Carbon-supported iron-ionic liquid: An efficient and recyclable catalyst for benzylation of 1,3-dicarbonyl compounds with alcohols

Article abstract of DOI:10.1039/c4gc00755g

The effect of the addition of ILs with non-coordinating anions on the iron-catalyzed benzylation of 1,3-dicarbonyl compounds with alcohols under solvent free conditions has been evaluated. Among them, the presence of those containing the bistriflimide anion was found to be crucial for selectivity towards the benzylated product in a homogenous reaction. Therefore, the catalytic activity of Fe(OTf)₃-N₄₁₁₁NTf₂ combination supported on carbon materials with different textural and chemical surface properties has been studied. In the heterogeneous phase, reaction selectivity was also enhanced by the addition of the IL. However, substantial differences between the activities of different Fe-IL/carbon material catalytic systems were observed, indicating the influence of carbon support properties. With regards to selectivity, the best results were obtained using carbon supports with low microporosity. Moreover, the presence of oxygen functional groups on the carbon surface improved catalyst recycling. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00755g

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Page 1 of 17
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Carbon-supported iron-ionic liquid: an efficient and recyclable
catalyst for benzylation of 1,3-dicarbonyl compounds with alcohols
P. Moriel, and A. B. García *
Instituto Nacional del Carbón (CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain
Combining chemical strategies: a robust, heterogeneous and recyclable catalytic
system: iron-ionic liquid/carbon material as solid Lewis acid for direct benzylation of
1,3-dicarbonyl compounds with alcohols.

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Carbon-supported iron-ionic liquid: an efficient and recyclable
catalyst for benzylation of 1,3-dicarbonyl compounds with alcohols
P. Moriel, and A. B. García *
Instituto Nacional del Carbón (CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain
Abstract
The effect of the addition of ILs with nonꢀcoordinating anions on the ironꢀcatalyzed
benzylation of 1,3ꢀdicarbonyl compounds with alcohols under solvent free conditions
have been evaluated. Among them, the presence of those with the bistriflimide anion
was found to be crucial to the selectivity towards the benzylated product in the
homogenous reaction. Therefore, the catalytic activity of the Fe(OTf)3ꢀN4111NTf2
combination supported on carbon materials with different textural and chemical
surface properties has been studied. In the heterogeneous phase, the reaction
selectivity was also enhanced by the addition of the IL. However, substantial
differences between the activities of the different FeꢀIL/carbon material catalytic
systems were observed, indicating the influence of the carbon support properties. As
regards selectivity, the best results were obtained by using the carbon supports with
low microporosity. Moreover, the presence of oxygen functional groups on the
surface of the carbon surface improved the catalyst recycling.
1

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Introduction
In the last decade, iron salts have emerged as powerful Lewis acid catalysts for
organic synthesis.1 An example is the carbonꢀcarbon bond formation in the reaction
between an active methylene compound and an alcohol. Direct substitution of a
hydroxyl group is an attractive saltꢀfree and atomꢀeconomic process with water being
the only byꢀproduct. However, due to the poor leaving ability of this functional group,
harsh reaction conditions and large amount of expensive, toxic and moisture
sensitive reagents are required. These drawbacks can be solved through the use of
catalytic amounts of FeCl3 or Fe(ClO4)3.2,3 Moreover, iron salts are inexpensive, easy
to handle and environmentꢀfriendly.
Ionic liquids (ILs) are another topic of great attention in chemistry because of a
number of interesting tailored properties.4 Specifically the use of ILs in catalysis
allows the recycling of the catalysts, and in many cases, improves dramatically their
activities. For example, Lewis acidic metal salts often become much more reactive in
ILs containing nonꢀcoordinating anions as a result of anion exchange between the
Lewis acid and the IL.5 Consequently, more electrophilic Lewis acidic species
generated in situ accelerate the catalytic reactions. Additionally, because of the polar
nature of ILs, the activation barrier of the reactions can be reduced by the
stabilization of the polar transition states or reaction intermediates. However, the
immobilization of catalysts in ILs usually brings about mass transfer limitations and
cumbersome workꢀup procedures due to the high viscosity of many ILs. These
drawbacks can be overcome by supporting both the catalyst and the IL on the
surface of a solid. Supported Ionic Liquid Phase Catalysts (SILPC) combine the
benefits of ILs and heterogeneous catalysts, such as flexibility, good solubility of the
2

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catalytically active species, as well as ease of handling, separation and recycling.6
Generally, SILPC are composed of three elements: a porous solid support, an IL (a
small amount of IL is dispersed on the support surface) and the active catalyst.
