5,5′-Dimethyl-3,3′-azoisoxazole as a new heterogeneous azo reagent for esterification of phenols and selective esterification of benzylic alcohols under Mitsunobu conditions

Article abstract of DOI:10.1039/c004357e

5,5′-Dimethyl-3,3′-azoisoxazole is used as a new efficient heterogeneous azo reagent for the highly selective esterification of primary and secondary benzylic alcohols and phenols with aliphatic and aromatic carboxylic acids via Mitsunobu protocols. The reaction is highly selective for primary benzylic alcohols versus secondary ones, aliphatic alcohols and also phenols. The isoxazole hydrazine byproduct can be simply isolated by filtration and recycled to its azoisoxazole by oxidation.

Full text of DOI:10.1039/c004357e

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
5,5¢-Dimethyl-3,3¢-azoisoxazole as a new heterogeneous azo reagent for
esterification of phenols and selective esterification of benzylic alcohols under
Mitsunobu conditions†
Nasser Iranpoor,* Habib Firouzabadi* and Dariush Khalili
Received 17th March 2010, Accepted 17th June 2010
DOI: 10.1039/c004357e
5,5¢-Dimethyl-3,3¢-azoisoxazole is used as a new efficient heterogeneous azo reagent for the highly
selective esterification of primary and secondary benzylic alcohols and phenols with aliphatic and
aromatic carboxylic acids via Mitsunobu protocols. The reaction is highly selective for primary benzylic
alcohols versus secondary ones, aliphatic alcohols and also phenols. The isoxazole hydrazine byproduct
can be simply isolated by filtration and recycled to its azoisoxazole by oxidation.
that do not require chromatographic purification of the desired
Introduction
product.12 Besides, polymer-supported reagents,13 fluorous azo
There are a number of chemical transformations that are highly
and phosphine reagents whose by-products could be separated
profitable from a synthetic point of view, but often suffer
either by fluorous flash chromatography or fluorous solid-phase
separation,14 using acidic or basic workup of reaction mixture15
and ring opening metathesis (ROM)16 are other assortments that
provide solutions for the separation problem in the Mitsunobu
reaction. However, the precious nature of many of these reagents,
in connection with the scarcity of their commercial availability,
addition of time and limitation of scope probably precludes
their use in usual organic synthesis. The mentioned laborious
purification encountered with this reaction and motivation for
extension of this important reaction in our research group,17
prompted us to design new heterogeneous azo reagents that
suggest an attractive approach to solve the separation problem
in the Mitsunobu reaction. After an extensive survey of the
literature, we were unable to gain any examples detailing the use
of heterogeneous azo reagents in the Mitsunobu reaction. This
work is the first successful attempt to apply these reagents that
are heterogeneous in essence in the Mitsunobu reaction. So, we
herein report our observation on the synthesis and use of the new
azo compound 5,5¢-dimethyl-3,3¢-azoisoxazole, for the prepara-
tion of a series of benzylic and phenolic esters in Mitsunobu
reaction.
from difficult purification steps. Infamous in this respect is
the Mitsunobu reaction. In the Mitsunobu reaction, using the
combination of a phosphine and azodicarboxylates activates
alcohols (ROH) for attack by a range of relatively acidic pronu-
cleophiles (NuH) to form the dehydrated coupled products (Nu–
R).1 Truly, the Mitsunobu reaction is a versatile method for the
in situ conversion of aliphatic alcohols into alkylating agents
under mild conditions. Although the Mitsunobu reaction was
originally used for esterification,2 a wide range of compounds
that include amines,3 azides,4 ethers,5 cyanides,6 thiocyanides,7
thioesters,8 and thioethers9 can be synthesized using similar
protocols. This reaction historically faced purification challenges
and often haunts the chemist in the isolation of the desired
product. The separation of the reagent-derived byproducts, here,
phosphine oxide and hydrazinodicarboxylate, from the desired
condensation product is almost invariably the most time and
resource consuming part of the Mitsunobu reaction. Due to the
wide scope of the Mitsunobu reaction, a set of strategies were
elaborated which provide quicker work-up procedures instead of
using the tedious and time demanding classical chromatographic
methods for the separation of products and spent reagents, and
introduce alternatives to DEAD and triphenylphosphine (TPP).
