Cucurbit[5]uril-mediated electrochemical hydrogenation of α,β-unsaturated ketones

Article abstract of DOI:10.1007/s10847-019-00944-4

The potential of cucurbit[5]uril to be used as inverse phase transfer catalyst in electrocatalytic hydrogenation of α,β-unsaturated ketones is illustrated. The interaction behavior among isophorone and cucurbit[5]uril was also investigated using cyclic voltammetry and UV/vis absorption spectroscopy. The results concerning to both techniques revealed an enhancement in the intensity of the absorption peak and also in the current cathodic peak of isophorone in presence of cucurbit[5]uril. This achievement is related to the increase of the isophorone solubility in the medium being an indicative of a host-guest complex formation. The electrochemical hydrogenation of isophorone using cucurbit[5]uril was more efficient than others well-stablish methodologies. Regarding to (R)-(+)-pulegone and (S)-(+)-carvone, the use of cucurbit[5]uril leads to an increase of 17% and 9%, on average, respectively, in the yields when compared to the control reaction. The efficiency of selective C=O bond hydrogenation of 1-acetyl-1-cyclohexene was evaluated. The presence of cucurbit[5]uril increased by 12% the hydrogenations yields of 1-acetyl-1-cyclohexene when compared to the control reaction. In this sense, these results open up an opportunity to carry out electrocatalytic reactions within the cucurbit[5]uril environment.

Full text of DOI:10.1007/s10847-019-00944-4

Journal of Inclusion Phenomena and Macrocyclic Chemistry
https://doi.org/10.1007/s10847-019-00944-4
ORIGINAL ARTICLE
Cucurbit[5]uril‑mediated electrochemical hydrogenation
of α,β‑unsaturated ketones
1
1
1
Ayrlane Sales · Isabela Andrade de Oliveira e Castro · Frederico Duarte de Menezes ·
1
1
Thiago Matheus Guimarães Selva · Márcio Vilar
Received: 29 July 2019 / Accepted: 26 August 2019
©
Springer Nature B.V. 2019
Abstract
The potential of cucurbit[5]uril to be used as inverse phase transfer catalyst in electrocatalytic hydrogenation of α,β-
unsaturated ketones is illustrated. The interaction behavior among isophorone and cucurbit[5]uril was also investigated using
cyclic voltammetry and UV/vis absorption spectroscopy. The results concerning to both techniques revealed an enhancement
in the intensity of the absorption peak and also in the current cathodic peak of isophorone in presence of cucurbit[5]uril.
This achievement is related to the increase of the isophorone solubility in the medium being an indicative of a host-guest
complex formation. The electrochemical hydrogenation of isophorone using cucurbit[5]uril was more eꢀcient than others
well-stablish methodologies. Regarding to (R)-(+)-pulegone and (S)-(+)-carvone, the use of cucurbit[5]uril leads to an
increase of 17% and 9%, on average, respectively, in the yields when compared to the control reaction. The eꢀciency of
selective C=O bond hydrogenation of 1-acetyl-1-cyclohexene was evaluated. The presence of cucurbit[5]uril increased by
1
2% the hydrogenations yields of 1-acetyl-1-cyclohexene when compared to the control reaction. In this sense, these results
open up an opportunity to carry out electrocatalytic reactions within the cucurbit[5]uril environment.
Keywords Cucurbit[5]uril · Electrocatalytic hydrogenation · Inverse phase transfer catalyst · Isophorone · Voltammetry
Introduction
cyclodextrins water-soluble. The size of their nanometric
hydrophobic cavity allows the formation of inclusion com-
plexes with diꢁerent hydrophobic substances [10].
It is well known that cavitands such as cyclodextrins [1–3],
cucurbit[n]urils [4, 5], pyragallol[n]arenes [6], calix[n]
arenes [7], cryptophanes [8, 9], can form a host-guest
inclusion complex. One of the most popular examples of
this kind of cavitands is the cyclodextrins. The chemistry
of cyclodextrins has already been extensively reported [2,
Both cavities of the cyclodextrins diꢁers in size and
chemical nature, with the smaller cavity lined by primary
hydroxyl groups and the larger one by secondary hydroxyl
groups (Fig. 1a and Table 1). In this sense, through the host-
guest, it is possible to enhance the solubility of hydrophobic
molecules as well to improve and/or accelerate some organic
reaction [2, 3, 15, 20–22].
