Manganese-mediated acetylation of alcohols, phenols, thiols, and amines utilizing acetic anhydride

Article abstract of DOI:10.1080/00397911.2019.1650282

Manganese(II) chloride-catalyzed acetylation of alcohols, phenols thiols and amines with acetic anhydride is reported. This method is environment-friendly and economically viable as it involves inexpensive, relatively benign catalyst, mild reaction condition, and simple workup. Acetylation is performed under the solvent-free condition at ambient temperature and acetylated products obtained in good to excellent yields. Primary, secondary heterocyclic amines, and phenols with various functional groups are smoothly acetylated in good yields. This method exhibits exquisite chemoselectivity, the amino group is preferentially acetylated in the presence of a hydroxyl/thiol group.

Full text of DOI:10.1080/00397911.2019.1650282

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Manganese-mediated acetylation of alcohols,
phenols, thiols, and amines utilizing acetic
anhydride
Isha Jain, Ramandeep Sharma & Payal Malik
To cite this article: Isha Jain, Ramandeep Sharma & Payal Malik (2019): Manganese-
mediated acetylation of alcohols, phenols, thiols, and amines utilizing acetic anhydride, Synthetic
Communications, DOI: 10.1080/00397911.2019.1650282
To link to this article: https://doi.org/10.1080/00397911.2019.1650282
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https://doi.org/10.1080/00397911.2019.1650282
Manganese-mediated acetylation of alcohols, phenols,
thiols, and amines utilizing acetic anhydride
Isha Jain, Ramandeep Sharma, and Payal Malik
1Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, India
ABSTRACT
ARTICLE HISTORY
Received 16 May 2019
Manganese(II) chloride-catalyzed acetylation of alcohols, phenols
thiols and amines with acetic anhydride is reported. This method is
environment-friendly and economically viable as it involves inexpen-
sive, relatively benign catalyst, mild reaction condition, and simple
workup. Acetylation is performed under the solvent-free condition at
ambient temperature and acetylated products obtained in good to
excellent yields. Primary, secondary heterocyclic amines, and phenols
with various functional groups are smoothly acetylated in good
yields. This method exhibits exquisite chemoselectivity, the amino
group is preferentially acetylated in the presence of a hydroxyl/thiol
group.
KEYWORDS
Acetylation; acetic
anhydride; manganese(II)
chloride; solvent-free;
chemoselectivity
GRAPHICAL ABSTRACT
Introduction
Functional group protection is the most fundamental strategy for the synthesis of poly-
functional molecules.[1] Acetylation is a versatile method for masking of hydroxyl, sulf-
hydryl and amine groups.[2] Acetic anhydride is most commonly used acetylating
agent.[3] Triethylamine, pyridine and 4-dimethylaminopyridine (DMAP) are employed
as base catalysts for the acetylation of alcohols, phenols, and amines.[4] Various metal
chlorides,[5] perchlorates,[6] triflates[7] and nitrates[8] have been used as active catalysts
for acetylation. Though metal triflates and perchlorates demonstrated high catalytic effi-
ciency for the acetylation reactions but a few limitations are associated with these meth-
ods, such as handling of hazardous perchlorates, the high price of metal triflates and
some triflates required dry reaction conditions.[9]
CONTACT Payal Malik
msg.payal@gmail.com
Department of Chemistry, Sant Longowal Institute of Engineering
and Technology, Longowal 148106, India.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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I. JAIN ET AL.
Scheme 1. Synthetic strategies for acetylation reactions.
Although the past two decades have witnessed remarkable advances in protection and
deprotection chemistry,[10] from sustainability viewpoint still, there is a need for an eco-
nomical, mild, and eco-friendly protocol for acetylation. In this regard, the third most
abundant biocompatible transition metal in the Earth’s crust, manganese (Mn) is a very
attractive metal component for the development of cost-effective benign catalytic proc-
esses.[11] Recently, Mn salts are considered as an alternative to iron salts which resulted
in an upsurge in Mn-chemistry.[12] Mn(CH3COO)3 and Mn(III) complexes are used as
catalysts for acetylation of alcohols and amines (Scheme 1a).[13] However, high tempera-
ture, use of solvent, elaborated ligands and excess of Ac2O make these methods less
suitable. Herein, we describe a mild and simple protocol for acetylation of alcohols,
phenols, thiols and amines using acetic anhydride as an acetylating agent in presence of
readily available, moisture stable MnCl2ꢀ4H2O as catalyst under the solvent-free condi-
tion at ambient temperature (Scheme 1b).
Results and discussion
In initial attempts, acetylation of alcohol was carried out using benzyl alcohol (1 equiv.)
as a model substrate with acetic anhydride (1.1 equiv.), in the presence of various Mn
salts, as a catalyst at room temperature under solvent-free condition (Table 1). The cata-
lytic efficiency of MnBr2, MnCl2ꢀ4H2O, and MnO2 was tested for acetylation reaction,
MnCl2ꢀ4H2O provided best results, 96% benzyl acetate was obtained within 1 h, in case
of MnBr2 and MnO2, moderate yields were achieved. Moreover, in the absence of
MnCl2ꢀ4H2O, the reaction was sluggish and only 15% product was obtained after 24 h
(Table 1, entry 6). Catalyst loading was optimized using a different amount of
MnCl2ꢀ4H2O, the reaction was slowed down when 0.5 mol% catalyst was used, whereas
the increased catalyst ratio i.e. 1.5 mol% does not show a considerable effect on reaction
time and product yield (Table 1, entries 4 and 5). The stoichiometric ratio between the
reactants is very important from a green chemistry viewpoint. To minimize waste of
reagents and make the purification process simpler we performed reaction using an
equimolar amount of benzyl alcohol and acetic anhydride under the same conditions,
desired product yield was reduced to 85%. To observe the solvent effect in this process,

