (M = Ag, Au; X = Cl, Br).[a]
teristic of fast exchange equilibrium of the carbene moiety
with the metal center.[11] The non-observance of Ag-carb-
ene coupling in the NMR time-scale indicates that these
complexes are useful carbene transfer reagents.[12] The carb-
enic carbon resonances in complexes 7 and 8 appeared as
broad singlets, in this case characteristic of a slow exchange,
that could be related to a higher steric hindrance in these
complexes induced by the aromatic substituents.
M
R
Complex
Ha
Hb
Cc
Ag
Au
Ag
Au
Ag
Au
Ag
Au
CH2CO2Et
CH2CO2Et
CH3
CH3
Mesityl
Mesityl
2,6-iPr2C6H3
2,6-iPr2C6H3
5
9
7.10
7.10
7.02
7.00
6.85
6.92
7.04
7.04
7.10
7.10
7.06
7.04
6.96
6.94
7.24
7.24
183.9
174.4
183.1
173.1
173.5
174.1
183.6
173.8
6
Synthesis of the Gold Complexes (NHC)AuCl
10
7
11
8
Once the silver complexes were prepared, we applied the
methodology, which was described by Lin[8] and was further
developed by the group of Nolan,[13] for the synthesis of the
gold analogues by a transmetalation process. The com-
plexes (NHC)AgX 5–8 were treated with AuCl(SMe2) as
the gold source (Scheme 4). In all cases, the reaction took
place at room temperature over 3 h affording the corre-
sponding complexes 9–12 with the general formula (NHC)
AuCl in 60–67% yields.
12[b]
[a] CDCl3 was the solvent used in all cases. [b] For complex 12, a
mixture of CDCl3 and CD2Cl2 was employed to ensure complete
dissolution.
Catalytic Experiments
As mentioned in the Introduction, we have previously
reported the catalytic capabilities of a series of copper[3a,3c]
and gold[3b–3d] complexes containing N-heterocyclic carb-
ene ligands (NHC), to induce the carbene insertion into
the carbon–hydrogen bond of different hydrocarbons, using
ethyl diazoacetate (EDA) as the carbene source. We have
now screened the potential of complexes 9–12 for such
transformations, wondering about the role of donor atoms
in the N-pendant groups of the NHC ligands. As represen-
tative hydrocarbons, we have chosen benzene and n-hexane
for Csp2–H and Csp3–H functionalization, respectively.
The reaction of benzene and ethyl diazoacetate in the
presence of the complexes 9–12 and NaBAr4Ј as halide
scavenger led to mixtures of ethyl phenylacetate and the cy-
Scheme 4. Synthesis of the gold complexes 9–12.
The novel AuI–NHC complexes were characterized by cloheptatriene shown in Scheme 5. Ethyl phenylacetate
spectroscopic methods and elemental analysis. The 1H arose from the gold-catalyzed formal insertion of the carb-
NMR spectra of complexes 10–12 containing the asymmet- ene :CHCO2Et into the aromatic C–H bond, whereas the
ric NHC ligands showed two distinct resonances at low cycloheptatriene was the product of a Buchner reaction also
field for the two backbone-ring protons (6.92–7.24 ppm), as mediated by the metal center. In all cases EDA was con-
well as signals characteristic of their corresponding side- sumed quantitatively, although benzene was functionalized
chain R groups. As expected, in the case of the C2-symmet- with a variable yield of 11–66% (the remaining initial diazo
ric complex 9, bearing two CH2CO2Et substituents, the compound was converted into a mixture of diethyl fumarate
backbone hydrogens appear as a single resonance centered and maleate). This is in contrast with the catalytic activity
at δ = 7.10 ppm (Table 2). The carbenic carbons in these of the parent IPrAuCl,[3b] which displayed a very high
compounds give rise to resonances in the 173.1–174.4 ppm chemoselectivity toward benzene. This difference could be
interval, similar to those reported for IPrAuCl and the result of a certain deactivation of the metal center due
IMesAuCl (175.1 and 173.4 ppm, respectively).[13] As al- to the presence of the carboxylate groups. Actually, the less
ready mentioned for the silver analogues, there is no signifi- active catalyst is that bearing two carboxylate fragments, 9,
cant electronic effect of the substituents in the electronic whereas the three compounds with one carboxylate group
