Direct oxidation of secondary alcohol to ester by performic acid

Article abstract of DOI:10.1039/c3gc41198b

The reaction pathways and kinetics of the oxidation reactions of 1-phenylethyl alcohol (PEA), 1-(3,4-dimethoxyphenyl)ethanol (MVA), 1-(4-hydroxy-3-methoxyphenyl)ethanol (HMOPE), 1-(3-aminophenyl)ethanol (APE), 1-(4-methylphenyl)ethanol (MPE) and cyclohexanol with performic acid (PFA) were investigated in formic acid solvent. An unexpected new reaction pathway, from which the secondary alcohols can be directly oxidized to corresponding esters, was found. The reaction products (esters and ketones) of oxidation of PEA, MVA, HMOPE, APE, MPE and cyclohexanol were detected at different reaction times. The reaction rate constants, k₁, k₂, k₃ and power orders α, β, γ of PFA concentration for three oxidation reactions pathways: alcohol to ester, alcohol to ketone and ketone to ester were obtained, respectively. These findings might provide a new insight into the technology of lignin degradation.

Full text of DOI:10.1039/c3gc41198b

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Direct oxidation of secondary alcohol to ester by
performic acid†
Cite this: DOI: 10.1039/c3gc41198b
Xiao-Hong Li, Xiang-Guang Meng,* Ying Liu and Xiao Peng
The reaction pathways and kinetics of the oxidation reactions of 1-phenylethyl alcohol (PEA), 1-(3,4-
dimethoxyphenyl)ethanol (MVA), 1-(4-hydroxy-3-methoxyphenyl)ethanol (HMOPE), 1-(3-aminophenyl)-
ethanol (APE), 1-(4-methylphenyl)ethanol (MPE) and cyclohexanol with performic acid (PFA) were investi-
gated in formic acid solvent. An unexpected new reaction pathway, from which the secondary alcohols
can be directly oxidized to corresponding esters, was found. The reaction products (esters and ketones)
of oxidation of PEA, MVA, HMOPE, APE, MPE and cyclohexanol were detected at different reaction times.
The reaction rate constants, k1, k2, k3 and power orders α, β, γ of PFA concentration for three oxidation
reactions pathways: alcohol to ester, alcohol to ketone and ketone to ester were obtained, respectively.
These findings might provide a new insight into the technology of lignin degradation.
Received 20th June 2013,
Accepted 5th September 2013
DOI: 10.1039/c3gc41198b
www.rsc.org/greenchem
phenyl)ethanol (MPE), 1-(3-aminophenyl)ethanol (APE) and
cyclohexanol as the models of lignin, to investigate their selec-
Introduction
Alcohol oxidation is a key reaction in organic synthesis and tive oxidation to esters by performic acid which can be readily
will likely play a significant role in the development of value- prepared by the cheap reagents H2O2 and formic acid. It is of
added chemicals from biomass.1–4 In previous reports, alcohol great significance that these alcohols can be rapidly oxidized
can be only oxidized into the corresponding aldehydes5–8 or into esters in relatively short time. Its advantages relate to the
ketones,9–12 and aldehydes and ketones can be further oxi- following three aspects: performic acid is a green oxidant;
dized into organic acids13–16 or esters.17–20 In this work, an formic acid can be reused by distillation; the generated ester
unexpected new reaction pathway, from which secondary can be easily hydrolyzed.
alcohols can be directly oxidized into esters, was found. The
discovery is of vital importance to organic synthesis and degra-
dation of lignin. Lignocellulose is a renewable natural biomass
Results and discussion
which can be converted into chemicals and biofuel.21 The
degradation of lignin and its separation from cellulose are two
of the most difficult problems encountering the production of
biofuel. Lignin contains lots of units of –OH and/or –OCH3
substituted aromatic propyl alcohol. Recently we proposed a
new route to degrade lignin under mild conditions by the
selective oxidation of aromatic alcohols to ketones and after-
wards to esters.22 Here a more convenient route, in which aro-
matic alcohols can be directly oxidized to esters by performic
acid, and then the esters can easily hydrolyze to phenols and
alkyl carboxylic acids, was proposed.
