Mild, rapid, and inexpensive microwave-assisted synthesis of allylic and propargylic esters

Article abstract of DOI:10.1080/00397911.2011.640970

A variety of allylic and propargylic esters were rapidly prepared via microwave heating of their corresponding mixed anhydride derived from pivaloyl chloride. The reaction conditions were modified to account for the sterics of the alcohol and the electronics of the carboxylic acid. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.640970

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Mild, Rapid, and Inexpensive Microwave-
Assisted Synthesis of Allylic and
Propargylic Esters
Monica A. Gill a & Jeffrey M. Manthorpe a
a Department of Chemistry, Carleton University, Ottawa, Ontario,
Canada
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To cite this article: Monica A. Gill & Jeffrey M. Manthorpe (2013): Mild, Rapid, and Inexpensive
Microwave-Assisted Synthesis of Allylic and Propargylic Esters, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:10, 1460-1468
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Synthetic Communications1, 43: 1460–1468, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.640970
MILD, RAPID, AND INEXPENSIVE MICROWAVE-
ASSISTED SYNTHESIS OF ALLYLIC AND
PROPARGYLIC ESTERS
Monica A. Gill and Jeffrey M. Manthorpe
Department of Chemistry, Carleton University, Ottawa, Ontario, Canada
GRAPHICAL ABSTRACT
Abstract A variety of allylic and propargylic esters were rapidly prepared via microwave
heating of their corresponding mixed anhydride derived from pivaloyl chloride. The reaction
conditions were modified to account for the sterics of the alcohol and the electronics of the
carboxylic acid.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Allylic esters; esterification; microwave-assisted synthesis; mixed anhydride;
propargylic esters
INTRODUCTION
The formation of esters is one of the most established and thoroughly studied
reactions in organic chemistry.[1] Widely used techniques employ carbodiimides,
phosphorous reagents such as BOP-Cl, activated esters, acid chlorides, or anhydrides.
Allylic esters are a particularly important class of compounds because of their utility
in an array of transformations, including allylic rearrangements,[2] such as the Claisen
rearrangement,[3] metal-catalyzed allylic alkylations,[4] including decarboxylative
allylations,[5] cycloadditions,[6] epoxidations,[7] and dihydroxylations.[8] However,
allylic esters can be difficult to prepare[9] and handle because of their acid sensitivity
and propensity toward nucleophilic substitution.[10]
Received November 7, 2011.
Address correspondence to Jeffrey M. Manthorpe, Carleton University, Department of Chemistry,
203 Steacie Building, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada. E-mail: jeffrey_manthorpe@
carleton.ca
1460

