Accelerated decarboxylation of 1,3-dimethylorotic acid in ionic liquid

Article abstract of DOI:10.1016/j.bioorg.2006.01.001

The solvent effect of ionic liquids on the decarboxylation of 1,3-dimethylorotic acid and its analogue in ionic was investigated. The rate acceleration observed was proposed to be a result of the stabilization of the zwitterionic intermediates by the charged groups available in these special solvents.

Full text of DOI:10.1016/j.bioorg.2006.01.001

Bioorganic Chemistry 34 (2006) 99–104
www.elsevier.com/locate/bioorg
Accelerated decarboxylation of 1,3-dimethylorotic
acid in ionic liquid
Freeman M. Wong, Weiming Wu ¤
Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, CA 94132, USA
Received 16 November 2005
Available online 24 February 2006
Abstract
The solvent eVect of ionic liquids on the decarboxylation of 1,3-dimethylorotic acid and its ana-
logue in ionic was investigated. The rate acceleration observed was proposed to be a result of the sta-
bilization of the zwitterionic intermediates by the charged groups available in these special solvents.
© 2006 Elsevier Inc. All rights reserved.
Keywords: Decarboxylation; Orotic acid; Ionic liquid
1. Introduction
The decarboxylation of 1,3-dimethylorotic acid (1) and its analogues has been studied in
much detail as a model for the enzymatic decarboxylation catalyzed by orotidine-5Ј-mono-
phosphate decarboxylase [1–9]. 1,3-Dimethyluracil (2, Scheme 1) is formed as the sole
product from the decarboxylation of acid 1 at elevated temperatures. The mechanistic
studies involve the investigation of the nature and stability of the intermediate as well as
the eVects of factors such as solvent and hydrogen bonds.
The vastly diVerent rates observed in the decarboxylation of acids 1, 3, and 4 (acid 4
reacts about 3000 times faster than 1 or 3) have provided a unique opportunity to investi-
gate the factors important for the rate of the reactions [1,7]. Recent work has demonstrated
that the stability of carbanions formed from the decarboxylation of these three acids does
not play an important role in determining the rate of decarboxylation [6,7]. A two-step
*
Corresponding author. Fax: +1 415 338 2384.
E-mail address: wuw@sfsu.edu (W. Wu).
0045-2068/$ - see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.bioorg.2006.01.001

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F.M. Wong, W. Wu / Bioorganic Chemistry 34 (2006) 99–104
O
O
CH3
O
CH3
O
N
N
N
CO2H
N
H
CH3
1
CH3
2
Scheme 1.
O
O
CH3
N
O
N
CO2H
O
N
CO2H
N
CO2H
CH3
CH3
3
CH3
4
1
O
OH
OH
CH3
O
CH3
CH3
N
N
N
k
K
+
N
+
N
–
–
-CO2
N
CO2H
O
CO2
O
CH3
1
CH3
CH3
5
6
dP/dt = Kk[1]/(K +1) When K << 1, kob = Kk
Scheme 2.
mechanism has been proposed to account for the large diVerence in rate constants mea-
sured for acids 1, 3, and 4 as shown in Scheme 2. In this mechanism, an equilibrium leading
to the formation of a zwitterionic structure such as 5 precedes the loss of CO2 and forma-
tion of the carbanion (e.g., 6) [1,7]. When the zwitterionic structure is much less stable than
the acid, i.e., equilibrium constant K¿ 1, the observed rate constant is simply the product
of the K and the rate constant k for the second step, loss of CO2 [1,7].
Theoretical calculations on the relative stability of the zwitterionic structures from acids
1, 3, and 4 have shown that the zwitterionic structure derived from acid 4 is indeed much
more stable than those from acids 1 and 3, providing support for the proposed mechanism
[7]. In this report, we present kinetic results for decarboxylations in ionic liquid solvents
that provide further support for the mechanism.
2. Materials and methods
2.1. Kinetic measurements
The progress of the reaction was followed by NMR spectrometry. A series of NMR
tubes (each containing 7 mg of substrate and 200 mg of solvent) was heated in an oil-bath
at 206°C for time intervals of 2–20min. DMSO-d6 (1ml) was added to the NMR tubes to
form a homogeneous solution. The percentage of substrate remaining was calculated from
the integration of corresponding peaks of the substrate and product. The logarithm of the
fractions of substrate remaining was plotted over time and excellent Wrst-order kinetics was

F.M. Wong, W. Wu / Bioorganic Chemistry 34 (2006) 99–104
101
Fig. 1. NMR spectra and data analysis for the decarboxylation of acid 3.
observed. The NMR spectra and data analysis of the decarboxylation of acid 3 in imidazo-
lium ionic liquid was shown in Fig. 1 as an example. The reaction was monitored until
there was less than 20% substrate remaining.
2.2. Chemicals
Acid 1 was prepared through the methylation of orotic acid (from Sigma) as reported by
Curran and Angier [10]. The same method was used to prepare 3 from 2-pyridone-6-carbox-
ylic acid (from Aldrich). Acid 4 was prepared from 4-pyrone-2-carboxylic acid (from TCI
America) as reported by Beak and Siegel [1]. The identity and purity of the synthesized com-
pounds was veriWed by their 1H NMR spectrum, mass spectrum, and melting point.
3. Results and discussion
It has been reported that the observed rate constant for the decarboxylation of
1,3-dimethylorotic acid does not change much in diVerent solvent due to cancellation of