density around the carbenic carbon. Nevertheless, the sig- showed similar activities toward benzene (55–66%). With
nals for the gold compounds appeared to be shifted upfield regard to the regioselectivity between the formation of
by 10 ppm (Table 2) compared with those for each silver phenylacetate and cycloheptatriene, again a decrease from
analogue, possibly a result of the different effect of the that of IPrAuCl (75:25) was observed. Interestingly, com-
metal center in the π-back donation, as proposed by plexes 11 and 12, having a mesityl and a 2,6-diisoprop-
Herrmann and coworkers.[11b]
ylphenyl substituent, respectively, at one of the nitrogen
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Catalytic Hydrocarbon Functionalization with Gold Complexes
atoms were less productive toward the insertion product
The incorporation of the carbene group to benzene or to
than 10, with a simple methyl group at that position. Since hexane takes place by different mechanisms. As shown in
NMR spectroscopic data for the series of four complexes Scheme 7 (a), it has been proposed that benzene undergoes
showed little or no difference for the carbenic carbon atom, an electrophilic attack of the metallacarbene to give a Whe-
the observed differences in catalytic behavior should be due land-like intermediate[14] from which both the insertion or
to local effects at the metal center, in two senses: the pos- the cycloheptatriene products evolve. The reverse process
sible coordination of the carboxylate group and/or the ste- (gold-catalyzed retro-Buchner reaction) has recently been
ric effect of the substituents at the NHC ring (see below).
observed from cycloheptatrienes and cationic AuI com-
plexes.[15] In the case of a saturated C–H bond (Scheme 7,
b), a concerted process in which the C–H bond acts as a
(poor) nucleophile and interacts with the carbenic carbon
atom occurs.[16] In the system we describe herein, it could
be thought that a transient carboxylate coordination would
affect the catalytic process. In our previous work with
IPrAuCl and related catalysts these transformations are
likely to occur through linear, two-coordinate intermediates
of the type NHC–Au=C(H)CO2Et+. At variance with those
catalysts, the system reported herein could proceed through
the intermediacy of a gold–carbene tricoordinate species
(Scheme 7, c). The additional electron density from oxygen
coordination would decrease the electrophilicity of the gold
atom, decreasing the catalytic activity. The proximity of the
carbene group to the second substituent of the NHC ligand
would also explain the observed steric effect in the hexane
case: the 2,6-diisopropylphenyl substituent seems to dis-
favor the functionalization of the primary sites in this sys-
tem.[3c]
Scheme 5. Reaction of benzene and ethyl diazoacetate in the pres-
ence of 9–12 and NaBArЈ4 as the catalyst precursor (5% catalyst
loading with respect to EDA).
When n-hexane was treated with ethyl diazoacetate at
room temperature in the presence of 9–12 and NaBArЈ, the
formation of a mixture of three products derived from the
formal insertion of the carbene unit into the C–H bonds of
primary as well as of the two distinct secondary sites were
observed (Scheme 6). A 1:1 mixture of hexane and dichlo-
romethane was employed as the reaction solvent, since the
catalysts were insoluble in the hydrocarbon itself. In ad-
dition, a gradual decomposition was observed during the
process (24 h), explaining the lack of complete consumption
of the diazo compound. However, the results deserve some
comments, particularly regarding the regioselective func-
tionalization of primary versus secondary sites.
Scheme 7. The accepted pathways for the functionalization of C–
H bonds of benzene (a) and alkanes (b) by carbene transfer from
diazo compounds. (c) Proposed tricoordinate intermediate in the
case of complexes 9–12 and NaBArЈ4 as the catalyst precursor.
