Reactivity of electrogenerated N-heterocyclic carbenes in room-temperature ionic liquids. Cyclization to 2-azetidinone ring via C-3/C-4 bond formation

Article abstract of DOI:10.1002/adsc.200800049

The intrinsic chemistry of imidazolium-based room-temperature ionic liquids, related to the acidity of the C-2 imidazolium cation, can be modified via cathodic cleavage of the C-2/hydrogen bond. N-Heterocyclic carbenes, electrogenerated by electrolysis of

Full text of DOI:10.1002/adsc.200800049

FULL PAPERS
DOI: 10.1002/adsc.200800049
Reactivity of Electrogenerated N-Heterocyclic Carbenes in
Room-Temperature Ionic Liquids. Cyclization to 2-Azetidinone
Ring via C-3/C-4 Bond Formation
Marta Feroci,a,* Isabella Chiarotto,a Monica Orsini,b Giovanni Sotgiu,a
and Achille Inesia,*
a
Dipartimento di Ingegneria Chimica Materiali Ambiente, Università “La Sapienza”, via Castro Laurenziano 7, 00161
Roma, Italy
Fax : (+39)-06-49766749; e-mail: marta.feroci@uniroma1.it or achille.inesi@uniroma1.it
Dipartimento di Elettronica Applicata, Università di Roma Tre, via Vasca Navale 84, 00146 Roma, Italy
b
Received: January 23, 2008; Revised: March 6, 2008; Published online: May 16, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
À
Abstract: The intrinsic chemistry of imidazolium- din-2-one ring via C-3/C 4 bond formation. The
based room-temperature ionic liquids, related to the electrosynthesis of b-lactams 2a–k has been achieved
acidity of the C-2 imidazolium cation, can be modi- under mild conditions, elevated yields and avoiding
fied via cathodic cleavage of the C-2/hydrogen bond. the use of toxic, volatile, molecular solvents.
N-Heterocyclic carbenes, electrogenerated by elec-
trolysis of imidazolium-based room-temperature
ionic liquids, are stable bases that are strong enough Keywords: carbenes; cyclization; electrochemistry;
to deprotonate bromoamides 1a–k yielding the azeti- imidazolium-based RTILs; b-lactams
Introduction
design of task-specific ionic liquids, possessing partic-
ular reactivity, has attracted much attention.[3]
Room-temperature ionic liquids (RTILs), due to their
Owing to a high ionic conductivity and a wide elec-
low vapour pressure, chemical and thermal high sta- trochemical potential window, RTILs have been used
bility, solvating ability, non flammability and possible as electrolytes instead of conventional organic sol-
recyclability, have received great attention as poten- vents.[4] The wide electrochemical potential window
tial “green” solvents in clean organic synthetic pro- allows us to extend the investigations to more positive
cesses to replace classic, toxic and volatile molecular and negative potentials with respect to those carried
solvents. Air- and moisture-stable RTILs (salts of out in molecular solvents containing massive quanti-
large organic cations, that is, tetraalkylammonium, ties of supporting electrolytes. The use of RTILs has
1,3-dialkylimidazolium, N-alkylpyridinium, N-alkyl- been reported in studies concerning organic electro-
thiazolium, N,N-ÀdialkylÀpyrazoliun, etc. with inorganic synthesis and evaluation of the redox behaviour of
counterions BF4 , PF6 , ZnCl3 , CF3SO3 , etc) are electroactive substrates.[5] The electrochemical behav-
molten salts with melting points close to room tem- iour of imidazolium-based RTILs was investigated by
perature.[1] In the past, relatively little attention has cyclic voltammetry and electrolyses under galvano-
been paid to the reactivity of these peculiar solvents. static control. The cathodic reduction of imidazolium
Recently, the “non-innocent” nature of specific cation to carbene and dihydrogen (vide infra)[6] and
RTILs has been emphasized by several authors. Ac- the basic nature of N-heterocyclic carbenes, stabilized
cordingly, the possible utilization of RTILs as cata- by the presence of two adjacent N atoms, have been
lysts or reagents, in addition to the role of mere sol- extensively proved.[2b,7]
À
À
vent, has been extensively investigated.[1b,2] As regards
Consequently, the reactivity of the imidazolium-
the imidazolium-based RTILs (the most extensively based RTILs, related to the acidity of the C-2 hydro-
studied class of ionic liquids), one of the key proper- gen of the imidazolium cation, may be strongly modi-
ties is their Brønsted acidity. This specific reactivity is fied by inducing the presence of carbene by cathodic
À
related to the acidity of the C-2 hydrogen of the 1,3- cleavage of the C H bond.
dialkylimidazolium cation.[2b] In this context, the
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Marta Feroci et al.