SYNTHESIS OF ALLYLIC AND PROPARGYLIC ESTERS
1461
RESULTS AND DISCUSSION
As part of an ongoing research project, we required a facile route to allylic
esters. We had ready access to ethyl ester 1 (Scheme 1)[11] and we first investigated
the possibility of a titanium(IV) isopropoxide–catalyzed transesterification. As can
be seen from Table 1, there was a considerable solvent effect in the reaction, with
tetrahydrofuran (THF) promoting allyl ester formation but toluene suppressing it.
Interestingly, the formation of isopropyl ester 6 was also observed and in some cases
was the major product, particularly in toluene. Frustratingly, we were never able to
obtain a sufficient conversion to the allyl ester to make this a viable pathway, even
with very large excesses of allyl alcohol (3). Similar observations have been reported
previously.[12] Attempts to transesterify the corresponding sulfone 2 offered no
improvement. Use of boric acid catalysis[13] returned only starting material.
Ethyl ester 1 was cleanly saponified to afford a-thiacarboxylic acid 8, which was
used as our prototypical substrate (Scheme 2) as we sought to find a straightforward
esterification method. To our consternation, this turned out to be far more challeng-
ing than anticipated. Our initial investigation (Table 2) explored the use of acid chlor-
ide 9 (entries 1 and 2) with heterogeneous and homogeneous bases, which afforded
only modest yields of 4. We also explored the esterification of acid 8 using diisopro-
pylcarbodiimide (DIC). In the presence of catalytic amounts of dimethylaminopyri-
dine (DMAP), DIC afforded 4 as a minor constituent of a complex mixture. The
use of DIC with stoichiometric DMAP did not produce 4. 2-Chloro-N-methylpyridi-
nium iodide (Mukaiyama’s reagent)[14] afforded only a small amount of the desired
product in a complex mixture.
Given the failure of coupling agents, we then decided to explore the use of com-
mercially available chloroformates to form mixed anhydrides, rationalizing that
attack would occur preferentially on the carboxylate carbonyl (rather than on the car-
bonate carbonyl) of the mixed anhydride. In practice, ethyl chloroformate afforded
only poor amounts of 4 when catalytic amounts of DMAP were used, and without
DMAP only the ethyl ester 1 was observed, even in the presence of excess allyl alcohol
(entries 3 and 4). Surprisingly, even the more electron-poor phenyl chloroformate
heavily favored the phenyl ester 10 (entries 5 and 6). Use of di-tert-butyldicarbonate
produced significant quantities of the highly hindered tert-butyl ester (entries 7 and 8).
Given the propensity of other nucleophiles to interfere with our desired acyla-
tion, we then opted to explore the use of a ‘‘true’’ mixed anhydride and drive the
carbonyl regioselectivity via sterics. Thus we selected pivaloyl chloride (PivCl) as
our mixed anhydride precursor. Unlike other acid chlorides used for mixed anhydride
prepartion, such as 2,4,6-trichlorobenzoyl chloride,[15,16] PivCl is very inexpensive
(ꢀ$18.50=mol), and the tert-butyl group provides significant steric hindrance. This
Scheme 1. Preparation of esters 1 and 2.

1462
M. A. GILL AND J. M. MANTHORPE
Table 1. Transesterification of esters 1 and 2
Starting
material
Equiv. 3
Catalysta
Solvent
Temp. (ꢁC)
Time
1=2: 4=5: 6=7b
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
5
10
5
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
B(OH)3
THF
THF
65
65
18 h
18 h
8:71:21
7:67:26
17:0:83
PhMe
PhMe
THF
110
110
18 h
21 h
10
5
14:3:83
20:59:21
150 (mwave)
150 (mwave)
170 (mwave)
40
10 min
20 min
10 min
18 h
5
THF
THF
19:53:28
17:54:29
5
5
CH2Cl2
THF
14: 55: 31
0: 0: 100
18:7:75
10
5
65
110
21 h
21 h
PhMe
PhMe
THF
10
5
110
21 h
11:5:84
12:60: 28
15:59:26
150 (mwave)
170 (mwave)
170 (mwave)
110
10 min
10 min
20 min
5 h
5
THF
THF
PhMe
5
1.5
Partial decomposition
100:0:0
aCatalyst loading was 50 mol% for Ti(O-i-Pr)4 and 10 mol% for B(OH)3.
bRatios determined by integration of 1H NMR spectra of crude reaction mixtures.
approach led to clean formation of the allyl ester, both with and without catalytic
amounts of DMAP (entries 9–11). This tactic presented several advantages: the major
by-products (base, base hydrochloride, and pivalic acid) all can be readily removed by
extraction or facile chromatography, and by using a mixed anhydride, we do not lose
any of our desired acid component, unlike when a symmetrical anhydride is used.[10a]
This has obvious implications for the use of complex and precious carboxylic acids.
The undesired allylic pivalate 12 is formed by either incomplete mixed anhydride
formation or addition of the alcohol to the more hindered carbonyl and can be
removed by evaporation or chromatography. We evaluated the rate of mixed an
hydride formation by combining acid 8, PivCl, and pyridine in an NMR tube at
0.5 M in CDCl3 and observed that both 8 and PivCl were consumed within 5 min, thus
indicating that mixed anhydride formation is rapid and formation of 12 is due to
diminished regioselecitivity.
Scheme 2. Preparation of acid 8 and acyl chloride 9.

