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Green Chemistry
DOI: 10.1039/C5GC01142F
of imidazolium compounds.43
Fig. 2 Cyclic voltammograms of pyrene (top, black solid line), MeIm
(top, black dotted line), pyrene + 6 eq. MeIm (middle, blue solid line),
Decreasing the amount of MeIm from 6 to 3 equivalents does
55 not change the yield of the reaction and only 2.35 Faradays were
required (entry 3, Table 1). Under these conditions, but at lower
concentration ([pyrene] = 4.8×10ꢀ4 M, Fig. 3), the UVꢀvis.
spectrum of the reacting solution has been monitored as a
function of the electrolysis progress. The initial pyrene signature
−
pyrene + 6 eq. MeIm + 1.5 eq. HBF4 (middle, blue dotted line), 1+,BF4
(bottom, red solid line) in CH3CN 0.1 M TEABF4 (WE: Pt Ø = 2 mm, ν =
5
100 mV s−1).
After addition of 6 equivalents of MeIm to the pyrene solution,
the cyclic voltammogram shows a new oxidation peak at Epa
=
1.71 V (Fig. 2, middle, blue solid curve).This redox system does
not correspond to the direct oxidation of MeIm since this latter is
10 irreversibly oxidized at higher potential (Epa = 1.85 V (Fig. 2, top,
60 with band maxima at
progressively substituted by a new bathochromically shifted
spectrum appearing at = 265, 275, 314, 326 and 341 nm which
λ = 262, 273, 305, 319 and 334 nm is
λ
corresponds to 1+,BF4 . The presence of 7 isosbestic points at
λ =
−
black dotted curve)). After addition of 1.5 equivalents of HBF4,
−
1.5 equivalents of MeIm are protonated (MeImꢀH+,BF4 ).
263, 268, 309, 313, 322, 330 and 337 nm confirms the simple
−
65 transformation of the initial pyrene into 1+,BF4 , with no
Irreversible reduction of this species is clearly seen at Epc = −0.76
V (Fig. 2, middle, blue dotted curve). Thus, MeImꢀH+,BF4− is the
15 first reactant to be reduced, leading to MeIm and H2.
Consequently, in a single compartment electrochemical cell,
oxidation of pyrene will happen at the anode while reduction of
intermediate observed between.
Following the conditions of entry 3, the electrosynthesis was
attempted with undistilled CH3CN and under air (entry 4, Table
1). These conditions have been tested since they are easy to
70 implement and they could be easily scaled up for industrial
applications. Moreover, it is expected that MeImꢀH+,BF4− will be
reduced at a higher potential than oxygen and thus this latter will
not interfere. Besides, production of potentially recoverable
hydrogen gas during the electrosynthesis will progressively drive
75 away the initial air atmosphere. Though full conversion of the
initial pyrene reactant was not reached and number of abstracted
electrons was higher than theoretically expected (3.25 F), the
yield remained very good (84%).
−
MeImꢀH+,BF4 will proceed at the cathode. Theoretically, the
anodic nucleophilic substitution reaction requires abstraction of 2
20 molar equivalents of electrons from pyrene.31,42 This reaction
−
starts with the formation of pyrene cation radical (Pyr•+,BF4 ,
Scheme 1) which is then attacked by MeIm nucleophile. This
−
cationic intermediate (PyrꢀMeIm•+, BF4 , Scheme 1) is oxidized
−
and releases one proton providing 1+,BF4 . In return, the cathodic
25 reaction must involve 2 F and thus two equivalents of MeImꢀ
−
H+,BF4 must be reduced at the cathode. As only one molar
equivalent of protons is generated during the formation of
1+,BF4 , initial addition of at least one equivalent of H+ is needed
to preclude other reactions than reduction of protons to happen at
30 the cathode.
−
2.00
1.75
1.50
1.25
s
b
1.00
A
0.75
0.50
0.25
0.00
Scheme
1 Proposed reaction mechanism for the electrochemical
−
formation of 1+,BF4 .
260
280
300
320
340
360
380
Bulk electrolyses were first performed with 6 equivalents of
35 MeIm (entry 2, Table 1) in a three electrodes/two compartments
configuration under potentiostatic conditions (Eapp = 1.40 V).
3.25 Faradays (F) were necessary for full conversion of pyrene.
In these conditions a yield as high as 92% was obtained denoting
the perfect regioselectivity of this anodic nucleophilic
40 substitution. It should be noted that no other pyreneꢀbased
λ
/ nm
80 Fig. 3 Electrolysis of a 4.8×10ꢀ4 M solution of pyrene with 3 equiv. of 1ꢀ
MeIm and 1.5 equiv. of HBF4 followed by UVꢀvis spectroscopy (l = 2
mm, 0.1 M TEABF4 in CH3CN, Eapp = 1.40 V vs. SCE, −2.20 F, WE: Pt
wire, WE and counterꢀelectrode are in the same compartment).
An experiment with only 2 equiv. of MeIm and 1 equiv. of
85 tetrafluoroboric acid was attempted but full conversion of the
pyrene was not possible even after reaching the theoretical value
of 2 F (entry 5, Table 1). Nevertheless, a correct yield of 75%
was obtained.
An upscale of the reaction was tested in the same conditions as
90 entry 3 (entry 6, Table 1) but on 3.500 g of pyrene at higher
concentration. After abstraction of only 2.05 molar equivalents of
electrons, very close to the theoretical value of 2 F, a 95% yield
was reached denoting the even higher efficiency of this
electrosynthesis on larger scale.
1
product was detected on the H NMR spectrum of the crude
solution. Besides, the cyclic voltammogram of the electrolyzed
crude solution do not show any trace corresponding to the initial
unsubstituted pyrene oxidation confirming that this species was
45 fully exhausted. Furthermore, new irreversible oxidation (Epa
=
1.54 V) and reduction (Epc = −1.79 V) peaks which corresponds
to the oxidation and reduction of the electrogenerated 1+,BF4− are
clearly seen at the end of the electrolysis (see Fig. 2, bottom, red
−
curve, for the cyclic voltammogram of pure 1+,BF4 ). The peak
50 located at Epc = −1.79 V reasonably corresponds to the reduction
95
For industrial applications, the cell setup must be as simple and
as cheap as possible. Thus, a one compartment cell containing
of the imidazolium moiety of 1+,BF4 since this potential value
−
falls within the common potential range found for the reduction
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De Robillard, Guillaume