These efforts were latterly reviewed.10 For example, Sugimura
and co-workers recently published the details of the preparation
and handling of di-2-methoxyethyl azodicarboxylate (DMEAD)
in esterification reactions. The hydrazine of DMEAD can be
mostly removed by a simple extraction with neutral water.11 The
ferrocenyl-tagged triphenylphosphine reagent has been used to-
gether with di-tert-butylazodicarboxylate in Mitsunobu reactions
Results and discussion
We have recently reported the first use of azo pyridines for the
Mitsunobu reaction.18 Among them, the use of 4,4¢-azopyridine
offers rapid purification by simple filtration of the produced
insoluble hydrazine. Furthermore, we have already found that N-
alkyl salts of azopyridines have lower reactivity compared to that
of azo pyridines in esterification processes. This study points to the
great electronic effect of the pyridinium rings on the basicity of the
obtained adduct from the reaction of PPh3 and the azo reagent.
Based on this observation, we aimed to investigate whether aryl
azo compounds including two heteroatoms such as N, O or S
might behave similarly to N-alkyl pyridinium salts or not. Among
the reported methods for the preparation of azo compounds19
from deactivated aromatic amines, the cheap and readily available
Department of Chemistry, College of Sciences, Shiraz University, Shi-
raz 71454, Iran. E-mail: iranpoor@susc.ac.ir, firouzabadi@susc.ac.ir;
Fax: (+)98 711 2280926; Tel: +(98)7112284822
† Electronic supplementary information (ESI) available: Detailed experi-
mental procedure, more examples, characterization data for azo reagent
2d and all ester products, and copies of 1H or 13C NMR spectra. See DOI:
10.1039/c004357e
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Table 1 Synthesis of symmetrical azo compounds 2a–d
azobenzothiazole 2b and 2c gave the desired ester in 13 and
15% yields. In the case of using 2a, no reaction took place
(Table 2, entry 1). This difference in reactivity could be due to
the presence of more electronegative atoms (N and O) in 2d
which makes its azo group more prone to electrophilic addition
of PPh3. Therefore, 5,5¢-dimethyl-3,3¢-azoisoxazole was selected
as the reagent of choice for transformation of acids/alcohols to
their corresponding esters under Mitsunobu reaction conditions
offering improved reactivity and yields. Initially, we used different
organic solvents such as CH2Cl2, CHCl3, Et2O, THF and CH3CN
to find the most suitable one. The best results were achieved using
acetonitrile in terms of time and yield. Moreover, we found that in
acetonitrile, 5,5¢-dimethyl-3,3¢-azoisoxazole 2d is not soluble and
its hydrazine can easily be removed from the reaction mixture by
simple filtration. This hydrazine byproduct can be simply recycled
to its azo compound 2d by its oxidation with iodosobenzene
diacetate in DMSO at 60 ◦C for 10 h.
Further optimization studies on the esterification of benzyl
alcohol with benzoic acid revealed that using 1.1 equiv. of both 2d
and PPh3, 1.3 equiv. of alcohol and 1.0 equiv. of acid gave a quan-
titative conversion of the acid/alcohol to benzyl benzoate. In an
attempt to convert primary and secondary aliphatic alcohols such
as 2-phenylethanol, 1-octanol and 2-octanol under Mitsunobu
conditions to their esters, we observed that no reaction had taken
place. Consequently, azo 2d can be used as a selective reagent for
esterification of only benzylic alcohols in Mitsunobu processes.
We also probed the effect of sequential addition of azo, PPh3
and acid in our study. Treating the preformed azo 2d/PPh3 adduct
sequentially with a solution of acid followed by addition of alcohol
gave nearly identical results to the addition of PPh3 to the mixture
of acid/azo 2d, followed by addition of alcohol and no distinct
Entry
ArNH2
ArN NAr
Time/h
Yield (%)
1
2
3
4
1a
1b
1c
1d
2a
2b
2c
2d
1
1
1
1
15
20
35
60
sodium hypochlorite (bleach),20 has been found to be the most
suitable one. Our initial investigation centered on the synthesis of
a series of new symmetrical heteroaromatic azo compounds. The
symmetrical azo compounds 2a–d (see Scheme 1 for structures)
were synthesized by the coupling of their readily available aromatic
amines 1a–d using 6–14% aqueous solution of NaOCl. We started
with the coupling of 2-aminobenzimidazole 1a to produce 2,2¢-
azobenzimidazole 2a, but the isolated yield for the synthesis of 2a
was quite low (15%, Table 1).