SYNTHESIS OF ALLYLIC AND PROPARGYLIC ESTERS
Table 2. Attempted esterification of 8 and 9
1463
Starting
Entry material
Coupling
agent
Equiv.
3
Conditions
Result
1
2
3
9
9
8
None
3
3
3
K2CO3 (5 equiv.);
CH2Cl2 (0.6 M),
0ꢂ23 ꢁC, 18 h
Py (2 equiv.), DMAP
(0.1 equiv.), CH2Cl2
(0.6 M), 0ꢂ23 ꢁC, 18 h
4 (40% yield)
None
Variable yields
of 4 (40–60%)
Ethyl chloroformate
(1.0 equiv.)
Py (2 equiv.), DMAP
(0.1 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h
4:1 14:86
1 only
4
5
6
8
8
8
Ethyl chloroformate
(1.0 equiv.)
3
3
3
Py (2 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h
Py (2 equiv.), CH2Cl2
Phenyl chloroformate
(1.0 equiv.)
4:10 10:> 90 þ
(0.1 M), 0ꢂ23 ꢁC, 4 h
Py (2 equiv.), DMAP
(0.1 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 4 h
Py (2 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h impurities
Py (2 equiv.), DMAP
(1.0 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h
Py (2 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h
Py (2 equiv.), DMAP
(1.0 equiv.), CH2Cl2
(0.1 M), 0ꢂ23 ꢁC, 18 h
unidentifiable impurities
Phenyl chloroformate
(1.0 equiv.)
4:10 12:88 þ unidentifiable
impurities
7
8
8
8
Boc2O
(1.1 equiv.)
3
3
4:11 70:30 þ unidentifiable
Boc2O
(1.1 equiv)
4:11 9:91 þ unidentifiable
impurities
9
8
8
Pivaloyl chloride
(1.0 equiv.)
3
3
4:8 74:26
10
Pivaloyl chloride
(1.0 equiv.)
4:8 79:21
11
8
Pivaloyl chloride
(1.0 equiv.)
3
Py (2 equiv.), DMAP
(1.0 equiv.), CH2Cl2
(0.1 M), reflux, 18 h
74% isolated 4
At this point we optimized the reaction conditions (Table 3). A brief solvent
screen showed no significant advantages to other solvents, and we therefore opted
to stay with CH2Cl2; however, we did seek to shorten the reaction time. Given that
the boiling point of CH2Cl2 is 40 ꢁC, there is little room for accelerating the reaction
by increasing the temperature. However, microwave heating in a sealed vial provides
a rapid and simple approach to decreasing the reaction time.
One of the impressive features of this protocol is that it is effective with no
excess of allyl alcohol; only 1 equivalent of acid, PivCl, and alcohol can be used
without detriment to the regioselectivity (entries 1 and 2). In addition, the absence

