Sn(II)-catalyzed β-citronellol esterification: A Bronsted acid-free process for synthesis of fragrances at room temperature

Article abstract of DOI:10.1039/c4cy01069h

Simple SnCl₂·2H₂O was demonstrated to be able to catalyze β-citronellol esterification with acetic acid at room temperature under solvent-free conditions, achieving high conversion and ester selectivity (ca. 88% and 99%, respectively). Tin(ii) chloride is a stable and water-tolerant Lewis acid that is commercially available and less corrosive than Bronsted acid catalysts. This selective process is an attractive alternative to the mineral acid-catalyzed process because it avoids product neutralization common in those reactions. The effects of main reaction parameters such as reactant stoichiometry, temperature, solvent, and catalyst concentration were assessed. Among the tin catalysts evaluated, SnCl₂ was the most active and selective. Moreover, SnCl₂ was as active as sulfuric and p-toluenesulfonic acid catalysts, the Bronsted acids investigated herein, with additional advantages of being a solid and less corrosive catalyst.

Full text of DOI:10.1039/c4cy01069h

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Journal Name
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ARTICLE
Sn(II)ꢀcatalyzed
βꢀcitronellol
esterification:
a
Brønsted acidꢀfree process for synthesis of fragrances
at room temperature
Cite this: DOI: 10.1039/x0xx00000x
M.J. da Silva*, A.A. Julio and K. T. dos Santosa
Received 00th January 2012,
Accepted 00th January 2012
Simple SnCl2·2H2O was demonstrated to be able to catalyze βꢀcitronellol esterification with
acetic acid at room temperature under solventꢀfree conditions, achieving high conversion and
ester selectivity (ca. 88 and 99 %, respectively). Tin (II) chloride is a stable and water tolerant
Lewis acid, commercially available and lesser corrosive than Brønsted acid catalysts. This
selective process is an attractive alternative to the mineral acidꢀcatalyzed process because
avoid products neutralization common in those reactions. Effects of main reaction parameters
such as reactants stoichiometry, temperature, solvent, and catalyst concentration were assessed.
Among tin catalysts evaluated, SnCl2 was the most active and selective. Moreover, SnCl2 was
DOI: 10.1039/x0xx00000x
www.rsc.org/
as active as sulfuric and
pꢀtoluenesulfonic acid catalysts, Brønsted acids investigated herein,
with addition advantages of being a solid and less corrosive catalyst.
solvent and at room temperature. We paid special attention to assess
Introduction
factors driving esterification selectivity and to optimizing the
reaction conditions. Remarkably, at room temperature the SnCl2
catalyst promoted esterification of β citronellol with HOAc achieved
a very high selectivity (ca. 99 %) with ca. 88 % of conversion,
within shorter reaction time than those reported in the literature
[21,22]. To the best of our knowledge, this is the first report of
terpenic alcohol esterification reaction catalyzed by SnCl2.
βꢀCitronellol is the acyclic terpenic alcohol extensively
used and is a valuable raw material for the industries of perfumes,
beverage, food, and pharmaceuticals [1ꢀ3]. βꢀcitronellyl acetate is an
important flavor ingredient and fragrance being industrially
produced by enzymatic processes or via Brønsted acidꢀcatalyzed
reactions [4,5]. Indeed, the synthesis of citronellyl acetate could be
even more attractive when carried out via environmentally benign
processes, under solventꢀfree conditions and where steps of products
neutralization are avoided and the catalyst recovery facilitate,
minimizing the generation of salts and effluents [6].
Among catalysts employed in esterification reactions, Lewis acids
have significant advantages compared to the liquid Brønsted acid
catalysts, such as its low corrosiveness and great water tolerance
[7,8]. On the other hand, enzymatic catalysts commonly used in
esterification are expensive and require rigid control of temperature
and acidity of reaction medium. Actually, enzymatic catalysts have
been intensively used in the terpenic alcohol esterification, however,
their high cost and the difficulty of catalyst recovery are negative
aspects of these processes, that compromised their application in a
large scale [9,10]. On this regard, tin (II) chloride is an inexpensive
and easily handling solid catalyst that could be more potentially
active in the acidꢀcatalyzed reactions [11ꢀ15].