Oxidation of 2-aminothiazole 1b as a starting amine yielded 2,2¢-
azothiazole 2b in low yield (20%, Table 1). Similarly, treatment
of 2-aminobenzothiazole 1c and 3-amino-5-methyl-isoxazole 1d
with sodium hypochlorite at 0 ◦C afforded 2,2¢-azobenzothiazole
2c and 5,5¢-dimethyl-3,3¢-azoisoxazole 2d in 35% and 60% yield,
respectively. We then studied the applicability of these azo
compounds (2a–d) for esterification of benzyl alcohol with benzoic
acid as a model reaction. The results of esterification obtained
with these azo reagents in the presence of triphenylphosphine in
refluxing acetonitrile are shown in Table 2.
When 5,5¢-dimethyl-3,3¢-azoisoxazole 2d was used, a clear
difference in efficiency was observed. Indeed the use of azo 2d
provided the benzyl benzoate 3g in 89% yield. In contrast, 2,2¢-
Scheme 1
Table 2 Azo compounds 2a–d for esterification of benzyl alcohol with benzoic acid under Mitsunobu conditions
Entry
Azo
Time/h
Isolated yield of 3g (%)
1
2
3
4
2a
2b
2c
2d
24
24
24
6.5
0
13
15
89
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Scheme 2
improvement in yield was observed. With the optimization studies
completed, we examined a variety of primary and secondary
benzylic alcohols with electron-donating and -withdrawing groups
on the phenyl fragment and various aromatic and aliphatic acids
(Scheme 2, Table 3).
The reaction proved to be quite efficient with various acids and
phenols. The results are summarized in Table 4.
The conversion of phenol to phenyl benzoate was chosen to
optimize the reaction conditions. Consistent with our previous
optimization reactions, we selected refluxing acetonitrile as the
solvent. Subjecting 1.2 equivalents of phenol to 1.0 equivalent
of acid and 1.2 equivalents each of PPh3 and 5,5¢-dimethyl-3,3¢-
azoisoxazole 2d resulted in 70% conversion to phenyl benzoate
4a and incomplete consumption of azo 2d and PPh3. Decreasing
the number of equivalents of azo 2d and PPh3 to 1.1 and
increasing phenol to 1.3 equiv. led to the clean conversion of
phenol to 4a, isolated in 85% yield after purification. These
reaction conditions were applied to a variety of substrates
(Table 4).
After the successful esterification of the simple phenol (entry
1), we found that a variety of substitution patterns perform
the reaction in good to high yields. Consequently, we found
that the overall yield in this reaction depends mostly on the
electronic nature of substituents on the phenol in such a manner
that electron-donating groups increase the yield of esterification
(Table 4, entries 2–4 compared to entries 5 and 6). The lowered
reactivity of ortho-substituted alcohols is attributed to the in-
creased steric hindrance around the –OH functional group. To
investigate this further in the reaction of phenols with acids, a
set of reactions using more encumbered phenols were performed.