3
, 10–15]. Cyclodextrins are natural products composed
of d-glucopyranosyl units having a toroidal shape with a
hydrophilic exterior and lipophilic interior cavity (Fig. 1a
and Table 1) [10, 16–19]. This amphiphilic character makes
The guest hydrophobicity plays also an important role in
the stability of the host-guest complexes. Cucurbit[n]urils
is not an exception of this rule. Cucurbit[n]urils belongs to
a series of macromolecules ligands having an inner cavity
and, therefore, with a potential host-guest binding capacity.
Cucurbit[n]urils are pumpkin-shaped, highly symmetrical
and rigid macromolecules, with negatively charged carbonyl
rims and a lipophilic cavity (Fig. 1b) [4, 18, 23]. They are
constructed of n-glycouril units bounded in rigid arrange-
ment by methylene bridges. The number of glycouril units
determines the portal size and the available volume of
the cucurbit[n]urils (Table 1). They exhibit an equatorial
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10847-019-00944-4) contains
supplementary material, which is available to authorized users.
*
Márcio Vilar
marciovilar@recife.ifpe.edu.br
1
Departamento de Sistemas, Processos e Controles
Industriais, Instituto Federal de Educação, Ciência e
Tecnologia de Pernambuco, Cidade Universitária, Recife,
PE CEP 50740-540, Brazil
Vol.:(0
1
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3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
Fig. 1 Structural comparison of cucurbit[n]urils and cyclodextrins: a d-glucopyranosyl units representation and general structure of cyclodex-
trins and b n-glycouril units and general representation of cucurbit[n]urils pumpkin-shape
Table 1 Structural parameters
Entry Properties
Cyclodextrins (CDs)
Cucurbit[n]uril (CB[n])
and properties of cucurbit[n]
urils and cyclodextrins [11,
αCD
βCD
γCD
CB[5] CB[6] CB[7]
CB[8]
8
1
6–19]
1
2
3
4
5
6
Number of units
6
7
8
5
6
7
Molecular weigh (anhydrous) 972.85 1134.99 1297.14 830.7 996.84 1162.98 1329
Portal diameter (Å)a
4.7
5.3
174
7.9
6.0
6.5
262
7.9
7.5
8.5
427
7,9
2.4
4.4
82
3.9
5.8
164
9.1
5.4
7.3
279
9.1
6.9
8.8
479
9.1
b
Inner cavity diameter (Å)
Cavity volume (Å )
Cavity height (Å)
3
9.1
a
b
Narrowed rim of CDs; wider rim of CDs
symmetry plane, making both cavity openings identical
Fig. 1b). Their total depth is 9.1 Å but shown variations
[26] on dipolar cycloadditions catalyzed by cucurbit[6]uril.
Otherwise, cucurbit[7]uril has been employed to catalyze
photolysis of bicyclic azoalkanes by transition metal ions
coordinated to the cucurbit[7]uril rim [27]. Additionally,
Barooah et al. [28] reported the manipulations of the pho-
tochemical reactivity of coumarin derivatives in presence
of cucurbit[8]uril. Although cucurbit[5]uril shares similar
binding properties with other members of the cucurbit[n]
urils family, it small inner volume (Table 1) greatly limits
its use in host-guest chemistry when compared to the larger
homologues.
(
in cavity width among their diꢁerent analogues (Table 1).
The ꢂrst production of cucurbit[n]urils was reported by
Behrend and coworkers in 1905 [24]. The authors reported
the synthesis of a pumpkin-shaped molecule with six gly-
couril unit named as cucurbit[6]uril. In the early 2000 s
cucurbit[n]urils with different sizes (n=5,7,8), namely
respectively, cucurbit[5]uril, cucurbit[7]uril, cucurbit[8]uril
have been synthesized by Kim et al. [25]. The potential of
CB[n] catalyze chemical reactions starting from Mock work
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
Accordingly to the green chemistry principles, a non-
toxic, non-inꢃammable, abundantly available, inexpensive
and of lower risk solvent to human health should be pre-
ferred in organic reactions [29]. In this sense, water should
be the ꢂrst choice among the chemists not only because it
is environmentally acceptable but also due to the mecha-
nistic point of view [30]. However, the most of substrates
used in organic or electroorganic synthesis are insoluble in
water leading to an overall yield drop of the reactions mak-
ing it use limited. One approach described in the literature to
overcome this limitation is the use of inverse phase transfer
catalyst (IPTC) in which the low soluble substrate is trans-
ferred by the IPTC from the organic into the aqueous phase
through the host-guest complexation.
