Tandem oxidation–Wittig reaction using nanocrystalline barium manganate (BaMnO4); an improved one-pot protocol

Article abstract of DOI:10.1016/j.tetlet.2016.07.027

A one-pot, tandem oxidation–Wittig procedure has been developed in which the reacting components are generated in situ from alcohols, triphenyl phosphine, and ethyl bromoacetate using barium manganate as a mild oxidizing agent without the addition of an external base.

Full text of DOI:10.1016/j.tetlet.2016.07.027

Tetrahedron Letters 57 (2016) 3773–3775
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Tandem oxidation–Wittig reaction using nanocrystalline barium
manganate (BaMnO4); an improved one-pot protocol
b,
a
c
Mohammad Gholinejad a, , Habib Firouzabadi , Maedeh Bahrami , Carmen Nájera
⇑
⇑
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Gava Zang, Zanjan 45137-66731, Iran
b The Late Professor Ali Akbar Moshfegh Laboratory, Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
c Departamento de Química Orgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-pot, tandem oxidation–Wittig procedure has been developed in which the reacting components
are generated in situ from alcohols, triphenyl phosphine, and ethyl bromoacetate using barium man-
ganate as a mild oxidizing agent without the addition of an external base.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 11 May 2016
Revised 23 June 2016
Accepted 7 July 2016
Available online 9 July 2016
Keywords:
Tandem
Oxidation
Wittig
BaMnO4
Base-free
Alcohols are readily available chemicals which are used for the
synthesis of important materials through their facile conversion
into other classes of compounds such as aldehydes and ketones.
The stereoselective conversion of carbonyl compounds into olefins
via the Wittig reaction is widely employed. In a traditional Wittig
reaction, aldehydes are usually added to a mixture of the phospho-
nium halide salt in the presence of a base.1 It is well noted that
handling aldehydes may raise problems due to their volatility, tox-
icity, and their tendency for polymerization.2 In recent years, par-
ticular attention has been paid to one-pot, multi-step reactions
which shorten a synthetic pathway, and bring about significant
time–cost benefits and enhancement in reaction efficiency.3 Along
this line, one-pot, tandem oxidation–Wittig reactions via the oxi-
dation of primary alcohols to aldehydes, followed by in situ reac-
tion with phosphorus ylides are an effective strategy for the
efficient generation of alkenes.4 Paying attention to this approach,
Ireland and Norbeck,5 reported a procedure in which an aldehyde
was generated in situ by the Swern oxidation of an appropriate
alcohol, which subsequently underwent reaction with the Wittig
reagent to produce an olefin. Using this method, the isolation of
the aldehyde was removed from the protocol and as a conse-
quence, the product yield was noticeably improved.5 After the
introduction of this protocol, several oxidation methods such as
Dess–Martin periodinane,6 tetrapropylammonium perruthenate
(TPAP),7 ortho-iodoxybenzoic acid (IBX),8 BaMnO4,9 MnO2,10
PCC,11 in situ aerobic oxidation using a ruthenium catalyst,12,13
SO3Py,14 and a TEMPO–BAIB15 system were employed for the
one-pot Wittig reaction. In all of the reported reactions, stabilized
phosphonium ylides were employed and added to the reaction
mixture. Using stabilized ylides, which are limited in number,
limits the application of the reported methods. Choudary and
co-workers also reported the application of nanocrystalline
magnesium oxide as a heterogeneous, solid base catalyst for the
one-pot Wittig reaction. However, this method was performed
under non-oxidative conditions and aldehydes were used as the
starting materials.16
In order to expand this methodology, we report a one-pot,
tandem oxidation–Wittig procedure in which the reacting compo-
nents are generated in situ from alcohols, triphenyl phosphine, and
ethyl bromoacetate using BaMnO4 as a mild oxidant without the
addition of an external base. The procedure is highly stereoselec-
tive, producing the corresponding olefins in high yields.
17
To optimize the reaction, a model reaction using BaMnO4
(1.2 mmol), benzyl alcohol (1 mmol), PPh3 (1.2 mmol) and ethyl
bromoacetate (1.2 mmol) in different solvents at 80 °C was stud-
ied. It was found that CH3CN was a suitable solvent for the reaction
(Table 1). The reaction proceeded well and after 24 h the olefin was
isolated in 90% yield with an E/Z ratio of 98:2 as determined by 1H
NMR spectroscopy.
⇑
Corresponding authors. Tel.: +98 024 3315 3231; fax: +98 024 3315 3232
(M.G.); tel.: +98 713 2284822; fax: +98 713 2280926 (H.F.).
E-mail addresses: gholinejad@iasbs.ac.ir (M. Gholinejad), firouzabadi@chem.
susc.ac.ir (H. Firouzabadi).
http://dx.doi.org/10.1016/j.tetlet.2016.07.027
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

