In situ generated Ph3P(OAc)2as a novel reagent for the efficient acetylation of alcohols and thiols at room temperature

Article abstract of DOI:10.1016/j.tetlet.2013.01.068

Ph₃P, Br₂, and ammonium acetate are used for the in situ generation of Ph₃P(OAc)₂, which was characterized by different NMR techniques. The Ph₃P(OAc)₂generated was used as a novel and efficient reagent for the acetylation of alcohols and thiols in acetonitrile at room temperature under homogeneous conditions. This reaction was also performed under heterogeneous conditions using 1,3,2,4- diazadiphosphetidine as an easily prepared, stable, and heterogeneous P(III) compound.

Full text of DOI:10.1016/j.tetlet.2013.01.068

Tetrahedron Letters 54 (2013) 1813–1816
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Tetrahedron Letters
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In situ generated Ph3P(OAc)2 as a novel reagent for the efficient acetylation
of alcohols and thiols at room temperature
⇑
⇑
Nasser Iranpoor , Habib Firouzabadi , Elham Etemadi Davan
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Ph3P, Br2, and ammonium acetate are used for the in situ generation of Ph3P(OAc)2, which was character-
ized by different NMR techniques. The Ph3P(OAc)2 generated was used as a novel and efficient reagent for
the acetylation of alcohols and thiols in acetonitrile at room temperature under homogeneous conditions.
This reaction was also performed under heterogeneous conditions using 1,3,2,4-diazadiphosphetidine as
an easily prepared, stable, and heterogeneous P(III) compound.
Received 9 October 2012
Revised 29 December 2012
Accepted 16 January 2013
Available online 23 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Alcohol
Thiol
Acetylation
Bromine
Ammonium acetate
Triphenylphosphine
1,3,2,4-Diazadiphosphetidine
Due to the abundance of esters in natural compounds, they are
considered important functional groups in organic synthesis.1 Esters
are synthesized through the reaction of alcohols with carboxylic
acids2 or acylating/formylating agents, or by transesterification.3
Conversion of alcohols into their acetates in the presence of acetic
anhydride or acetyl chloride as acetylating agents has been studied
in the presence of a variety of catalysts such as DMAP,4 tert-Bu3P,5
F-DMAP,6 Al(OTf)3,7 TiCl3(OTf),8 polymer-supported gadolinium
triflate,9 Mg(NTf2)2,10 RuCl3,11 ZrOCl2Á8H2O,12 Mg(ClO4)2,13
have reported the use of azopyridines as alternative reagents for
esterification reactions.27
In the absence of any Mitsunobu reagent, there are very limited
reports on the direct esterification of alcohols via carboxylic salts,
28
with examples including ROH/RCO2K/Ph3P/CCl4 and ROH/TsIm/
RCO2Na/Et3N/TBAI.29 The former method28 uses a combination of
excess Ph3P (2.0 equiv with respect to the alcohol) and CCl4 at
55–60 °C. Apart from the disadvantage of using CCl4, this method
suffers from a lack of stereoselectivity and the reaction of second-
ary alcohols occurs with both inversion and racemization.
The acylation of thiols, themselves an important functionality in
many biologically active compounds,30 has been performed with
different acylating agents catalyzed by a variety of catalysts such
as, [Cp2Ti(OSO2C8F17)2],31 Cu(OTf)2,32 Cp2Zr(OPf)2,33 bro-
modimethylsulfonium bromide,34 InCl3,35 and Ag(OTf).36 However,
as far as we are aware, the acylation of thiols in the presence of hal-
ogen sources and Ph3P has not been reported so far.
