Electrosynthesis of cyclic carbamates from aziridines and carbon dioxide

Article abstract of DOI:10.1039/a910285j

A new and selective synthesis of five-membered ring cyclic carbamates involving nickel-catalyzed CO₂ incorporation into aziridines under mild electrochemical conditions was carried out in good yields.

Full text of DOI:10.1039/a910285j

Electrosynthesis of cyclic carbamates from aziridines and carbon dioxide
Patricia Tascedda and Elisabet Dun˜ach*
Laboratoire de Chimie Bioorganique, Associe´ au CNRS, Universite´ de Nice-Sophia Antipolis, 06108 Nice Cedex 2,
France. E-mail: dunach@unice.fr
Received (in Cambridge, UK) 23rd December 1999, Accepted 2nd February 2000,
Published on the Web, 1st March 2000
A new and selective synthesis of five-membered ring cyclic
carbamates involving nickel-catalyzed CO2 incorporation
into aziridines under mild electrochemical conditions was
carried out in good yields.
two regioisomers 2a and 3a in 83% yield and 60% aziridine
conversion (Table 1, entry 1). The major isomer corresponds to
the incorporation of CO2 at the less hindered side of the
monosubstituted aziridine. No reaction occurred in the absence
of current and in the absence of the nickel catalyst a very low
yield of cyclic carbamates was obtained ( < 10%).
From a mechanistic point of view, oxidation of the magne-
sium rod to Mg2+ ions in solution takes place at the anode. At
the cathode, the reduction of the Ni(II) complex to Ni(0) occurs
reversibly at 21.2 V vs. SCE. Cyclic voltammetry showed that
upon addition of 1a to the Ni(II)–bipy solution, the reduction
peak of the Ni(II)/Ni(0) transition at 21.2 V became irreversible.
This was taken to indicate that the electrogenerated Ni(0)
reacted with 1a by a possible insertion into the C–N bond of the
aziridine ring. The oxidative addition of Ni(0) complexes to
oxirane rings has already been reported.20
Within our ongoing interest in carbon dioxide incorporation into
small heterocyclic ring systems,1,2 we present here our results
on carbon dioxide incorporation into aziridines for the synthesis
of 2-oxazolidones. 2-Oxazolidones are an important class of
heterocyclic five-membered ring compounds.3 These cyclic
carbamates show good antibacterial properties4,5 and are widely
used in pharmaceutical chemistry.6
Their synthesis generally proceeds through the condensation
of 1,2-aminoalcohols with toxic carbonyl derivatives such as
phosgene7 or cyanic acid (from urea decomposition).8 The
direct addition of CO2 to b-amino alcohols at high pressure and
temperature has been described in the patent literature.9 Other
methods of synthesis of 2-oxazolidones include the reaction of
epoxides with cyanuric acid10 or isocyanates11 or the reaction of
alkenes with N-bromosuccinimide and isocyanates.12 2-Oxa-
zolidones can also be obtained from a-ketols and isocyanates or
chloroformates.13
Table 1 Ni-catalyzed electrochemical CO2 incorporation into aziridinesa
Conver- Carbamate Ratio
Conditionsc sion (%) yieldd (%) 2+3
Entry Aziridine Catalystb
The direct functionalization of aziridines to cyclic carba-
mates has been reported via flash vacuum pyrolysis at 600 °C in
