Aromatic aldehyde-catalyzed gas-phase decarboxylation of amino acid anion via imine intermediate: An experimental and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2013.06.046

It is generally appreciated that carbonyl compound can promote the decarboxylation of the amino acid. In this paper, we have performed the experimental and theoretical investigation into the gas-phase decarboxylation of the amino acid anion catalyzed by the aromatic aldehyde via the imine intermediate on the basis of the tandem mass spectrometry (MS/MS) technique and density functional theory (DFT) calculation. The results show that the aromatic aldehyde can achieve a remarkable catalytic effect. Moreover, the catalytic mechanism varies according to the type of amino acid: (i) The decarboxylation of α-amino acid anion is determined by the direct dissociation of the C-C bond adjacent to the carboxylate, for the resulting carbanion can be well stabilized by the conjugation between α-carbon, C=N bond and benzene ring. (ii) The decarboxylation of non-α-amino acid anion proceeds via a SN2-like transition state, in which the dissociation of the C-C bond adjacent to the carboxylate and attacking of the resulting carbanion to the C=N bond or benzene ring take place at the same time. Specifically, for β-alanine, the resulting carbanion preferentially attacks the benzene ring leading to the benzene anion, because attacking the C=N bond in the decarboxylation can produce the unstable three or four-membered ring anion. For the other non-α-amino acid anion, the C=N bond preferentially participates in the decarboxylation, which leads to the pediocratic nitrogen anion.

Full text of DOI:10.1016/j.molstruc.2013.06.046

Journal of Molecular Structure 1049 (2013) 149–156
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Aromatic aldehyde-catalyzed gas-phase decarboxylation of amino acid
anion via imine intermediate: An experimental and theoretical study
Zhang Xiang
Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Using MS/MS and DFT to study decarboxylation of amino acid anion catalyzed by aromatic aldehyde.
Decarboxylation of -amino acid anion is determined by direct dissociation of adjacent CAC bond.
Dissociation of CAC bond is promoted by conjugation between -carbon, C@N bond and benzene ring.
Decarboxylation of non- -amino acid anion proceeds via a SN2-like transition state.
Dissociation of CAC bond and attacking of carbanion to C@N bond or benzene occur synchronously.
a
a
a
a r t i c l e i n f o
a b s t r a c t
Article history:
It is generally appreciated that carbonyl compound can promote the decarboxylation of the amino acid. In
this paper, we have performed the experimental and theoretical investigation into the gas-phase decar-
boxylation of the amino acid anion catalyzed by the aromatic aldehyde via the imine intermediate on the
basis of the tandem mass spectrometry (MS/MS) technique and density functional theory (DFT) calcula-
tion. The results show that the aromatic aldehyde can achieve a remarkable catalytic effect. Moreover, the
Received 16 January 2013
Received in revised form 15 June 2013
Accepted 17 June 2013
Available online 24 June 2013
catalytic mechanism varies according to the type of amino acid: (i) The decarboxylation of
anion is determined by the direct dissociation of the CAC bond adjacent to the carboxylate, for the result-
ing carbanion can be well stabilized by the conjugation between -carbon, C@N bond and benzene ring.
(ii) The decarboxylation of non- -amino acid anion proceeds via a SN2-like transition state, in which the
a-amino acid
Keywords:
Tandem mass spectrometry
Density functional theory
Amino acid
a
a
dissociation of the CAC bond adjacent to the carboxylate and attacking of the resulting carbanion to the
C@N bond or benzene ring take place at the same time. Specifically, for b-alanine, the resulting carbanion
preferentially attacks the benzene ring leading to the benzene anion, because attacking the C@N bond in
the decarboxylation can produce the unstable three or four-membered ring anion. For the other
Decarboxylation reaction
Aromatic aldehyde catalysis
non-
a-amino acid anion, the C@N bond preferentially participates in the decarboxylation, which leads
to the pediocratic nitrogen anion.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
substances in the synthesis of biologically active compounds
[10–14]. Therefore, the investigation into the decarboxylation of
the amino acids seems to be of great significance. Early research
mainly focuses on the radical-induced decarboxylation of amino
acids [15–17]. It is evident that this process is important for biolog-
ical systems considering many well-established enzymatic or
metabolic pathways of radical generation ‘‘in vivo’’. Chemically,
decarboxylation appears to be initiated by oxidative attack.