The textural properties of mesoporous materials, such as a large surface area to
disperse the active phases and a proper pore size distribution to facilitate the
diffusion of reactants and products, make them very attractive as catalyst supports.
Carbon materials have been scarcely studied even though showing some
advantages over other traditional catalyst support like silica, alumina, metal oxides or
polymers.7 These materials have high corrosion resistance, good thermal and
mechanical stability, ease of handling, and low cost of manufacture. Furthermore,
chemical properties of the surface can be modified to control polarity, hydrophobicity
and acidꢀbase character. Recently, mesoporous carbon materials have been
successfully employed in our research group as support for different types of
catalyst, namely Ru and Pd nanoparticles,8 ILs derived from choline and amino acids
9 and lipases CALA and CALB.10
Results and discussion
Based on the background above described, the effect of the addition of ILs with
nonꢀcoordinating anions on the ironꢀcatalyzed benzylation of 1,3ꢀdicarbonyl
compounds with alcohols under solvent free conditions have been evaluated in this
work. Different mesoporous carbon materials have been tested as support for the
immobilization of FeꢀIL catalysts. The influence of the textural and surface chemical
properties of the carbon materials on the catalytic activity of the supported FeꢀIL has
3

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also been discussed. Furthermore, the recyclability and substrate scope of the
carbonꢀsupported FeꢀIL systems have also been studied.
The reaction between acetylacetone (1a) and 1ꢀphenylethanol (2a) in a
homogeneous phase using FeCl3, Fe(ClO4)3 or Fe(OTf)3 as metal source, different
ionic liquids (ILs) and reaction conditions was firstly investigated (Table 1). The
selected ILs were 1ꢀbutylꢀ3ꢀmethylimidazolium cation (BMIM) with different nonꢀ
coordinating anions (PF6, BF4, OTf and NTf2) and butyltrimethylammonium
bis(trifluoromethylsulfonyl)imide (N4111NTf2) whose apparent degree of dissociation is
larger than the corresponding IL with an aromatic cation.11
Most of the catalytic combinations promote the formation of 3ꢀ(1ꢀ
phenylethyl)pentaneꢀ2,4ꢀdione (3a) to some extent. Apart from the desired product,
relatively high amounts of bis(diphenylmethyl) ether (4a) and, in a low quantity (0ꢀ
10%), styrene (5), 4ꢀphenylpentanꢀ2ꢀone (6a),12 1ꢀphenylethyl acetate (7a) and some
oligomers derived from styrene (8) were also detected (see Scheme S1 and Table
S1 in the Supporting Information for more details).13 As seen in Table 1, the best
results were obtained when lessꢀnucleophilic, i.e. more Lewic acidic, counterꢀanions
of the metal salt (perchlorate and triflate) and the IL (bistriflimidate) were used
(entries 2 and 8). The addition of ILs with bistriflimide anion was crucial to the
selectivity towards the benzylated product (3a) (entry 1). However, any general trend
was observed as regards the cation of the IL. Therefore, the formation of
Fe(ClO4)x(NTf2)3ꢀx or Fe(OTf)x(NTf2)3ꢀx appears to be the most catalytically active iron
species.[5] Moreover, the catalyst selectivity was observed to improve by increasing
the amount of IL (entries 3ꢀ4 and 7ꢀ8).