Thus, the present study describes using of SnCl2 as
catalyst under mild reaction conditions (ca. temperature and
pressure room) in the
β citronellyl acetate synthesis in absence
of solvent. Herein, we investigated effects of main reaction
parameters in the conversion and selectivity, in addition to
compare the catalytic activity of Lewis or Brønsted acids.
Experimental
Chemicals
All chemicals are commercially available and utilized without prior
handling. SnCl2.2H2O (95 % wt.), SnF2, SnBr2 and Sn(CH3COO)2
(99 % wt), were purchase from Sigma Aldrich. Brønsted acid
catalysts, H2SO4 (98% wt.) and pꢀtoluenesulfonic acid (99 % wt.)
were acquired from Aldrich and Proquimios, respectively. βꢀ
citronellol (99 % wt.), was purchased from SigmaꢀAldrich and used
without as received.
The development of selective processes to functionalize
monoterpenes based on commercial and simple metal catalysts is a
goal that has been pursued by our research group [16,17]. Recently,
tin (II) halide catalysts were successfully tested in the ketalization
and esterification reactions of fatty acids and glycerol [18,19].
Earlier, tin catalysts were successfully used in transesterification
reactions of vegetal oil [20]. Nevertheless, any work describing its
use in the terpenic alcohols esterification until this moment. Herein,
we wish describe a simple and efficient Sn(II)ꢀcatalyzed βꢀ
citronellol esterification process with HOAc in the absence of
Catalytic runs
Catalytic runs were performed in a glass reactor (50 mL) equipped
with a magnetic stirrer and sampling septum. Typically, βꢀcitronellol
and HOAc were dissolved in an adequate molar ratio (15 mL
solution), and then the reaction was initiated by the adding of Sn(II)
catalyst (ca. 1 to 10 mol %).
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Reaction progress was followed by GC analyses of aliquots taken at presence or absence of catalyst; by simplification, only the main
regular time intervals in a Shimadzu GC 2010 instrument, FID, fitted results are displayed in Table 1.
with Carbowax 20M capillary column. Toluene was the internal
standard. To calculate the reaction conversions, were compared the
corresponding product chromatographic peak areas with the
calibrating curves. To adjust the concentration to the calibration
curve we diluted all the aliquots with acetonitrile.
Despites the acidity of HOAc, and even using a slight excess them in
relation to βꢀcitronellol (i.e. molar ratio of 2:1), any ester formation
was detected in absence of catalyst, regardless reaction temperature
studied (i.e. 298 or 333 K, Table 1).
On the other hand, in presence of SnCl2 the βꢀcitronellol
esterification reactions became catalytic at room temperature or
when the reaction it were heated to 333 K (Runs 3, 4, Table 1).
Products identification
The reaction products were analyzed on a Shimadzu MSꢀQP 2010
ultra mass spectrometer instrument operating at 70 eV, coupled with
a Shimadzu 2010 GC. We isolated major product by column
chromatography using silica gel (60G). The 1H and 13C NMR spectra
was recorded on the Mercuryꢀ300 Varian Spectrometer at 300 and
75 MHz respectively, in CDCl3 solution using TMS as internal Run
standard. FTꢀIR spectroscopy analyses carried out in Varian 660 FTꢀ
Table 1. Temperature effects in the SnCl2ꢀcatalyzed βꢀcitronellol
esterification with HOAca
T
(K)
SnCl2
(mol %)
Convers.
(%)
Selectivity
(%)
olig.
0
0
1
0
0
Ni
0
0
IR Spectrometer. The spectroscopic data of the βꢀcitronellyl acetate
(Fig. 1) are summarized as follow:
1
2
298
333
0
0
0
3
4
298 10
333 10
42
21
90
40
2
2
8
58
aReaction conditions: βꢀcitronellol (7.8 mmol), HOAc (15.6
mmol), CH3CN solution (15 mL), 6 h. b(1) = βꢀcitronellyl
acetate; ni = complex mixture of nonꢀidentified products.
Fig.1. βꢀcitronellyl acetate
We verified that an increase of temperature affected the
conversion and reaction selectivity. Noticeably, the undesirable
formation of oligomers (ca. 58 % selectivity, Table 1) in the
reactions performed at 333 K compromised the ester selectivity.