14
15
H5PV2Mo10O40
,
(NH4)2.5H0.5PW12O40
,
ZnO,16 ATPB (acetonyltri-
phenylphosphonium bromide),17 BiFeO3,18 and [TMBSA][HSO4].19
The use of ethyl acetate for the acetylation of alcohols has been
carried out with catalysts or reagents such as PPh3/CBr4,20 H6 [PMo9-
V3O40],21 dodeca-tungsto(molybdo)phosphoric acid,22 [PCl3Àn
(SiO2)n],23 TiCl3(OTf),8 and N-heterocyclic carbene catalysts.24
-
The reaction of alkyl halides with metal salts of carboxylic acids
to produce the corresponding esters has been found to be unsuc-
cessful due to side reactions.20 However, due to the presence of a
wide variety of alcohols in comparison to alkyl halides, esterifica-
tion of carboxylic salts with alcohols is a more attractive strategy.
For this purpose, Mitsunobu conditions have been employed as an
important method for the activation of alcohols using Ph3P/DEAD,
or DIAD/sodium25 or zinc26 carboxylate. In order to remove the
problems encountered with the handling of DEAD, recently, we
In continuation of our work on the reactions of alcohols, thiols,
and amines in the presence of Ph3P/Br2 and nucleophiles,37 we
decided to study the use of the Ph3P/Br2/NH4OAc mixed reagent
system for the acetylation of alcohols.
Wefirstused a combinationof Ph3P, Br2, and NH4OAcfor theacet-
ylation of 3-phenylpropanol as a model study. The optimized condi-
tions were found to be PPh3/Br2/NH4OAc/alcohol (1.25:1.25:2.87:1)
in CH3CN at room temperature. The reaction was complete within
0.3 h and the corresponding acetate was obtained in 90% yield along
with the formation of Ph3PO (Table 1, entry 1).
⇑
Corresponding authors. Tel.: +98 711 2284822; fax: +98 711 2280926.
E-mail addresses: iranpoor@susc.ac.ir (N. Iranpoor), firouzabadi@susc.ac.ir
(H. Firouzabadi).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.068

1814
N. Iranpoor et al. / Tetrahedron Letters 54 (2013) 1813–1816
Table 1
Ph
N
Ph
N
Ph
Optimization of the P(III) source for the acetylation of 3-phenylpropanol
PhHN
Ph
NHPh
NHPh
PhHN
NHPh
N
N
P
P
P
P
P
N
Entry P(III) source Molar (equiv) Time (h) Conversion (%) Yield (%)
Ph
Ph
1
2
3
4
Ph3P
P1
P2
1.25
0.62
0.41
0.31
0.3
0.7
0.7
0.7
100
100
100
100
90
90
89
92
P1
P2
Ph
N
N
Ph
N
PhHN
NHPh
N
P3
P
P
P
P
N
Ph
Ph
P3
The structure of Ph3P(OAc)2 was determined by different spec-
troscopy techniques. The 1H NMR spectrum of this reagent in
CD3CN showed a singlet at 1.85 ppm for the two methyls of the
acetoxy groups. Its 13C NMR spectrum showed two singlets for
the methyl and carbonyl groups of the two acetoxy functionalities
at 20.9 and 172.0 ppm, respectively.38 The NMR data obtained
showed that both acetoxy groups were in identical environments,
which could be due to the possibility of rapid exchange processes
at room temperature. The 31P–{1H} spectral signal of Ph3P(OAc)2
appeared as a singlet at 45.0 ppm.
In order to perform the reaction under heterogeneous condi-
tions, which allows easy separation of the produced phosphine
oxide, we decided to replace PPh3 with phosphazane oligomers,
which we previously used as ligands for C–C coupling reactions.39
We employed 1,3,2,4-diazadiphosphetidine dimer [(PhNH)PNPh]2
(P1) and trimer [(PhNH)PNPh]3 (P2) along with dinuclear
[(PhNH)P2(NPh)2]2NPh (P3)40 as sources of P(III) (Fig. 1) in conjunc-
tion with Br2 and NH4OAc under the optimized reaction conditions.
Although the three phosphazanes P1–P3 have similar connectiv-
ities, they showed different solubilities in organic solvents. Com-
pounds P1 and P2 are soluble, however, P3 was insoluble in
solvents such as ethyl acetate, acetonitrile, diethyl ether, and
CH2Cl2.