Recently, carbonyl-induced decarboxylation of amino acids has
attracted a lot of interest. Wolfenden’s group has reported the
ability of acetone to promote the decarboxylation of 2-aminomal-
onate [18]. Of various methods available, the decarboxylation of
tryptophan in the presence of ketone catalyst is the simplest way
to the synthesis of tryptamine, which is a very important starting
material in the synthesis of various indole alkaloids [13].
Decarboxylation is a chemical reaction that releases carbon
dioxide. Specifically, the loss of a carboxyl group is normally
described as the direct conversion of its conjugate base, a carbox-
ylate, to carbon dioxide and a carbanion, followed by protonation
of the carbanion. Due to their importance, the reactions of this kind
have received a great deal of attention in chemistry and biochem-
istry [1–9].
Amino acids play central roles both as building blocks of
proteins and as intermediates in metabolism. Decarboxylation of
the amino acids is one of the effective methods for obtaining a
number of important amino compounds which are versatile
E-mail address: zhangxiang@mail.zjgsu.edu.cn
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.06.046

150
Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
(a)
COO-
R1
COOH
R1
R1
ESI(-)
R2
-CO2
R2-CHO
H2O
R2
N
R1
R2
R2
N
N
H2N
COOH
(b)
H2N
-CO2
R2
N
ESI(-)
COOH
R2-CHO
H2O
R2
N
COO-
N
n
n
n
COOH
n
R2= aryl; n=1, 2, 4
R1=-CH3, PhCH2-;
Scheme 1. Decarboxylation of the
a
-amino acid and non-a-amino acid anions catalyzed by the aromatic aldehyde.
Certain progress has been made in the carbonyl-catalyzed
ture: ꢁ240 °C; Sheath gas flow rate: 9.28 L/min; Tube lens: 55 V.
In the MS/MS experiments, the carboxylate anions have been
isolated monoisotopically in the ion trap and collisionally activated
by the same collision energies. Helium (99.99%) was used as the
trapping and collision gas.
decarboxylation of amino acids, however, to the best of our knowl-
edge, most of the investigations concentrate in the liquid reactions.
The rate of the decarboxylation is determined by the catalyst and
pH value of the solution. However, the carbonyl-catalyzed
decarboxylation of amino acids in the gas phase has received
considerably less attention. This makes us decide to explore the
gas-phase decarboxylation reactions of the amino acids with the
assistance of aromatic aldehyde from both experimental and com-
2.3. Computational methods
All structures were computed on the basis of the hybrid density
functional theory (M06-2X) [22] and the 6-31+G(d, p) basis set
which were implemented in Gaussian 09 program package [23].
All the gas phase minima and transition structures (here also re-
ferred to as transition states) were characterized by frequency
analysis. Frequency calculations identify minimum structures with
all real frequencies, while transition states with only one imagi-
nary frequency. Zero point energy (ZPE) corrections were applied
at the same level [24]. To confirm the transition states connecting
the designated intermediates, intrinsic reaction coordinate (IRC)
calculations were carried out. Furthermore, to obtain more reliable
energetic data, higher-level single-point energy calculations were
performed at M06-2X/6-311++G(3df, 2p) level by using the
M06-2X/6-31+G(d, p) optimized geometries.
putational perspective, as is shown in Scheme 1.
such as -alanine -phenylalanine and etc, and non-
such as b-alanine, -aminobutyric acid and -aminocaproic acid,
a
-Amino acids,
L
L
a-amino acids,
c
e
have been selected as the object of study. Herein, two items should
be emphasized. First, as this work focuses on the gas-phase decar-
boxylation of amino acid anion via imine intermediate, only aro-
matic aldehyde has been selected as the catalyst. The reason is
that the use of aliphatic aldehyde probably leads to the problem
of obtaining additional intermediates (enamines) which would
complicate the mechanistic study of the reactions involved. Sec-
ondly, the tandem mass spectrometry in the negative ion mode
has been used to study the decarboxylation of the amino acid
anion.