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Table 1. Acetylation of benzyl alcohol using different salts under solvent/solvent-free conditionsa.
CH2OH
CH2OAc
Mn salt
rt
Ac2O
+
Entry
Mn salt (mol%)
MnBr2 (1)
MnO2 (1)
MnCl2ꢀ4H2O (1)
MnCl2ꢀ4H2O (0.5)
MnCl2ꢀ4H2O (1.5)
-
MnCl2ꢀ4H2O (1)
MnCl2ꢀ4H2O (1)
MnCl2ꢀ4H2O (1)
Time (h)
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
9
3.5
2
1
1
1
-
-
-
-
-
71
55
96
40
96
15
67
53
78
24
-
2
2
2
EtOAc
Et2O
THF
aReaction conditions: Alcohol (1 equiv.), acetic anhydride (1.1 equiv.), and Mn salt at 25 ꢂC; bIsolated yield.
the acetylation was subsequently carried out in several solvents (e.g., EtOAc, Et2O, and
THF), and the results were compared with the solvent-free condition. It was found that
the presence of solvent does not improve the product yields or reaction time.
After optimization of the reaction conditions, various primary, secondary, tertiary
and benzylic alcohols were subjected to acetylation and corresponding acetate deriva-
tives were obtained in good to excellent yields (Table 2). Electron withdrawing groups
like –NO2 and –Cl retard the rate of acetylation reaction. Acetylation is relatively slow
in case of long-chain aliphatic alcohols as compared to benzylic alcohols (Table 2,
entries 1, 4, and 5). It was observed that acid-labile furfuryl alcohol provided better
yields in THF (donor solvent), under solvent-free condition some complex by-product
was also obtained along with the expected product (Table 2, entry 7). Acid-sensitive,
chiral secondary and tertiary alcohols were successfully acetylated in high yields without
any side reactions (Table 2, entries 8–17). Particularly, (ꢁ)-menthol, (ꢁ)-borneol and
(ꢁ)-linalool retained their stereochemical configurations during the acetylation reaction,
no racemization or epimerization was observed.
In addition, to extend the substrate scope this strategy was used for acetylation of
various phenols, thiophenols and primary, secondary heterocyclic amines and corre-
sponding acetate derivatives were obtained in good to excellent yields (Table 2, entries
18–32). No fries rearrangement was observed in the case of phenols. Acetylation reac-
tion of aliphatic thiol was slow as compared to aromatic thiols (entries 27 vs. 25 and
26). Primary amines acetylated faster than the secondary amines. Notably, this method
is highly chemoselective, 2-aminophenol and 2-aminothiophenol gave corresponding
acetamides in excellent yields keeping the –OH and –SH functionalities unaltered
(Table 2, entries 22 and 23). We performed an intermolecular competitive acetylation
reaction, an equimolar mixture of aniline and phenol/thiophenol, was reacted with 1.1
equiv. of acetic anhydride in presence of MnCl2ꢀ4H2O, aniline was acetylated selectively
leaving the phenol/thiophenol unreacted. These excellent selectivities would make this
strategy attractive for protecting group chemistry.
To gain insights into the mechanism of acetylation reaction, controlled experiments
were performed. It has been observed that MnCl2 catalyzed coupling reactions proceed
through the SET mechanism.[11] Having that in mind, we carried out a radical