In order to assess the validity of the above proposal, we
have carried out DFT calculations [DFT, B3LYP, 6-31G(d)
(C, O, N, H), and SDD (Au)] on the model carbenes 13
(with CH2 as the carbene unit, Figure 1) and 14 (with
CHCO2Me as the carbene unit, Figure 1). Identical minima
were calculated when solvent corrections (CH2Cl2) were in-
cluded in the calculations. The NBO analysis of 13 and 14
failed to locate any significant bonding interaction between
AuI and the ester group. For comparison, we have included
the structure of 15 with two phenyl substituents at the
Scheme 6. Reaction of hexane and ethyl diazoacetate in the pres-
ence of 9–12 and NaBArЈ4 (5% catalyst loading with respect to
EDA).
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A. M. Echavarren, P. J. Pérez et al.
FULL PAPER
All the reagents were purchased from commercial sources and used
without further purification. Ligands 1 and 2 were prepared follow-
ing literature procedures.[4] Elemental analyses were carried out
with a Perkin–Elmer EA 2400 analyzer.
NHC, which shows a similar natural atomic charge at the
AuI center. The Au–O distances found fall within the range
4.4–4.5 Å, indicating that this secondary interaction be-
tween AuI and the ester at the arm is, at most, very weak.
Therefore, our proposal of a transient tricoordinate species
seems to be disfavored on the basis of such calculations.
N-(Ethylacetyl)-NЈ-(mesityl)imidazolium Bromide (3): N-Mesityl-
imidazole (600 mg, 3.22 mmol), ethyl bromoacetate (0.39 mL,
3.54 mmol), and THF (1 mL) were placed in a flask. The reaction
mixture was stirred for 24 h at 130 °C. Volatiles were removed un-
der vacuum and the residue was washed with diethyl ether and
dried under vacuum (935 mg, 97%). C16H21BrN2O2 (353.25): calcd.
1
C 54.40, H 5.99, N 7.93; found C 54.45, H 6.02, N 7.90. H NMR
(400 MHz, CDCl3): δ = 1.32 (t, J = 7 Hz, 3 H, CH3CH2O), 2.10
(s, 6 H, o-CH3), 2.34 (s, 3 H, p-CH3), 4.28 (q, J = 7.2 Hz, 2 H,
CH3CH2O), 5.89 (s, 2 H, CH2CO2Et), 7.01 (s, 2 H, CH, arom.),
7.15 (d, J = 1.6 Hz, 1 H, CH, imidazole), 7.86 (d, J = 2 Hz, 1
H, CH, imidazole), 10.17 (s, 1 H, NCHN) ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = 14.3 (CH3CH2O), 17.8 (o-CH3), 21.3 (p-
CH3), 50.9 (CH2CO2Et), 63.0 (CH3CH2O), 122.7, 125.3, 129.9,
130.8, 134.5, 139.1, 141.5 (NCHN), 166.6 (C=O) ppm.
N-(2,6-Diisopropylphenyl)-NЈ-(ethylacetyl)imidazolium Bromide (4):
N-(2,6-Diisopropylphenyl)imidazole (641 mg, 2.81 mmol), ethyl
bromoacetate (0.34 mL, 3.10 mmol), and THF (1 mL) were placed
in a flask. The reaction mixture was stirred for 24 h at 130 °C.