The oxidations of PEA, MVA, HMOPE, MPE, APE and cyclo-
hexanol by performic acid were investigated in formic acid at
30 °C. The contents of alcohol, ketone and ester in reaction
solution were quantitatively detected by HPLC and GC. From
Fig. 1 it can be seen that the corresponding esters could be
generated in relatively short time. The experimental results
were listed in Table 1. It is noted that acetophenone could not
be oxidized by performic acid under this work’s conditions.
However from Fig. 1 and Table 1 we found that the products of
oxidation of PEA consisted of phenyl acetate, acetophenone
and 1-phenylethyl formate, and the major product was phenyl
acetate. The experimental phenomena indicated that PEA
could be directly oxidized to phenyl acetate by performic acid.
Further it can be seen that the –OCH3 and/or –OH substituted
aromatic alcohols MVA and HMOPE could be quickly con-
verted to the corresponding esters within 15 min under this
work’s conditions. Similarly MPE and APE could also be
entirely converted into the corresponding esters within 4 h.
Cyclohexanol, a typical aliphatic secondary cyclitol, was also
Herein, we selected secondary alcohols 1-phenylethyl
alcohol (PEA), 1-(3,4-dimethoxyphenyl)ethanol (MVA), 1-(4-
hydroxy-3-methoxyphenyl)ethanol
(HMOPE),
1-(4-methyl-
Department of Chemistry, Key Laboratory of Green Chemistry and Technology,
Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064,
P. R. China. E-mail: mengxgchem@163.com
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc41198b
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Fig. 1 The HPLC profiles of secondary alcohols PEA, MVA and cyclohexanol oxidation reaction solution. [a] PEA, reaction for 25 h, detected at λ = 280 nm; [b] MVA,
reaction for 15 min, detected at λ = 280 nm; [c] cyclohexanol, reaction for 65 min, detected at λ = 254 nm.
entirely oxidized into ester within 65 min (Fig. 1). Thus, two replace performic acid (Fig. 3). From Fig. 3, we found that the
parallel reaction pathways from a secondary alcohol to ketone products of oxidation of PEA by perbenzoic consisted of only
and ester were suggested, as illustrated in Scheme 1.
both phenyl acetate and acetophenone, and the major product
In order to investigate the reaction kinetics, we detected the was phenyl acetate. Moreover, it was also found that per-
variations of the measured concentrations of alcohols, ketones benzoic acid could not oxidize acetophenone to phenyl acetate
and esters with reaction time (Fig. 2). According to eqn (4)–(7) under this work’s conditions. This indicated that PEA could be
in the ESI,† the reaction rate constants k*, k′ , k′ and k′ can be directly oxidized to phenyl acetate by perbenzoic acid in the
1
2
3
obtained by fitting the experimental data in Fig. 2. From same way as by performic acid. According to eqn (4), (5), (11)
Fig. 2, it can be seen that the theoretical curves were in good and (12) in the ESI,† the reaction rate constants k′ and k′ can
1
2
agreement with the experimental data. The multiple corre- be obtained by fitting the experimental data in Fig. 3. From
lation coefficients were all more than 0.97. Thus from eqn Fig. 3, it can be seen that the theoretical curves were also in
(8)–(10) (ESI†), the values of k1, k2, k3, α, β and γ were obtained good agreement with the experimental data. The multiple cor-
by fitting the straight lines of ln k′ vs. ln cP (Fig. 4S in the relation coefficients were all more than 0.97. Thus from
