Insight into the Mechanism of the Pechmann Condensation Reaction Using NMR

Article abstract of DOI:10.1021/acs.joc.5b01802

The mechanism of the Pechmann condensation is still controversial despite the technological and biochemical importance of coumarins. Here, we present NMR evidence for a mechanism featuring the sequence of initial electrophilic aromatic substitution followed by transesterification and a final dehydration. This mechanism has been convincingly defined and supported by the temporal evolution of two key intermediates which could be purified and identified.

Full text of DOI:10.1021/acs.joc.5b01802

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Brief Communication
Insight into the Mechanism of the Pechmann
Condensation Reaction Using NMR
Stephen Tyndall, Koon Fai Wong, and Melissa Ann VanAlstine-Parris
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b01802 • Publication Date (Web): 02 Sep 2015
Downloaded from http://pubs.acs.org on September 5, 2015
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Insight into the Mechanism of the Pechmann Condensation Reaction Using NMR
Stephen Tyndall, Koon Fai Wong and Melissa A. VanAlstine-Parris*
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Department of Chemistry, Adelphi University 1 South Avenue, Garden City, NY 11530, USA
*Corresponding author email: MVanAlstine@adelphi.edu
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ABSTRACT: The mechanism of the Pechmann condensation is still controversial despite the
technological and biochemical importance of coumarins. Here we present NMR evidence for a
mechanism featuring the sequence of initial electrophilic aromatic substitution, followed by
transesterification and a final dehydration. This mechanism has been convincingly defined and
supported by the temporal evolution of two key intermediates which could be purified and
identified.
The von Pechmann condensation reaction,1 which allows the synthesis of coumarins by reaction
of phenols with β-keto esters, has received significant attention as a synthetically useful strategy
in organic synthesis. The Pechmann reaction is a one-pot reaction that is believed to proceed
through three steps; electrophilic aromatic substitution (EAS), transesterification (TE, -EtOH)
and dehydration (-H2O) but the order of the steps are unknown (Scheme 1).2 While many
scientists have proposed mechanisms for this reaction, starting with resorcinol and ethyl
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acetoacetate, (Scheme 1, R=CH3), no one has been able to support their mechanism with
identification of intermediates.3–5
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Scheme 1. Possible Mechanisms for the von Pechmann Condensation Reaction
Tyagi et al6 used GC to monitor the reaction of 7-substituted 4-methylcoumarins for conversion
and selectivity. However, they did not report detecting any intermediates, only byproducts that
were formed at higher reaction time and temperature. In 2010, Calvino-Casilda et al7 used
Raman to monitor the reaction of resorcinol and ethyl acetoacetate in real time. They were
unable to observe any intermediates. Daru and Stirling3 performed a theoretical study on the
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Pechmann reaction of 7-hydroxy-4-methylcoumarin and determined that all three paths are
relatively equivalent and therefore possible.
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The Pechmann reaction is most commonly used to combine activated phenols with ethyl
acetoacetate to form 4-methylcoumarin derivatives. When ethyl 4,4,4-trifluoroacetoacetate is
used as the β-keto ester to form 7-hydroxy-4-trifluoromethylcoumarin (HFC), the reaction tends
to be sluggish, either the yield is low,8 or the reaction requires a higher catalyst load.9 Previously,
in trying to understand the lower yield of the trifluoromethyl derivatives, a possible intermediate,
4,7-dihydroxy-4-(trifluoromethyl)chroman-2-one 1b (Figure 1), was isolated. It was determined
that if the reaction was heated to a higher temperature (>80°C), this compound 1b was not
isolated.10 Preliminary studies on the viability of using 19F and 1H NMR to monitor this reaction
are described below.
Figure 1. Intermediates isolated in the synthesis of 7-hydroxy-4-trifluoromethylcoumarin.