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b-Lactams, natural and synthetic compounds con- tions, without any addition of bases, in suitable classi-
taining the azetidin-2-one skeleton, are well known cal organic solvents (CH3CN, EtCN, DMSO, DMF) –
for their biological activity[8] and their utility as versa- as supporting electrolyte solutions.[12]
tile chiral building blocks in asymmetric synthesis (b-
At present, we are engaged in a search for new
lactam synthon methodology[8a,b,9]). Therefore, investi- electrochemical procedures that may be carried out
gations concerning new approaches to the synthesis of while avoiding the use of classic, volatile and noxious
the azetidin-2-one ring may be of great interest. In organic solvents.[5a] In this context, the reactivity of
addition to the frequently employed Staudinger reac- some bromo amides was studied by us in electrolyzed
tion (the [2 + 2] ketene-imine cycloaddition), many imidazolium-based RTILs, that is, in RTILs contain-
other chemical strategies for the synthesis of b-lac- ing a carbene center.[13] The aim of this investigation
tams have been set up.[10] Base-promoted, in classical was to set-up the cyclization of bromo amides to b-
organic solvents (MeCN, DCM), stereoselective cycli- lactams in electrochemically modified RTILs, thus
zation of N-(p-methoxybenzyl)-N-(2-chloro)propion- avoiding the use of toxic and volatile classical molecu-
ylamino acid derivatives to 1,3,4,4,-tetrasubstituted b- lar solvents. The recycle ability of the RTILs was
lactams have been discussed by R. Gonzµles-MuÇiz carefully investigated.
et al.[11] Recently, the electrochemically induced cycli-
À
zation of haloamides to b-lactams (via C-4 H depro-
tonation) has been achieved by us under mild condi- Results and Discussion
1-Butyl-3-methyl-1H-imidazolium tetrafluoroborate
([bmim]BF4) was initially electrolyzed, under galvano-
static conditions in a divided cell, in the absence of
bromo amide. After the consumption of a prefixed
number of Faradays, the current was switched off and
bromoamide (1a, taken as model compound,
Scheme 1) added to the cathodic compartment. Work-
up of the cathodic solution (extraction with diethyl
ether) afforded b-lactam 2a.
This result is consistent[6] with the monoelectronic
cathodic reduction of the bmim+ cation 3a to 1-butyl-
3-methylimidazol-2-ylidenecarbene 3b and dihydro-
gen [Scheme 2, reaction (1)]. In [bmim]BF4,
N-heterocyclic carbenes are bases that are stable and
À
strong enough to deprotonate the C-4 H group of
bromo amide 1a subsequently added to the cathodic
solution (e.g., in DMSO or in H2O, pKa of bmim+
cation was found to be in the range 21–24[2b]). The re-
Scheme 1.
Scheme 2.
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Reactivity of Electrogenerated N-Heterocyclic Carbenes
FULL PAPERS
sulting carbanion undergoes cyclization to afford the bromo amide is cathodically reducible at a potential
azetidine ring via intramolecular halogen displace- more positive with respect to the one of the imidazoli-
ment of the C-3 Br bond and C-3 C-4 bond forma- um cation [bmim]+ 3a. The voltammetric curves of 1a
tion [Scheme 2, reaction (2)].
À
À
in [bmim]BF4 (vitreous carbon cathode, n=0.20 VsÀ1,
In order to verify the influence of various parame- t=458C) show an irreversible reduction peak related
À
ters on the yields of the isolated b-lactam, several to the two electron cathodic cleavage of the C-3 Br
electrolyses were carried out using different numbers bond (Figure 1; Ep=À1.58 V vs. Ag/AgCl). Voltam-
of Faradays per mole of bromo amide supplied to the metric curves, recorded after pre-fixed intervals of
electrode (Q), different cathodic materials and differ- time Dt since of the addition of 1a to the electrolyzed
ent
imidazolium-based
RTILs
([bmim]BF4, [bmim]BF4, show that the peak current decreases on
[bmim]PF6, [bmim]CH3SO4). The yields of isolated 2a increasing of Dt (i.e., on decreasing of the concentra-
are affected by Q (i.e., by the number of equivalents tion of substrate 1a; Figure 1, curves c–e). According-
of electrogenerated carbene 3b; Table 1, entries 1–4); ly, the reaction time, related to the overall process of
the cathodic material, modifying the current efficien- cyclization of bromo amide 1a to b-lactam 2a, was
cy, may be of the utmost importance (TabÀle 1, en- evaluated as the Dt value suitable for the disappear-
À
tries 4–8À). The nature of the anions BF4 , PF6 , ance of the reduction peak (Dt=3 h 5 min).