the presence of ethyl chlorocarbonate14 and by reaction with
CO2 at high pressure (50 atm, 60 °C) in the presence of
tetraphenylantimony halides.15
We describe now a new and selective method of 2-oxa-
zolidone synthesis using a simple electrochemical procedure, by
which the cyclic carbamates 2 and 3 can be obtained in good
yields from the direct reaction of substituted aziridines 1 with
CO2. Carbon dioxide insertion into the C–N bond of the
aziridine ring takes place under very mild conditions, at room
temperature and with atmospheric carbon dioxide pressure
(Scheme 1). This new carboxylation reaction widens the field of
utilization of CO2 as a C-1 building block for organic
chemicals.16
The reaction was catalyzed by a Ni(II) complex (10 mol%)
and was carried out in a single-compartment cell fitted with a
consumable magnesium anode and an inert cathode (e.g.
stainless steel). Electrolyses were conducted at constant in-
tensity in DMF as the solvent, and [Ni(bipy)3][BF4]2 (bipy =
2,2A-bipyridine) was shown to be an efficient catalyst for this
carboxylation. This same Ni(II)–bipy complex has already been
used as a catalyst in other carbon dioxide incorporation
reactions (e.g. into alkynes17 or diynes18).
1
2
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
Ni–bipy
Ni–bipy
Ni–bpy
Ni–bipy
Ni–bipy
Ni–bipy
Mg/st. steel 60
KBr
83
100
88
60+40
75+25
50+50
66+34
76+24
73+27
75+25
67+33
78+22
86+14
84+16
Mg/Ni
KBr
Mg/C
KBr
Al/st. steel
KBr
20
45
30
3
4
50
5
Mg/st. steel 30
NBu4BF4
Mg/st. steel 45
KCl
85
6
75
7
Ni–cyclam Mg/st. steel 85
94
KBr
8
Ni–cyclam Mg/st. steel 65
93
KBr
9
Ni–cyclam Mg/st. steel 99
94
KBr
10
11
Ni–cyclam Mg/st. steel 99
100
100
KBr
Ni–cyclam Mg/st. steel 63
KBr
a General electrochemical conditions: into a single-compartment cell21 of
ca. 50 mL capacity fitted with the corresponding couple of electrodes, 40
mL of freshly distilled DMF, the supporting electrolyte (2 3 1022 M), the
Ni(II) catalyst (0.3 mmol) and aziridine 1 (3 mmol) were introduced. The
solution was stirred at room temperature and CO2 was bubbled into it at
atmospheric pressure. The electrodes were connected to a stabilized
constant current supply (Sodilec, EDL 36.07) and electrolyzed at 60 mA
(corresponding to a current density of ca. 0.3 A dm22) for 7 h, until the
consumption of 1 was negligible. The reaction mixture was then stirred at
50 °C overnight, hydrolyzed with aqueous HCl and extracted with ethyl
acetate. The crude product was analyzed by GC, NMR and mass
spectrometry, purified by column chromatography on silica gel and the
identity of the carbamate products compared to the known compounds.
b Ni–bipy = [Ni(bipy)3][BF4]2, Ni–cyclam = [Ni(cyclam)]Br2; both are 10
mol% with respect to the aziridine 1. c The experimental conditions relate to
the anode/cathode couple of electrodes and the supporting electrolyte (2 3
1022 M). d The carbamate yields of 2 + 3 are calculated on the basis of
converted aziridine.
The electrochemical carboxylation of the N-Boc protected
aziridine 1a19 led to the cyclic carbamates as a 60+40 mixture of
Scheme 1
DOI: 10.1039/a910285j
Chem. Commun., 2000, 449–450
This journal is © The Royal Society of Chemistry 2000
449