In this regard, 2,6-dimethylphenol, 2-isopropylphenol and 2-tert-
butylphenol (Table 4, entries 7–9) were selected as more sterically
demanding phenols in this study. Conversion of these phenols
into their esters 4g–4i was noticeably lower as determined by
isolated yields after column chromatography. These results show
that, similar to DEAD and DIAD, azo reagent 2d is also sensitive
to steric factors.25
Since esterification reactions of 4-nitrobenzoic acid are usually
studied in Mitsunobu reactions,21 we first applied our reaction con-
ditions to this substrate. In its reactions, both electron-rich (–OMe
and –Me) and electron-poor (–Cl and –NO2) benzylic alcohols
gave their corresponding esters in high yields. The applicability of
the azo 2d was also briefly probed by the reaction of the less acidic
nucleophile, benzoic acid, with a series of benzyl alcohols that
have different electronic and steric effects. There is little difference
among donating groups such as –OMe and –Me in time and yield
of esterification (Table 3, entries 8, 9 and 10). In addition, steric
effects via introduction of an ortho group to the benzyl alcohol
were examined. The results (entry 8 compared to 9) indicate that
the yield of the ester formation dropped slightly as the bulk of the
alcohol increases by ortho substituent. The stereospecificity of the
reaction with 5,5¢-dimethyl-3,3¢-azoisoxazole 2d was also studied
using (R)-1-phenylethanol as a chiral secondary alcohol. By this
method, the corresponding esters (S)-1-phenyl-1-ethyl benzoate
3m was obtained in moderate yield (55%) with perfect inversion of
stereochemistry. The configuration of the product was established
by comparison of its optical rotation with the literature.22 The
reaction was also be carried out successfully when we used a
less electron-deficient acid (lower pKa) such as 4-methylbenzoic
acid as a nucleophile. Further results, which illustrate the scope
of this reagent, showed that aliphatic acids such as propanoic
acid (Table 3, entries 18 and 19), crotonic acid (Table 3, entry
20) and undec-10-enoic acid (Table 3, entry 21) could be cleanly
benzylated to afford the expected products 3r–3u. Owing to the
importance and application of fatty acid esters in industrials,23 we
applied successfully our reaction conditions for the esterification
of stearic and oleic acids with benzyl alcohol (Table 3, entries 22
and 23).
As far as we know, none of the reported methods on Mitsunobu
esterification using DEAD, and also our previous report using
azopyridines,18 have shown any selectivity between different classes
of alcohols, so we hoped to observe some selectivity with our new
heterogeneous azo compound 2d. To investigate the selectivity of
esterification of alcohols using 5,5¢-dimethyl-3,3¢-azoisoxazole 2d,
we set up some experiments using typical reaction conditions with
equimolar amounts of different alcohols and observed excellent
selectivity for esterification of benzylic alcohols versus aliphatic
ones, primary benzylic alcohols vs. secondary ones and also
benzylic alcohols vs. phenols. The results are summarized in
Table 5.
Generally, the phenolic –OH group also participates as an acidic
component in the Mitsunobu reaction, but it is surprising that
very few reports have focused on the deliberate use of phenols as
a nucleophile in the Mitsunobu reaction.18,24 To gain insight into
the efficiency of azo reagent 2d, various phenols with electron-
withdrawing and electron-donating groups were subjected to
the Mitsunobu esterification in association with aliphatic and
aromatic acids (Scheme 3).
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Table 3 Esterification of various benzylic alcohols under Mitsunobu reaction promoted by 2d
Entry
1
RCO2H
ROH
Product
Time/h
5
Yield (%)
89
2
3
4
5
6
4.5
5.5
6.5
6.5
8
89
86
83
85
80
7
8
6.5
6
89
87
9
5.5
6.5
8
90
85
83
77
10
11
12
9
13
11
55
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Table 3 (Contd.)
Entry
14
RCO2H
ROH
Product
Time/h
7
Yield (%)
82
15
16
17
6.5
8
86
82
73
9
18
19
20
7.5
9.5
9.5
85
80
73
21
22
23
9
9
8
70
70
71
Scheme 3
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Table 4 Synthesis of phenolic esters using 5,5¢-dimethyl-3,3¢-azoisoxazole 2d
Entry
1
Acid
Phenol
Product
Time/h
9.5
Yield (%)
85
2
3
8.5
9
88
88
4
5
10
86
80
10.5
6
7
12
70
64
11.5
8
9
11
63
59
11.5
10
7.5
96
11
12
8.5
90
85
10.5
13
11
83
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Table 5 Selective conversion of various alcohols with benzoic acid in
binary mixtures to corresponding esters using 2d under Mitsunobu
conditionsa
Experimental
General procedure for the synthesis of azo compounds
Time/ Yield
h
Azo compounds (2a–d) were prepared by oxidative coupling
of their corresponding amines (1a–d) by sodium hypochlorite
solution. A 50 mL aliquot of a cold solution of heterocyclic
aromatic amine (25 mmol) in THF was added dropwise to mixture
of 120 mL of sodium hypochlorite and 30 mL of water. The mixture
was stirred at 0 ◦C as a colored precipitate formed. Filtration
was performed a few minutes after the end of addition. The azo
participates were collected and were used in our reactions without
any purification.