4

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Table 1 Ironꢀcatalyzed reaction of acetylacetone (1a) and 1ꢀphenylethanol (2a)[a, b, c]
Entry
Iron source
Fe(OTf)3
IL
Conv. (%)[d] Yield (%)[e]
1
2
3
ꢀ
100
100
100
23
97
67
Fe(ClO4)3.xH2O
Fe(OTf)3
BMIM NTf2
BMIM NTf2
BMIM
4
Fe(OTf)3
100
78
[f]
NTf2
5
6
FeCl3.6H2O
Fe(ClO4)3.xH2O
Fe(OTf)3
BMIM NTf2
N4111 NTf2
N4111 NTf2
100
100
100
100
100
100
100
100
58
48
69
79
86
44
46
22
15
5
7
[f]
8
Fe(OTf)3
N4111 NTf2
N4111 NTf2
BMIM OTf
BMIM OTf
BMIM OTf
BMIM BF4
BMIM BF4
BMIM BF4
BMIM PF6
BMIM PF6
BMIM PF6
9
FeCl3.6H2O
Fe(ClO4)3.xH2O
Fe(OTf)3
10
11
12
13
14
15
16
17
18
FeCl3.6H2O
Fe(ClO4)3.xH2O
Fe(OTf)3
80
10
0
FeCl3.6H2O
Fe(ClO4)3.xH2O
Fe(OTf)3
13
96
13
24
13
100
91
FeCl3.6H2O
[a] Selected data from the optimization of the reaction conditions. [b] General reaction
conditions: 0.5 mmol of 2a; 4 equiv. of 1a; 4 mol% Fe; 77 mg of IL; 60 ºC for 3.5 h. [c] After
3.5 h the yields of bis(diphenylmethyl) ether (4a) and styrene (5) were lacking or remained
constant as determined by GC. See Scheme S1 and Table S1 (Supporting information) for
more details. [d] GC conversion of 2a. [e] GC yield of 3a. [f] 100 mg of IL was used.
5

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On the basis of these results, to evaluate the catalytic activity of FeꢀIL systems in
heterogeneous phase, Fe(OTf)3 was selected as the iron source to avoid the
14
explosiveness of perchlorate salt
and N4111NTf2 as IL. Therefore, the Fe(OTf)3ꢀ
N4111NTf2 combination was immobilized on four carbon materials (MBꢀ0, MBꢀLTA,
MBꢀ2000 and MBꢀ2000ꢀLTA) by an impregnation method, and they were employed
as catalysts for the model reaction. For comparison, the catalytic activity of Fe(OTf)3
supported on MBꢀ2000 and MBꢀ2000ꢀLTA was also studied.
Before discussing the results of the catalytic activity of the carbonꢀsupported FeꢀIL
systems, the textural parameters (surface area and pore volume) and surface
chemical analyses (total oxygen) of the carbon materials supports (Table 2) are
briefly considered. The starting MB shows relatively high microꢀ and mesoporous
volumes (MBꢀ0). Heating treatment of MBꢀ0 at 2000 ºC remarkably reduced the BET
surface area, mainly at expense of the micropore volume, and the surface oxygen
(MBꢀ2000). Plasma oxidation treatment (LTA) of MBꢀ0 and MBꢀ2000 did not
significantly affect the textural parameters; however, the amount of oxygen on the
surface increased drastically (MBꢀLTA and MBꢀ2000ꢀLTA).
Table 2 Textural parameters and XPS oxygen analysis of the carbon materials
SBET
Vt
VDRN2
VMESO
Micro
(%)[b]
44
O
Material
MBꢀ0
(m2gꢀ1) (cm3gꢀ1) (cm3gꢀ1) (cm3gꢀ1)[a]
(at.%)
1294
1251
211
1.141
1.111
0.676
0.719
0.506
0.490
0.081
0.083
0.635
0.621
0.595
0.636
5.0
MBꢀLTA
44
25.8
1.0
MBꢀ2000
12
MBꢀ2000ꢀLTA 223
12
32.9
[a] Mesopore volume : Vt – VDRN2. [b] Percentage of micropore volume (VDRN2) over the total
N2 pore volume Vt (at p/p0 = 0.99).
6

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Substantial differences between the catalytic activity of the supported Fe(OTf)3ꢀ
N4111NTf2 systems were observed, indicating a significant influence of the carbon
materials properties (Table 3). Thus, whereas Fe(OTf)3ꢀN4111NTf2/MBꢀ0 and
Fe(OTf)3ꢀN4111NTf2/MBꢀLTA did not catalyze the formation of 3a, the reaction
proceeded smoothly in the presence of Fe(OTf)3ꢀN4111NTf2/MBꢀ2000 and Fe(OTf)3ꢀ
N4111NTf2/MBꢀ2000ꢀLTA leading to almost quantitative yields of 3a. J. J. Rodríguez et
al. found that depending on the available pores in a solid support, ILs tend to fill
micropores firstly, then mesopores and lately macropores.15 Consequently, the
access of the substrate molecules to the catalyst on MBꢀ0 and MBꢀLTA was more
limited compared to MBꢀ2000 and MBꢀ2000ꢀLTA that presented lower microporosity
(Table 3). In heterogeneous phase, the selectivity of the formation of 3a was also
enhanced by the addition of the IL, although to a lesser extent (entries 5 and 6). It
should be also remarked that the catalyst loading in the heterogeneous reaction was
lower than in the homogeneous one. Therefore, the addition of N4111NTf2 and its
subsequent immobilization on an appropriate carbon support improved drastically the
selectivity of Fe(OTf)3 towards benzylated product, from 23 to 98% of yield (entries 1
and 8, Table 1 and entry 3, Table 3).