Conversely, oligomers were not formed when the reaction was
carried out at 298 K. Oligomers are not detectable by GC
analysis because high molar weight. However, a checking of
mass balance proven that oligomers were formed, comparing
GC peaks area of formed products against GC peak area of βꢀ
citronellol consumed. In addition, after reaction solution
cooling the oligomers were precipitated as a white solid and
analyzed by FTꢀIR spectroscopy, who allow its identification.
1H NMR (300 MHz, CDCl3ꢀd1): δ (integration, multiplicity,
coupling constant, attribution); 0.90 (d, 3H, 9 – CH3); 1.13 – 1.53
(m, 5H, 2 – CH2, 3 – CH and 4 – CH2); 1.60 (s, 3H, 10 – CH3); 1.68
(s, 3H, 8 – CH3); 1.92 – 2.04 (m, 2H, 5 – CH2); 2.04 (s, 3H, 2’ –
OCCH3); 4.06 – 4.12 (m, 2H, 1 – OCH2); 5.05 – 5.10 (m, 1H, 6 –
CH).
13C NMR (75 MHz, CDCl3ꢀd1): δ (integration, multiplicity, coupling
constant, attribution); 17.63 (C10); 19.38 (C2’); 21.05 (C9); 25.35
(C5); 25.71 (C8); 29.42 (C3); 32.37 (C2); 36.94 (C4); 63.02 (C1);
124.52 (C6); 131.34 (C7); 171.26 (C1’).
(FTꢀIR) ν(cmꢀ1): 2962; 2922; 1743; 1455; 1367; 1239; 1055.
45
40
298 K no catalyst)
35
GCꢀMS (m/z/ relative intensity): 198/0.05; 138/23; 123/31;
109/17; 95/57; 81/77; 69/69; 55/42; 41/100.
333 K no catalyst
30
333 K SnCl2 (10 mol%)
298 K SnCl2 (10 mol%)
25
20
15
10
5
Results and discussion
General aspects
Recently, we described the use of tin halides as catalysts in the
esterification reactions of glycerol as well as free fatty acids [17,18].
In those works, among tin catalysts assessed, SnCl2 was always the
most effective catalyst. Thus, inspired by these findings, we
investigate the catalytic activity of SnCl2⋅2H2O in the terpenic
alcohols esterification reactions. βꢀcitronellol was selected as model
molecule and initially the reactions were carried out in CH3CN
solutions.
0
0
1
2
3
4
5
time (h)
Fig. 2. Effects of temperature on SnCl2ꢀcatalyzed βꢀcitronellol
esterificationa
aReaction conditions: βꢀcitronellol (7.8 mmol), HOAc (15.6 mmol),
CH3CN solution (15 mL), 6 h.
Effect of temperature in the SnCl2ꢀcatalyzed βꢀcitronellol
esterification with HOAc
The increase of the conversion rate with the decreasing of
temperature suggest an exothermal character for this process (Fig.
2). Although this aspect was not been further described in literature,
it was described another kind of reaction involving alcohols and
The SnCl2ꢀcatalyzed βꢀcitronellol esterification with HOAc was
accomplished at different temperatures (ca. 298 to 333 K) in
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4CY01069H
carbonylic reactant (i.e. glycerol and ketone), and the same
observation was find out [23].
B
Linear Fit of B
y = a + b*x
No Weighti
Equation
Weight
0.26
0.24
0.22
0.20
0.18
0.16
0.14
Thermodynamic studies
Residual
Sum of
8.68462E-
5
To calculate the equilibrium constant for the βꢀcitronellol
esterification reaction with HOAc (Keq) some considerations
deserves be highlighted. Firstly, it should be considered that it is
independent of HOAc concentration (due to its excess). In addition,
we must take into account that the reaction products (i.e. water and
βꢀcitronellyl acetate) are formed with the same concentration. Thus,
after these simplifications, we can write the equation for equilibrium
constant as follow:
Squares
0.97795
Adj. R-Squ
Value Standard Er
Intercept
Slope
1.2031
-0.312
0.08682
0.02701
B
B
Keq = [βꢀcitronellyl acetate]2/[βꢀcitronellol]
Equation 1
3.00
3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
1/T (10-3)
The initial concentration of βꢀcitronellol was equal to 0.873 mol Lꢀ1.