CH3SO4 does not play a significant role (Table 1, en-
After the work-up of the cathodic solution,
tries 4, 9 and 10). The best result was obtained in the [bmim]BF4 was recovered and reused for five subse-
electrolysis of [bmim]BF4, Q=2.0 Faradays molÀ1, Pt quent syntheses. In each case, b-lactam 2a was isolated
cathode (Table 1, entry 4), producing b-lactam 2a in without any decrease of the yield (82, 94, 92, 89, 93%).
82% yield. The remarkable consumption of 2.0 Fara-
To test the generality of this electrochemical proce-
days molÀ1 of 1a, with respect to the monoelectronic dure, concerning the use of electrogenerated carbene
cathodic reduction of 3a to 3b [Scheme 2, reaction as basic promotor of the cyclization of bromo amides
(1)], may be related to the current yield of the elec- to b-lactams, the reactivity of carbene was investigat-
trochemical step as well as to the yield of the follow- ed versus bromo amides 1b–f containing different sub-
ing deprotonation process.
stituents. The investigation was carried out under the
Next, the progress of the cyclization reaction may optimized conditions reported for Table 1, entry 4. b-
be observed (and the reaction time evaluated) by vol- Lactams 2b, 2c, 2e, 2f and 2h–k were isolated in very
tammetric analysis of the electrolyzed solutions of elevated yields, 81–92% (Table 2, entries 2, 3, 5, 6, 8–
À
[bmim]BF4 containing 1a. The C Br group of the 11). Bromo amides 2d and 2g, containing a tertiary
Table 1. Reactivity of electrogenerated carbene 1-butyl-3-
methyl-1H-imidazol-2-ylidene 3b versus bromo amide 1a in
RTILs.[a] Effect of Q,[b] cathode material and nature of the
anion on the yield of b-lactam 2a.
Entry RTIL
Cathode Q Yield
Recovered
1a [%]
[%][c]
of 2a
1
2
3
4
5
6
7
8
9
C
Pt
Pt
Pt
Pt
Cu
Ti
Al
C
1.0 23
1.3 41
1.8 74
2.0 82[d]
2.0 72
2.0 14
2.0 57
2.0 33
2.0 76
2.0 78
64
46
18
70
54
Pt
10
[a]
Experimental conditons: electrolyses carried out under
galvanostatic conditions (I=25 mAcmÀ2, divided cell, Pt
anode, 458C) in RTIL with subsequent addition of 1a.
Number of Faradays per mol of 1a supplied to the elec-
trodes.
Figure 1. Cyclic voltammetric curves for [bmim]BF4 (curve
a), electrolyzed [bmim]BF4 (see text, curve b), electrolyzed
[bmim]BF4 containing bromo amide 1a (c=0.15 mol dmÀ3,
curves c-e). The curves were recorded 15 min (curve c),
45 min (curve d) and 1 h 45 min (curve e) after the addition
of 1a to the electrolyzed [bmim]BF4 (vitreous carbon cath-
ode, n=0.20 VsÀ1, reference Ag/AgCl, T=458C).
[b]
[c]
[d]
Isolated yields, based on starting 1a.
Further increases in the number of Faradays per mol of
1a did not result in an increase in the yield of 2a.
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Marta Feroci et al.
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Table 2. Synthesis of b-lactams 2a–k via reaction of electro-
generated carbene 1-butyl-3-methyl-1H-imidazol-2-ylidene
3b with bromo amides 1a–k in [bmim]BF4.[a]
stated concentration. In electrochemically activated
RTILs, N-heterocyclic carbenes are stable bases that
are strong enough to deprotonate bromo amides 1a–k
À
yielding the azetidin-2-one ring via C-3 C-4 bond for-
Entry
Bromo amide
b-Lactam
Yield [%][b]
mation. The synthesis of b-lactams 2a–k has been car-
ried out under mild conditions, in elevated yields and
avoiding the use of toxic classical organic solvents as
well as bases and catalysts.
