An efficient method for selective oxidation of 1,2-diols in water catalyzed by Me2SnCl2

Article abstract of DOI:10.1039/c3ra42754d

Dimethyltin(iv)dichloride-catalyzed selective oxidation of 1,2-diols in water was achieved using dibromoisocyanuric acid (DBI) or Br₂ as oxidants. The catalyst activates the 1,2-diol moiety through the formation of stannylene acetal in addition to enhancing selectivity. Various cyclic and acyclic 1,2-diol substrates have been selectively oxidized affording α-hydroxyketones in good to excellent yields. This method is safe and simple in operation.

Full text of DOI:10.1039/c3ra42754d

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An efficient method for selective oxidation of 1,2-diols
in water catalyzed by Me2SnCl23
Cite this: RSC Advances, 2013, 3, 19247
Julius M. William, Masami Kuriyama and Osamu Onomura*
Received 5th June 2013,
Accepted 8th August 2013
DOI: 10.1039/c3ra42754d
www.rsc.org/advances
Dimethyltin(IV)dichloride-catalyzed selective oxidation of 1,2-
diols in water was achieved using dibromoisocyanuric acid (DBI)
or Br2 as oxidants. The catalyst activates the 1,2-diol moiety
through the formation of stannylene acetal in addition to
enhancing selectivity. Various cyclic and acyclic 1,2-diol substrates
have been selectively oxidized affording a-hydroxyketones in
good to excellent yields. This method is safe and simple in
operation.
far, a handful of methods on selective oxidation of 1,2-diols in
organic solvents have been presented using organotin catalysts,15
chiral copper compounds,16 and electrochemical methods.15,17
But to the best of our knowledge no report on selective oxidation
in water has been presented. Our most recent report on selective
monobenzoylation of 1,2-diols in water,18 and other reports on
selective monobenzoylation in organic solvents,19 greatly moti-
vated this investigation. In these reports, Me2SnCl2 catalyst
activates the 1,2-diol moiety through the formation of stannylene
acetal and also facilitate selectivity. We envisioned that if we
introduce a suitable oxidizing agent to a stannylene acetal in
water, then selective oxidation may be achieved.
Introduction
Organic reactions in water as a solvent or co-solvent have attracted
a lot of attention recently. This is because of the unique properties
of water and its key role as a solvent for green chemistry.1 With
increasing regulatory pressure targeting organic solvents, the
development of nonhazardous alternatives is of great importance.2
Selective oxidation of 1,2-diols to their corresponding a-hydro-
xyketones is a basic and an important organic reaction.3 This is
because a-hydroxyketone units are found in some antitumor
antibiotics and important natural products like olivomycin A,4
kurasoin A and B.5 However, the few methods available for
selective oxidation of 1,2-diols to a-hydroxyketones,6–11 suffer from
drawbacks such as relatively low yields and side reactions
accompanying C–C bond cleavage.10 Use of Bu2SnO,12 has been
recognized as a highly selective method. It proceeds in two stages:
formation of stannylene acetal b by azeotropic condensation of
1,2-diol 1 with Bu2SnO followed by the oxidation of b by Br2
(Scheme 1).
Results and discussion
We began our investigation on catalytic selective oxidation of 1,2-
diols in water using cis-cyclooctane-1,2-diol 1a as a model
substrate and halogen cation ‘‘source’’ oxidants. After a series of
optimization studies, we found that monoselective oxidation of 1a
proceeded efficiently in the presence of 10 mol% of Me2SnCl2, 1.2
equiv. potassium carbonate, and 1.5 equiv. Br2 in water at low
temperature (0 uC) under shielding the light for 1 h to afford 2a in
(85%) yield (entry 1, Table 1).
Temperature optimization experiments revealed that 25 uC
was the optimum temperature for oxidation of 1a using Br2,
affording (91%) yield (entry 1–3). Reducing the amount of Br2 to
1.0 equiv. led to a drop in yield (78%) (entry 4). Commonly known
mild oxidizing agents, N-bromosuccinimide (NBS) and
N-iodosuccinimide (NIS) afforded lower yields at 0 uC (entries 5
and 6), while N-chlorosuccinimide (NCS) only afforded a trace
amount of the desired product (entry 7). Dibromoisocyanuric acid
This method has been applied to the synthesis of (+)-
spectinomycin13 and to the oxidation of sugars into oxo-sugars.14
Unfortunately, it is unfavourable from an environmental viewpoint
and also requires more than 1 equimolar amount of Bu2SnO. So
Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki 852-8521, Japan. E-mail: onomura@nagasaki-u.ac.jp; Fax: +81 95 819
2476; Tel: +81 95 819 2429
Electronic supplementary information (ESI) available: Experimental procedures
3
and characterization data. NMR spectra for all novel compounds. See DOI: 10.1039/
c3ra42754d
Scheme 1 Oxidation of 1,2-diols via stannylene acetal.