It is well known that the C@N bond widely exists in the
organocatalyses [19–21], because the formation of the C@N bond
can effectively increase the electrophilicity of the corresponding
carbonyl. In this paper, the influence that the C@N bond and ben-
zene ring exert on the dissociation of the CAC bond adjacent to the
carboxylate and the nature of the decarboxylation will be explored
by tandem mass spectrometry technique and DFT calculations.
3. Results and discussion
Several typical amino acids have been selected as the objects of
the investigation. Thereinto,
sent -amino acids, while non-
-aminobutyric acid and -aminocaproic acid. The examination
L
-alanine and
L-phenylalanine repre-
a
a-amino acids contain b-alanine,
c
e
of the direct decarboxylation of these amino acid anions without
any catalyst has been performed first. The deprotonated amino
acids can be prepared by electrospray ionization of a 3:1 (v/v)
acetonitrile/water solution containing these amino acids in the
negative ion mode. Then the collision-induced dissociation (CID)
experiments of these carboxylate anions will provide the informa-
tion on the decarboxylation in the gas phase. It is found that the
direct decarboxylation of these amino acid anions is hard to take
place. In order to avoid the case that the collision energy is not suf-
ficient to strike the CAC bond to lose CO2, collision energy scanning
within the range of possibilities has been performed; however, the
fragment ion due to the loss of CO2 has never been found. As the
decarboxylation of the carboxylate belongs to SE1 reaction [25–
27] (see Eqs. (1) and (2)), the rate-determining step is the dissoci-
ation of the adjacent CAC bond, which leads to the carbanion and
CO2. Thus, it is difficult for amino acid anion to lose CO2, maybe
because the producing carbanion is very unstable.
2. Experimental
2.1. Material
L
-alanine,
-aminocaproic acid, benzaldehyde, cinnamaldehyde, 4-methoxy-
benzaldehyde and 4-(dimethylamino)-benzaldehyde were
L-phenylalanine, b-alanine, c-aminobutyric acid,
e
obtained from the Sigma-Aldrich Company. These chemical
reagents with >95% purity have been used without further
purification.
2.2. Mass spectrometry
The mass spectral data were acquired on a LCQ ion trap mass
spectrometer from ThermoFinnigan (San Jose, CA, USA) equipped
with an electrospray ionization (ESI) interface operated in the
negative ion mode. ESI of a CH3CN/H2O (3:1) solution containing
amino acid (about 500
hyde was carried out at a flow rate of 15
R—X s!low Rꢂ þ Xþ
ð1Þ
ð2Þ
l
M) and a stoichiometric amount of alde-
SE1
l
L/min into the ESI
source. The operation parameters are listed as follows: spray tip
potential ꢁ3000 V; capillary voltage: ꢁ20 V; capillary tempera-
fast
R
ꢂ þ Yþ ! R—Y

Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
151
Fig. 1. MS/MS spectra of the carboxylate anions (from a-amino acids) with imine structures.
The next section focuses on the influence that the aromatic
aldehyde exerts on the decarboxylation via the C@N bond. The
carboxylate anion with imine structure (see Scheme 1a) can be
prepared by electrospray ionization of a 3:1 (v/v) acetonitrile/
water solution containing the amino acid and a stoichiometric
amount of aromatic aldehyde in the negative ion mode. Then the
collision-induced dissociation (CID) experiment of this carboxylate
anion with the same collision energy as that of the amino acid an-
ion will provide the information on the decarboxylation catalyzed
by the aldehyde in the gas phase, as is shown in Fig. 1. Different
from that of the amino acid anions, the decarboxylation of the car-
boxylate anions with imine stuctures seems very easy and the MS/
MS spectra are very concise, for basically no other fragment ions
a-carbon, C@N bond and benzene ring, which can stabilize the
resulting carbanion due to the loss of CO2.