ESI†). The calculated results were listed in Table 2. It was eqn (8) and (9) (ESI†), the values of k1, k2, α and β were
noted that the reaction of PEA with performic acid was obtained by fitting the straight lines of ln k′ vs. ln cP (Fig. 6S in
obviously different in comparison with the other oxidations of ESI†). The calculated results were listed in Table 2. To better
alcohols (Fig. 1 and Fig. 5S in the ESI†). We found that PEA understand the comprehensive reaction kinetics, we carried
could rapidly react with formic acid to generate 1-phenylethyl out alone the “one step” oxidation reactions of ketones
formate (Fig. 1). Thus the kinetic rate constant for the oxi- (DMOPEN, HMOPEN, APEN, MPEN and cyclohexanone) with
dation of PEA by PFA was difficult to calculate. Further it was PFA. It was found that ln(c0(ketone)/ct(ketone)) vs. reaction
found that PEA can’t react with benzoic acid to generate 1-phe- time were all straight lines (r > 99), as seen in Fig. 3S in the
nylethyl benzoate, therefore in order to avoid the interference ESI.† This indicated that they were also first-order with respect
of an esterification reaction, we chose perbenzoic acid to to the concentrations of ketones. Thus, based on the
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Table 1 The oxidation of secondary alcohols to esters and ketonea
that the amount of PFA involved in the oxidation of ketone to
ester was greater than one molecule, which was in agreement
with our previous work.22 Further, it is possible that performic
acid is involved in the formation of a hydrogen bond with
itself as described by Yamabe et al.23 Similarly, β > 1, which
implies that PFA might play several roles in the reaction
process to directly oxidize alcohol to ester, such as abstracting
H atom, attacking C atom and forming a hydrogen bond with
itself or formic acid. The reaction mechanism will be further
investigated in our next work.
Conversion
(%)
Yield
(%)
Substrate
Product
Time
15 min
100
>98
13 min
100
>98
39.8
28.5
Conclusions
In summary, the oxidation reactions of secondary alcohols
PEA, MVA, HMOPE, MPE, APE and cyclohexanol with perfor-
mic acid were investigated in detail. Two parallel reaction path-
ways were found. A secondary alcohol could be oxidized to not
only ketone but also ester. Furthermore, the oxidation of alco-
hols to esters by PFA was relatively fast and it was a predomi-
nant pathway in the reaction kinetics. The experimental
analyses and kinetic calculations provided clear evidence for
the new reaction pathway. Performic acid could play a complex
role in the oxidation reaction. This finding might also provide
a new convenient method and technology for the degradation
of lignin and the synthesis of chemicals.
25 h
93
24.7
>98
4 h
100
3.5 h
100
100
>98
>98
65 min
Experimental
Materials
a c0(alcohol) = 0.02 M, c0(PFA) = 0.37 M, 30 °C.
1-Phenylethyl alcohol (PEA), 1-(3,4-dimethoxyphenyl)ethanol
(MVA), 1-(3-aminophenyl)ethanol (APE), 1-(4-methylphenyl)-
ethanol (MPE), cyclohexanol, 1-(3,4-dimethoxyphenyl)etha-
none (DMOPEN), 1-(3-aminophenyl)ethanone (APEN), 1-(4-
methylphenyl)ethanone (MPEN), acetophenone (AP), cyclo-
hexanone, phenyl acetate, 1-(4-methylphenyl)acetate, 30%
hydrogen peroxide, benzoic acid, formic acid (FA) were all
analytical grade and purchased from commercial sources. The
formic acid was dried by B2O3 and further purified by distilla-
tion. All other chemicals and solvents used were analytical
grade with the same purification extent.
Scheme 1 The reaction pathways proposed for alcohol oxidation by performic
acid.