3774
M. Gholinejad et al. / Tetrahedron Letters 57 (2016) 3773–3775
Table 1
ties (entries 1–10, 13–17). Sterically hindered benzylic alcohols
containing ortho-substituents reacted well and produced the
desired olefins (entries 7, 10, 13, 16) in high yields (83–86%).
3-Phenyl-1-propanol was reacted under the optimized conditions
and afforded a 78% isolated yield with 100% (E)-selectivity (entry
11). Cinnamyl alcohol also produced the corresponding
conjugated diene in 91% yield with absolute (E)-stereoselectivity
(entry 18).
Optimization of the reaction conditions for the reaction of benzyl alcohol and ethyl
bromoacetatea
CO2Et
BaMnO4 (1.2 eq)
OH
+
Br
CO2Et
PPh3 (1.2 eq), solvent
80 °C
Entry
Solvent
Yieldb (%)
We believe that the nanocrystalline structure of basic BaMnO4
(see TEM image, average size 30–40 nm with high surface area,
Fig. 1) acts as a base in the reaction.16
1
2
3
4
5
6
THF
H2O
65
5
94
90
54
73c
CH3CN
Toluene
1,4-Dioxane
CH3CN
In order to further explore the scope and limitations of the
method, we performed the reaction of 1-phenylethanol with
Ph3P and ethyl bromoacetate under the optimized reaction condi-
tions. However, the reaction did not proceed cleanly and the
obtained ketone was recovered from the reaction mixture. Further-
more, we also studied the reaction of 2-bromopropionate, benzyl
alcohol, and Ph3P under the optimized reaction conditions. The
reaction proceeded sluggishly and after 24 h only 28% of the
product was observed (Scheme 1).
It is worth noting that the substitution reaction between ethyl
bromoacetate and Ph3P proceeds much faster than oxidation of
the Ph3P; therefore, there is no Ph3P left in the reaction mixture
for further oxidation into Ph3PO. Also, the oxidation of benzyl alco-
hol in the presence of Ph3P in CH3CN at 80 °C for 2 h was studied
a
Reaction conditions: ROH (1 equiv), BaMnO4 (1.2 equiv), PPh3 (1.2 equiv), ethyl
bromoacetate (1.2 equiv), solvent (5 mL).
b
GC yields with similar E/Z stereoselectivity.
Reaction performed at 60 °C.
c
Using the optimized condition, different primary benzylic
alcohols as well as 3-phenyl-1-propanol and cinnamyl alcohol
were examined (Table 2). Reactions of benzylic alcohols contain-
ing electron donating and electron withdrawing groups
proceeded efficiently and the desired alkenes were obtained in
high to excellent yields (81–93%), with excellent stereoselectivi-
Table 2
Tandem oxidation–Wittig reaction of different alcohols using BaMnO4 in the absence of an external basea
BaMnO4 (1.2 eq)
CO2Et
RCH2OH
+
Br
CO2Et
R
PPh3 (1.2 eq), CH3CN
24 h, 80 °C
Entry
1
ROH
Product
Yield (%)
E/Z
Entry
10
ROH
Product
CO2Et
Yield (%)
83
E/Z
CO2Et
OH
Me
OH
OH
OH
1a
90
88
98:2
1j
94:6
Me
Me
Me
CO2Et
CO2Et
OH
2
3
1b
1c
96:4
93:7
11
12
1k
78
89
100
Me
Cl
Me
CO2Et
OH
CO2Et
86
1l
90:10
Cl
CO2Et
CO2Et
OH
OH
OH
4
5
1d
1e
93
91
99:1
99:1
13
14
1m
1n
86
90
90:10
99:1
O2N
Br
Cl
O2N
Br
Cl
CO2Et
CO2Et
CO2Et
OH
OH
OH
Br
Br
CO2Et
OH
OH
6
7
8
1f
90
85
84
99:1
99:1
95:5
15
16
17
1o
1p
1q
81
84
88
96:4
92:8
99:1
Cl
Cl
Cl
Cl
OMe
OMe
CO2Et
CO2Et
CO2Et
1g
1h
Cl
OH
NO2
OH
Cl
CO2Et
NO2
OPh
NO2
OPh
NO2
CO2Et
CO2Et
OH
OH
9
1i
89
99:1
18
1r
91
100
a
Reaction conditions: ROH (1 equiv), BaMnO4 (1.2 equiv), PPh3 (1.2 equiv), ethyl bromoacetate (1.2 equiv), CH3CN (5 mL), 80 °C.

M. Gholinejad et al. / Tetrahedron Letters 57 (2016) 3773–3775
3775
improved one-pot protocol with respect to the previously reported
procedures in which the oxidation of the alcohols was performed
before using a commercially available phosphonium halide salt.
The procedure is highly stereoselective and produces the respected
products as (E)-isomers in high yields.
Acknowledgments
The authors are grateful to Institute for Advanced Studies in
Basic Sciences (IASBS) Research Council, Shiraz University and Iran
National Science Foundation (INSF-Grant number of 94010666) for
support of this work. C. Nájera is also thankful to The Spanish Min-
isterio de Economia
y Competitividad (MINECO) (projects
CTQ2013-43446-P and CTQ2014-51912-REDC), FEDER, the Gener-
alitat Valenciana (PROMETEOII/2014/017) and the University of
Alicante for financial support.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.07.
027.
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Herein, we present a base-free protocol in which primary alco-
hols, triphenyl phosphine, and ethyl bromoacetate were reacted in
a tandem oxidation–Wittig reaction using BaMnO4. This is an