Table 2 shows the equilibrium constants for each reaction at the
different temperatures studied. The Equation 2 is true when the
enthalpy variation is constant with temperature.
Fig. 3. Linear plot –RlnKeq versus 1/T
__
ꢀS
R
ꢀH
RT
lnKeq =
Effect of reactants molar ratio in the SnCl2ꢀcatalyzed βꢀ
citronellol esterification
Equation 2
When performed at room temperature, the only product obtained in
the βꢀcitronellol esterification with HOAc was βꢀcitronellyl acetate
(Fig.4), always with selectivity equal or higher than 95 %,
determined via GCꢀMS analyses.
Table 2. Values of enthalpy and entropy variation for the SnCl2ꢀ
catalyzed βꢀcitronellol esterification reactions with HOAc
Temperature Equilibrium ꢀRlnKeq
ꢀH
ꢀS
(K)
constant
(Keq)
SnCl2
O
OH
+
+
H2O
(kJ molꢀ1)
(kJ molꢀ1)
CH3COOH
O
298
308
318
328
0.161
0.113
0.073
0.046
0.150
0.178
0.215
0.252
Fig. 4. SnCl2ꢀcatalyzed βꢀcitronellol esterification with HOAca
aReaction conditions: βꢀcitronelol:HOAc proportion 1: 2; SnCl2 (10
mol %); 298 K; 6 h.
ꢀ0.312
0.0012
The reactants stoichiometry is a key aspect in this reaction and its
effect was investigated in the range of 1:1 to 1:10 (Table 3).
Although the βꢀcitronellol:HOAc molar ratio has been increased to
1:10, only a poor conversion (ca. lower than 5 %) was reached in
catalyst absence. Conversely, in the SnCl2ꢀcatalyzed reactions the
conversion drop to almost 90 % when the proportion of reactants
was equal or higher than 1: 6. Any decreasing of ester selectivity
was observed in all these reactions at room temperature. Selectivity
of βꢀcitronellyl acetate remained equal or higher than 95 %
aReaction conditions: βꢀcitronellol (7.8 mmol), HOAc (15.6 mmol),
CH3CN solution (15 mL), 6 h.
From data shown in Table 2, we could build the linear plots of ln Keq
versus ln 1/T (Fig. 3) and by using the Equation 2 the respective
values of ꢀS and ꢀH were calculated.
independently of reactants molar ratio employed
.
Whilst the slope of the curve give the value of ꢀH and the Y axis
intercept provide ꢀS. The values for enthalpy and entropy variations
were found to be equal to ꢀ0.312 kJ molꢀ1 and 0.0012 kJ molꢀ1,
respectively (Fig.3).
The kinetic curves displayed in Fig. 5 show that both initial rate and
final conversion were almost the same in the reactions with molar
ratio between 1:6 and 1:10. Thus, this was the proportion selected to
perform the reactions in absence of solvent. We verified that without
solvent and with molar ratio equal to 1:6, the reaction behavior was
the same in terms of conversion and selectivity (not showed in
Figure 4 by simplification).
The negative value ꢀH confirm the exothermic character of reaction,
that explain the increased observed in conversion when the
temperature was decreased of 333 to 298 K (Fig.2).
In addition to the recovery problems and toxicity, some enzymatic
catalysts are sensible to acetic acid presence, that become the choice
of the adequate solvent an crucial aspect for those reactions [24.25].
However, in the SnCl2ꢀcatalyzed reaction described herein, the
reactants themselves were used as liquid phase and thus this problem
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DOI: 10.1039/C4CY01069H
was easily circumvented.