According to the description above, it is easy for the
a-amino
acid anion to lose CO2 with the assistance of the aromatic
aldehyde. Then, we wonder whether the similar situation happens
to the non-a-amino acid anion. The decarboxylation of b-alanine
anion with imine structure (see Scheme 1b) has been investigated
first, in which the C@N bond is at the b-position. The MS/MS spec-
trum of the corresponding carboxylate anion with imine group
(from benzaldehyde) is shown in Fig. 2a. The corresponding
fragment ion due to the loss of CO2 at m/z 132 is observed, which
indicates that the aromatic aldehyde can also effectively promote
the decarboxylation of b-alanine anion. Analogously, the decarbox-
have been found except the one due to the loss of CO2. For
L
-ala-
ylation of the c-aminobutyric acid and e-aminocaproic acid anions
nine, benzaldehyde and cinnamaldehyde have been tested and
MS/MS spectra of the corresponding carboxylate anions with imine
functional groups are depicted in Fig. 1a and b. The corresponding
fragment ions due to the loss of CO2 at m/z 132 and 158 are all
observed, which means that aromatic aldehyde can effectively
catalyzed by benzaldehyde was tested with MS/MS method, as are
shown in Fig. 2b and c, respectively. The corresponding fragment
ions due to the loss of CO2 at m/z 146 and 174 are all observed.
It is deduced that the aromatic aldehyde can also achieve a good
catalytic effect on the decarboxylation of the non-
anion.
a-amino acid
promote the decarboxylation of the
Analogously, for -phenylalanine, the MS/MS spectra of the
corresponding carboxylate anions with imine groups are shown
in Fig. 1c, d, and f, respectively. Herein, benzaldehyde,
a-amino acid anions.
L
To further understand the mechanism of the decarboxylation
catalyzed by the aromatic aldehyde, theoretical calculations have
been performed. The electronic structure method of choice has
been density functional theory (DFT), in particular, the M06-2X hy-
brid functional. Recently, Brill’ group has presented a DFT study of
the decarboxylation of the amino acid [28], which indicates that
DFT is well fitting for the investigation into the decarboxylation.
The next section will focus on the mechanistic study on the
decarboxylation of amino acid anion catalyzed by the aromatic
aldehyde via the C@N bond.
e
4-methoxybenzaldehyde, 4-(dimethylamino)benzaldehyde and
cinnamaldehyde have been tried. The corresponding fragment ions
due to the loss of CO2 at m/z 208, 238, 251 and 234 are all found.
Compared with the direct decarboxylation of
ylalanine anions, it is evident that the aromatic aldehyde can
promote the decarboxylation of these -amino acid anions. The
reason might be that the conjugation system is formed between
L-alanine and L-phen-
a

152
Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
Fig. 2. MS/MS spectra of the carboxylate anions (from non-a-amino acids) with imine structures.
Fig. 3. (a) The barrier for the decarboxylation of the
L-alanine and
L-phenylalanine anions; (b) The activation enthalpies for the decarboxylation of the L-alanine and L-
phenylalanine anions catalyzed by various of aldehydes (kcal/mol) (Å).
For the
decarboxylation is the dissociation of the adjacent CAC bond (see
Scheme 1a). Thus, uncatalyzed -alanine and -phenylalanine an-
ions were examined first. Unfortunately, the transition states for
the corresponding CAC bond dissociation in these -amino acid
a
-amino acid anion, the rate-determining step in the
of CO2, as is shown in Fig. 3a. The barriers for the loss of CO2 from
the -alanine and -phenylalanine anions are as high as 70.5 and
60.4 kcal/mol, respectively, which are consistent with the MS/MS
experiments. When the amino group in the -amino acid reacts
L
L
L
L
a
a
with different types of aldehydes to give the imines, the transition
anions cannot be located. However, we can also study the decar-
boxylation in kinetics by using restricted structural optimization.