Figure 1. Structures of the phosphazanes.
Table 2
Optimization of the halogen and acetate sources in the acetylation reaction with 3-
phenylpropanol in acetonitrile at room temperature
Entry Halogen source P(III)
Acetate source Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
Br2
Br2
I2
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
NH4OAca
NH4OAca
NH4OAca
NH4OAca
NH4OAca
NH4OAca
NH4OAca
NH4OAca
NH4OAcb
NH4OAcb
0.3
0.7
5
7
7.5
9
8
10
4
4
4
4
90
92
91
92
90
90
80
78
I2
NCS
NCS
NBS
NBS
Br2
Br2
Br2
Br2
c
Ph3P
P3
—
—
—
c
10
11
12
Ph3P or P3 NaOAcd
Ph3P or P3 KOAcd
c
c
—
a
b
c
NH4OAc was dissolved in CH3CN on refluxing.
NH4OAc was added to the mixture without prior dissolution in hot CH3CN.
After 4 h, the reaction was incomplete and 3-phenylpropyl bromide was
obtained in 50–60% yield.
d
NaOAc and KOAc were added to the mixture without prior dissolution in hot
CH3CN.
The amount of each phosphazane was determined on the basis
of the number of phosphorus atoms they contained. Therefore,
0.62, 0.41, and 0.31 molar equivalents of P1–P3 provided equal
numbers of P(III) sites for the reaction with one equivalent of the
alcohol. As the results in Table 1 show, all three reactions were
complete within 0.7 h and the isolated yields were found to be
nearly the same. However, the reactions of P1 and P2 are homoge-
neous but with P3, the reaction occurs heterogeneously, which pro-
vides a simpler work-up. On the basis of the higher atom economy
with P3 compared to the other phospazanes, the heterogeneous
nature of its reaction, and also the higher yield for its preparation,
this P(III) compound was selected as the most suitable.
The reaction of benzyl alcohol (Table 3, entry 8), under the opti-
mized conditions, produced benzyl bromide as the major product.
In order to decrease the formation of this bromination product,
especially in the reaction of benzylic alcohols, we studied the use
of NBS as the halogen source and found that on performing the
reaction in an ice bath, acetylation of benzyl alcohol occurred pre-
dominantly and the amount of bromide by-product was reduced.
Table 3
Acetylation of alcohols in the presence of NH4OAc
With these results in hand, next we optimized the order of addi-
tion using Ph3P and also P3 as the sources of P(III). The best result
was obtained when ammonium acetate was dissolved in refluxing
CH3CN and then was added to a stirred solution of a mixture of
PPh3/Br2 or P3/Br2 in CH3CN at room temperature. We observed
the formation of 3-phenylpropyl acetate after 0.3 and 0.7 h respec-
tively, in 90% and 92% isolated yields (Table 2, entries 1 and 2). The
reaction of P3 occurs heterogeneously with 0.31 equiv, however in
the case of Ph3P, 1.25 equiv of PPh3 was required and the reaction
was homogeneous. The reaction with P3 is superior to that using
PPh3 in terms of the isolation of its phosphine oxide and the stoi-
chiometry of the reagent. The use of sodium or potassium acetates,
which are insoluble in CH3CN, even under reflux conditions did not
produce any product. We were also interested to see if the reaction
proceeded in the presence of other halogen sources. We used NBS,
NCS, and I2 instead of Br2, and observed that the reaction time in
the presence of Br2 was much shorter and the yield was higher (Ta-
ble 2, entries 3–8).
