Exclusive aromatic vs aliphatic C-H bond functionalization by carbene insertion with gold-based catalysts

Article abstract of DOI:10.1021/om200206m

The direct functionalization of aromatic C-H bonds by carbene insertion from diazo compounds catalyzed by gold complexes with N-heterocyclic ligands is described. The reaction is completely selective toward the C sp²-H bonds, other C sp³-H bonds remaining unreacted. A study with several NHC ligands in Au(I) and Au(III) complexes has been performed. The potential application of this strategy to give profen derivatives has also been explored.

Full text of DOI:10.1021/om200206m

ARTICLE
pubs.acs.org/Organometallics
Exclusive Aromatic vs Aliphatic CꢀH Bond Functionalization
by Carbene Insertion with Gold-Based Catalysts
Ivan Rivilla, B. Pilar Gꢀomez-Emeterio, Manuel R. Fructos, M. Mar Díaz-Requejo,* and
Pedro J. Pꢀerez*
Laboratorio de Catꢀalisis Homogꢀenea, Departamento de Química y Ciencia de los Materiales,
Unidad Asociada al CSIC, Centro de Investigaciꢀon en Química Sostenible (CIQSO), Universidad de Huelva,
21007-Huelva, Spain
S Supporting Information
b
ABSTRACT: The direct functionalization of aromatic CꢀH bonds by carbene insertion
from diazo compounds catalyzed by gold complexes with N-heterocyclic ligands is
described. The reaction is completely selective toward the Csp2ꢀH bonds, other Csp3ꢀH
bonds remaining unreacted. A study with several NHC ligands in Au(I) and Au(III)
complexes has been performed. The potential application of this strategy to give profen
derivatives has also been explored.
’ INTRODUCTION
The functionalization of carbonꢀhydrogen bonds throughout
a formal metal-mediated carbene insertion using diazo com-
pounds as the carbene source (Scheme 1a) constitutes a
synthetic tool that has attracted growing interest in the last
few decades.1,2 Both the inter-3 and intramolecular2 versions
have been described for Csp3ꢀH bonds. While the function-
alization of aromatic Csp2ꢀH bonds with this strategy has
been somewhat developed for intramolecular processes, the
intermolecular version is still quite rare (Scheme 1b). Exam-
ples are reduced to independent work by Shechter4 and
Livant5 with dirhodium acetate as the catalyst and elaborated
diazo compounds leading to the direct functionalization of
benzene or anisole. After those reports, we described6 the first
example of this intermolecular aromatic CꢀH bond function-
alization by carbene insertion with commercial ethyl di-
azoacetate (EDA), based in a gold catalyst. The complex
IPrAuCl (1; IPr = 1,3-bis(diisopropylphenyl)imidazol-2-
ylidene) in the presence of 1 equiv of NaBAr04 (Ar0 =
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) as halide
scavenger promoted the conversion of benzene and ethyl
diazoacetate (EDA) into a 3:1 mixture of ethyl 2-phenyl-
acetate and ethyl cyclohepta-2,4,6-trienecarboxylate (eq 1), at
variance with other reported catalytic systems that exclusively
provided the cycloheptatriene product.7 The aromatic CꢀH
bond functionalization of toluene was also described using
EDA as carbene source and the same gold catalytic precursor
(eq 2). In this case, we found a 4:1 mixture of products
derived from the formal insertion of the carbene group into
the aromatic CꢀH bond and the cycloheptatriene derivative,
respectively.
In a separate work from our laboratory, complexes with the
composition (NHC)AuCl were found to catalyze the functionaliza-
tion of alkane CꢀH bonds by carbene insertion.8 A significant effect
of the NHC ligand was observed in both the chemo- and regios-
electivity of the reaction. On the basis of this, we have wondered
about the existence of a similar effect in the functionalization of
aromatic CꢀH bonds. In this contribution, we describe the catalytic
Scheme 1. Functionalization of CꢀH Bonds by Carbene
Insertion from Diazo Compounds
Received: March 7, 2011
Published: April 27, 2011
r
2011 American Chemical Society
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|

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capabilities of a series of Au(I) and Au(III) complexes toward the
aromatic CꢀH bond functionalization using several diazo com-
pounds as the carbene source. We have found that the appropriate
choice of metal oxidation state, NHC ligand, and diazo source allows
a certain control in the selectivity of this transformation, with a
preferential insertion of the carbene moiety into the Csp2ꢀH bonds,
even when other Csp3ꢀH bonds were available. We have also applied
this methodology to the synthesis of several profen derivatives.