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F.M. Wong, W. Wu / Bioorganic Chemistry 34 (2006) 99–104
the solvent eVects on the equilibrium constant, K, and the rate constant for the loss of CO2,
k [1]. As solvent polarity increases, the equilibrium constant for zwitterion formation
increases, but the rate constant for the loss of CO2 of the zwitterion decreases [1,11]. Since
the observed rate constant for the reaction is equal to the product of K and k, the lack of a
solvent eVect is considered to reXect the compensatory changes in the two steps [1].
Ionic liquids, on the other hand, may prove useful in changing the equilibrium constant
for the formation of the zwitterion. Most of the measurements of the polarity of ionic liq-
uid solvents have placed them to be comparable to or slightly more polar than those of
polar aprotic solvents such as dimethyl sulfoxide (DMSO) or tetramethylenesulfone (sulfo-
lane) [12–14]. For example, the dielectric constants of DMSO and sulfolane (46.7 and 44,
respectively) are in the range (40–55) reported for imidazolium-based ionic liquids [12].
However, the positively charged heterocyclic ring structures (imidazolium and pyridinium)
may stabilize the zwitterion structure via electrostatic interactions.
The rate constants for decarboxylation of acids 1 and 3 were shown in Table 1. Unfortu-
nately, the data for acid 4 were not available. As discussed above, acid 4 decarboxylates
about 3000 times faster than 1 or 3, consequently the reaction temperatures for convenient
measurement are much lower (»120 °C), at which a homogeneous solution of acid 4 in
ionic liquid cannot be obtained. The two ionic liquid solvents employed are 1-butyl-3-
methylimidazolium tetraXuoroborate ([bmim]+[BF4]¡) and 1-butyl-4-methylpyridinium
tetraXuoroborate ([bmp]+[BF4]¡). The rate constants in sulfolane were available from our
previous study [7].
The utilization of ionic liquids as solvents for the decarboxylation of orotic acid ana-
logues provided a rate enhancement of about 3-fold. Rate acceleration of this size may not
look large, however, the absence of solvent eVect when other solvents were employed
makes this result rather interesting and signiWcant [1]. Furthermore, the rate acceleration
corresponds to about 1kcal/mol reduction in activation energy, ꢀꢀGÐ, at the reaction tem-
perature. It should be pointed out that this change in activation energy compares favorably
to the contribution provided by hydrogen bonds to the pyrimidine carbonyl oxygens, as
demonstrated by quantum mechanical calculation in quite non-polar medium and by a
mutagenesis study on orotidine 5Ј-monophosphate decarboxylase [9,15]. Therefore, the
modest eVect of ionic liquid solvents on the reaction rate is meaningful and important.
Ionic liquids have been shown to catalyze reactions that involve charge development in the
transition state [16].
The eVect of solvents on the decarboxylation of orotic acids can be complicated due
to the compensatory consequences on the equilibrium constant, K, and the rate constant
for the loss of CO2, k [1]. The rate constant k in ionic liquid is likely to be similar to or
smaller than that in sulfolane due to the relatively similar polarity of the ionic liquids
and sulfolane. The polarity eVect on the equilibrium constant K is likely to be similar as
well. Furthermore, even if the polarity of ionic liquids and sulfolane are diVerent, the
Table 1
Rate constants of orotic acid analogues in selected solvents at 206 °C
Substrate
Rate constant (s¡1
)
Sulfolane
[bmim]+[BF4]¡
[bmp]+[BF4]¡
Acid 1
Acid 3
7.5 § 1.3 £ 10¡4
1.2 § 0.2 £ 10¡3
2.1 § 0.2 £ 10¡3
2.1 § 0.3 £ 10¡3
2.9 § 0.8 £ 10¡3
3.1 § 0.8 £ 10¡3

F.M. Wong, W. Wu / Bioorganic Chemistry 34 (2006) 99–104
103
OH
OH
CH3
O
CH3
O
N
N
+
+
–
–
N
CO2
N
CO2
N
CH3
CH3
–
H3C
R
+
N
H3C
R
BF4
–
N
BF4
+
Scheme 3.
polarity eVect on K and k should cancel each other out [1]. The near identical results in
two ionic liquids (Table 1) might have underscored this point. However, the ion-pair
nature of ionic liquid solvents may be ideal in stabilizing the zwitterion structure 5,
which has a large degree of charge separation. For example, the ion pairs in ionic liquids
can form electrostatic interactions with the dipole of zwitterion 5 as speculated in
Scheme 3. It has been previously shown that ionic liquids accelerate the rates of reac-
tions where the starting materials are charge-neutral and the formation of the activated
complex involves development of charges [16]. This interaction will stabilize the zwitter-
ion and thus increase the equilibrium constant K and consequently the observed rate
constant. The overall rate acceleration is quite modest since the interaction described is
random and not pre-organized.
Our results have demonstrated that the rate for the decarboxylation of orotic acid and
analogues is enhanced when ionic liquids are utilized as reaction solvent. The rate enhance-
ment could be a result of the stabilization of the zwitterionic intermediate by the ion pairs
in ionic liquids. Therefore, the results reported here can be viewed as further support for
the mechanistic proposal for the decarboxylation of orotic acid analogues that involves the
formation of a zwitterion as an intermediate [1,7].
Acknowledgments
This investigation was supported by the National Institutes of Health, MBRS SCORE
Program—Grant #5 S06 GM52588. We thank Professor Scott Gronert at SFSU for help-
ful discussions. We also thank Mark Anderson, Sha Jones, Emily Nieder, and Dustin San-
chez for preparation of the substrates.
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