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Table 3 Catalytic activity of carbonꢀsupported Fe(OTf)3ꢀN4111NTf2 and Fe(OTf)3
systems.[a,d]
Catalyst
Conv. (%)[b]
Yield (%)[c]
Entry
1
58
53
4
4
Fe(OTf)3ꢀN4111 NTf2/MBꢀ0
Fe(OTf)3ꢀN4111 NTf2/MBꢀLTA
Fe(OTf)3ꢀN4111 NTf2/MBꢀ2000
Fe(OTf)3ꢀN4111 NTf2/MBꢀ2000ꢀLTA
Fe(OTf)3/MBꢀ2000
2
3
4
5
6
100
100
100
100
98
96
65
86
Fe(OTf)3/MBꢀ2000ꢀLTA
[a] General reaction conditions: 0.5 mmol of 2a; 4 equiv. of 1a; 286 mg of Fe(OTf)3ꢀ
N4111NTf2/MB (35 wt% IL; 3.5 wt% iron salt) (3.5 mol% Fe); 60 ºC for 3.5 h. [b] GC conversion
of 2a. [c] GC yield of 3a. [d] See Table S2 in Supporting Information for secondary products
yield.
To reinforce the advantage of these heterogeneous catalyst systems, the
recyclability of Fe(OTf)3ꢀIL/MBꢀ2000 and Fe(OTf)3ꢀIL/MBꢀ2000ꢀLTA was studied. In
first attempts, second runs of both provided high conversions of alcohol 2a, but low
selectivity for benzylated product 3a. Therefore, it was decided to quantify the
amount of iron supported after the first cycle of the reaction by ICPꢀMS. As shown by
the data collected in Table 4, ICPꢀMS analyses of carbon supports obtained after
ether workꢀup revealed that, in both cases, most of the Fe(OTf)3ꢀN4111NTf2 was
dissolved in the organic phases. However, when the heterogeneous catalyst systems
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were washed with Et2O or Hexane, most of the catalyst remained on the support.
Consequently, removing the 1,3ꢀdicarbonyl compound is necessary for avoiding
losing catalyst during the reaction workꢀup.16 Thus, this strategy was used in
recycling experiments. The results, summarized in Table 5, are in agreement with
ICPꢀMS analyses. Therefore, Fe(OTf)3ꢀBMIMNTf2, a bit more soluble in Et2O,
supported on MBꢀ2000 and MBꢀ2000ꢀLTA could be used without loss of activity for
only two cycles (entries 2 and 4, Table 5). The best results were obtained with
Fe(OTf)3ꢀN4111NTf2/MBꢀ2000ꢀLTA that could be employed without loss of activity and
without harsher reaction conditions for 4 runs (entry 3, Table 5). This catalytic system
supported on MBꢀ2000 afforded moderated yields of 3a in the third cycle (entry 1,
Table 5). The difference observed between Fe(OTf)3ꢀN4111NTf2 supported on these
two carbon materials is linked to the surface properties. Probably, oxygen containing
functional groups on MBꢀ2000ꢀLTA would help to retain the iron saltꢀIL on the carbon
surface.