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
2a
2b
2c
2d
2e
2f
2g
2h
2i
82
91
81
59[c
87
81
Experimental Section
72[d]
92
Typical Experimental Procedure
9
89
10
11
1j
1k
2j
2k
87
84
The galvanostatic electrolysis was performed in a divided
glass cell (Pt spiral cathode and anode – apparent area
0.8 cm2; volume of catholyte and anolyte: 1.5 mL) using an
Amel Model 552 potentiostat equipped with an Amel
Model 572 integrator. Anolyte and catholyte were separated
through a G-5 glass septum. The electrolysis was carried
out, under a nitrogen atmosphere at 458C, at a constant
density current of 25 mAcmÀ2, with 1.5 mL of [bmim]BF4
(unless otherwise stated). The electrolysis was stopped after
passage of 97 C. At the end of the electrolysis, 0.5 mmol of
starting bromo amide was added and the catholyte was al-
lowed to stand at room temperature, under stirring, for 2 h.
The reaction mixture was then extracted with diethyl ether
(3”10 mL, allowing to stand under stirring with diethyl
ether 15 min each time). After removing the solvent from
the combined ethereal layers under reduced pressure, the
corresponding b-lactam was purified using flash chromatog-
raphy (n-hexane/ethyl acetate, 8/2) and characterized with
NMR and MS spectra.
In the case of bromo amide 1a, the catholyte (after extrac-
tion with diethyl ether) was kept (leaving it in the electroly-
sis cell) under vacuum for 2 h; then a voltammetric analysis
was carried out on it and a sample taken for a 1H NMR
spectrum. Having found no alteration in the ionic liquid, the
electrolysis was carried out for a second time on this solvent
and worked in the same way. This procedure was repeated
for five times (the yields in b-lactam are reported in the
text) and no alteration of the ionic liquid was evinced.
[a]
Experimental conditions: electrolyses carried out under
galvanostatic conditions (I=25 mAcmÀ2, divided cell, Pt
anode and cathode, T=458C) in [bmim]BF4 with subse-
quent addition of bromoamides 1a–k. Number of Fara-
days per mol of bromo amide supplied to the electrodes:
Q=2.0.
Isolated yields, based on starting bromo amide.
Bromo amide 1d (38%) was recovered.
Bromo amide 1g (25%) was recovered.
[b]
[c]
[d]
À
C Br bond, were converted in 59 and 72% yield, re-
spectively; substrates 1d and 1g were recovered (38
and 25%, respectively, Table 2, entries 4 and 7).
Last, we thought a comparison between the electro-
chemical methodology in ionic liquids and the classi-
cal procedures (employing organic solvents and
bases) could be helpful, with reference to the cycliza-
tion of halo amides. Accordingly, we have evaluated
the reactivity of two specific substrates (1a and 1h) in
organic solvents (benzene and acetonitrile) versus
normal, commercially available bases (Et3N and
Cs2CO3, see Supporting Information).
b-Lactam 2a was isolated from MeCN solutions
containing substrate 1a and Cs2CO3 or Et3N (yields in
2a: 81 and 90%, respectively, versus 82–94% in RTIL,
see above); b-lactam 2h was isolated from a benzene
solution containing 1h and Et3N (yield in 2h: 82%;
lit.[14] 75% versus 92% in RTIL, see Table 2, entry 8).
Therefore, our method for the electrochemical syn-
thesis of b-lactams in ionic liquids, while avoiding the
use of toxic and environmentally unfriendly organic
solvents, does not involve any decrease in the yields
of the isolated products.
Supporting Information
Experimental details and spectral characterization data for
all compounds are given in the Supporting Information.
Acknowledgements
The authors thank PRIN 2006, MIUR and CNR (Rome) for
financial support and Mr. Marco Di Pilato for his support in
the experimental part of this work.
Conclusions
A simple electrochemical procedure, that is, an elec-
trolysis under galvanostatic conditions, is able to
modify the reactivity of imidazolium-based ionic liq-
uids causing the presence in this solvents of 2-butyl-3-
methylimidazol-2-ylidenecarbene 3b at a previously
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