View Article Online
On the other hand, electrogenerated Ni–bipy complexes are
also known to catalytically react with CO2 to form its radical
anion,17,22 which, under the reaction conditions, undergoes
reductive dimerization to oxalate.
In the electrocarboxylation of aziridines (Scheme 1), there is
a competition for the electrogenerated Ni(0) complexes between
two processes: (i) Ni(0) insertion into the aziridine ring followed
by further CO2 uptake and ring closure to the carbamate and (ii)
CO2 reduction to oxalate. This competition, which was very
dependent on the reaction conditions, accounts for the limited
aziridine conversion rates in some cases.
The electrochemical method described here employs very
mild experimental conditions (CO2 pressure of 1 atm, 20 °C), as
compared to existing methods, which require harsh conditions
or the use of toxic starting materials such as phosgene. It is also
worth noting that this reported method utilizes carbon dioxide
as the starting C-1 carbon source, and constitutes a new example
in the field of green and catalytic chemistry.
Notes and references
The influence of several factors such as the nature of the
electrodes, the supporting electrolyte and of the ligand(s)
attached to nickel were examined in the electrocarboxylation of
1a. As regards the electrode, a magnesium/stainless steel couple
afforded the best results, as compared to the use of carbon fiber
or nickel foam as cathodes (compare Table 1, entries 1–3).
When an aluminium anode was used (with several cathode
materials) low aziridine conversions and low carbamate yields
(e.g. Table 1, entry 4) were observed, with CO2 electroreduction
being favored. Among supporting electrolytes, KBr, KCl and
NBu4BF4 were tested with Mg/stainless steel electrodes. The
use of NBu4BF4 (Table 1, entry 5) afforded a good yield of
carbamate but a low conversion of 1a while KCl led to a
conversion of 45% (Table 1, entry 6); KBr afforded the best
results (Table 1, entry 1).
1 P. Tascedda and E. Dun˜ach, J. Chem. Soc., Chem. Commun., 1995,
43.
2 P. Tascedda, M. Weidmann, E. Dinjus and E. Dun˜ach, Appl.
Organomet. Chem., 2000, in press.
3 M. E. Dyen and D. Swern, Chem. Rev., 1967, 197.
4 S. Sato, Y. Yoshida and S. Kuwahara, Jpn. J. Microbiol., 1960, 4,
419.
5 D. F. Kefauver and I. Drupa, Antibiot. Chemother., 1960, 10, 688.
6 G. Chakraborty, Indian J. Pediat., 1964, 28, 357; R. M. Stabler,
J. Parasitol., 1957, 43, 280.
7 H. L. Crowther and H. McCombie, J. Chem. Soc., 1913, 103, 27; D.
Ben-Ishai, J. Am. Chem. Soc., 1956, 78, 4962.
8 W. J. Close, J. Am. Chem. Soc., 1951, 73, 95.
9 J. W. Lynn, US Pat. 2 975 187, 1961; A. B. Steele, US Pat. 2 868 801,
1959.
10 R. W. Cummins, J. Org. Chem., 1963, 28, 85.
11 S. Knapp, P. J. Kukkola, S. Sharma, T. G. Murali Dhar and A. B. J.
Naughton, J. Org. Chem., 1990, 55, 5700.
12 C. Heathcock and A. Hassner, Angew. Chem., 1963, 75, 344.
13 W. H. Pirkle and A. K. Simmons, J. Org. Chem., 1983, 48, 2520.
14 M. R. Banks, J. I. G. Cadogan, I. Gosney, K. G. Hodgson and D. E.
Thomson, J. Chem. Soc., Perkin Trans 1, 1991, 961.
15 R. Nomura, T. Nakano, Y. Nishio, S. Ogawa, A. Ninagawa and H.
Matsuda, Chem. Ber., 1989, 122, 2407.
16 A. Behr, Carbon Dioxide Activation by Metal Complexes, VCH,
Weinheim, 1988; T. Inui, M. Anpo, K. Izui, S. Yanagida and T.
Yamaguchi, Advances in Chemical Conversions for Mitigating Carbon
Dioxide, Studies in Surface Science and Catalysis, Elsevier, Am-
sterdam, 1998.
17 S. De´rien, E. Dun˜ach and J. Pe´richon, J. Am. Chem. Soc., 1991, 113,
8447.
18 S. De´rien, J. C. Clinet, E. Dun˜ach and J. Pe´richon, J. Org. Chem., 1993,
58, 2578.
19 Aziridine 1a was prepared according to: P. Wessig and J. Schwarz,
Synlett., 1997, 893.
20 R. J. De Pasquale, J. Chem. Soc., Chem. Commun., 1973, 157; M.
Weidmann, Doctoral Thesis, Universitat Jena, Germany, 1997.
21 J. Chaussard, J. C. Folest, J. Y. Ne´de´lec, J. Pe´richon, S. Sibille and M.
Troupel, Synthesis, 1990, 5, 369.
In the related CO2 incorporation into oxirane rings for the
synthesis of cyclic carbonates,2 we showed that both 2,2A-bipy
and cyclam ligands on nickel (cyclam
= 1,4,8,11-tetra-
azacyclotetradecane) were effective for the CO2 insertion
reaction. In the electrocarboxylation of 1a, the use of [Ni(cy-
clam)]Br2 as the catalyst afforded 2a and 3a in 94% yield with
85% aziridine conversion (Table 1, entry 7). With the Ni–
cyclam catalytic system the electroreduction of carbon dioxide
was almost completely inhibited, in favor of aziridine carbox-
ylation. The observed regioselectivity 2a+3a was 75+25.
The reaction was then extended to the carboxylation of
several monosubstituted aziridines 1b–e with [Ni(cyclam)]Br2
as the catalyst. Thus, aliphatic, aromatic or ether substituted N-
Boc aziridines (Table 1, entries 8–10) led regioselectively to the
corresponding carbamates in good yields and excellent conver-
sions (Table 1, entries 9, 10). The electrocarboxylation of the
non-protected N–H aziridine 1e was also efficient and led to a
84+16 regioisomeric ratio of 2e+3e in quantitative yield and
with a 63% conversion (Table 1, entry 11).
In conclusion, a novel catalytic system for incorporation of
CO2 into aziridines for the synthesis of cyclic carbamates was
developed. Use of stable and readily available Ni(II) complexes
of cyclam or bipyridine ligands and application of simple
preparative electrochemical conditions, led to efficient catalytic
insertion of the carbon dioxide into the C–N bond of aziridines
with regioselectivities of 60–86%.
22 L. Garnier, Y. Rollin and J. Pe´richon, New J. Chem., 1989, 13, 53; P.
Daniele, G. Ugo, G. Bontempelli and M. Fioriani, J. Electroanal.
Chem., Interfacial Electrochem., 1987, 219, 259.
Communication a910285j
450
Chem. Commun., 2000, 449–450