Volatiles were removed under vacuum, and the residue was purified
by silica gel chromatography (CH2Cl2/CH3OH 10:1) to give 4 as a
brown solid (818 mg, 74%). C19H27BrN2O2 (395.33): calcd. C
57.72, H 6.88, N 7.09; found C 57.91, H 6.97, N 7.17. 1H NMR
(400 MHz, CDCl3): δ = 1.15 [d, J = 6.8 Hz, 6 H, CH(CH3)2], 1.24
[d, J = 6.8 Hz, 6 H, CH(CH3)2], 1.33 (t, J = 7 Hz, 2 H, CH3CH2O),
2.30–2.41 [m, 2 H, CH(CH3)2], 4.30 (q, J = 7.2 Hz, 2 H,
CH3CH2O), 5.93 (s, 2 H, CH2CO2Et), 7.16 (br. s, 1 H, CH, imid-
azole), 7.30 (d, J = 7.6 Hz, 2 H, CH, arom.), 7.53 (t, J = 7.8 Hz, 1
H, CH, arom.), 7.76 (br. s, 1 H, CH, imidazole), 10.31 (s, 1 H,
Figure 1. Minimum structures for model cationic intermediates 13,
14, and 15 [DFT, B3LYP, 6-31G(d) (C, N, O, H), and SDD (Au),
CH2Cl2] showing selected bond lengths (boldface) and natural
atomic charges (italics).
NCHN) ppm. 13C{1H} NMR (100 MHz, CDCl3):
δ = 14.2
(CH3CH2O), 24.5 [CH(CH3)2], 24.6 [CH(CH3)2], 28.7 [CH(CH3)2],
50.9 (CH2CO2Et), 62.9 (CH3CH2O), 123.7 (CH, imidazole), 124.8
(CH, arom.), 125.3 (CH, imidazole), 125.4 (CH, arom.), 130.3 (CH,
arom.), 132.1 (CH, arom.), 139.3 (CH, arom.), 145.6 (NCHN),
166.5 (C=O) ppm.
Conclusions
We have prepared a series of imidazolium salts with their
corresponding silver and gold (NHC)MX derivatives (M =
Ag, Au; X = Cl, Br) with a CH2CO2Et pendant group. The
gold complexes have been tested as catalysts for the func-
tionalization of benzene and hexane by means of the metal-
catalyzed diazo compound (ethyl diazoacetate) decomposi-
tion and subsequent carbene (CHCO2Et) transfer. Al-
though catalytic activities are moderate, the observed trends
suggest a somewhat distinct behavior of these gold com-
plexes when employed as catalysts compared to the pre-
vious NHCAuCl (NHC) IPr, IMes, ItBu, IAd catalyst pre-
cursors for the same transformations. Work aimed to ascer-
tain the origin of such differences as well as to design other
ligands in which the electronic and steric effect would en-
hance the catalytic activity is currently underway in our
laboratories.
[N,NЈ-Bis(ethylacetyl)imidazolin-2-ylidene]silver Chloride (5):
A
Schlenk flask was charged with 1 (99 mg, 0.36 mmol), silver oxide
(100 mg, 0.434 mmol), and CH2Cl2 (15 mL). The reaction mixture
was stirred at room temperature for 14 h in the dark and then fil-
tered through Celite. The solvent was removed under vacuum to
give 5 as a white solid (120.6 mg, 87%). C11H16AgClN2O4 (383.58):
1
calcd. C 34.44, H 4.20, N 7.30; found C 34.18, H 4.24, N 7.69. H
NMR (400 MHz, CDCl3): δ = 1.24 (t, J = 7.2 Hz, 6 H, CH3CH2O),
4.17 (q, J = 7.2 Hz, 4 H, CH3CH2O), 4.92 (s, 4 H, CH2CO2Et), 7.10
(s, 2 H, CH, imidazole) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ
= 14.3 (CH3CH2O), 52.9 (CH2CO2Et), 62.5 (CH3CH2O), 122.9
(CH2, imidazole), 167.7 (C=O), 183.9 (C-carbene) ppm.
[N-(Ethylacetyl)-NЈ-(methyl)imidazolin-2-ylidene]silver Chloride (6):
As with the procedure described for 5, a mixture of 2 (100 mg,
0.49 mmol) and silver oxide (137 mg, 0.59 mmol) in CH2Cl2
(15 mL) was stirred at 40 °C for 7 h in the dark and then filtered
through Celite. The solvent was removed under vacuum to give 6
as a white solid (92.9 mg, 60%). C8H12AgClN2O2 (311.51): calcd.