standard substrate. Moreover, diꢁerent organic substrates
were tested, taking into account structural variations. Finally,
this study is relevant from both electrocatalytic, supramo-
lecular and green standpoint because there are no studies
reporting the use cucurbit[5]uril as IPTC on the ECHSA
methodology.
Experimental
Chemicals
Reagent-grade chemicals were used without further puriꢂca-
tion. Isophorone (97%), (R)-(+)-Pulegone (97%), 1-acetyl-
1-cyclohexene (97%), 1-cyclohexyl-ethanol (97%) and
(S)-(+)-carvone (96%) were purchase from Sigma Aldrich.
Diethyl ether and ammonium chloride (supporting elec-
trolyte) were purchased from Vetec and used as received.
Sodium sulphate was purchased from Acros. Cucurbit[5]
uril, α- and β-cyclodextrin (Aldrich) were used as received.
Deionised water, obtained by a Mili-Q system, was used as
solvent in all experiments.
The ꢂrst use of IPTC agent has been reported by Zahalka
et al. [31] and Harada and co-workers [32]. Baur et al. [33]
reported the use of calix[n]arenes as IPTC in Suzuki cou-
pling reaction of phenyl boronic acid and iodobenzene in
aqueous solution. The authors show that the eꢀciency of the
tested calix[n]arenes was higher than that for β-cyclodextrin.
Mortreux and coworkers use modified β-cyclodextrin
(
per(2,6-di-O-methyl)-β-cyclodextrin) as IPTC in rhodium-
catalyzed hydroformylation of a variety of water-insoluble
terminal oleꢂns achieving high yields and selectivity [20].
In the electrocatalysis ꢂeld, Maiorova et al. [34] have
applied β-cyclodextrin in the electrochemical hydrogenation
Procedures
UV/vis spectroscopy
(ECH) of acetophenone and methyl-4-isobutylacetophenone
in both diꢁerent solutions (water and water:ethanol) reach-
ing high conversions (90%). Farnia et al. [35] studied the
electrochemical reduction of acetophenones in aqueous alka-
line solutions, in absence and presence of β-cyclodextrin,
leading to D,L and meso pinacols (head-to-head dimers) as
products. Amatore et al. [36] described the electrochemical
behaviors of a variety of acetophenones in the presence of
α-, β-, and methyl-β-cyclodextrin, in diꢁerent solvents. The
authors showed that, in dimethylformamide, cyclodextrin
act as weak proton donors towards the electrogenerated ace-
tophenone anion radical favoring the head-to-tail coupling.
Vilar and Navarro [2] studied the ECH of benzaldehyde in
water, using a nickel sacriꢂcial anode (SA) (henceforward
referred as ECHSA) and β-cyclodextrin as IPTC. The results
showed that using β-cyclodextrin the yield of benzyl alcohol
was 27% higher than that obtained in the absence of the
IPTC. Thus, it should be possible to anticipate that other
cavitands with similar dimensions and hydrophobicity inner
properties can be used as IPTC. Even though cucurbit[5]uril
is not big enough to accommodate large molecules into its
cavity, its small size may be used to solve the mass-transport
limitations in electrocatalytic reactions.
UV/vis spectrums were recorded (Cary 50 Probe) with a
full-length range of 190–350 nm for aqueous solutions of
−
1
the isophorone (6.1 mmol L ) in absence and presence
of diꢁerent concentrations of cucurbit[5]uril, from 3.5 to
−
1
7.4 mmol L . The resulting solutions were irradiated by
ultrasound for 5 min and then added to a quartz cuvette with
1.0 cm optical path length and taken to record the spectrums.
Electrochemical experiments
The electrochemical experiments were run with a potentio-
stat/galvanostat Autolab PGSTAT101 (Eco Chemie, Utretch,
The Netherlands) controlled by a personal computer running
NOVA software (version 1.11.2).