The reaction of ethyl 4,4,4-trifluoroacetoacetate and resorcinol with iodine catalyst in toluene
can be monitored by 19F NMR. At the start of the reaction there are two main peaks (-75.8 and -
79.9 ppm) and a few minor peaks in the 19F NMR spectra coming from tautomerization of ethyl
4,4,4-trifluoroacetoacetate (Figure 2A).11,12 As the reaction proceeds, the β-keto ester peaks are
significantly reduced. Initially a peak at -82.3 ppm appears (Figure 2B). When the temperature is
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increased above 80°C, a second peak appears at -83.7 ppm, resulting from intermediate 1b
(Figure 2C). After the reaction is heated overnight, the peak at -82.3 ppm decreases dramatically
and the product (HFC) peak at -66.2 ppm is a major peak (Figure 2D).
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Figure 2. 19F NMR spectra from the synthesis of HFC at A) start of the reaction, B) 2 h, 70 °C
C) 5 h, 90 °C and D) overnight, 90 °C.
Observation of this new peak at -82.3 ppm in the 19F NMR spectra of the reaction led us to
investigate the structure of this compound. Through monitoring the reaction over time at various
temperatures we were able to tailor the reaction conditions to enhance the production of this new
compound (Table 1). Running the reaction between 50 – 80 °C yielded this new compound as
the major component. This compound is heat labile therefore identification depended heavily on
NMR experiments (1H, 13C and 2D (HSQC and HMBC) to determine the structure of
intermediate 2b (Figure 1).
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Table 1. Optimization Conditions for 2b
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time
(h)
1
T
(°C)
50
50
60
60
60
70
70
70
80
80
80
yielda (%)
2b
1
1b
0
HFC
0
0
0
0
0
0
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2
8
0
1
38
80
87
81
86
80
82
82
81
0
2
3
3
4
1
4
2
8
0
0
3
18
7
1
1
1
1
2
11
17
3
aDetermined by 19F NMR
Characterization of compound 2b by 1H-13C HSQC revealed the single carbon-13 signal at δC
38.4 ppm correlated to the diastereotopic methylene protons at δH 2.90 and 3.68 ppm. The ethyl
ester signals at δH 1.04 and 3.93 ppm corresponded to carbon signals δC 13.8 and 59.6 ppm,
respectively. The aromatic protons at δH 6.22, 6.23 and 7.24 ppm corresponded to carbon signals
δC 106.2, 102.9 and 130.1 ppm, respectively. The three hydroxyl peaks at δH 6.69, 9.33 and 9.69
ppm did not correlate to any carbon peaks, as expected.
Monitoring the reaction by 19F NMR limits this method to only fluorinated compounds. The
reaction can also be monitored by 1H NMR due to a number of distinct resonances displayed by
each component in solution. At the start of the reaction there are a grouping of quartets at 4.0-4.2
ppm from the tautomerization of ethyl 4,4,4-trifluoroacetoacetate. As compound 2b forms, a pair
of doublets are observed at 2.9 and 3.5 ppm arising from the diastereotopic methylene protons
and a quartet at 3.9 ppm from the ethyl ester. Compound 1b has a pair of doublets at 3.1 and 3.3
ppm, arising from its diastereotopic methylene protons. As compound 2b is converted to
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compound 1b, ethanol is produced, which can be observed by a quartet around 3.4 ppm. All
peaks are well resolved and therefore can be integrated to measure conversion. The solvent peaks
from toluene do not interfere with any of these peaks but water can interfere with one of the
doublets from compound 1b.
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In the range of 9 – 11 ppm, the aromatic hydroxyl peaks can be used to monitor the progress of
the reaction. Resorcinol has a single hydroxyl peak at 9.1 ppm. Compound 2b has two phenolic
peaks at 9.3 and 9.7 ppm. Compound 1b has a single aromatic hydroxyl around 10.1 ppm. HFC
has a phenolic peak around 11 ppm. Caution should be used in this range because the peaks arise
from acidic protons where the peak position and width is highly concentration dependent.
Building upon the success of monitoring the synthesis of HFC starting with ethyl 4,4,4-trifluoro-
acetoacetate, we attempted to apply this method to the synthesis of 7-hydroxy-4-methylcoumarin
with ethyl acetoacetate. The reaction with ethyl acetoacetate proceeds quicker and with less
catalyst.9 Even at lower temperatures, approaching the solubility limit of resorcinol, well defined
intermediates were not observed. Our results are consistent with previous experimental
literature.7,6 The theoretical study done with ethyl acetoacetate by Daru and Stirling3 shows that
in the Gibbs free energy profile the intermediates are not very stable and not separated from the
next step by a large energy barrier, therefore less likely to be observable.