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Table 1 Selective oxidation of 1,2-diol 1a in water using various oxidants
Table 2 Selective oxidation study by varying various parameters
Yield (%)a
Entry
Variation from the ‘‘standard’’ conditions
Yield (%)a
Entry
Oxidant (equiv.)
Temp. (uC)
1b
2b
3b
4b
5b
6b
7b
8b
9c
Under O2
Under Ar
90
73
78
63
54
40
43
65
,1
76
60
1
2
3
4
5
6
7
8
Br2 (1.5)
Br2 (1.5)
Br2 (1.5)
Br2 (1.0)
NBS (2.0)
NIS (2.0)
NCS (2.0)
DBIb (1.0)
DBIb(1.5)
DBDMHc (1.5)
DIDMHd (1.5)
H2O2 (2.0)
0
85
91
89
78
40
29
trace
90
83
86
45
0
25
210
25
0
0
0
0
0
0
0
0.05 equiv. Me2SnCl2
0.01 equiv. Me2SnCl2
Absence of Me2SnCl2 for 1 h
Absence of Me2SnCl2 for 5 h
Absence of K2CO3
Without shielding the light
Absence of Me2SnCl2
0.05 equiv. Me2SnCl2
Absence of K2CO3
9
10c
11c
10
11
12
0
a
b
c
Isolated yield. 1.5 equiv. Br2 was used. 1.0 equiv. DBI was used.
a
b
c
Isolated yield. Dibromoisocyanuric acid. 1,3-Dibromo-5,5-
d
dimethylhydantoin. 1,3-Diiodo-5,5-dimethylhyndantoin.
Me2SnCl2, although at the expense of slight erosion in selectivity
(entry 1). Dialkyltin dichloride catalysts gave moderate yields of
mono-oxidation products (entries 2–4), while dialkyltin oxide and
Cu salts did not show impressive activities (entries 5–7 and 8–9).
We next explored the substrate applicability to the oxidation
system involving Me2SnCl2 as the catalyst and Br2 or DBI as the
oxidants. Generally, selective oxidation using 1.5 equiv. Br2 and 1.0
equiv. DBI gave comparable yields for all substrates screened
(Table 4). All meso- and trans-cyclic-1,2-diols, underwent selective
oxidation affording mono-oxidized products in moderate to
excellent yields (entries 1–6). A number of acyclic-1,2-diols were
selectively oxidized at elevated temperature of 50 uC,22 affording
corresponding a-hydroxy-ketones in excellent to moderate yields
(entries 7–9). Next, we carried out selective oxidation of 1,2-diols
having both primary and secondary alcohols. When 1,2-diols 1k,
1l, 1m, 1n, and 1o were treated under these catalytic conditions,
(DBI), first reported by Gottardi20 as a mild but more effective
brominating agent than NBS,21 afforded the desired product in an
excellent yield of 90% with 1 equiv. (entry 8). Increasing the
amount of DBI to 1.5 equiv. led to the formation of some diketone
product, resulting in a drop in yield of the desired product (entry
9). 1.5 equiv. 1,3-dibromo-5,5-dimethylhydantoin (DBDMH) gave
comparable results. However, a closely related 1,3-diiodo-5,5-
dimethylhydantoin (DIDMH) was not efficient in effecting this
oxidation (entries 10 and 11). Efforts to use H2O2 as oxidant failed
as it did not oxidize our reaction.
We next investigated the effect of a number of different
reaction parameters on the efficiency of selective oxidation of 1,2-
diol 1a in water with Br2 or DBI. The results are summarized in
Table 2.
With bromine as the oxidant, the 90% yield was maintained
under oxygen atmosphere, while in argon there was a drop to 73%
(entries 1 and 2). Reducing the catalyst loading to 0.05 equiv. and
0.01 equiv. led to a drop in yields (entries 3 and 4). As expected,
poor yield was obtained in the absence of the catalyst (entry 5).