Cyclic voltammetry
The cyclic voltammetry experiments were carried out at
room temperature in a standard undivided three-electrode
cell with a capacity of 10 mL. The working, reference and
counter electrodes were a glassy carbon electrode (ϕ =
−
1
3 mm), a home-made Ag|AgCl|KCl (3 mol L ) electrode
[37] and a platinum coiled wire, respectively.
In this context, we describe the ꢂrst use of cucurbit[5]uril
in the ECHSA of α,β-unsaturated ketones in water using an
undivided electrochemical cell. Isophorone (IUPAC name:
Electrochemical studies were performed by using a
−
1
0.2 mol L of NH Cl aqueous solution as supporting elec-
4
3
,5,5-trimethyl-2-cyclohexen-1-one) was adopted as the
trolyte. The glassy carbon working electrode was polished to
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
a mirror-like using an alumina suspension (0.3 μm particle
size) with a microcloth polishing pad before and between
each cyclic voltammetry measurement. This procedure was
performed to ensure the removal some possible adsorbed
species. The potential range studied in cyclic voltamme-
try experiments started from 0 to −1.65 V at a scan rate of
temperature program started from 60 °C for 2 min followed
−1
by a 10 °C min ramp up to 220 °C, where this temperature
was held for 2 min. For 1-acetyl-1-cyclohexene, comparisons
with authentic samples were performed by GC in order to
identify the hydrogenation products. Toluene (0.067 mmol)
was used as an internal standard.
−
1
1
00 mV s .
GC-MS measurements was conducted on a Shimadzu
GC-MS model GC17A, a 70 eV ion trap, ꢂtted with a 30 m
capillary TGMS - 5 (HP) column. The temperature program
The presence of cucurbit[5]uril toward the electrochemi-
cal reduction of isophorone was evaluated. For this purpose,
−
1
−1
1
0 mL of an aqueous solution of isophorone (2.6 mmol L )
started from 60 °C for 3 min followed by a 10 °C min ramp
−
1
−1
in 0.2 mol L NH Cl was prepared. The resulting solu-
up to 240 °C and 60 °C min ramp up to 3000 °C, where
4
tion was stirred for 24 h and added to the electrochemical
this temperature was held for 4 min.
system. Diꢁerent dispersions of cucurbit[5]uril ranging
−
1
from 0.614 to 1.32 mmol L were prepared directly in
−1
0
.2 mol L of NH Cl. The cyclic voltammetry experiments
Results and discussion
4
of the cucurbit[5]uril dispersions in presence of isophorone
−
1
(
2.6 mmol L ) were conducted as mentioned before.
UV/vis spectroscopy behavior of isophorone
in presence of cucurbit[5]uril
Electrochemical hydrogenation procedure
UV/vis spectra of cucurbit[5]uril and isophorone were
recorded at room temperature (Fig. 2a). Isophorone show
A nickel rod was used as sacriꢂcial anode (0.1 dm diam-
2
eter; 0.078 dm area). Another Ni bar (0.1 dm diameter;
an absorption band (λ ) at 240 nm which corresponds to
max
2
0
.078 dm area) was also used as the working electrode.
the n → π* and π* → π* transitions of the chromophores
C=O and C=C, respectively of the α,β-unsaturated carbonyl
system of isophorone [38, 39]. As can be notice in Fig. 2a,
b and Table 2 (spectra 2–7), an enhancement in the absorp-
tion intensity of isophorone was observed due to the pres-
ence of cucurbit[5]uril. A hypsochromic shift of 1 nm was
Deionised water was used as solvent in all electrochemical
hydrogenation experiments.