In more specific terms, according to restricting the length of the
CAC bond, we can calculate the corresponding barriers for the loss
states for the corresponding CAC bond dissociations in these
a-amino acid anions with the imine groups were investigated. For-
tunately, the transition state for the decarboxylation catalyzed by
the aromatic aldehyde can easily be located and the corresponding

Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
153
activation enthalpies are depicted in Fig. 3b. For
the activation enthalpy for decarboxylation decreases by at least
52.3 kcal/mol. On the other hand, for -phenylalanine (R1 = Ph),
L
-alanine (R1 = H),
In addition, the effect of the substituent in the p-position of the
benzene ring has been taken into account. The substituents contain
AN(CH3)2, AOCH3, AH, ANO2 and ACl. For the L-alanine, the
L
the corresponding activation enthalpy also decreases by at least
41.4 kcal/mol.
plotting of the activation enthalpies versus the Hammet constants
[29–31] rp for each substituent is given in Fig. 4a and poor
Fig. 4. (a) The plotting of the activation enthalpies versus rp for the decarboxylation of
L
-alanine catalyzed by the aromatic aldehydes; (b) The plotting of the activation
p for the decarboxylation
-phenylalanine catalyzed by the
enthalpies versus rꢂ for the decarboxylation of
L
-alanine catalyzed by the aromatic aldehydes; (c) The plotting of the activation enthalpies versus
-phenylalanine catalyzed by the aromatic aldehydes; (d) The plotting of the activation enthalpies versus rꢂ for the decarboxylation of
aromatic aldehydes.
r
of
L
L

154
Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
correlation coefficient (r2 = 0.710) is obtained. However, when
r
p is
we wonder how the aromatic aldehyde catalyzes the decarboxyl-
ation. During the process of searching for the transition states for
the decarboxylations, it is found that the C@N bond or benzene
ring participate in the decarboxylation. The decarboxylation
proceeds via a SN2-like transition state (see Eq. (3)), in which the
dissociation of the CAC bond adjacent to the carboxylate and
attacking of the resulting carbanion to the C@N bond or benzene
ring take place at the same time, as is shown in Scheme 2. The cor-
responding optimized structures of the transition states are de-
picted in Fig. 5. With b-alanine for example (Scheme 2a), when
the dissociation of the C4AC5 bond takes place, it is found that C5
anion can also attack the C@N bond or benzene ring. Specifically,
four pathways are assumed, namely Pathway i (N-attack), ii (C1-at-
tack), iii (C2-attack) and iv (C3-attack). The corresponding activa-
tion enthalpies are 55.1, 70.7, 55.2 and 53.9 kcal/mol,
respectively. Obviously Pathway iv is the most favorable and the
product is the benzene anion. In other words, the benzene ring is
involved in the decarboxylation.
replaced by substituent conatant rꢂ, good correlation coefficient
(r2 = 0.969) has been obtained, as is shown in Fig. 4b. It can be
deduced that there exists a direct resonance interaction between
substituent and the anionic reaction center. In other words, in
the transition states or the decarboxylation products, the carbon
adjacent to the phenyl bears parts of negative charge, which is
from the resonance. Simialr condition happens to the decarboxyl-
ation of L-phenylalanine catalyzed by the aromatic aldehydes.
Fig. 4c shows the plotting of the activation enthalpies versus the
Hammet constants rp and the corresponding correlation coeffi-
cient (r2) is only 0.709, while that of the activation enthalpies ver-
sus the substituent constants rꢂ shows good correlation coefficient
(r2 = 0.955). Furthermore, in Fig. 4a–d, the slopes are all negative,
which suggests that the electron-withdrawing group can promote
the decarboxylation, while the electron-donating group has the
opposite effect. The reason might be that electron-withdrawing
group increases the stability of the carbanion.
According to the analyses above, for the decarboxylation of
a
-amino acid anion catalyzed by the aromatic aldehyde, the
producing carbanion can be well stabilized by the conjugation be-
tween -carbon, C@N bond and benzene ring. In other words, the
negative charge can be dispersed via the resulting -system. On
the other hand, for the non- -amino acid anion, as there exists
no conjugation between -carbon, C@N bond and benzene ring,
R—X þ Nuꢂ ! ½Nuꢂ ꢃ ꢃ ꢃ R ꢃ ꢃ ꢃ Xꢄ ! RNu þ Xꢂ SN2
a
p
Analogously, for
c-aminobutyric acid or e-aminocaproic acid,
a
four pathways have also been assumed (Scheme 2b and c). DFT
calculations show that the most favorable pathways are Pathways
a
Fig. 5. Optimized structures of the transition states for the decarboxylations of the non-a-amino acid anions catalyzed by the benzaldehyde (Å).

Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
155
(a)
CO2
+
(i)
(ii)
+
CO2
CO2
N
(iv)
N
3
2
H≠i = 55.1
H≠ii = 70.7
Δ
Δ
Δ
(iii)
(i)
N
COO-
5
(iii)
1
(iv)
4
N
(ii)
CO2
+
H≠iv = 53.9
+
+
H≠iii = 55.2
Δ
N
(b)
CO2
N
(viii)
(v)
(vi)
+
CO2
CO2
3
(vii)
N
H≠vi = 52.4
H≠v = 64.2
COO-
N
Δ
Δ
(v)
(vi)
2
4
5
1
(vii)
(viii)
N
CO2
+
+
H≠vii = 60.6
H≠viii = 57.7
Δ
Δ
N
N
(c)
CO2
+
CO2
+
(xii)
(ix)
(x)
(xi)
N
3
2
H≠ix = 58.7
COO-
4
N
Δ
(ix)
H≠x = 55.6
Δ
5
1
(x)
(xii)
(xi)
N
CO2
+
CO2
+
H≠xii = 59.1
H≠xi = 68.0
Δ
Δ
N
Scheme 2. Proposed pathways for the decarboxylations of the non-a-amino acids catalyzed by the benzaldehyde. (a) b-alanine; (b)
c
-aminobutyric acid; (c)
e
-aminocaproic
acid (kcal/mol).
vi (for
c-aminobutyric acid) and x (for
e
-aminocaproic acid),
2. The decarboxylation of a-amino acid anion is determined by
respectively. Different from b-alanine, in Pathways vi and x, the
C@N bond participates in the decarboxylation and the correspond-
ing products are both nitrogen anions. According to the DFT calcu-
lations, two items can be concluded: (a) Except b-alanine, for the
the direct dissociation of the CAC bond adjacent to the
carboxylate. The catalytic mechanism of the aromatic aldehyde
is that the producing carbanion can be well stabilized by the
conjugation between
for the negative charge can be dispersed via the resulting
-system.
3. The decarboxylation of non-
a-carbon, C@N bond and benzene ring,
decarboxylation of the other non-a-amino acid anion catalyzed
by the aromatic aldehyde, the carbanion due to the dissociation
of the CAC bond adjacent to the carboxylate attacks the C@N bond
to produce pediocratic cyclic nitrogen anion, which can promote
the decarboxylation. (b) For b-alanine, in Pathways i or ii, attacking
the C@N bond leads to the unstable three- or four-membered ring
product. Thus, the benzene ring participates in the decarboxylation
to form pediocratic benzene anion.
p
a
-amino acid anion proceeds via a
SN2-like transition state, in which the dissociation of the CAC
bond adjacent to the carboxylate and attack of the resulting
carbanion to the C@N bond or benzene ring take place at the
same time. Specifically, for b-alanine, the resulting carbanion
preferentially attacks the benzene ring leading to the benzene
anion, because attacking the C@N bond in the decarboxylation
can produce the unstable three or four-membered ring anion.
4. Conclusions
For the other non-a-amino acid anion, the C@N bond preferen-
tially participates in the decarboxylation, which leads to the
pediocratic nitrogen anion.
On the basis of the tandem mass spectrometry (MS/MS) tech-
nique and DFT calculations, the present paper has carried out the
experimental and theoretical investigation into the decarboxyl-
ation of the amino acid anion catalyzed by the aromatic aldehyde.
The following conclusions can be made from our results:
Acknowledgement
1. Whether for a-amino acid or non-a-amino acid anion, both MS/
The author gratefully acknowledges financial support from the
start-up costs to introduce research personnel of Zhejiang Gong-
shang University (1110XJ2010044, 1110XJ2110044).
MS experiments and DFT calculations show that the aromatic
aldehyde can effectively promote its decarboxylation.

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Z. Xiang / Journal of Molecular Structure 1049 (2013) 149–156
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