’ RESULTS AND DISCUSSION
Catalytic Activity of NHC-Containing Au(I) and Au(III) Com-
plexes. A series of complexes of general formula (NHC)AuCl
and (NHC)AuBr3 were prepared following literature methods,9,10
using the set of NHC ligands shown in Scheme 2. We have first
targeted benzene as the substrate to be functionalized using EDA as
the carbene source (see eq 1) with an array of 10 gold complexes as
the catalyst precursors and using 1 equiv of NaBAr04 to remove the
chloride ligand in situ and generate the cationic Au(I) or Au(III)
catalytic species.6,11 The reactions were performed using benzene
as the solvent, at room temperature, and adding the EDA in one
portion, avoiding the use of slow addition devices, a technique
frequently employed in these transformations involving diazo
compounds. The results are shown in Table 1, from which we
can extract some interesting observations. First, most complexes
induced high to very high conversions, intended as the percentage
of EDA converted into products. It is worth mentioning that a
secondary, nondesired reaction leading to the formation of diethyl
fumarate (DEF) and diethyl maleate (DEM) usually constitutes
the main drawback for diazo compound based transformations,
although in our case the amounts of DEF and DEM are not high
(with the exception of the IMes derivative). Second, the insertion
product, i.e., ethyl 2-phenylacetate, was the major product in all
cases, the highest selectivity toward this compound being provided
by the IPrAu-containing catalyst.
Scheme 2. NHC Ligands Employed in This Work
Table 1. Reaction of Benzene and EDA in the Presence of (NHC)AuCl and (NHC)AuBr3 as Catalyst Precursorsa
(NHC)AuCl
(NHC)AuBr3
Conv.
(%)b
% Insertion
Productc
%Addition
Productc
Conv.
(%)b
%Insertion
Productc
%Addition
Productc
Entry
NHC
1
2
3
4
5
IPr
SIPr
IMes
IAd
99
99
64
99
90
75
65
70
45
65
25
35
30
55
35
98
81
85
85
99
60
56
54
55
69
40
44
46
45
31
ItBu
a Reaction conditions: catalyst loading 0.025 mmol, 0.5 mmol of diazo compound in 3 mL of benzene, room temperature. b Percentage of initial EDA
converted into products. The remaining initial EDA (up to 100%) was converted into diethyl fumarate and maleate. c See eq 1.
Table 2. Reaction of Toluene and EDA in the Presence of (NHC)AuCl and (NHC)AuBr3 as Catalyst Precursorsa
(NHC)AuCl
(NHC)AuBr3
Entry
NHC
Conv. (%)b
% Insertion Productc
% Addition Productc
Conv. (%)b
% Insertion Productc
% Addition Productc
o
m
p
o
m
p
1
2
3
4
5
IPr
99
64
72
89
95
80
21
78
21
72
27
78
16
60
25
20
20
28
22
40
75
90
87
96
97
>99
22
-
46
48
44
47
44
33
31
29
37
31
50
50
44
47
42
28
31
30
28
31
SIPr
IMes
IAd
ItBu
>99
19
-
70
30
30
23
26
80
25
77
27
a Reaction conditions: catalyst loading 0.025 mmol, 0.5 mmol of diazo compound in 3 mL of the substrate at room temperature. b Percentage of initial
EDA converted into products. The remaining initial EDA (up to 100%) was converted into diethyl fumarate and maleate. c See eq 2.
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In order to gain information about the effect of the presence of
alkylic CꢀH bonds as subtituents in the aromatic ring, we have also
studied toluene as the substrate. In the previously reported catalytic
systems for carbene transfer from diazo compounds to toluene, or
other alkyl-aromatic substrates, the reaction provided mixtures of
the functionalization product derived from the insertion into the
alkyl group,7b,12,13 as well as the cycloheptatrienes (eq 3).
with slight variations. It seems that for this diazo compound the
o:m:p ratio is not significantly influenced by the catalyst.