Effect of catalyst concentration in the SnCl2ꢀcatalyzed βꢀ
itronellol esterification
In all runs catalyzed by Sn(II), the high initial rate of βꢀcitronellol
esterification allows to reach a maximum conversion within first
thirty minutes of reaction, staying almost constant from this point
forward regardless catalyst concentration (Figure 5)
Table 3. Effect of reactants stoichiometry in the SnCl2ꢀcatalyzed βꢀ
citronellol esterification with HOAca,b
Run
HOAc: βꢀcitronelol
molar ratios
Conversions
(%)
0.1 mol %
2.5 mol %
5.0 mol %
7.5 mol %
10.0 mol %
100
80
60
40
20
0
SnCl2ꢀ
Blankꢀ
catalyzed
reactions
reactions
1
2:1
4:1
43
69
88
89
89
0
0
2
3
4
5
6:1
0
0
30
60
90
time (min)
120
150
180
8:1
<5
5
10:1
Fig. 6. Effect of SnCl2.2H2O concentration in the βꢀcitronellol
aReaction conditions: βꢀcitronellol (7.8 mmol), CH3CN solution (15
mL), SnCl2 (10 mol %), 298 K, 3 h. βꢀcitronellyl acetate selectivity
esterification with HOAc.a
b
aReaction conditions: βꢀcitronellol (13.1 mmol), HOAc (78.6 mmol),
298 K, 3 h.
determined via GC was equal or higher than 95 % in all catalytic
runs.
From rate constant measured within first thirty minutes of reaction
(i.e. mmol βꢀcitronellol converted/ 1800 s) for each run with a
specific catalyst concentration (ca. 0.10 to 10 mol %) it was possible
to build linear plot (Fig.6) and determine the reaction order in
relation to the SnCl2 concentration.
100
80
A high linear coefficient (R2 = 99.9 %) clearly show that a first order
dependence in relation to SnCl2 concentration determined by slope
of curve (ca. 1.2) may be suggested for these reactions (Fig.7).
60
40
1:2
1:4
1:6
1:8
20
3
1:10
C1
Linear Fit of C1
2
0
0
30
60
90
120
150
180
time (min)
1
0
Fig. 5. Kinetic curves of SnCl2ꢀcatalyzed βꢀcitronellol esterification
with HOAc using different molar ratios between reactants.
-1
-2
aReaction conditions: βꢀcitronellol (7.8 mmol), SnCl2 (10 mol %) 3
h.
-3
-9,0 -8,5 -8,0 -7,5 -7,0 -6,5 -6,0 -5,5 -5,0 -4,5
The reason of the fast deactivation of catalyst was investogated. We
performed experiments using different initial concentrations of water
(ca. 0.1 to 10 mol % in relation to the catalyst). We verified that
beginning reactions with water concentration higher than 10 mol %
resulted in a noticeable decreased of the initial rate. Starting a
reaction with 1:6 molar ratio of βꢀcitronellol: HOAc and an initial
amount of water equal to 10 mol %, resulted in the lowering
conversion rate after 30 minutes for 40 %. Contrarily, in the
reactions in absence of water, conversions near to 90 % were
obtained (Fig. 5). Besides to shift equilibrium toward reactants,
water may coordinate to the tin(II) cations, compromising its
activity.
ln SnCl2 concentration
Fig. 7. Determination of order in relation to the SnCl2 catalyst
concentration
A first order dependence have been also verified in SnCl2ꢀcatalyzed
reaction, in which the substrates were glycerol and HOAc [26].
Thus, it is suggestive that in the determining step of reaction rate
only one tin atom should be involved (Scheme 1).
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4CY01069H
X
X
Sn2+
O
100
ROH
SnX2
- SnX2
298 K
O
90
H2O
+
+
OH
OR
298 K
Sn(OAc)2
SnF2
80
O
O
R
H
70
60
50
SnBr2
SnCl2
OH
PTSA
H2SO4
Scheme 1. Mechanism proposal of tinꢀcatalyzed esterification of
40
30
20
10
0
alcohols with HOAc
Although there are different approaches in literature on regards of
carbonyl group activation could be made by tin coordinated to the
anionic ligands or partially dissociated, it is reasonable suppose that
the carbonyl polarization become more favorable its attack by
hydroxyl group as described in the Scheme 1 [27].
0
20
40
60
80
100
120
140
160
180
200
time (min)
Table 4. Effect of SnCl2.2H2O concentration in the βꢀcitronellol
esterification with HOAca
Fig. 8. Effect of catalyst nature in βꢀcitronellol esterification with
HOAca,b
Run
SnCl2
Conversion
(%)
Product selectivity
(%)
aReaction conditions: βꢀcitronellol (13.1 mmol), HOAc (78.6 mmol),
Lewis acid catalyst (10 mol % of Sn2+ in relation to βꢀcitronellol);
Brønsted acid catalyst (10 mol % of H+ in relation to βꢀcitronellol), 3
h, 25 °C.