Experimental Section
1
C 30.84, H 3.88, N 8.99; found C 30.81, H 3.94, N 9.13. H NMR
General: All preparations and manipulations were carried out un-
der an oxygen-free nitrogen atmosphere using Schlenk techniques.
NMR experiments were carried out with a Varian Mercury
400 MHz. GC data were collected with a Varian 3900 instrument.
(400 MHz, CDCl3): δ = 1.31 (t, J = 7.2 Hz, 3 H, CH3CH2O), 3.85
(s, 3 H, CH3N), 4.25 (q, J = 7.2 Hz, 2 H, CH3CH2O), 4.90 (s, 2 H,
CH2CO2Et), 7.02 (d, J = 1.2 Hz, 1 H, CH, imidazole), 7.06 (d, J
= 2 Hz, 1 H, CH, imidazole) ppm. 13C{1H} NMR (100 MHz,
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Catalytic Hydrocarbon Functionalization with Gold Complexes
CDCl3): δ = 15.1 (CH3CH2O), 39.9 (CH3N), 53.5 (CH2CO2Et), (400 MHz, CDCl3): δ = 1.32 (t, J = 7.2 Hz, 3 H, CH3CH2O), 2.01
63.3 (CH3CH2O), 123.5 (CH, imidazole), 123.7 (CH, imidazole),
168.7 (C=O), 183.1 (C-carbene) ppm.
(s, 6 H, CH3), 2.32 (s, 3 H, CH3), 4.28 (q, J = 7.2 Hz, 2 H,
CH3CH2O), 5.09 (s, 2 H, CH2CO2Et), 6.92 (d, J = 1.7 Hz, 1 H,
CH, imidazole), 6.95 (s, 2 H, CH, arom.), 7.24 (d, J = 1.7 Hz, 1 H,
CH, imidazole) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 14.3
(CH3CH2O), 18.0 (CH3), 21.4 (CH3), 52.1 (CH2CO2Et), 62.7
(CH3CH2O), 122.1, 122.7, 129.7, 134.6, 135.0, 140.0, 167.1 (C=O),
173.4 (C-carbene) ppm.
[N-(Ethylacetyl)-NЈ-(mesityl)imidazolin-2-ylidene]silver Bromide (7):
A mixture of 3 (116 mg, 0.33 mmol) and silver oxide (93 mg,
0.40 mmol) in CH2Cl2 (15 mL) was stirred at room temperature for
24 h in the dark. After filtration with Celite and solvent removal,
complex 7 was isolated as an off-white solid (93 mg, 60%). 1H
NMR (400 MHz, CD2Cl2): δ = 1.30 (t, J = 7 Hz, 3 H, CH3CH2O),
1.82 (br. s, 6 H, CH3), 2.38 (s, 3 H, CH3), 4.25 (q, J = 7 Hz, 2 H,
CH3CH2O), 5.02 (s, 2 H, CH2CO2Et), 6.85–6.99 (m, 2 H, CH,
arom., CH, imidazole), 7.35–7.38 (m, 2 H, CH, arom., CH, imid-
[N-(2,6-Diisopropylphenyl)-NЈ-(ethylacetyl)imidazolin-2-ylidene]gold
Chloride (12): A method of preparation similar to that used above,
with 8 as the starting material, gave 12 as a yellowish solid (120 mg,
67%). C19H26AuClN2O2·CH2Cl2 (631.5): calcd. C 38.0, H 4.43, N
4.43; found C 39.25, H 4.53, N 4.43. 1H NMR (400 MHz, CD2Cl2):
δ = 1.06 [d, J = 6.8 Hz, 6 H, CH3(CH3)2], 1.20 [d, J = 6.8 Hz, 6
H, CH3(CH3)2], 1.22–1.26 (m, 2 H, CH3CH2O), 2.36 [sept, J =
6.8 Hz, 2 H, CH(CH3)2], 4.23 (q, J = 7.2 Hz, 2 H, CH3CH2O), 5.02
(s, 2 H, CH2CO2Et), 7.04 (d, J = 1.9 Hz, 1 H, CH, imidazole), 7.24
(d, J = 1.9 Hz, 1 H, CH, imidazole), 7.31–7.34 (m, 2 H, CH,
arom.), 7.42 (t, J = 7.6 Hz, 1 H, CH, arom.) ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = 13.8 (CH3CH2O), 24.0 [CH(CH3)2], 24.2
[CH(CH3)2], 28.3 [CH(CH3)2], 52.0 (CH2CO2Et), 62.4
(CH3CH2O), 121.7 (CH, imidazole), 123.7 (CH, imidazole), 124.1
(CH, arom.), 130.6 (CH, arom.), 145.9 (CH, arom.), 168.9 (C=O),
175.5 (C-carbene) ppm.