The observation and isolation of compounds 2b and 1b imply that the synthesis of HFC by the
Pechmann condensation reaction proceeds through an electrophilic aromatic substitution
reaction, followed by transesterification ending in dehydration (Path C). To determine if this path
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applies to all coumarins synthesized by the Pechmann condensation reaction the synthesis of a
wide variety of coumarins will be monitored. It is possible that the strong electron withdrawing
effects of the CF3 group may force the reaction to take a specific mechanistic path. A large
number of catalysts have been shown to be effective for this reaction and different types of
catalyst may proceed through a different mechanistic path. Therefore the role of all types of
catalysts used for the Pechmann reaction will be further investigated.
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NMR is increasingly being used in process development as a way to provide in situ analysis and
real time data on the progress of a reaction and identify undesired byproducts.13 Our experiment
adds to the literature on NMR reaction monitoring by observing the formation and disappearance
of intermediates to determine the mechanism of the reaction.14 Monitoring a reaction by NMR
also helps to identify reaction conditions that yield each intermediate and can be used for kinetic
studies.15
EXPERIMENTAL SECTION
The procedure has been modified from previous literature.10 Iodine (317 mg, 1.25 mmol, 25
mol%) was added into a mixture of resorcinol (550 mg, 5 mmol), ethyl 4,4,4-trifluoroaceto-
acetate (880 µL, 6 mmol) and trifluorotoluene (60 µL, 0.5 mmol) in toluene (1 mL). The reaction
was heated and stirred. A 50 µL sample was removed at designated time periods, diluted with
600 µL DMSO-d6 and NMR spectra was acquired. Upon completion, the reaction mixture was
cooled to room temperature, diluted with 10 mL of ethyl acetate, and washed with 10 mL of
distilled water. The combined organic layers were dried over anhydrous sodium sulfate and the
solvent removed under vacuum. Purification was performed via column chromatography
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(hexane/ethyl acetate, 8:2). 1H, 13C{1H}, 19F, 1H-13C HSQC and 1H-13C HMBC NMR spectra
were obtained for both intermediates.
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Ethyl 3-(2,4-dihydroxyphenyl)-4,4,4-trifluoro-3-hydroxybutanoate (2b). The reaction was
heated at 60°C and stirred for 3 h. Clear oil (0.94 g, 63%). Rf = 0.33 (hexane/EtOAc, 1:1) IR
(film) ν 3360, 2988, 1713, 1627, 1603, 1167 cm-1 1H NMR (500 MHz, DMSO) δ 1.04 (t, J =
7.10 Hz, 3H, CH3), 2.90 (d, J = 15.9 Hz, 1H), 3.68 (d, J = 15.9 Hz, 1H), 3.93 (q, J1=7.10 Hz,
J2=2.10 Hz, 2H, CH2 ), 6.23 (m, 2H), 6.69 (s, 1H, OH), 7.24 (d, 1H, J = 9.2 Hz), 9.33 (s, 1H,
OH), 9.69 (s, 1H, OH). 13C{1H} NMR (125 MHz, DMSO) δ 13.8, 38.4, 59.6, 74.7 (q, J = 28.6
Hz), 102.9, 106.2, 112.3, 125.6 (q, J = 28.6 Hz), 130.1, 156.1, 158.3, 168.5. 19F NMR (DMSO) δ
-82.3 ppm. Sample thermally unstable for MS.
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ACKNOWLEDGMENT
This research was supported by Faculty Development Grants and a Frederick Bettelheim
Research Award from Adelphi University to MVP. We are grateful to Dr. Brian Stockman for
his NMR expertise and Dr. András Stirling for our discussions.
ASSOCIATED CONTENT
Supporting Information
Copies of 1H, 13C, 19F, HSQC and HMBC NMR spectra of compounds 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
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