Extending the reaction time to 5 h in the absence of Me2SnCl2,
resulted in the formation of the diketone and associated drop in
the yield of the monoketone (entry 6). Similarly, absence of the
base (potassium carbonate) or performing the reaction in presence
of light led to drop in yields (entries 7 and 8). In the reaction with
DBI as the oxidant but without the catalyst, essentially no reaction
was observed (entry 9). As with bromine as the oxidant, reducing
the catalyst loading to 0.05 equiv., also led to a drop in yield (76%)
(entry 10). Similarly, the yield dropped in the absence of the base
(entry 11).
Table 3 Screening of catalyst with DBI as oxidant
Entry
Catalyst
Yield (%)a
1
2
3
4
5
6
7
8
9
Me2SnBr2
n-Oct2SnCl2
n-Bu2SnCl2
Ph2SnCl2
n-Bu2SnO
Ph2SnO
Me2SnO
CuCl2
Cu(OTf)2
87
61
68
57
23
58
30
15
10
Lack of reaction with DBI in the absence of Me2SnCl2
prompted us to study the effects of other organotin compounds
as potential catalysts in reactions involving different 1,2-diols with
1.0 equiv. of this oxidant. The results are summarized in Table 3.
Me2SnBr2 was found to be effective as a catalyst in place of
a
Isolated yield.
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Table 4 Scope of substrates with Br2 or DBI as oxidants
Yield (%)a
Br2 DBI
Entry 1,2-diol
Temp (Time)
0 uC (1 h)
Product
1
2
3
4
5
6
7
b: n = 3
93
91
0 uC (2 h)
c: n = 2
Scheme 2 Competition reaction between cis-1,2-cyclooctanediol and cycloocta-
72
69
nol.
0 uC (3 h)
d: n = 1
55
57
25 uC (0.5 h)
0 uC (2 h)
a,e: n = 4
81
c,f: n = 2
73 71
d,g: n = 1
85
0 uC (3 h)
57
92
57
91
50 uC (1 h)
8
9
50 uC (1 h)
50 uC (1 h)
2h
89
56
88
59
10
11
12
13
14
0 uC (2 h)
rt (2 h)
82
86
34
69
85
87
82
37
67
80
Scheme 3 Proposed reaction pathway for selective oxidation using Me2SnCl2 as a
catalyst with Br2 or DBI as the oxidant.
rt (3 h)
other hand, in the absence of Me2SnCl2, 2q was predominantly
obtained. These results clearly demonstrate the role of Me2SnCl2
in the activation of the hydroxyl groups of 1,2-diols and the
versatility of the DBI as a promising oxidant for mono-ols in water.
Finally a proposed reaction pathway for selective oxidation of
1,2-diols 1 is shown in Scheme 3. Chelate complex A23 formed
from the interaction of 1 with Me2SnCl2 or related complex A9
(activated intermediates), could be oxidized by (Br+) generated
from the oxidant in water leading to 2.
0 uC (1 h)
0 uC (1 h)
15
0 uC (2 h)
60
56
Conclusions
a
Isolated yield.
A catalytic process for selective oxidation of 1,2-diols in water
under mild conditions has been achieved. This method not only
provides a new approach to an environmentally benign process for
selective oxidation 1,2-diols but also introduces an efficient
oxidant (DBI) with superior results compared to NBS, which has
commonly been used for selective oxidation in organic solvents.
The method may also have potential in selective oxidation of
specific hydroxyl groups in polyols. Selective oxidation using safer
catalysts and enantioselective oxidation of 1,2-diols in water is
currently ongoing in our lab.
the secondary alcohols were selectively oxidized preferentially
affording products 2k, 2l, 2m, 2n, and 2o respectively in moderate
to excellent yields (entries 10–14). Additionally, 1,2,6-hexanetriol
1p underwent selective oxidation at the secondary hydroxyl group
of the 1,2-diol moiety affording 2p in good yield (entry 15).
Competitive reaction between 1,2-diol 1a and mono-ol 1q was
next tried (Scheme 2). In the presence of Me2SnCl2, 1a and 1q were
oxidized to 2a and 2q in the ratio of 74 : 26 respectively. On the
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Acknowledgements
This research was supported by MEXT KAKENHI Grant
Number 23105539 as Grantin-Aid for Scientific Research on
Innovative Areas from The Ministry of Education, Culture,
Sports, Science and Technology, JSPS KAKENHI Grant Number
24590012 as Grant-in-Aid for Scientific Research (C) from The
Japan Society for the Promotion of Science, Research Grant for
Pharmaceutical Sciences from Takeda Science Foundation,
and the President’s Discretion Fund of Nagasaki University.
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19250 | RSC Adv., 2013, 3, 19247–19250
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