azole) ppm. 13C{1H} NMR (100 MHz, CD2Cl2):
δ = 13.9
(CH3CH2O), 17.3 (CH3), 20.9 (CH3), 52.5 (CH2CO2Et), 62.3
(CH3CH2O), 122.6, 122.9, 128.8, 129.2, 134.8, 135.1, 135.3, 167.4
(C=O), 183.8 (br. s, C-carbene) ppm.
[N-(2,6-Diisopropylphenyl)-NЈ-(Ethylacetyl)imidazolin-2-ylidene]-
silver Bromide (8): Following the above protocol, 4 (51 mg,
0.13 mmol) and silver oxide (35 mg, 0.15 mmol) were stirred at
40 °C for 12 h in the dark in CH2Cl2 (15 mL) as the solvent. Fil-
tration through Celite prior to removal of volatiles afforded com-
1
plex 8 as an off-white solid (58.7 mg, 90%). H NMR (400 MHz,
CDCl3): δ = 1.13 [d, J = 7.2 Hz, 6 H, CH(CH3)2], 1.23 [d, J =
7.2 Hz, 6 H, CH(CH3)2], 1.35 (t, J = 7.2 Hz, 3 H, CH3CH2O), 2.40
[sept, J = 7.2 Hz, 2 H,CH(CH3)2], 4.31 (q, J = 7.2 Hz, 2
H,CH3CH2O), 5.03 (s, 2 H, CH2CO2Et), 7.04 (br. s, 1 H, CH, imid-
azole), 7.25–7.27 (m, 3 H, CH, arom., CH, imidazole), 7.47 (t, J =
7.6 Hz, 1 H, CH, arom.) ppm. 13C{1H} NMR (100 MHz, CDCl3):
δ = 14.07 (CH3CH2O), 24.3 [CH(CH3)2], 24.6 [CH(CH3)2], 28.2
[CH(CH3)2], 52.6 (CH2CO2Et), 62.6 (CH3CH2O), 121.9 (CH, imid-
azole), 122.0 (CH, imidazole), 124.3 (CH, arom.), 130.6 (CH,
arom.), 134.4 (CH, arom.), 145.7 (CH, arom.), 167.2(C=O), 183.6
(br. s, C-carbene) ppm.