Entry Binary mixture
Product: conversion (%)b
(%)c
1
2
3
4
Benzyl alcohol
1-Octanol
Benzyl alcohol
2-Octanol
Benzyl alcohol
1-Phenylethanol
1-Phenylethanol
Benzyl benzoate: 100
1-Octyl benzoate: 0
Benzyl benzoate: 100
2-Octyl benzoate: 0
Benzyl benzoate: 100
1-Phenylethyl benzoate: 4
1-Phenylethyl benzoate:
100
1-Octyl benzoate: 0
1-Phenylethyl benzoate:
100
2-Octyl benzoate: 0
Benzyl benzoate: 100
7
85
6.5
7
81
84
11
70
1-Octanol
1-Phenylethanol
5
6
7
11
6
69
89
General procedure for the synthesis of benzylic esters using azo (2d)
2-Octanol
Benzyl alcohol
4-Methoxy phenol 4-Methoxypheny benzoate:
0
4-Hydroxy benzyl
alcohol
To a flask containing a stirred mixture of acid (1 mmol) and azo
compound (1.1 mmol) in refluxing acetonitrile (4 mL) was added
PPh3 (1.1 mmol). Benzylic alcohol (1.3 mmol) was then added to
the reaction mixture. The reaction was monitored by TLC in n-
hexane–ethyl acetate (9 : 1). After completion of the reaction, the
solvent was evaporated and diethyl ether (15 mL) was added. The
insoluble hydrazine was filtered off and the filtrate was evaporated
to give a viscous oil. The purification was achieved using a short
column of silica gel eluted with n-hexane–ethyl acetate (9 : 1).
5.5
85d
a The molar ratio of Ph3P/2d/binary mixture/acid is 1.1/1.1/1.3/1.0.
b Analysis by 1H-NMR and GLC. c Isolated yield. d Only one product
was isolated.
General procedure for the synthesis of phenolic esters using azo (2d)
In Scheme 4, we offer a plausible mechanistic pathway and
intermediates involved in the Mitsunobu reaction using azo 5,5¢-
dimethyl-3,3¢-azoisoxazole 2d based on the experimental data.
In addition to the observed optical rotation which supports
the inversion of configuration, the isolation of triphenyl oxide and
hydrazine by-products are considered as evidence for the proposed
mechanism.
Acid (1 mmol) and azo 2d (1.1 mmol) were placed in a one-
necked round bottom flask, acetonitrile (4 mL) was added to the
mixture and heated at 80 ◦C. Triphenyl phosphine (1.1 mmol)
and then phenol (1.3 mmol) were added to the reaction mixture.
It was stirred for several hours. The progress of the reaction was
monitored by TLC. After disappearance of the starting material,
the solvent was evaporated. Diethyl ether (15 mL) was added and
Scheme 4
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the solid hydrazine was filtered off. The residue was subjected
to flash chromatography over silica gel by using n-hexane–ethyl
acetate as eluent to give the corresponding ester.
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Conclusions
In this study, we have introduced a new heterogeneous azo
reagent 5,5¢-dimethyl-3,3¢-azoisoxazole for the highly selective
synthesis of benzylic and phenolic ester. The easy synthesis of this
reagent in one step, facility in product isolation, separation of the
hydrazine by-products and its recyclability are the most important
advantages of this new azo reagent. In comparison with DEAD
and azopyridines, we have demonstrated that combination of 5,5¢-
dimethyl-3,3¢-azoisoxazole with PPh3 is a highly selective system
for the preparation of esters of primary benzylic alcohols in the
presence of secondary ones and also phenols.
Acknowledgements
We thank the Shiraz University Research Council for the financial
support of this study.
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