Diazo Compound Screening. The lack of observation of a
significant effect of the NHC ligand in the o:m:p regioselectivity in
the reaction of EDA with toluene led us to employ a set of different
diazo compounds. In the first series of experiments, benzene was
reacted with ethyl 2-diazopropionate, ethyl R-diazo-R-(trimethy-
lsilyl)acetate and ethyl 3,3,3-trifluoro-2-diazopropionate in the
presence of catalytic amounts of IPrAuCl þ NaBAr04 (eq 4). As
inferred from the results shown in Table 3, this transformation
strongly depended on the nature of the diazo compound.
Following an experimental procedure similar to that noted for
benzene, we have carried out a study with the 10 catalyst
precursors (Table 2). To our delight, we have observed that in
no case did functionalization of the methyl group take place. On
the other hand, the formal insertion of the carbene group into the
aromatic CꢀH bond was the preferred path. We have also found
that the Au(III) complexes bearing IPr or SIPr ligands exclusively
provided the insertion product, with none of the seven-mem-
bered cycle. The presence of the methyl group in the aromatic
ring also induces the formation of the three ortho, meta, and para
isomers. Table 2 also contains the regioselectivities observed,
Whereas the previously reported reaction with EDA led to the
consumption of the diazo compound at room temperature after a
few hours, the use of ethyl 2-diazopropionate originated a change
in the course of the reaction: after 3 days at 60 °C, only 20% of the
initial diazo compound was consumed, with 10% of the corre-
sponding insertion product being obtained. Similar results were found
with the trifluoromethyl diazo analogue ethyl 3,3,3-trifluoro-2-
diazopropionate (Table 3, entry 3). However, a quite interesting
result was obtained with ethyl R-diazo-R-(trimethylsilyl)acetate
as the carbene source: after 24 h at room temperature, quantitative
conversions into a 85:15 mixture of the respective insertion and
addition products were observed (Table 3, entry 4). No products
derived from the coupling of the carbene groups were detected.
The study was expanded to toluene as the substrate, but also
introducing the variable of the oxidation state of gold. Thus, both
IPrAuCl and IPrAuBr3 complexes were employed as the catalyst
precursors, in the presence of 1 equiv of NaBAr04. Table 4 contains the
Table 3. Reaction of Benzene and Different Diazo Com-
pounds in the Presence of IPrAuCl þ NaBAr04 as the Catalysta
temp (°C)/ conversn amt of insertion amt of addition
entry
Rb
time (h)
(%)c
product (%)
product (%)
1
2
3
4
H
20/6
>99
20
75
10
25
nd
nd
15
CH3
CF3
60/72
60/3
15
>99
85
Me3Si
60/24
>99
a Reaction conditions: catalyst loading 0.025 mmol, 0.5 mmol of diazo
compound in 3 mL of the substrate. b R as in eq 3. c Percentage of initial
diazo compound converted into products. The remaining initial EDA
(up to 100%) was converted into diethyl fumarate and maleate.
Table 4. Reaction of Toluene with Several Diazo Compounds in the Presence of IPrAuCl and IPrAuBr3 as the Catalyst
Precursorsa,b
a Reaction conditions: catalyst loading 0.025 mmol, 0.5 mmol of diazo compound in 3 mL of toluene, temperature 60 °C, except for EDA (room temperature).
b Reaction times: EDA, 12 h; ethyl 2-diazopropionate, 72 h; ethyl 3,3,3-trifluoro-2-diazopropionate, data correspond to 3 h, catalyst decomposition being
observed after that time; ethyl 2-(trimethylsilyl)-2-phenylacetate, 24 h. c Determined by NMR with internal standard at the end of the reaction. The ratio
corresponds to the ortho:meta:para distribution of products. d Based on the initial diazo compound, the remaining up to 100% corresponded to fumarate or
maleate derivatives or unreacted diazo compound. e Distribution of products; nd = not detected; nr = no carbene transfer reaction observed.