In order to evaluate both the cyclodextrin and cucurbit[5]
uril inꢃuence on the electrocatalytic system, aqueous stock
solutions of α-cyclodextrin and β-cyclodextrin and cucur-
bit[5]uril were prepared. A 4.0 mL aliquot of the stock solu-
tion was mixed with 1.0 mmol of the substrate and stirred for
−
1
observed in presence of cucurbit[5]uril (3.50 mmol L or
0.57 equivalents) into isophorone solution (Table 2, entry
−
1
1
h. After that, the electrolytic cell containing both the work-
2 and Fig. 2a) which persisted up to 7.40 mmol L (1.21
equivalents) of cucurbit[5]uril. These ꢂndings and observa-
tion are an indicative of the host-guest inclusion formation
due to the change of environment around the chromophore
group of isophorone.
ing electrode (Ni) and the sacriꢂcial anode (Ni), were ꢂlled
−
1
with 6.0 mL of the supporting electrolyte (0.2 mol L ). At
this point, the ꢂnal concentrations of species in solution
−
1
were: 1.2 mmol L of α-cyclodextrin or β-cyclodextrin or
cucurbit[5]uril. Electrolysis was carried out by using a gradi-
−
2
ent current density initially at 350 mA dm . After passage
Electrochemical behavior of isophorone in absence
and presence of cucurbit[5]uril
−
1
of the ꢂrst half-charge (96.50 C or 1.0 F mol ), the current
−
2
density was decreased to 300 mA dm (passing the second
−
1
half-charge; 48.25 C or 0.5 F mol ), and successively to
The electrochemical reduction behavior of isophorone in
absence and presence of cucurbit[5]uril was evaluated using
cyclic voltammetry technique at glassy carbon electrode.
Figure 3 displays the cyclic voltammograms of isophorone
−
2
−1
2
50 mA dm (third half-charge; 24.12 C or 0.25 F moL )
−
2
and 200 mA dm (fourth half-charge; 12.06 C or 0.125 F
−
1
mol ).
After the electrolysis, the water solution was extracted
with diethyl ether and dried with Na SO . Product analysis
−
1
−1
(2.6 mmol L ) aqueous solution containing 0.2 mol L of
NH Cl, as supporting electrolyte, in absence and presence
2
4
4
was performed primarily by gas chromatography with ꢃame
ionization (GC-FID) or mass spectral detection (GC-MS).
GC-FID measurements were conducted on a Varian 3380
GC gas chromatograph equipped with a ꢃame ionization
detector. This GC-FID instrument was operated with a 30
m × 0.25 mm column, with a 0.25 μm dimethylpolysiloxane
lining and a guard column. 2 μL sample was injected at a
temperature of 250 °C in split mode with a 200:1 ratio. The
of increased amounts of cucurbit[5]uril, recorded on glassy
−
1
carbon electrode at scan rate of 100 mV s .
It is possible to observe an irreversible reduction signal
−
1
at −1.57 V vs. Ag|AgCl|KCl (3 mol L ). The signal around
− 0.65 V is related to the reduction of dissolved oxygen.
From the data it is possible to observe an increase of the
current of cathodic peak with the increase of the concentra-
−
1
tion of cucurbit[5]uril until 0.746 mmol L . Furthermore,
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
Fig. 2 a Absorption spectra (λ
−
1
=
240 nm) for 6.1 mmol L
of isophorone with diꢁerent
equivalents of cucurbit[5]uril:
a) 0.0; (b) 0.57 (3.5 mmol L 1);
−
(
−
1
(
c) 0.68 (4.2 mmol L );
−1
(
d) 0.80 (4.9 mmol L ); (e)
−
1
1
.01 (6.2 mmol L ); (f) 1.08
−1
(
(
6.6 mmol L ) and (g) 1.21
−
1
7.4 mmol L ). b Absorp-
tion spectra changes in the
presence of diꢁerent equiva-
lents of cucurbit[5]uril: ((a)
0
.0 (λ=242 nm); (b) 0.57
−1
(
(
3.5 mmol L ; λ=241 nm);
−1
c) 0.68 (4.2 mmol L
;
λ=241 nm); (d) 0.80
−
1
(
4.9 mmol L ; λ=241 nm);
−1
(
e) 1.01 (6.2 mmol L
;
λ=241 nm); (f) 1.08
−
1
(
6.6 mmol L ; λ=241 nm)
−1
and (g) 1.21 (7.4 mmol L
λ=241 nm)
;
Table 2 UV–vis spectral data of 6.1 mmol L− isophorone in water
1
with increasing concentration of cucurbit[5]uril at 298 K
Spectra
[CB[5]] (mmol L−1
)
Absorbance
a.u.)