1464
M. A. GILL AND J. M. MANTHORPE
Table 3. Optimization of esterfication of 8
Equiv.
ROH
Equiv.
Py
Temp
(ꢁC)
Time
(min)
Conc.
8 (M)
Ratio
4:12
Isolated
yield (%) 4
Entry
Additive
1
2
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.25
1.25
10 mol% DMAP
10 mol% DMAP
2
2
2
2
2
2
2
1
2a
2
2
2
2
2
2
100
100
100
120
100
100
100
100
100
60
10
10
10
10
10
10
10
10
10
180
90
45
30
90
180
0.1
0.1
0.1
0.1
0.25
0.5
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
70:30
71:29
79:21
74:26
72:28
77:23
81:19
61:39
11:89
84:16
89:11
83:17
86:14
86:14
81:19
n.d.
n.d.
58
3
No
No
No
No
No
No
No
No
No
No
No
No
No
4
n.d.
66
5
6
69
60
7
8
50
9
n.d.
78
10
11
12
13
14
15
80
80
83
81
80
80
79
74
80
68
a2-Chloropyridine was used in place of pyridine.
of catalytic amounts of DMAP improved the regioselectivity (entries 2 and 3).
Increasing the reaction temperature from 100 to 120 ꢁC led to a modest decrease
in regioselectivity (entries 3 and 4). The optimal reaction concentration (entries 3
and 5–7) was found to be 0.5 M. Even though the regioselectivity was slightly greater
at 1.0 M, the isolated yield of 4 was greatest at 0.5 M. 2-Chloropyridine was found to
be insufficiently basic to effect efficient mixed anhydride formation and thus yielded
primarily 12 via simple acylation of 3 by pivaloyl chloride. A screen of temperature
and reaction time (entries 6 and 10–15) revealed that 80 ꢁC for 90 min was optimal
for both yield and regioselectivity.
With our optimized conditions in hand, we began to explore the scope of the
reaction (Table 4). The reaction proved to be readily scalable to at least 2.0 g
(6.6 mmol) of 8 (entry 2). In addition, a variety of allylic alcohols were successfully
coupled with acid 8, including 2- and 3-substituted primary alcohols (entries 3–5),
as well as secondary and tertiary allylic alcohols (entries 6 and 7); however, the more
electron-poor or sterically encumbered alcohols required longer reaction times to
obtain comparable yields. We were also able to employ a-thiophenylacetic acid 13
(entries 8–10) as the carboxylic acid component under the same reaction conditions.
When we moved to electronically neutral acid 14, no desired product was obtained
(entry 11). Fortunately this was readily remedied by changing the base from pyridine
to DMAP and increasing the reaction temperature to 100 ꢁC (entry 12). Similarly,
benzoic acid 16 afforded poor results with pyridine, but again, replacement of pyri-
dine with DMAP and increasing the reaction temperature resulted in increased yield.

SYNTHESIS OF ALLYLIC AND PROPARGYLIC ESTERS
Table 4. Scope of esterification protocol
1465
Entry
R1CO2H
R2OH
Base
Conditions
Product
Yield (%)
1
2
8
8
3
3
Py
Py
80 ꢁC, 45 min
80 ꢁC, 45 min
4
4
81
86a
3
8
Py
80 ꢁC, 90 min
17
80
4
5
8
8
Py
Py
80 ꢁC, 45 min
80 ꢁC, 90 min
18
19
75
76
6
8
Py
80 ꢁC, 120 min
20
78
7
8
8
Py
Py
80 ꢁC, 120 min
80 ꢁC, 90 min
21
22
73
76
13
3
9
13
Py
Py
80 ꢁC, 90 min
23
73
50
10
13
80 ꢁC, 180 min
24
11
12
14
14
3
3
Py
DMAP
80 ꢁC, 90 min
100 ꢁC, 45 min
25
25
0
25b
13
14
DMAP
100 ꢁC, 45 min
26
75
14
15
16
15
16
16
3
3
3
DMAP
Py
DMAP
100 ꢁC, 45 min
80 ꢁC, 90 min
80 ꢁC, 45 min
27
28
28
76
32
63
(Continued )