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Table 5. Reaction of Isobutylbenzene with Several Diazo Compounds in the Presence of IPrAuCl and IPrAuBr3 as the Catalyst
Precursorsa,b
a Reaction conditions: catalyst loading 0.025 mmol, 0.5 mmol of diazo compound in 3 mL of isobutylbenzene, temperature 60 °C. b Reaction times:
EDA, 12 h; ethyl 2-diazopropionate, 72 h; ethyl 3,3,3-trifluoro-2-diazopropionate, data correspond to 3 h, catalyst decomposition being observed after
that time; ethyl 2-(trimethylsilyl)-2-phenylacetate, 24 h. c Determined by NMR with internal standard at the end of the reaction. The ratio corresponds
to the ortho:meta:para distribution of products. d Based on the initial diazo compound, the remaining up to 100% corresponded to fumarate or maleate
derivatives or unreacted diazo compound. e Distribution of products; nd = not detected; nr = no carbene transfer reaction observed.
conversions and distributions of products observed. It is noteworthy
that in no case was the methyl group of toluene functionalized, again
demonstrating the selectivity of this catalytic system toward aromatic
CꢀH bond functionalization. EDA as well as the SiMe3-containing
diazo compound provided high to very high conversions, whereas the
CH3-orCF3-derived diazo reagents displayed a much lower reactivity,
in spite of the fact that the reactions were performed at 60 °Cexceptin
the case of EDA. In the case of the more reactive reagents, the Au(III)
catalyst precursor gave exclusively the insertion product, the cyclo-
heptatriene being not detected in the reaction mixture (some
fumarate and maleate were also present due to the side reaction).
Finally, the o:m:p selectivity did not vary significantly either with the
catalyst or with the carbene source.
On the Way to Profen Derivatives: Isobutylbenzene as
Substrate. Once we learned that other diazo compounds were
compatible with this gold-based catalytic system, we focused our
attention on the expansion of this methodology. As shown in
Scheme 3, profens such as ibuprofen could be ideally prepared by
the direct insertion of a given carbene unit into the CꢀH bond of an
aromatic hydrocarbon.
acetate provided the best conversions, the diazo reagent being
entirely consumed and the coupling products remaining below
10%. On the other hand, ethyl 2-diazopropionate and ethyl 3,3,3-
trifluoro-2-diazopropionate gave low to moderate conversions.
Reactions were carried out at 60 °C for 24 h with the more reac-
tive diazo compounds. The chemoselectivity, intended as the
ratio of insertion to addition products, is favored when using the
Au(III) catalyst, that induced the completely selective formation
of the insertion products with EDA or Me3SiC(N2)CO2Et. It is
noteworthy that in no case did the insertion of the carbene group take
place into the carbonꢀhydrogen bonds of the alkyl substitutents. As
mentioned in the Introduction, this is at variance with other
reported catalytic systems for carbene transfer from diazo
compounds that, when applied to alkyl-aromatic substrates, led
to the functionalization of the alkyl group,7b,11,12 as well as to the
cycloheptatrienes. In our case, the selectivity induced by the gold
catalyst toward aromatic functionalization is complete with RC-
(N2)CO2Et (R = H, SiMe3). In addition, the complex IPrAuBr3
also enhanced the formation of the para isomer, which reached
63% and 56% for EDA and the trimethylsilyl derivative.
Some Comments about the Mechanism Governing This
Transformation. In the case of the functionalization of Csp3ꢀH
bonds by this methodology, it is commonly accepted that such a
transformation occurs after the interaction of a metallacarbene
intermediate with the CꢀH bond through a transition state
involving exclusively a carbenic carbon atom and the CꢀH bond
(Scheme 4).14 This is in good agreement with the experimental
Scheme 3. Strategy for the Synthesis of Ibuprofen by
Carbene Insertion
Scheme 4. Commonly Accepted Explanation for the Func-
tionalization of Csp3ꢀH by Carbene Insertion
Given the results presented above, we decided to study the use
of several diazo compounds using isobutylbenzene as an ibupro-
fen precursor. Again, IPrAuCl and IPrAuBr3 were employed as
the catalyst precursors. The results of this study are shown in
Table 5. Ethyl diazoacetate and ethyl R-diazo-R-(trimethylsilyl)-
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observation that the observed trend in reactivity, in all reported
systems, follows the order tertiary > secondary > primary sites:
i.e., the bond dissociation energy (BDE) order.15 However, this
assumption can be hardly applied to the results described in this
contribution. In the molecule of isobutylbenzene, quite distinct
CꢀH bonds, relative to BDE values, are present: aromatic (ca.
115 kcal/mol), benzylic (ca. 90 kcal/mol), aliphatic tertiary (ca.