λ
(nm)
max
(
at 298 K
1
2
3
4
5
6
7
0.00
3.50
4.20
4.90
6.20
6.60
7.40
2.36
2.45
2.46
2.48
2.51
2.54
2.56
242
241
241
241
241
241
241
the reduction potential of isophorone does not shift signiꢂ-
cantly in presence of cucurbit[5]uril at diꢁerent dispersions
concentrations when compared to the cyclic voltammogram
of isophorone in absence of cucurbit[5]uril. Accordingly to
Zuman and coauthors [40–42], for α,β-unsaturated ketones
it was recognized that the products of their two-electron
and two-proton reduction in aqueous media are saturated
ketones. This reaction course was later conꢂrmed for diꢁer-
ent α,β-unsaturated ketones [40, 41] and no exception was
reported until now. In this sense, the results concerning to
Fig. 3 may suggest that, in presence of cucurbit[5]uril, the
redox center of isophorone could be located outside of the
cavity. [43]. On the other hand, the current cathodic peak
decreased in the higher cucurbit[5]uril dispersion concen-
Fig. 3 Cyclic voltammograms of isophorone [blue dashed line, (a)
−1
−1
2
.6 mmol L ] and 0.2 mol L NH Cl aqueous solution recorded at
4
−1
glassy carbon electrode at 100 mV s . In the presence of cucurbit[5]
−1
−1
−1
uril: (b) 0.614 mmol L , (c) 0.746 mmol L and (d) 1.32 mmol L
(Color ꢂgure online)
.
molecules can reach the glassy carbon electrode surface.
As expected, and corroborating the previously discussion
concerning to UV-vis spectra (Fig. 2a, b), these preliminary
observation open up an opportunity to perform electrocata-
lytic reactions within the conꢂned cavity of cucurbit[5]uril.
The cucurbit[5]uril eꢀect on ECHSA of isophorone
−
1
tration (1.32 mmol L ). This behavior may be due to the
fact that cucurbit[5]uril dispersed in solution could increase
the isophorone solubility, therefore, a greater amount of
Before discussing the cucurbit[5]uril influence in the
ECHSA process, we would like to make some considerations
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
about general aspect of the ECH. According to Lessard [44],
the ECH involves the generation of chemisorbed hydrogen,
by reduction of water or hydronium ions (Eq. (1)). The next
step is the reaction of the adsorbed organic substrate with
the chemisorbed hydrogen (Eq. (5)), followed by the desorp-
tion of the hydrogenated product (Eq. (6)). In this sense, the
eꢀciency of the ECH is directly proportional to the ratio
of the catalytic hydrogenation rate (Eqs. (4)–(6)) over the
hydrogen generation rate (Eqs. (2) and (3)).
pharmaceutical intermediate used as a solvent for vinyl res-
ins [45], whereas a selective hydrogenation of the C=O bond
leads to isophorol (unsaturated alcohol, UOL). It is know
that conventional processes of isophorone hydrogenation
produces a low overall conversion with a high selectivity to
dihydroisophorone or a high overall conversion with poor
selectivity for this product [45–48]. In this work, isophorone
was selectively hydrogenated by ECHSA producing dihy-
droisophorone in good yields (Table 3, entries 2–4) after the
−
1
passage of 2 F mol .
+
−
H O + e + M → MH + H O
3
ads
2
(1)
(2)
(3)
(4)
(5)
(6)
As can be seen, the use of cucurbit[5]uril furnish a slight
gain of 8% (Table 3, entry 4), on average, in the hydro-
genation yield when compared to the same reaction in the
absence of the IPTC (Table 3, entry 1). Furthermore, the
ECHSA in presence of cucurbit[5]uril was more eꢀcient
than that electrochemical systems using water:methanol
2
MHads → M + H2
+
H O + MH + e → M + H + H O
−
3
ads
2
2
Y = Z + M → M(Y = Z)ads
(
Table 3, entries 5 and 6) [49]. Indeed, accordingly to Pem-
2
MHads + M(Y = Z)ads → M(YH − ZH)ads
berton et al. [43] in catalysis within conꢂned environments
the background reactivity outside the supramolecular reac-
tion cavity must be lower than the reactivity inside of the
supramolecular vessel. Thus, cucurbit[5]uril presents the
ability of scavenger the isophorone into a conꢂned envi-
ronment increasing it solubility and, as a consequence, the
probability of collision between both the isophorone and
hydrogens adsorbed in the cathode matrix.