Table 4 ICPꢀMS analyses of Iron [a]
Fe(OTf)3ꢀN4111NTf2
Fe(OTf)3ꢀBMIMNTf2
MBꢀ2000 MBꢀ2000ꢀLTA MBꢀ2000
MBꢀ2000ꢀLTA
0.35
Freshly prepared
0.35
0.25
0.35
0.09
0.34
0.22
0.31
0.07
0.32
0.18
0.34
Washed with
Et2O[b]
0.21
0.33
Washed with
Hexane [b]
After reaction and
workꢀup with Et2O
[a] Percentage of iron. [b] 286 mg of catalyst supports were washed with 3 x 5 mL of solvent,
filtered and dried under reduced pressure prior to elemental analyses
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Table 5 Recycling study of FeꢀIL catalysts supported on carbon materials [a, c]
Entry
1
Carbon support
MBꢀ2000
FeꢀIL
Run Yield (%)[b]
1
2
3
4
1
2
3
1
2
3
4
5
1
2
3
98 (100)
96 (100)
72(100)
50 (98)
96 (100)
93 (100)
52 (99)
96 (100)
95 (100)
92 (100)
93 (100)
47 (99)
95 (100)
97 (100)
56 (100)
Fe(OTf)3ꢀN4111NTf2
2
3
4
Fe(OTf)3ꢀBMIMNTf2
Fe(OTf)3ꢀN4111NTf2
Fe(OTf)3ꢀBMIMNTf2
MBꢀ2000ꢀLTA
[a] General reaction conditions: 0.5 mmol of 2a; 4 equiv. of 1a; 286 mg of Fe(OTf)3ꢀ
N4111NTf2/MB (35 wt% IL; 3.5 wt% iron salt) (3.5 mol% Fe); 60 ºC for 3.5 h. 1a was removed
under reduced pressure before reaction mixture workꢀup. [b] GC yield of 3a. GC conversion
of 2a in brackets. [c] See Table S3 in Supporting Information for secondary products yield.
Having these efficient experimental conditions in hand, we studied the scope of the
reaction regarding both coupling partners, the 1,3ꢀdicarbonyl compound (1) and the
alcohol (2), using Fe(OTf)3ꢀN4111NTf2/MBꢀ2000ꢀLTA as catalytic system.
Representative examples are reported in Table 6.
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Table 6 Catalytic activity of Fe(OTf)3ꢀN4111NTf2/MBꢀ2000ꢀLTA in the reaction of 1,3ꢀ
dicarbonyl compounds (1) and alcohols (2)[a]
Entry Nucleophile (1) Alcohol (2) Product (3) Time (h)
Yield (%)
1
2
3
4
5
6
3.5
3.5
3.5
24
99 (100)
99 (100)
45 (74)[c, d]
99 (100)
O
O
Ph
Ph
3e
O
O
O
47 (100)[c, d]
98 (100)
1c
24
24
[a] General reaction conditions: 0.5 mmol of 2a; 4 equiv. of 1a; 286 mg Fe(OTf)3ꢀ
N4111NTf2/MBꢀ2000ꢀLTA (35 wt% IL; 3.5 wt% iron salt) (3.5 mol% Fe); 60 ºC. [b] GC yield of
3a. GC conversion of 2a is shown in brackets. [c] Mixture of diastereomers ≈ 1:1, determined
by 1H NMR analysis. [d] Secondary products are shown in Scheme S2 (Supporting
information).
The reactions of 2,4ꢀpentadione (1a) with diphenylmethanol (2b) and an allyl
alcohol (2c) proceeded smoothly to give the benzylated products 3b and 3c with
excellent yield and high selectivity (entries 1 and 2, Table 6). Benzylation of 1,3ꢀ
difunctionalized compounds with higher pKa values (a cyclic βꢀdiketone 1b and a βꢀ
ketoester 1c)17 with diphenylmethanol (2b) afforded quantitative yields, but longer
times of reaction were needed (entries 4 and 6, Table 6). The reactions with alcohol
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2a were lack of selectivity, giving yields of 45 and 47% of the desired benzylated
product 3d and 3f, respectively. In these cases, the carbocation generated from 2a
was too unstable to overcome higher activation energy barrier of the benzylation
reaction with 1b and 1c (entries 3 and 5, Table 6).3d
Conclusions
Different strategies have been combined to achieve the benzylation of 1,3ꢀ
difunctionalized compounds with alcohols by an efficient, economic and environmentꢀ
friendly process. The developed catalyst system consists of immobilizing Fe(OTf)3
solved in a minimal amount of N4111NTf2 on a mesoporous carbon material with
suitable textural and surface chemical properties by an impregnation method,
avoiding cumbersome procedures and large amount of byꢀproducts and solvent
waste. This carbonꢀsupported catalyst can be recycled without loss of activity and
without harsher reaction conditions for 4 runs. Furthermore, the reactions can be
carried out under air atmosphere and solventꢀfree conditions without any special
precautions and the immobilized catalyst system can be stored for months in open air
without loss of catalytic activity.
