Efficient reductive deoximation by tungsten(VI) chloride (WCl6) or molybdenum(V) chloride (MoCl5) in the presence of Zn powder in CH3CN

Article abstract of DOI:10.1246/bcsj.75.1761

Tungsten(VI) chloride (WCl₆) or molybdenum(V) chloride (MoCl₅) in the presence of zinc powder in CH₃CN provides an efficient and facile procedure for the deprotection of oximes to their corresponding aldehydes and ketones in high yields.

Full text of DOI:10.1246/bcsj.75.1761

c. Jpn.
e Chemical © 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1761–1764 (2002) 1761
ciety of Ja-
n
e Chemical
ciety of Ja-
Efficient Reductive Deoximation by Tungsten(Ⅵ) Chloride (WCl ) or
6
n
Molybdenum(Ⅴ) Chloride (MoCl ) in the Presence of Zn Powder in
5
CH CN
3
6 5
Deoximation with WCl /Zn or MoCl /Zn
H. Firouzabadi et al.
0
2
Habib Firouzabadi,* Arezu Jamalian, and Babak Karimi
6
6
1
4
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
SJA8
0
9-2673
(
Received October 30, 2001)
3
0
0
86
6 5 3
Tungsten(Ⅵ) chloride (WCl ) or molybdenum(Ⅴ) chloride (MoCl ) in the presence of zinc powder in CH CN pro-
1
2
vides an efficient and facile procedure for the deprotection of oximes to their corresponding aldehydes and ketones in
high yields.
0.2
Oximes are highly stable and readily prepared compounds
which are used extensively for the protection of carbonyl
1
groups, for the purification and characterization of carbonyl
2
compounds, and also in the preparation of amides via Beck-
3,4
mann rearrangement.
Oximes represent a series of deriva-
tives for the classical identification of carbonyl compounds, as
5
exemplified in the synthesis of erythromycin derivatives.
Since oximes could be prepared from non-carbonyl starting
materials, such as the Barton reaction, their conversion to car-
6
Scheme 1.
4
/NaI system has been reported. In this procedure
bonyl functionality is important from a synthetic point of view.
Although quite a number of methods for converting oximes to
carbonyl compounds are available, the discovery of newer effi-
cient methods is of practical importance. Some of the methods
3
3
by the TiCl
the ratio of the reagents is 1:2:2, and also requires successive
washings with an aqueous solution of sodium thiosulfate and
water in order to destroy any liberated iodine in the reaction
process. In continuation of our interest to develop new appli-
cations of WCl and MoCl in organic synthesis, we now re-
7
8
reported for deoximation involve periodic acid, PCC, PCC/
9
10
2 2
H O , triethylammonium chlorochromate, bispyridinesilver
11
12
13
14
6
5
permanganate,
Raney nickel,
CrO
3
/TMCS,
2 4
N O ,
15
16
17
port on new reductive deoximation reactions by the in situ gen-
eration of low-valent tungsten and molybdenum species from
WCl and MoCl in the presence of zinc powder (Scheme 1).
Dowex-50, dimethyldioxirane, t-BuOOH, (NH ) S O /
4 2 2 8
21
1
8
19
20
silica, Mn(OAc)
3
,
4
N-haloamides, NaIO /silica, Des-
2
2
23
24
6
5
Martin periodinane, NaBiO
gel, Zn/AcOH, alumina supported KMnO .
4
3
/silica, IBX, Zn(NO
3
2
) /silica
25
26
27
The prepara-
Results and Discussion
tion of carbonyl compounds from their oxime derivatives by
conventional methods have several limitations; i.e. the reagents
used are often hazardous along with reactions requiring long
reaction times or reflux temperatures. In addition, some of the
methods cited in the literature do not describe the deoximation
Various types of benzaldehyde oximes were efficiently con-
verted to their benzaldehydes with WCl (1 equiv) and Zn
6
powder (3 equiv) in good-to-excellent yields (Table 1, entries
1–7). The deprotection of different types of ketoximes were
also achieved in moderate-to-good yields (Table 1, entries 8–
10). This method was also applied for the deprotection of an
acyloin ketoxime (Table 1, entry 11); only 50% of the corre-
sponding acyloin was obtained. This method was also suc-
cessfully applied for the deoximation of oximes derived from
aldehydes and ketones bearing enolizable hydrogens (Table 1,
16,28,29
30
of aldoximes,
or they give low yields of aldehydes.
Some of the oxidative deoximation of oximes are accompanied
8
by the further oxidation of carbonyl compounds. Moreover,
Des-Martin periodinane causes an explosion under excessive
2
2
heating during preparation.
Low-valent oxophilic d-block metals have become impor-
tant in the deoxygenation of different types of organic sub-
entries 8,10, 12–14). We have also applied MoCl (0.6–0.8
5
strates. Recently, we have reported that WCl
6
/NaI and WCl
6
/
equiv) and Zn powder (2–3 equiv) for the deoximation of dif-
Zn are efficient reagents for the coupling of sulfonic and
sulfinic acid derivatives. We have also reported that MoCl in
5
ferent aromatic and saturated aliphatic oximes to their corre-
sponding carbonyl compounds in good-to-excellent yields
(Table 2). The deprotection of an acyloin ketoxime (Table 2,
3
1
the presence of NaI or Zn powder can be used for the coupling
of sulfonic and sulfinic acid derivatives, the deoxygenation of
entry 10) was also tried with a lesser amount of MoCl /Zn
5
3
2
sulfoxides and the reductive coupling of sulfonyl chlorides.
Deoximation by the in situ low-valent generation of titanium
compared with WCl /Zn, that produced the corresponding acy-
6
loin in 70% (Table 2, entry 10), and was higher than that ob-