Catalytic Experiments. (a) Benzene: An equimolar mixture of the
gold complex 9–12 and NaBArЈ4 (0.025 mmol) was dissolved in
neat benzene (1–3 mL). After 15 min of stirring, EDA (0.5 mmol)
was added in one portion. After 12 h of additional stirring, the
mixture was analyzed by GC. The volatile components were re-
moved, and the residue analyzed by NMR spectroscopy to identify
the products.[3b]
(b) n-Hexane: An equimolar mixture of the gold complex 9–12 and
NaBArЈ4 (0.025 mmol) was dissolved in a mixture of CH2Cl2
(5 mL) and n-hexane (5 mL). After 15 min of stirring, EDA
(0.5 mmol) was added with the aid of a syringe pump (dissolved in
5 mL of CH2Cl2 and hexane, 1:1) for 24 h. The mixture was ana-
lyzed by GC and NMR spectroscopy.[3c]
[N,NЈ-Bis(ethylacetyl)imidazolin-2-ylidene]gold Chloride (9): A mix-
ture of the silver complex 5 (102 mg, 0.33 mmol) and (dimethyl
sulfide)gold(I) chloride (115 mg, 0.39 mmol) in dichloromethane
(50 mL) was stirred for 3 h at room temperature. After filtration,
activated carbon was added to the filtrate and the mixture was
filtered through Celite. The volatiles were then removed under re-
duce pressure to afford complex 9 as a white solid (30.1 mg, 64%).
C11H16AuClN2O4 (472.05): calcd. C 27.95, H 3.41, N 5.93; found
Computational Details: Calculations were performed with the
GAUSSIAN 09 series of programs.[17] The geometries were opti-
mized at the DFT level using the B3LYP functional.[18] The
LANL2DZ basis set, which includes the relativistic effective core
potential (ECP) of Hay and Wadt and employs a split-valence
(double-ζ) basis set, was used for Au.[19]
1
C 27.71, H 3.49, N 6.66. H NMR (400 MHz, CDCl3): δ = 1.32 (t,
J = 7.2 Hz, 6 H, CH3CH2O), 4.26 (q, J = 7.2 Hz, 4 H, CH3CH2O),
5.00 (s, 4 H, CH2CO2Et), 7.10 (s, 2 H, CH, imidazole) ppm.
13C{1H} NMR (100 MHz, CDCl3): δ = 14.3 (CH3CH2O), 52.1
(CH2CO2Et), 62.7 (CH3CH2O), 122.3 (CH, imidazole), 166.9
(C=O), 174.4 (C-carbene) ppm.
Acknowledgments
We thank Ministerio de Ciencia
e Innovación (MICINN)
[N-(Ethylacetyl)-NЈ-(methyl)imidazolin-2-ylidene]gold Chloride (10):
Following the procedure described for 9, complex 10 was prepared
(CTQ2008-00042BQU, CTQ2010-16088/BQU, Consolider Ingenio
2010 grant number CSD-2006-0003), Junta de Andalucía (P07-
FQM-02870), the Agència de Gestió d’Ajuts Universitaris i de Re-
serca (AGAUR) (2009 SGR 47), and the ICIQ Foundation for fin-
ancial support. M. D. R. thanks the Ministerio de Educación y Ci-
encia (MEC) for a FPU fellowship.
from
6 and isolated as a white solid (75.1 mg, 60%).
C8H12AuClN2O2 (400.03): calcd. C 23.98, H 3.02, N 6.99; found C
23.61, H 3.21, N 6.88. 1H NMR (400 MHz, CDCl3): δ = 1.32 (t, J =
7.2 Hz, 3 H, CH3CH2O), 3.84 (s, 3 H, CH3N), 4.24 (q, J = 7.2 Hz, 2
H, CH3CH2O), 4.97 (s, 2 H, CH2CO2Et), 7.00 (d, J = 1.9 Hz, 1 H,
CH, imidazole), 7.05 (d, J = 1.9 Hz, 1 H, CH, imidazole) ppm.
13C{1H} NMR (100 MHz, CDCl3): δ = 14.3 (CH3CH2O), 38.8
(CH3N), 52.0 (CH2CO2Et), 62.8 (CH3CH2O), 122.1 (CH, imid-
azole), 167.5 (C=O), 173 (C-carbene) ppm.
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Eur. J. Inorg. Chem. 2012, 1380–1386
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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Received: October 21, 2011
Published Online: January 9, 2012
1386
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Eur. J. Inorg. Chem. 2012, 1380–1386