Stabilisation of alkylcarbamate anions using neutral hydrogen bond donors

Article abstract of DOI:10.1016/j.tetlet.2009.06.058

The interactions of a series of urea-based anion receptors and alkylcarbamate species formed by the reaction of carbon dioxide with primary amines have been investigated by ¹H NMR. Significant downfield shifts in the NH proton signals of the re

Full text of DOI:10.1016/j.tetlet.2009.06.058

Tetrahedron Letters 50 (2009) 4922–4924
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Stabilisation of alkylcarbamate anions using neutral hydrogen bond donors
*
Peter R. Edwards, Jennifer R. Hiscock, Philip A. Gale
School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The interactions of a series of urea-based anion receptors and alkylcarbamate species formed by the reac-
tion of carbon dioxide with primary amines have been investigated by 1H NMR. Significant downfield
shifts in the NH proton signals of the receptors in the presence of the alkylcarbamates were observed,
consistent with classical host:anion hydrogen-bonding. This observation demonstrates that neutral
hydrogen bond donor receptors can compete with ammonium cations to bind carbamates in DMSO-d6
solution.
Received 21 April 2009
Revised 1 June 2009
Accepted 12 June 2009
Available online 16 June 2009
Keywords:
Carbamate
Ó 2009 Elsevier Ltd. All rights reserved.
Carbon dioxide
Anion binding
Hydrogen-bonding
Carbon dioxide is a greenhouse gas, the anthropogenic emission
of which has grown significantly since the start of the industrial
revolution,1 causing a rapid increase in atmospheric carbon dioxide
levels.2 This is forecast to have negative climatic consequences.
Understandably, much work has been recently focussed upon
many differing approaches to the capture,3–9 activation or stabili-
sation10–12 and use13–16 of carbon dioxide with the aim of reducing
the atmospheric concentration or using it as a chemical feedstock.
Many industrial systems use aliphatic amines, which when ex-
posed to carbon dioxide, rapidly form alkylammonium alkylcarba-
mate salts.17–19 Weiss and co-workers have shown that, in
solution, such salts rapidly exchange the carbon dioxide between
two amines, such that only one species is observed on the NMR
timescale (Scheme 1).20
We hypothesised that our recently reported neutral oxo-anion
receptors21,22 3–6, (and previously unreported compound 2) which
have a very high affinity for oxo-anions (e.g., compound 4 binds
–OAc with a stability constant Ka >104 MÀ1 in DMSO-d6/0.5% H2O)
would bind, and hence stabilise alkylcarbamate anions via hydro-
gen-bonding interactions in organic solution. Proton NMR experi-
ments were used to assess the interaction.
tors 1–6, compared to the initial sample (Tables 1 and 2). The
downfield shift is consistent with host:anion hydrogen-bonding
interactions, as shown in Scheme 2. Model studies conducted by
passing CO2 through a solution of the receptor in the absence of
an amine showed no or insignificant shifts of the NH resonances
so ruling out bicarbonate production and binding under the exper-
imental conditions.
O
O
N
H
N
H
N
H
N
H
1
2
NH
NH
HN
HN
4
5
O
O
N
H
N
H
N
H
N
H
HN
O
O
N
H
N
H
N
H
N
H
Each receptor was dissolved in DMSO-d6 and the 1H NMR spec-
trum acquired. Two equivalents of n-butylamine or one equivalent
of 1,3-diaminopropane were then added, providing two amine
groups per urea and giving one carbamate group per urea once
the amine has reacted with CO2. Carbon dioxide was then bubbled
through each solution, before repeating the 1H NMR experiments.
This caused downfield shifts of the urea NH resonances for recep-
HN
NH
HN
6
3
All the observed chemical shift changes are smaller than those
seen upon addition of tetrabutylammonium acetate to solutions
of the receptors,21–23 presumably partly due to competition from
the strong electrostatic and hydrogen-bonding association between
the alkylcarbamate anion and alkylammonium cation. We at-
tempted to perform NMR titrations with alkylammonium carba-
* Corresponding author. Tel.: +44 23 8059 3332; fax: +44 23 8059 6805.
E-mail address: philip.gale@soton.ac.uk (P.A. Gale).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.058

P. R. Edwards et al. / Tetrahedron Letters 50 (2009) 4922–4924
4923
H
R
H
O
(a)
N+ R
H
N
O
O-
H
nBu
+H3N
N
H
N
H
H
H
CO2
4
2 eq.
N
R
N
N
H
H
O
R
N
H
O
H
N+ H
H
O-
O-
H
O
R
N
-O
nBu
H
N
+
nBu
Scheme 1. The reaction of primary amines with carbon dioxide.
+
nBu NH3
(b)
Table 1
Averaged downfield chemical shift changes/ppm for the one or two urea NH groups
labelled as downfield or upfield, (with errors/%) for 1–6 in the presence of 2 equiv n-
butylamine, satd with carbon dioxide
-O
O
O
N
2
N
N
H
N
H
H
Receptor
Urea NH (downfield resonance)
Urea NH (upfield resonance)
N
N
H
2. equiv.
4
H
1
2
3
4
5
6
0.11 (13)
0.26 (6)
0.27 (13)
0.71 (7)
0.67 (10)
0.63 (5)
O-
O
NH3
O-
0.28 (6)
0.30 (15)
O
H
0.68 (12)
N
H
+
NH3
Change in given chemical shift is the mean of 3 repeats.
NH3
Scheme 2. Proposed interactions between receptor 4 and the alkylcarbamate:
alkylammonium salt generated from (a) n-butylamine and CO2 and (b) 1,3-
diaminopropane and CO2.
Table 2
Averaged downfield chemical shift changes/ppm, for the one or two urea NH groups
labelled as downfield or upfield (with errors/%) for 1–6 in the presence of 1 equiv 1,3-
diaminopropane, satd with carbon dioxide
Acknowledgement
Receptor
Urea NH (downfield resonance)
Urea NH (upfield resonance)
1
2
3
4
5
6
0.08 (23)
0.28 (14)
0.37 (9)
0.70 (10)
0.50 (7)
0.56 (6)
The authors thank the EPSRC for funding (C-Cycle).
0.30 (15)
0.42 (10)
Supplementary data
0.51 (6)
Supplementary data (synthesis and characterisation data for
compound 2. NMR stack plots for compounds and carbamate salts)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.06.058.
Change in given chemical shift is the mean of 3 repeats.
mates, however, whilst we observed significant downfield shifts,
we could not reproducibly obtain stability constant values for car-
bamate binding. Loss of CO2 during the titration or incomplete car-
bamate formation may be the cause of this irreproducibility.
These studies show that increasing the number of hydrogen
bond donors in urea-based receptors from two to four causes a sig-
nificant increase in the urea NH downfield shift in the presence of
alkylcarbamate anions. This is indicative of the formation of a urea-
alkylcarbamate complex. In order to form this complex, the recep-
tor must compete with the primary ammonium cation generated
upon alkylcarbamate formation. If we look at the series of com-
pounds 1, 2 and 4 where we have two, three and four hydrogen
bond donors, respectively, the most downfield urea NH resonance
shifts by 0.11, 0.26 and 0.71 ppm, respectively, in the presence of
the carbamate formed from n-butylamine and carbon dioxide.
Whilst the value of the shift is not necessarily indicative of the
strength of the complex generated, analogy to our previous work
on carboxylate complexation by these receptors would suggest
that the diindolylurea 4 and dicarbazolyl urea 6 would bind the
carbamates most strongly.
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These results lead us to suggest that it is possible to stabilise
alkylcarbamate anions with simple neutral indolyl- and carbazol-
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