CH3CN
rt
R-OH + Br2 + P(III) + NH4OAc
R-OCOMe
R= alkyl, benzyl
P(III)= Ph3P or P3
Entry
1
Alcohol
P(III)
Time (h)
Yield (%)a
PhCH2CH2OH
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
0.3
0.7
0.3
0.7
0.3
0.7
0.35
0.75
0.33
0.7
5
91
90
90
92
95
93
90
88
2
3
4
5
6
7
8
PhCH2CH2CH2OH
CH3(CH2)14CH2OH
CH3(CH2)6CH2OH
C6H11CH2CH2OH
l-Menthol
90
91
88 (8)
84 (8)
87 (4)
85 (5)
40 (53)
34 (56)
7
CH3(CH2)5CH(OH)CH3
PhCH2OH
4.5
5.2
0.7
2
We next applied our optimized reaction conditions as a new ap-
proach for the acetylation of various primary and secondary alco-
hols in acetonitrile at room temperature41 and the results are
shown in Table 3.
a
The yields in parenthesis refer to the corresponding bromides.

N. Iranpoor et al. / Tetrahedron Letters 54 (2013) 1813–1816
1815
Table 4
Acetylation of benzylic alcohols in the presence of NH4OAc
CH3CN, rt
P(III)
+
R-OH
NBS
NH4OA
R-OCOMe
+
+
R = alkyl, benzyl
P(III) = Ph3P or P3
Entry
1
Alcohol
P(III)
Time (h)
Isolated yielda (%)
PhCH2OH
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
2
87 (3)
70 (26)
86 (6)
72 (20)
84 (8)
71 (24)
80 (10)
69 (25)
78 (15)
67 (28)
75 (14)
70 (19)
71 (17)
69 (22)
1.5
1.9
1.5
9
10
12
12
15
15
14
14
21
21
2
3
4
5
6
7
p-MeOC6H4CH2OH
p-ClC6H4CH2OH
p-NO2C6H4CH2OH
o-NO2C6H4CH2OH
PhCH(OH)CH3
Ph2CHOH
a
The yields in parenthesis refer to the corresponding bromides.
Table 5
Acylaton of thiols using Ph3P or P3.
Br2
PPh3
1)
PPh3PBr/Br
+
Entry
1
Thiol
P(III)
Time (h)
Yield (%)
(A)
PhCH2SH
Ph3P
Ph3Pa
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
Ph3P
P3
0.8
6
1
88
85
86
90
91
87
90
88
92
80
86
80
85
75
80
PPh3POAc/OAc
(B)
2 NH4OAc
XH
(A)
2)
+
_
2 NH4Br
2
3
4
5
6
7
CH3(CH2)2CH2SH
CH3CH2CH2SH
0.7
0.8
0.65
0.75
0.7
0.75
3.5
4.2
4.0
5.0
8.0
8.0
O
R
CH3(CH2)10CH2SH
CH3CH2CH2(SH)CH3
Cyclohexanethiol
CH3CH2(CH3)(SH)CH3
(B)
HOAc
+
+
R
X
3)
Ph3PO
Me
X: O, S
Scheme 1. The proposed mechanism for the acetylation of alcohols and thiols using
Ph3P/Br2/NH4OAc.
According to the obtained results from the acetylation of l-men-
thol and the thiols, the following reaction mechanism which shows
the retention of configuration is suggested (Scheme 1).
a
NBS was used instead of Br2 and an ice bath.
The isolation and characterization of Ph3PO and NH4Br can be
regarded as additional evidence for the proposed mechanism.
In conclusion, we have introduced Ph3P(OAc)2 as a new and eas-
ily prepared reagent for the acetylation of alcohols and thiols at
room temperature, homogeneously. The use of 1,3,2,4-diazadi-
phosphetidine (P3), instead of Ph3P, affords a similar reagent which
can be used for the same transformations under heterogeneous
conditions. Mechanistic studies showed that the reaction occurs
with retention of configuration. In addition, the use of 1,3,2,4-diaz-
adiphosphetidine (P3) as an easily prepared, stable, and heteroge-
neous P(III) reagent in these transformations, shows the
efficiency and advantages of this class of P(III) reagent in organic
synthesis.