95 kcal/mol), and aliphatic primary (ca. 98 kcal/mol). Since in
no case is the functionalization of other CꢀH bonds different
from the aromatics observed, and given that these are, by far, the
more stable bonds (in terms of BDE values) of the entire
molecule, it seems reasonable to propose that this transformation
cannot be explained by the pathway shown in Scheme 4. A
possible description of the mechanistic pathway could be based
in a previous work that employed rhodium diacetate catalysts for
intramolecular CꢀH insertion reactions. Padwa, Doyle, and co-
workers described16 the reaction shown in eq 5 as the result of the
electrophilic addition of the metallocarbene intermediate to the
aromatic ring, followed by a 1,2-hydride migration step. We
believe that a similar process could take place in our system,
therefore accounting for the observed exclusive selectivity toward
the aromatic CꢀH bonds.
’ CONCLUSION
We have found that the aromatic carbonꢀhydrogen bonds of
alkyl-substituted benzenes can be directly functionalized under
mild conditions using the methodology of carbene transfer from
several diazo compounds with gold-based catalysts, in a process
that discriminates between aromatic and aliphatic CꢀH bonds in
favor of the former. The precatalyst IPrAuBr3 has been found to
promote the best values of chemo- and regioselectivity. This
strategy has been applied for the direct synthesis of Profen
derivatives. Work aimed at developing a more efficient and
selective catalyst for this transformation is currently underway
in our laboratory.
’ EXPERIMENTAL SECTION
General Methods. All experiments requiring a dry atmosphere
were performed using conventional vacuum line and Schlenk techniques
or in a drybox. The reagents were purchased from Sigma Aldrich. The
complexes IPrAuCl and IPrAuBr3 were prepared following literature
procedures,9,10 as well as the diazo compounds17 and NaBAr04 (Ar0 =
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate).18 NMR solvents were
stored over molecular sieves under nitrogen. NMR data were recorded at
298 K using a Varian Mercury 400 instrument. 19F NMR chemical shifts
were referenced relative to an external CFCl3 standard. IR spectra were
recorded on a Varian 1000 FT-IR instrument. GC data were collected on
Varian 3900 and 3800 instruments.
General Catalytic Experiment. The catalyst (0.025 mmol) was
dissolved in 3 mL of the corresponding substrate (benzene, toluene, iso-
0
butylbenzene), and 1 equiv of NaBAr4 was added to the solution. After
15 min of stirring, RC(N2)CO2Et (R = H, Me, SiMe3, CF3; 0.5 mmol) was
added in one portion. After the mixture was stirred at 60 °C for 72 h (except
for R = H, which was performed at room temperature), the volatiles were
removed and the residue was dissolved in CDCl3. NMR studies revealed the
formation of two products: the cycloheptatrienes formed via the Buchner
reaction and phenyl acetates produced through the formal insertion of the
carbene into the aromatic CꢀH bond. See Tables 1ꢀ5 for results and the
Supporting Information for more details. We have not performed the
chromatographic separation of the ortho, meta, and para isomers, instead
determining their ratio by NMR studies.
The observation of the enhancement toward the para isomer
when using Au(III) also deserves some comment. The addition
of the halide scanvenger and subsequent interaction with ethyl
diazoacetate must afford transient goldꢀcarbene species
(Scheme 5). The geometry around the metal center must be
Scheme 5. Plausible Gold Carbene Intermediates
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, and tables giving
b
experimental details and spectroscopic data for the insertion and
addition compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
similar to that of the starting complexes: i.e., linear for Au(I)
and square planar for Au(III). The assignment of the trans
disposition of both the NHC ligand and the diazo-derived
carbene ligand is based on a previous observation from our
laboratory in a methylation reaction.11 On the basis of those
geometries, it is reasonable to propose that the interaction of
the carbene CRCO2Et with the aromatic ring would be more
constrained in the case of the square-planar geometry. How-
ever, the volume of the diazo compound also influences the
selectivity, the formation of the para isomer being reduced when
the size of the R substituent is increased. This is in agreement with
data in Table 4, where the para isomer is increased when moving
from Au(I) to Au(III) and decreased when moving from EDA to
Me3SiC(N2)CO2Et.
Corresponding Author
*E-mail: perez@dqcm.uhu.es (P.J.P.); mmdiaz@dqcm.uhu.es
(M.M.D.-R.).
’ ACKNOWLEDGMENT
We wish to thank the DGI (CTQ2008-00042BQU and
Consolider Ingenio 2010, Grant CSD2006-0003) and Junta de
Andalucía (P07-FQM-02870) for funding.
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