M(YH − ZH)ads → M + YH − ZH
The ratio (hydrogenation/hydrogen generation) is aꢁected
by the unsaturated organic substrate. For a given unsatu-
rated organic molecule, one of the factors which inꢃuence
its adsorption is concentration and/or solubility. Therefore,
any experimental alternative that results in the increase of
the solubility of an unsaturated molecule, will favors the
probability of reaction among both the adsorbed organic
molecule and hydrogen and, as a consequence, raise the
reaction yields.
As expected, the use of the β-cyclodextrin or
α-cyclodextrin in the ECHSA leads to higher hydrogena-
tion yields (68% and 59%, Table 3, entries 2 and 3, respec-
tively), which represents an increase of 19.5% and 10.5% in
the yields, respectively, when compared to that obtained in
the absence of the IPTC (Table 1, entry 1). The better perfor-
mance of both α- and β-cyclodextrin over cucurbit[5]uril is
related with the size of the cavity portal. Indeed, the smaller
size of the portal and inner cavity diameter of the cucur-
bit[5]uril (2.4 and 4.4 Å, Table 1, entries 3 and 4) seems
does not favor the complexation equilibrium, which is better
Thus, the results concerning the ECHSA of isophorone in
the presence of cucurbit[5]uril and cyclodextrins are sum-
marized in Table 3. Isophorone is a α,β-unsaturated ketone
widely used as solvent and it ECH proceeds given signiꢂ-
cant industrial products. The selective hydrogenation of
the C=C bond in the organic chain of isophorone leads to
dihydroisophorone (saturated ketone, SONE), an important
Table 3 Cucurbit[5]uril eꢁect on the ECHSA of isophorone
Entry
IPTC
Solvent
Cathode
Charge (F
[IPTC]
−1
Yield (%)
Products
SONE
Reference
ꢂnal
−
1
mol
)
(mmol L )
SOL
a
1
2
3
4
5
6
–
H O
Ni
Ni
Ni
Ni
2.0
2.0
2.0
2.0
2.0
2.0
–
48.5 ± 1.0
68.0 ± 1.5
59.0 ± 1.0
56.5 ± 1.0
13.0
48.5
68.0
59.0
56.5
13.0
27.0
0.0
0.0
0.0
0.0
0.0
6.0
This work
This work
This work
This work
[46]
2
βCDb
αCDb
CB[5]b
–
H O
1.2
1.2
1.2
–
2
H O
2
H O
2
H O/MeOH
C/poly PPV2+-Pdc
C/poly1.Ni(0)
2
–
H O/MeOH
–
33.0
[45]
2
a
Control reaction, i.e. without using IPTC: 10.0 mL of NH Cl solution (0.2 mol dm−3); isophorone (1 mmol); j = 350–250 mA dm−2; Ni cath-
4
ode; Ni sacriꢂcial anode; stirring. Reactions made in triplicate
b
Electrolysis conditions: 10.0 mL of NH Cl solution (0.2 mol dm−3); ISP (1 mmol); j = 350–250 mA dm−2; Ni cathode; Ni sacriꢂcial anode;
4
stirring. Reactions made in duplicate
c
Poly(pyrrole-vio1ogen) ꢂlm
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
supported by the larger portal and inner cavity diameter of
α-cyclodextrin (5.3 and 4.7 Å Table 1, entries 3 and 4) and
β-cyclodextrin (6.5 and 6.0 Å, Table 1, entries 3 and 4).
Based on the results and in the fact that catalysis within
cucurbit[n]urils can occur at the rim (carbonyl portal) [43] it
is reasonable to suggest that the ECHSA of isophorone may
occur accordingly to the Scheme 1.
(+)-isomenthone, (+)-neomenthol, (−)-menthol, (+)-neoi-
somenthol and (−)-isomenthol. In this work, (R)-(+)-pule-
gone was electrocatalytically hydrogenated by ECHSA in
presence of cucurbit[5]uril and, after the consumption of 4
−
1
F mol L , only (−)-menthone and (+)-isomenthone (satu-
rated ketones, SONE) were produced. The use of cucurbit[5]
uril leads to an increase of 17% in the hydrogenation yield
when compared to the control reaction (Table 4, entry 1).