The optimized ratio of 2:1.9:2.3:1 (Ph3P/NBS/NH4OAc/alcohol)
in CH3CN in an ice bath was found to be the most suitable for
the acetylation of benzylic alcohols homogeneously. In the case
of using P3, the optimized amount was found to be 0.5 molar
equivalents with respect to the alcohol. We then applied these con-
ditions for the acetylation of benzylic alcohols (Table 4).
Applying Ph3P(OAc)2 for the acetylation of l-menthol produced
menthyl acetate in 88% isolated yield. Based on the comparison of
the observed optical rotation of the obtained menthyl acetate
{[a
] = À70, c = 1, CHCl3} with the literature,23 the reaction was
found to occur with high retention of the configuration and optical
purity (98% ee). Here again, comparison of the 1H NMR spectrum of
the obtained product with the literature,42 confirmed that the reac-
tion proceeded with retention of configuration.
Acknowledgment
After successful conversion of the various alcohols into their al-
kyl acetates, we also investigated the applicability of this reaction
for the acetylation of thiols under the optimized conditions
(Table 5).
We gratefully acknowledge the financial support of this study
by Shiraz University Research Council.
According to the results in Table 5, thiols were also successfully
converted into their corresponding thioacetates in high yields.43 In
comparison with the reaction of benzyl alcohol (Table 3, entry 8),
the reaction of benzyl mercaptan with both Ph3P or P3 and Br2 or
NBS did not produce any benzyl bromide as a side product (Table 5,
entry 1).
References and notes
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a flask containing a stirred mixture of Br2
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(1.25 mmol, 0.0625 mL) and Ph3P (1.25 mmol, 0.327 g) in CH3CN (1.5 mL) at
room temperature. After stirring for 2.5 h at room temperature, the NH4Br
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was prepared in CD3CN. 1H NMR (CD3CN, 250 MHz): d 1.85 (s, 6H), 7.4–7.58
(m, 15H). 13C NMR (CD3CN, 62.5 MHz): d 20.9, 128.6, 131.4, 132.0, 132.8,
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31P NMR (CD3CN, 162 MHz): 45.0 ppm.
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The progress of the reaction was monitored by TLC (Table 3, entry 2). After
completion of the reaction (0.3 h) the reaction mixture was filtered to remove
the precipitated NH4Br followed by evaporation of the solvent. Column
chromatography of the crude mixture on silica gel using n-hexane/EtOAc
(3:1) as the eluent gave 3-phenylpropyl acetate in 90% yield (0.159 g). 1H NMR
(CDCl3, 250 MHz): d 1.88 (q, 2H, J = 7.5 Hz), 1.97 (s, 3H), 2.6 (t, 2H, J = 7.6 Hz),
4.0 (t, 2H, J = 6.5 Hz), 7.0–7.2 (m, 5H). 13C NMR (CDCl3, 62.5 MHz): d 20.9, 30.1,
32.1, 63.8, 126.0, 128.3, 128.4, 141.1, 171.1 ppm.
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43. Typical procedure for the conversion of benzyl mercaptan into benzyl thioacetate
using Ph3P/Br2/NH4OAc:
The same mixture of Ph3P, Br2, and NH4OAc was prepared as described above.
Addition of benzyl mercaptan (1.0 mmol, 0.11 mL) and monitoring the reaction
by TLC showed completion of the reaction after 0.8 h (Table 5, entry 1).
Removal of the produced NH4Br by filtration was followed by evaporation of
the solvent. Column chromatography of the crude mixture on silica gel using n-
hexane as the eluent, gave benzyl thioacetate in 88% yield (0.146 g). 1H NMR
(CDCl3, 250 MHz): d 2.3 (s, 3H), 4.1 (s, 2H), 7.26 (m, 5H). 13C NMR (CDCl3,
62.5 MHz): d 30.3, 32.1, 127.2, 128.4, 128.6, 137.5, 193.1 ppm.
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