1
762 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
6 5
Deoximation with WCl /Zn or MoCl /Zn
Table 1. Deprotection of Oximes with WCl and Zn Powder
6
3
in CH CN
a)
Entry
Substrate
Subst./
WCl /Zn
Time
min
Yield
%
6
1
2
1:1:3
1:1:3
15
91
Scheme 2.
20
88
ily hydrolyzed during the work-up to produce the correspond-
ing carbonyl compounds. We have shown this proposal by the
mechanism given above (Scheme 2). The work-up of the
3
4
5
1:1.4:4
1:1.2:4
1:1.1:4
16
15
20
95
93
92
deoximation reactions with WCl
ily proceed under both aqueous and non-aqueous conditions,
whereas, an aqueous work-up for similar reactions with TiCl
6 5
/Zn and MoCl /Zn could eas-
4
/
3
3
NaI is an obligatory process.
Conclusion
In this study, new applications of tungsten(Ⅵ) chloride
(
WCl ) and molybdenum(Ⅴ) chloride (MoCl ) were intro-
duced for the reductive deprotection of oximes to their parent
carbonyl compounds. We found that MoCl /Zn system is more
efficient than WCl /Zn for this purpose. The mildness of the
6
5
5
6
reaction conditions, easy and non-aqueous work-up, and high
yields of the products are worth mentioning concerning the
presented methods.
6
7
1:1:3
15
15
25
75
94
88
Experimental
1:1.2:4
1:1:3
General: All yields refer to isolated pure products, unless
otherwise stated. Because all of the products are known com-
pounds, elemental analyses are not given. Most of the products
were purified by column chromatography or recrystallization and
were identified by a comparison of their mp, bp, IR, MS, and
8
9
34
NMR with those reported for authentic samples. All of the sol-
vents were completely dried and redistilled.
General Procedure for the Deprotection of Oximes Using
1:1.2:4
1:1.1:4
40
20
93
75
the MoCl
sion of oxime (1 mmol) and zinc powder (2–3 mmol) in dry aceto-
nitrile (10 mL), MoCl (0.6–0.7 mmol) was added; the resulting
5
–Zn System (Non-Aqueous Work-Up). To a suspen-
5
1
0
mixture was stirred at room temperature. After completion of the
reaction (5–40 min), silica-gel (1 g) was added to the mixture and
the solvent was evaporated under reduced pressure. The resulting
powder was added on a silica-gel pad (3 cm thick) and washed
with petroleum ether (100 mL). Evaporation of the solvent afford-
ed the corresponding pure carbonyl compound (Table 2). In the
case of benzaldehyde oxime the yield was determined by its 2,4-
dinitrophenylhydrazone derivative.
1
1
1
2
1:1.2:4
1:1.2:4
15
25
50
73
1
1
3
4
1:1.2:4
1:1.1:4
25
30
77
70
General Procedure for the Deprotection of Oximes Using
the WCl
oxime (1 mmol) and zinc powder (3–4 mmol) in dry acetonitrile
10 mL), WCl (1–1.4 mmol) was added; the resulting mixture
was stirred at room temperature. After completion of the reaction
15–40 min), aqueous NaOH (10%, 25 mL) was added and the
6
–Zn System (Aqueous Work-Up). To a suspension of
(
6
a) Isolated yields.
(
6
tained in 50% using WCl /Zn . We believe that in these reac-
mixture was extracted with ether (2 × 25 mL). The organic layer
tions low-valent molybdenum or tungsten is responsible for a
deoxygenation reaction which occures occurring by an attack
of the low-valent metal to the oxygen of oximes. The generat-
was separated and dried over anhydrous Na SO . Removal of the
2
4
solvent gave an almost pure product. Further purification was
achieved by column chromatography on silica-gel using petroleum
ether as an eluent or by recrystallization from an appropriate sol-
5 3 2
ed imines in the presence of MoCl , MoCl , and ZnCl are eas-

H. Firouzabadi et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1763
Table 2. Deprotection of Oximes with MoCl
5
3
and Zn Powder in CH CN
a)
a)
Entry
Substrate
Subst./
Time
min
Yield
%
Entry
Substrate
Subst./
Time
min
Yield
%
5
MoCl /Zn
MoCl
5
/Zn
1
1:0.6:2
5
94
10
1:0.6:2
12
70
2
1:0.6:2
5
92
11
1:0.7:2
25
92
3
4
5
6
1:0.6:2
1:0.6:2
1:0.6:2
1:0.6:2
10
10
10
10
91
95
94
86
12
13
14
15
1:0.6:2
1:0.8:3
1:0.7:2
1:0.7:3
15
40
30
30
93
65
78
85
7
8
1:0.6:2
1:0.6:2
7
7
90
97
16
17
1:0.7:2
1:0.7:2
20
20
81
78
9
1:0.7:2
15
84
18
1:0.7:2
25
74
a) Isolated yields.
vent (Table 1).
ed by G. Lukacs, Springer-Verlag, Berlin (1993), Vol.2, pp. 121–
1
40.
5
The authors are thankful to Mrs. M. Hosseini for donating
some of the substrates, and also to the Shiraz University Re-
search Council for partial support of this work.
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