The (S)-(+)-carvone, a conjugated cyclohexenone con-
taining an exocyclic C=C bond can be also hydrogenated,
producing dihydrocarvone, carvotanacetone and carvomen-
thone (saturated ketones, SONE), dihydrocarveol and carveol
(unsaturated alcohols, UOL) and carvacrol (saturated alcohol,
SOL). As expected, the conjugated endocyclic C=C bond
was hydrogenated faster than the carbonyl group and dihy-
drocarvone was the main product in all experiments (Table 4,
ECHSA of α,β‑unsaturated ketones in presence
of cucurbit[5]uril
Table 4 shows the comparison between diꢁerent ECH pro-
cedures described in the literature and the ECHSA process
described in this work for diꢁerent α,β-unsaturated ketones.
(
R)-(+)-Pulegone is an exocyclic unsaturated terpenone
that can be hydrogenated, generating (−)-menthone,
Scheme 1 General cycle of
isophorone ECHSA mediated
by cucurbit[5]uril
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
1
3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
entry 4). The presence of cucurbit[5]uril leads to an enhance
of 9% in hydrogenation yields when compared to the control
reaction (Table 4, entry 3). The presence of cucurbit[5]uril
in the ECHSA process was more eꢀcient than others well
establish ECH methodologies described in the literature [48,
chemistry and its applications have barely been exploited.
In this work the interaction behavior of isophorone with
cucurbit[n]urils was studied by both cyclic voltamme-
try and UV/vis spectroscopy. Signiꢂcant enhancement in
both the absorbance peaks and the cathodic current peak
of isophorone was attributed to the host–guest complexa-
tion with cucurbit[n]urils. In the ECHSA methodology, the
use of cucurbit[n]urils as IPTC increase of the solubility of
α,β-unsaturated ketones favoring the probability of reaction
among the adsorbed substrate in the cathode matrix and the
chemisorbed hydrogen leading to an increase of the hydro-
genation yield. α-cyclodextrin and β-cyclodextrin showed
better results than those obtained with cucurbit[n]urils being
this behavior related with the size of the cavity portal of the
cavitands. However, the presence of cucurbit[n]urils in the
ECHSA process showed more eꢀcient and selective than
others well-establish ECH methodologies described in the
literature. In this sense, cucurbit[n]urils are primed to play
an active role in the development of new IPTC on the elec-
trocatalytic hydrogenation ꢂeld.
5
0] (Table 4, entries 5–7). The eꢀciency of selective (S)-(+)-
carvone hydrogenation was also evaluated by the relation of
(
SOL + SONE) and (SOL + UOL), where SOL, SONE and
UOL are the hydrogenations yield of the saturated alcohol,
saturated ketone and unsaturated alcohol. The (SOL+SONE)
represents the selectivity for C=C bond hydrogenation and the
(SOL+UOL) means selectivity for C=O bond hydrogenation.
From the data shown in Table 4 (entry 4), it is observed that
the use of cucurbit[5]uril leads to the selective hydrogenation
of the C=C bond. This happens, according to the literature,
ꢂrstly due to the thermodynamics favors the hydrogenation of
−1
the C=C bond over the C=O bond by c.a. 35 kJ mol [51] and
second, the C=C bond shows higher reactivity than the C=O
bond [52]. Conversely, in the absence of cucurbit[5]uril, the
selectivity for C=O hydrogenations increases being this fact
related to the extent of the reaction time and/or the catalyst
type employed in the experiments.
Acknowledgements The authors would like to acknowledge IFPE and
CNPq (456523/2014-6) granting for ꢂnancial support.
1-Acetyl-1-cyclohexene, which contains an exocyclic car-
bonyl was also hydrogenated in presence of cucurbit[5]uril
producing 1-acetylcyclohexane (SONE), as the major prod-
uct and, 1-cyclohexenyl-ethanol (UOL) and 1-cyclohexyl-
ethanol (SOL) as minor products (Table 4, entry 9). The
presence of cucurbit[5]uril increased by 12% the hydrogena-
tions yields when compared to the control reaction (Table 4,
entry 8). The eꢀciency of selective C=C bond hydrogena-
tion was also evaluated by the ratio of (SOL+SONE)/
Compliance with ethical standards
Conflict of interest The authors declare no conꢃict of interest.
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