Experimental section
Mesoporous activated carbon spheres (MBꢀ0), having a size distribution in the range
of 500ꢀ1000 ꢁm and derived from a phenolic resin, were used as starting support.
MBꢀ0 carbon material was heated at 2000 ºC under a flowing argon atmosphere for
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an hour in a electrical furnace, at a heating rate of 10 ºC/min (MBꢀ2000). Oxidation of
MBꢀ0 and MBꢀ2000 were carried out in a plasma reactor (MBꢀLTA and MBꢀ2000ꢀ
LTA). Batches of approx. 500 mg of carbon material were treated using oxygen
excited by a radiofrequency power of 75 W for 10 min.
Surface area, pore volume and pore size distribution of MBꢀ0, MBꢀ2000, MBꢀLTA
and MBꢀ2000ꢀLTA supports were derived from the N2 adsorption–desorption
isotherms performed at 77 K in a volumetric adsorption system. XꢀRay Photoelectron
Spectroscopy (XPS) surface chemical analyses of carbon supports were carried out
using Mg Ka Xꢀrays (1486.6 eV) at a power of 120 W and in a residual vacuum of 10ꢀ
7
Pa. The atomic percentages (atom.%) of the different elements present in the
approx. 10 nm upper layer were calculated from the survey spectra by considering
the integrated areas of the main XPS peaks. Representative samples of mesoporous
carbon supports (around 150 mg) were tested (more details about the carbon
supports characterization are provided in Supporting Information).
For the preparation of the carbonꢀsupported ironꢀionic liquid catalyst, the
corresponding ionic liquid (35 wt.%) and iron (III) triflate (3.5 wt%) dissolved in a
minimal amount of acetone were mixed with the carbon support in a screw top vial.
The mixture was shaken at room temperature for 2.5 h followed by a careful removal
of the solvent in a rotary evaporator. Then, the heterogeneous catalyst was dried at
60 ºC in a vacuum oven for 2 h. Catalyst loading was evaluated from the Fe
elemental content as analyzed by ICPꢀMS after wet digestion with aqua regia
(HCl:HNO3 3:1) in a microwave reactor .
All reactions were carried out under air atmosphere without any special
precautions. The reaction mixture in a flask was stirred at 60 ºC and 200 rpm, and
monitored by CG. After completion, the crude reaction mixture was allowed to cool to
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room temperature and taken up in diethyl ether (3 x 5 ml). The combined organic
extracts were dried over MgSO4, filtered and the solvent was evaporated under
reduced pressure; if necessary, the residue was purified by flash chromatography on
silica gel. In recycling experiments, it was necessary to remove 2,4ꢀpentadione under
reduced pressure before reaction workꢀup in Et2O (3 x 5 ml) to avoid losing catalyst.
Then, recovered catalyst was dried under reduced pressure and used in the next
cycle of the reaction. The products are well known in the literature and were
confirmed by GCꢀMS and 1H NMR spectroscopic techniques.18
Acknowledgements
Financial support from Asturias Government PCTI (under Project IB09ꢀ002C2) is
gratefully acknowledged. Patricia Moriel thanks the Spanish Research Council for
Scientific Research (CSIC) for a JAEꢀDoc contract, coꢀfunded by the European
Social Fund (ESF).
Notes and references
[1] a) L. X. Liu, Curr. Org. Chem., 2010, 14, 1099ꢀ1126; b) J. I. Padrón, V. S. Martín
in Top. Organomet. Chem. Vol. 33 (Ed.: B. Plietker), 2011, pp. 1ꢀ26; c) C. Bolm,
J. Legros, J. Le Paih, L. Zani, Chem. Rev., 2004, 104, 6217ꢀ6254.
[2] a) J. Kischel, K. Mertins, D. Michalik, A. Zapf, M. Beller, Adv. Synth. Catal.,
2007, 349, 865ꢀ870; b) Y. Yuan, Z. Shi, X. Feng, X. Liu, Appl. Organomet.
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[13] See Scheme S1 (Supporting information) for more details.
[14] Ferric perchlorate is stable in solid state, but is caused fire or violence explosion
when is heated and in contact with combustibles materials.
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[16] Hexane could not be used in the workꢀup of the reactions because is too nonꢀ
polar to take up the product of the reaction.
[17] pKa (1a) 8.94; pKa (1b) 10.87; pKa (1c) 10.67 (Scifinder database).
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16