Silver acetate mediated acetoxylations of alkyl halides

Article abstract of DOI:10.1055/s-0033-1338551

Silver acetate promotes the acetoxylation of alkyl halides under neutral reaction conditions. The reaction is applicable to primary and activated secondary alkyl halides, and 2,2-dibromoacetophenones for preparing the corresponding acetates in good yields. The presence of ester, amide, nitrile, hydroxy, and OTBDMS functions on the substrate is tolerated.

Full text of DOI:10.1055/s-0033-1338551

PRACTICAL SYNTHETIC PROCEDURES
▌165
practical synthetic procedures
Silver Acetate Mediated Acetoxylations of Alkyl Halides
Silver Acetate Mediated Acetoxylations
Roberto Nolla-Saltiel, Ulises Alonso Carrillo-Arcos, Susana Porcel*
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior s/n,
Ciudad Universitaria, 04510 México D.F., México
Fax +52(55)56162217; E-mail: sporcel@unam.mx
PSP
Received: 20.09.2013; Accepted after revision: 25.09.2013
No264
Abstract: Silver acetate promotes the acetoxylation of alkyl halides under neutral reaction conditions. The reaction is applicable to primary
and activated secondary alkyl halides, and 2,2-dibromoacetophenones for preparing the corresponding acetates in good yields. The presence
of ester, amide, nitrile, hydroxy, and OTBDMS functions on the substrate is tolerated.
Key words: alkyl halides, substitution, acetoxylation, silver acetate, isomerization
Y
Z
AgOAc
R
X
R
OAc
MeCN or DMF
63–100%
X = Br, Cl
Y = H, Br
Z = H, OAc
Scheme 1 Acetoxylation of alkyl halides with silver acetate
Acetates are cheap and efficient protecting groups of alco- in acetic acid.6 Closely related to these substitutions, the
hols. One of their advantages is that they can be gently re- transformation of primary iodides and bromides into alco-
moved by enzymes, which in addition opens the hols by oxygen transfer from bis(tributyltin)oxide with
opportunity to procure enantioenriched substrates.1 Usu- silver salts,7 and the replacement of a secondary iodide by
ally acetates are prepared starting from the corresponding OMe, or ONO3 with AgOTf and AgNO3 have also been,
alcohols;1 however, when the free hydroxyl group is not respectively, described.8 Taking into account these prece-
directly available an alternative way to introduce this dents we wanted to test the applicability of the substitu-
functionality is the replacement of a halogen by an ace- tion promoted by AgOAc under neutral conditions on a
toxy group. This second alternative has been less investi- wide range of alkyl halides.
gated, and typically is carried out by heating the
The difference in reactivity between AgOAc and NaOAc
on an activated α-bromo ketone 1 was examined first in a
corresponding alkyl halide in AcOH,2 or treating the alkyl
halide with KOAc or NaOAc in the presence of 18-crown-
polar aprotic solvent. Performing the reaction in acetoni-
6,3 or with KOAc in ionic liquids.4 The first method is
trile at room temperature with 1 equivalent of AgOAc, the
acetate 2 was cleanly obtained after 5 hours in 93% yield.
Increasing the temperature to 50 °C reduced the reaction
only suitable for substrates not sensitive to acid mediums,
the second one gives good yields just when the alkyl ha-
lide is activated, and the third one suffers from the incon-
time to 1.5 hours and the isolated yield was almost 100%
veniences associated to ionic liquids such as high cost and
(Scheme 2). Under these conditions, AcOH or less polar
viscosity. Given the utility of acetates and in order to fur-
solvents gave lower yield,9 whereas NaOAc instead of
ther develop methodologies that do not use alcohols as
AgOAc led only to 30% of conversion.
starting materials, we decided to explore the possibility of
synthesizing them by using AgOAc as the acetoxy suppli-
O
O
er under neutral conditions (Scheme 1). Silver salts have
been used in organic chemistry to facilitate substitution
reactions thanks to their high affinity for halogens.5 None-
theless, there exits only a few reports where silver salts
have been employed to promote the replacement of a hal-
ogen by an acetoxy group. In the major part of these re-
ports the main interest is not synthetic but mechanistic and
the transformation is achieved by adding AgOAc, Ag2O,
or Ag2CO3 to a solution of the corresponding alkyl halide
MOAc (1 equiv)
MeCN, 50 °C
OAc
Br
Cl
Cl
1
2
M = Ag, quant.
M = Na, 30%
Scheme 2 Acetoxylation of 2-bromo-4′-chloroacetophenone
With this result, the generality of the method was exam-
ined next on a variety of alkyl halides (Scheme 1, Table
1). In general, the reaction takes place very efficiently
with yields ranging from 63 to 100% in acetonitrile or
DMF for less reactive substrates (Table 1, entries 11–14).
SYNTHESIS 2014, 46, 0165–0169
Advanced online publication: 04.11.2013
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0033-1338551; Art ID: SS-2013-M0375-PSP
© Georg Thieme Verlag Stuttgart · New York