Preparation of performic acid (PFA)
H2O2 (30%, 0.2 ml) and formic acid (4.8 ml) were introduced
and stirred at constant 30 °C in a closed glass reactor. A reac-
tion sample was periodically drawn from the reactor. The con-
tents of performic acid and hydrogen peroxide were
determined according to literature methods24 (Fig. 1S in ESI†).
experimental data, k3 could be obtained by means of the one
step oxidation reaction. The calculated results were listed in
Table 1S in the ESI,† from which it can be seen that the values
of k3 were approximately consistent with those of the overall
reactions in Table 2. This indicated that Scheme 1 was reason-
able. From the calculated rate constants k1, k2 and k3 for the
oxidations of HMOPE, MVA, MPE, APE and cyclohexanol by
PFA, it can be found that k2 was approximately equal to k3,
however, k1 was much greater than k2. This indicated that the
oxidation of alcohols to esters by PFA was relatively fast and it
was a predominant pathway in the reaction kinetics. For the
power orders α, β and γ, it can be seen that γ > 1; this meant
Synthesis of 1-(4-hydroxy-3-methoxyphenyl)ethanone
(HMOPEN)
The synthesis of HMOPEN was a modification of the pre-
viously published literature methods.25,26 Acetic anhydride
(4 ml) was slowly added to a mixture of polyphosphoric acid
(45 g) and guaiacol (7.5 ml) and then stirred in an oil bath
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Fig. 2 The variations of concentration of alcohols, ketones and esters vs. time. (a) 10 °C, (b) 10 °C, (c) 30 °C, (d) 30 °C, (e) 30 °C; c0(alcohol) = 0.02 M, c0(PFA) = 0.37
M; solid line —: theoretical fitting curves based on eqn (5)–(7) in the ESI.†
Table 2 The calculated reaction rate constants k1, k2, k3 and the powers orders
α, β, γ for the oxidation of alcohols by performic acid
k2
k1
k3
(min−1 M−α
)
(min−1 M−β
)
(min−1 M−γ
)
α
β
γ
HMOPEa
MVAa
0.113
0.099
0.206
0.181
0.133
0.045
0.071
0.0015
0.101
0.090
0.064
0.017
0.021
0
1.04 1.24 1.21
1.06 1.28 1.24
1.03 1.51 1.04
0.93 1.16 1.08
0.98 1.21 1.05
Cyclohexanolb 0.043
APEb
MPEb
PEAc
0.024
0.041
0.0012
1.03 1.05
—
a 10 °C. b 30 °C, c0(alcohol) = 0.02 M, c0(PFA) = 0.37 M. c 50 °C,
c0(alcohol) = 0.02 M, c0(perbenzoic acid) = 0.37 M, —: k3 = 0, the γ
cannot be calculated.
until a purple-red liquid was obtained. Canary yellow precipi-
tate HMOPEN was formed immediately after a small amount
of ice water was added. The product was then extracted with
Et2O, washed with sodium bicarbonate and water, respectively,
then recrystallized from ethanol and dried in vacuo (yellowish
powder).
Fig. 3 The variations of concentration of PEA, AP and phenyl acetate vs. time
at 50 °C; c0(PEA) = 0.02 M, c0(perbenzoic acid) = 0.37 M; solid line —: theore-
tical fitting curves based on eqn (5), (11) and (12) in the ESI.†
dichloroethane. The extract was distilled and HMOPE was
obtained. Anal. Calcd for C9H12O3: C, 64.29; H, 7.14%; Found:
Synthesis of 1-(4-hydroxy-3-methoxyphenyl)ethanol (HMOPE)
1
C, 64.33; H, 7.17%. H NMR (300 MHz, CDCl3), 2.504 (–CH3),
HMOPE was synthesized according to the literature.27
HMOPEN (1.66 g) was added to absolute methanol (20 ml),
then sodium borohydride (0.84 g) was added slowly into the
mixture and the solution was stirred for 4 h in an ice water
bath. Then the suspension gradually became a clear solution.
3.995 (–OCH3), 7.455 and 7.473 (aromatic CH), 4.742 (CH),
9.385 (aromatic –OH), 6.532 (–OH).
Instruments and methods
Most of the solvent was removed by vacuum distillation, then Mass spectrometry was implemented with a Finnigan MAT
the reaction mixture was poured into iced water (20 ml) and 45001 mass spectrometer. Elemental analysis was imple-
adjusted to pH 6.0 with carbon dioxide and extracted with 1,2- mented with a Carlo Erba 1106 instrument. The typical
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21073126 and 21273156).
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