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I. JAIN ET AL.
Table 2. Acetylation of alcohols, phenols, thiols, and aminesa.
(continued)

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aReaction conditions: Alcohol (1 equiv.), acetic anhydride (1.1 equiv.), and 1 mol% MnCl2ꢀ4H2O at 25 ꢂC;
b
c
ꢃ
Monitored using TLC until the alcohol/phenol/amine was consumed; Isolated yield; Reaction was performed in THF.
inhibition experiment by reacting benzyl alcohol with acetic anhydride in the presence
of MnCl2ꢀ4H2O and 1 equiv. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a free
radical quencher, 60% benzyl acetate was obtained after 1 h. The results ruled out the
possibility of nonionic pathway.[5b]
In another controlled experiment, MnCl2ꢀ4H2O was reacted with acetic anhydride
at room temperature and the reaction was monitored by NMR spectroscopy. 13C
NMR measurements of the reaction mixture (suspension) revealed that a small broad
signal appears at d 168 ppm along with acetic anhydride signal (d 163 ppm). The
change in chemical shift and broadening of signal suggest that the catalyst is coordi-
nated with acetic anhydride, after addition of benzyl alcohol the signal was disap-
peared. There was no evidence for the formation of acetyl chloride. In addition, a
1
broad peak at d 11.5 ppm in the H NMR spectrum of the reaction mixture indi-
cates the formation of AcOH (all observed signals were broad due to the presence
of Mn). On the basis of NMR observations, we proposed acetylation mechanism
depicted in Scheme 2.
Conclusions
In summary, we have demonstrated that MnCl2ꢀ4H2O is an efficient catalyst for acetyl-
ation of alcohols, phenols, thiols, and amines under mild reaction conditions. All the
acetate derivatives were isolated in high yields. Main features of this process are the use
of stable, low cost, benign catalyst and the limited amount of acetic anhydride (1.1
equiv.), which makes the catalytic system more practical and environmentally viable.
This method shows a broad substrate scope and excellent chemoselectivity without any
ligand support.

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I. JAIN ET AL.
Scheme 2. Proposed mechanism for MnCl2ꢀ4H2O catalyzed acetylation of alcohol. H2O molecules are
omitted for simplification.
General procedure for acetylation
To a stirred mixture of alcohol/phenol/thiohenol/amine (1 mmol) and acetic anhydride
(1.1 mmol), 0.01 mmol of MnCl2ꢀ4H2O was added at room temperature. The reaction
mixture was stirred until alcohol/phenol/thiohenol/amine was consumed, the progress
of the reaction was monitored by TLC. The reaction mixture was quenched with satu-
rated aq. NaHCO3 and extracted with ethyl acetate (10 mL ꢄ 3). The organic layer was
dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude was
passed through a small pad of silica gel (eluent: hexane: ethyl acetate) to obtain pure
acetates (acetamides were precipitated out/crystallized direct from the reaction mixture)
1
and characterized by H NMR and IR spectroscopy. The data was found to be in accord
with previously reported acetates.
1
Characterization data and H NMR spectra can be found via the “Supplementary
Content” section of this article’s webpage.
Acknowledgments
The authors are grateful to Director, SLIET, Longowal for providing lab facilities and Panjab
University, Chandigarh for NMR measurements. Authors thank Prof. Harish Chopra, SLIET,
Longowal for his support.
ORCID
Payal Malik
http://orcid.org/0000-0001-6764-209X
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