166
R. Nolla-Saltiel et al.
PRACTICAL SYNTHETIC PROCEDURES
Table 1 Scope of the Acetoxylation Reaction with AgOAc (continued)
As in the case of 1, 2-bromoacetophenones 3a–d were
converted into their acetoxy derivatives 4a–d in excellent
yields (entries 1–4). α-Acetoxy ketones can be prepared
by a variety of methods but most of them involve the di-
rect oxidation of ketones10 or the oxidation of an enol
intermediate11 using an excess of the oxidant. Interesting-
ly, it was possible to perform a double substitution onto
2,2-dibromoacetophenones 3e–h to furnish the corre-
sponding 1,1-diacetates (entries 5–8). 1,1-Diacetates are
valuable protecting group of aldehydes under basic condi-
tions,12 they are usually synthesized from aldehydes13 and
to the best of our knowledge this is the first time that a
substitution over a 1,1-dibromide has been used as meth-
od for their preparation. Secondary α-bromo ketones were
also converted into α-acetoxy ketones, although higher
temperatures were required (entries 11 and 12). As ex-
pected primary and secondary benzyl bromides were
readily acetoxylated in good yields (entries, 9 and 10).
Primary substrates like bromooctane or 3-phenylpropar-
gyl bromide, were properly transformed into their acetoxy
derivatives using DMF as solvent (entries 13 and 14).
Nonetheless, cyclohexyl iodide and 3-bromocyclohexene,
both secondary alkyl halides, gave mainly the elimination
products under all the conditions examined.
Y
Z
AgOAc
R
X
R
OAc
4a–t
MeCN or DMF
3a–t
X = Br, Cl
Y = H, Br
Z = H, OAc
Entrya Product
Solvent Temp Time Yield
(°C) (h)
(%)
O
OAc
OAc
5
MeCN 70
15
98
4e
O
OAc
6
MeCN 70
20
20
93
99
OAc
Cl
4f
O
OAc
7
MeCN 70
OAc
Ph
4g
Table 1 Scope of the Acetoxylation Reaction with AgOAc
O
Y
Z
AgOAc
OAc
R
X
R
OAc
4a–t
MeCN or DMF
OAc
8
MeCN 70
15
96
3a–t
O
O
X = Br, Cl
Y = H, Br
Z = H, OAc
4h
Entrya Product
Solvent Temp Time Yield
OAc
(°C) (h)
(%)
9
MeCN 50
MeCN 50
4
2
90
91
O
4i
OAc
1
MeCN 50
2.5 94
OAc
10
4a
O
4j
OAc
O
OAc
2
MeCN 50
1.5 99
Ph
11
DMF
DMF
70
70
2
91
63
4b
4k
O
O
OAc
OAc
3
MeCN 50
1.5 98
12
36
O
O
4l
4c
OAc
O
6
13
DMF
DMF
70
25
24
2
93
OAc
4m
4
MeCN 50
2.5 90
MeO
OMe
OAc
OMe
Ph
14
100
4d
4n
Synthesis 2014, 46, 165–169
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Silver Acetate Mediated Acetoxylations
167
Table 1 Scope of the Acetoxylation Reaction with AgOAc (continued)
dergoes an electrophilic attack on the halogen group to
produce an allylic cation intermediate. Nonetheless it was
possible to obtain solely the linear acetoxy derivatives ei-
ther by adding 1 equivalent of Ph3P or with prolonged re-
action times. The presence of Ph3P decreases the
electrophilicity of the silver cation causing a milder inter-
action with the halogen group and thus inhibiting the for-
mation of the allylic cation. Under these conditions the
acetate anion likely replaces the halogen by an SN2 mech-
anism resulting in the exclusive formation of the linear de-
rivative. On prolonged reaction times silver ion itself
would promote an allylic isomerization reaction. This was
supported by the observation that the addition of 1 equiv-
alent of AgOAc to a 1:1 mixture of the linear 6 and
branched 7 acetoxy derivatives of cinnamyl bromide fur-
nished only the linear derivative 6 after 24 hours. As it has
been previously reported mainly for palladium14 and re-
cently for gold carbenes15 silver ions would promote the
rearrangement by π coordination with the olefin.
Y
Z
AgOAc
R
X
R
OAc
4a–t
MeCN or DMF
3a–t
X = Br, Cl
Y = H, Br
Z = H, OAc
Entrya Product
Solvent Temp Time Yield
(°C) (h)
(%)
O
OAc
15
MeCN 50
3
96
HO
4o
O
OAc
16
MeCN 70
MeCN 70
MeCN 50
2
90
93
95
TBDMSO
4p
AcO
O
MeCN, r.t
Br
OAc
Ph
Ph
Ph
OAc
O
17
24
16
5
6
7
4q
AgOAc, 1. 5 h
AgOAc, Ph3P (1:1), 2 h
AgOAc, 24 h
:
:
:
1
1
1
1
0.08
0
O
OAc
O
18b
MeCN, r.t
OAc
4r
Br
OAc
O
AcO
Et
8
9
10
N
19b
20c
MeCN 50
MeCN 50
48
60
96
91
AgOAc, 1.5 h
AgOAc, Ph3P (1:1), 2 h
AgOAc, 24 h
Et
:
:
:
1
1
1
0.23
0
0
4s
AcO
Scheme 3 Acetoxylation of geranyl and cinnamyl bromide
N
4t
In summary, we have found that silver acetate promotes
the substitution reaction of primary and activated second-
ary alkyl halides under soft and neutral reaction condi-
tions. The method described allows the preparation of 1,1-
diacetates and linear allylic acetoxy compounds by care-
ful tuning of the reaction conditions.
a Reaction conditions: R–X (0.1 M); R–X/AgOAc = 1:1. Unless oth-
erwise noted: R–X = R–Br.
b Reaction performed in the presence of Ph3P.
R–X/AgOAc/Ph3P = 1:2:1; R–X = R–Cl.
e R–X/AgOAc = 1:2; R–X = R–Cl.
Aiming at studying the compatibility of the method over
different functional groups, we tested the reaction in the
presence of a hydroxyl, an OTBDMS, an ester, an amide,
and a nitrile function. Satisfactorily the acetoxylation
worked efficiently without affecting the different func-
tionalities (entries 15–20). In the cases of ester 3r and
amide 3s, the addition of 1 equivalent of Ph3P was neces-
sary to accelerate the reaction (entries 18 and 19).
All reactions were carried out under a N2 atmosphere. Et2O and
CHCl3 were dried by standard methods and freshly distilled prior to
use. Anhydrous MeCN and DMF were purchased from Aldrich.
Commercial reagents were used as received without further purifi-
cation. Reactions containing AgOAc were protected from light in
order to prevent its decomposition. NMR spectra were recorded at
25 °C on a Jeol Eclipse 300 MHz spectrometer. High-resolution
mass spectra (HRMS) were recorded on a Jeol JMS-SX-102A spec-
trometer.
Finally we decided to examine the regioselectivity of the
substitution onto primary allylic substrates. At first in-
stance, the reaction of allylic substrates like cinnamyl or
geranyl bromides with silver acetate furnished a mixture
of linear and branched acetoxy derivatives (Scheme 3).
This is in line with an SN1 process where the silver ion un-
Acetoxylation of Alkyl Halides 3; General Procedure
To a suspension of AgOAc (42 mg, 0.25 mmol) in MeCN or DMF
(2 mL) was added the respective alkyl halide 3 (0.25 mmol) dis-
solved in the corresponding solvent (1 mL). The reaction mixture
was stirred at the temperature and time indicated in Table 1 until the
starting material was consumed. After completion of the reaction,
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 165–169