(mol %)
Ni
1
Olig
1
1
2
3
4
5
0.1
2.5
2
99
96
99
97
99
0
0
0
0
0
bSelectivity of βꢀcitronellyl acetate was higher than 98% in all runs,
except in the Sn(OAc)2ꢀcatalyzed reaction where it does not
obtained.
21
44
64
87
4
5.0
1
On the other hand, steric or electronic factors of anionic ligands may
change the catalytic activity of Lewis acid such as Sn(II) salts.
However, it was also important take in account the solubility of
catalysts. Sn(OAc)2 and SnBr2 were partially soluble, whereas to
SnCl2 and SnF2, who were completely solubilized. For these reason,
tin acetate or bromide were the lesser active catalysts than the
soluble salts. In addition, the lowest activity of Sn(OAc)2 may be
attributed to low lability of acetate ligand. This bidentate ligand
probabily hinder the activation of carbonyl group of HOAc by the
few molecules of Sn(OAc)2 soluble catalyst. In this sense, is
reasonable accept that the activation of carbonyl group of HOAc
depends on the Sn(II) cations concentration available in the
solution.
7.5
3
10.0
1
aReaction conditions: βꢀcitronellol (13.1 mmol), HOAc (78.6 mmol),
298 K, 3 h. b(1) = βꢀcitronellyl acetate; ni = complex mixture of nonꢀ
identified products.
It can be noted that decreasing of the catalyst concentration does not
result in any significant change on the reaction selectivity.
Regardless of SnCl2 concentration, the βꢀcitronellyl acetate
selectivity (1) remained higher than 96 % and with no oligomers
formation (Table 4). A highly selective conversion of βꢀcitronellol to
βꢀcitronellyl acetate was achieved using SnCl2 at 10 mol % within
reaction time shorter than those reported by the literature in
enzymatic processes [28]. Those authors suggested that HOAc had
an inhibitory effect on catalytic action of lipase assessed [29].
Conversely, the highest activity of SnCl2 may be attributed not only
to its high solubility as well as to the higher stability of chloride
anion, who is recognized as a leaving group better than fluoride
anion present in the SnF2. This result is in agreement with the
literature [25].
Lewis or Brønsted acidꢀcatalyzed βꢀcitronellol esterification with
HOAc
Finally, the most auspicious result is the high activity of SnCl2;
although it has been slightly lower than Brønsted acids studied, the
use of tin catalyst has significance handicaps such as to avoid
neutralization steps, generate less corrosion and being easily
handling because it is a solid compound.
The effect of catalyst nature (i.e. Brønsted or Lewis acids) was
assessed in the kinetic curves obtained are displayed in Fig. 8. In the
most of case, the acid strength is determining for the activity of
Brønsted acid catalysts. If considered that sulfuric and p-
toluenesulfonic acids were used at same hydrogenionic
concentration, the fact of reactions catalyzed by them have achieved
conversions almost equals (ca. 90%) could be attributed to probable
levelling effect caused by the composition of reaction medium.
Conclusions
An efficient process to synthesize terpenic esters (i.e. βꢀcitronellyl
acetate) was developed. Tin (II) chloride dihydrate, an inexpensive
and commercially available catalyst was used in environmentally
benign reaction conditions (i.e. in absence of solvent and at room
temperature). Tin(II) chloride catalyst efficiently promoted the βꢀ
citronellol esterification reaching the same conversion and
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selectivity (ca. 90 and 98 %, respectively) that corrosive Brønsted 15. J. Luo, J. Yu, R.J. Gorte, E. Mahmoud, D.G. Vlachos, M.A.
acid catalyst (i.e. H2SO4). In addition, among others tin catalysts Smith Catal. Sci. Technol., 2014, 4, 3074.
assessed herein, we find out that SnCl2 was most active and
selective. The comparison with others tin catalyst provide the
following order in terms of activity: SnCl2 > SnF2 > SnBr2
16. M. J. da Silva, D. M. Carari DM Catal. Lett. 2014, 144,:615.
>
Sn(OAc)2. Tinꢀcatalyzed esterification processes are lesser corrosive
than those catalyzed by Brønsted acids and faster and cheapier than
enzimatic processes reported in the literature.
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