1466
M. A. GILL AND J. M. MANTHORPE
Table 4. Continued
Entry
17
R1CO2H
R2OH
Base
Conditions
Product
Yield (%)
66
16
DMAP
100 ꢁC, 120 min
29
18
19
8
Py
80 ꢁC, 90 min
100 ꢁC, 90 min
30
31
74
71
15
DMAP
20
21
8
Py
80 ꢁC, 180 min
100 ꢁC, 180 min
32
33
56
68
15
DMAP
aReaction conducted using 2.0 g (6.6 mmol) of 8.
bThe modest yield is ascribed to the volatility of 25.
This methodology was successfully extended to incorporate the synthesis of
propargylic esters (entries 18–21). The conditions optimized for allylic esters required
little modification, as merely extending the reaction times produced the desired
materials in reasonable yields.
In conclusion, we have developed a simple and rapid microwave-assisted
method for the preparation of allylic and propargylic esters of a variety of carboxylic
acids. While esterification is a well-studied field, a review of the literature shows
that there are few esterification methods that use carboxylic acids and allylic or pro-
pargylic alcohols as starting materials, especially without excess of either the alcohol
or acid component.[1] This approach is advantageous because many allylic alcohols
are commercially available and can be used directly, rather than converted to allylic
halides or haloformates. We believe that the results reported herein constitute a use-
ful and practical contribution to the direct preparation allylic and propargylic esters,
two challenging classes of compounds.
EXPERIMENTAL
All reagents were purchased from Sigma-Aldrich (with the exception of 3,5-
bistrifluoromethylphenol, from Matrix Scientific) and were used as received, without
further purification. Microwave heating was performed in sealed vials in a Biotage
Initiator 2.5 instrument with the absorption level set to ‘‘normal’’. Methylene chlor-
ide was distilled from CaH2 prior to use. Crude products were analyzed by thin-layer
˚
chromatography (TLC) using glass-backed extra hard layer (60-A) TLC plates from

SYNTHESIS OF ALLYLIC AND PROPARGYLIC ESTERS
1467
Silicycle and visualized by fluorescence quenching under ultraviolet (UV) light and=
or staining using potassium permanganate. Flash chromatographic purification of
products was performed either on Silia-P Flash silica gel from Silicycle using a forced
flow of eluent by the method of Still et al.[17] or by automated chromatography on a
Biotage Isolera One equipped with a UV detector. Concentration in vacuo refers to
rotary evaporation at 40 ꢁC at the appropriate pressure. Yields refer to purified and
spectroscopically pure compounds unless explicitly indicated as crude. NMR spectra
were recorded on a Bruker Avance III 300 or Bruker AMX 400-MHz spectrometer.
Chemical shifts are reported in parts per million (ppm). Tetramethylsilane (TMS)
1
was used as the internal standard for H and 13C NMR spectra. 19F NMR spectra
are referenced to trifluorotoluene (ꢂ63.7 ppm). Infrared (IR) spectra were recorded
on a Varian 1000 Scimitar Series or an ABB Bomem MB series spectrometer.
Absorptions are given in wavenumbers (cmꢂ1). High-resolution mass spectrometry
(HRMS) was performed on a Kratos Concept-11A mass spectrometer with an elec-
tron beam of 70 eV at the Ottawa-Carleton Mass Spectrometry Center. Compound 1
was prepared via a literature method.[11]
General Microwave Esterification Procedure
A Biotage microwave vial was charged with a Teflon-coated stirbar, carboxylic
acid (1.0 equiv), and methylene chloride (0.5 M relative to carboxylic acid). The
vial was sealed with a crimp-top lid equipped with a septum. In sequence, pivaloyl
chloride (1.0 equiv), base (2.0 equiv), and alcohol (1.0 equiv) were added by syringe
through the septum. In the case of solid bases or alcohols, the final sealing of the tube
was delayed until addition was complete. The vial was heated for the indicated time at
the indicated temperature in the microwave reactor. The reaction mixture was then
diluted with diethyl ether, causing a white precipitate to form. This was washed with
ice-cold 0.1 M H2SO4. The layers were separated, and the aqueous phase was
extracted again with diethyl ether. The combined organic extracts were washed once
with brine, dried over MgSO4, filtered, and concentrated in vacuo. The crude product
was purified via flash chromatography using 5% EtOAc=hexanes.
Supporting Information Available
Detailed experimental procedures and characterization of acid 8 and esters 4
and 17 to 33 are available online in the Supplementary Information.
ACKNOWLEDGMENTS
This research has been supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canada Foundation for Innovation,
the Ontario Research Fund, and Carleton University.
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1468
M. A. GILL AND J. M. MANTHORPE
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