168
R. Nolla-Saltiel et al.
PRACTICAL SYNTHETIC PROCEDURES
Acknowledgment
sat. aq NH4Cl (20 mL) was added and the crude mixture was ex-
tracted with EtOAc (3 × 20 mL). When necessary, the product was
purified by column chromatography on silica gel (hexane–EtOAc).
This work was supported by CONACyT (project No. 153523),
DGAPA (project No.IB202212), and Instituto de Química
(UNAM). The authors would like to thank DGAPA for undergradu-
ate fellowship to Roberto Nolla-Saltiel, and Mª Angeles Peña-
González and Elisabeth Huerta-Salazar for technical support
(NMR).
2-Acetoxy-3′,4′-(methylenedioxy)acetophenone (4c)
Yield: 62.90 mg (98%); white solid; mp 75–77 °C.
IR (KBr): 1736, 1676, 1600 cm–1.
1H NMR (300 MHz, CDCl3): δ = 7.47 (d, J = 8.0 Hz, 1 H), 7.36 (s,
1 H), 6.83 (d, J = 8.1 Hz, 1 H), 6.03 (s, 2 H), 5.24 (s, 2 H), 2.20 (s,
3 H).
Supporting Information for this article is available online at
13C NMR (75 MHz, CDCl3): δ = 190.25, 170.53, 152.47, 148.46,
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
r
itmnatr
128.95, 124.05, 108.19, 107.62, 102.09, 65.83, 20.66.
HRMS-FAB: m/z calcd for C11H11O5 [M + H]+: 222.0528; found:
222.0527.
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1H NMR (300 MHz, CDCl3): δ = 7.60 (d, J = 8.9 Hz, 1 H), 6.67 (d,
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Synthesis 2014, 46, 165–169
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Silver Acetate Mediated Acetoxylations
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Synthesis 2014, 46, 165–169