Novel generation of selenoaldehydes through stannic chloride-induced unsymmetrical C-Se bond cleavage of bis(N,N-dimethylcarbamoylseleno)methanes

Article abstract of DOI:10.1016/j.tetlet.2005.03.153

Treatment of a benzene or a CH₂Cl₂ solution of bis(N,N-dimethylcarbamoylseleno)methanes with SnCl₄ afforded β-1,3,5-triselenanes, and the key intermediates, acylselonium ions and selenoaldehydes, were successfully trapped

Full text of DOI:10.1016/j.tetlet.2005.03.153

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3775–3778
Novel generation of selenoaldehydes through stannic
chloride-induced unsymmetrical C–Se bond cleavage of
bis(N,N-dimethylcarbamoylseleno)methanes
Kazuaki Shimada,* Yaling Gong, Hidenori Nakamura, Rei Matsumoto,
Shigenobu Aoyagi and Yuji Takikawa
Department of Chemical Engineering, Faculty of Engineering, Iwate University, Morioka, Iwate 020-8551, Japan
Received 19 January 2005; revised 9 March 2005; accepted 18 March 2005
Available online 8 April 2005
Abstract—Treatment of a benzene or a CH2Cl2 solution of bis(N,N-dimethylcarbamoylseleno)methanes with SnCl4 afforded b-
1,3,5-triselenanes, and the key intermediates, acylselonium ions and selenoaldehydes, were successfully trapped by using allyltri-
methylsilane or 2,3-dimethyl-1,3-butadiene to obtain the allylation products or the cycloadducts, respectively.
Ó 2005 Elsevier Ltd. All rights reserved.
Recently, various methodologies for the generation of
selenoaldehydes 3 have been reported in the light of
their structural interests and the potentiality as new
reactive intermediates for the organic reactions.1 How-
ever, few studies on the conversion of symmetrical dise-
leno- or ditelluroacetals into the corresponding
chalcogenoaldehydes or chalcogenoketones were
achieved due to the liability of the target species and
the lack of preparative methods of suitable dichalco-
genoacetals as the precursors of chalcogenoaldehydes.
In the course of our studies on the novel generation of
highly-reactive species related to higher-raw chalcogeno-
carbonyl compounds, we previously found a convenient
metrical C–Se bond cleavage to generate selenoaldehy-
des 3 via acylselonium ions A through the push–pull
type removal of N,N-dimethylselenocarbamate ion and
N,N-dimethylcarbamoyl cation.4 According to our
expectation as mentioned above, we started our explora-
tion on the reaction of 1 with a Lewis acid in the pres-
ence or absence of trapping agents. In this letter, we
describe a new and convenient method for generation
of selenoaldehydes 3 through the reaction of 1 with
SnCl4 under mild conditions as well as a novel stepwise
fragmentation pathway of 1 involving the in situ forma-
tion of key intermediates A as the precursors of 3.
preparation of stable ditelluroacetal derivatives
2
Bis(N,N-dimethylcarbamoyl)diselenide (4)5 was pre-
pared by treating DMF with sodium metal and elemen-
tal selenium at 100–110 °C followed by an aerobic
exposure at rt. Subsequently, stepwise treatment of a
DMF solution of diselenide 4 with NaH6 and a gem-
dihaloalkane (benzal bromide (5a), m-chlorobenzal bro-
mide (5b), p-chlorobenzal bromide (5c), 1,1-dibromo-
ethane (5d), dibromomethane (5e), and ethyl
through the reaction of N,N-dimethyltellurocarbamate
ions with gem-dihaloalkanes.1r,2 These results urged us
to the preparation and the further reaction of symmetri-
cal diselenoacetals 1 with a Lewis acid on the basis of the
coordinating interaction between the selenocarbamate
moiety and a soft Lewis acid.3 Actually, diselenoacetals
1 are expected to undergo Lewis acid-induced unsym-
R
R
Se
Lewis Acid
- Me2NCOSe-
- Me2NCO+
Me2NOC Se
Me2NOC
X
X
CONMe2
R
H
(X=Se)
1 (X=Se)
2 (X=Te)
3
A
Keywords: Bis(N,N-dimethylcarbamoylseleno)methane; Diselenoacetal; Selenoaldehyde; b-1,3,5-Triselenane; Acylselonium ion.
*
Corresponding author. Tel.: +81 19 621 6324; fax: +81 19 621 6323; e-mail: shimada@iwate-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.153

3776
K. Shimada et al. / Tetrahedron Letters 46 (2005) 3775–3778
dichloroacetate (5f), respectively) afforded highly air-
stable bis(N,N-dimethylcarbamoylseleno)methanes
(i.e., 1a: 60%, 1b: 37%, 1c: 21%, 1d: 59%, 1e: 31%, and
1f: 46%, respectively),2 which are regarded as symmetri-
cal diselenoacetals bearing a removable N,N-dimethyl-
carbamoyl group on each selenium atom.
tral pattern of 1a. These results excluded out the forma-
tion of a tight 1a–SnCl4 complex in the reaction mixture,
and a weak coordinating interaction between 1a and
SnCl4 involving an association–dissociation equilibra-
tion was suggested. Further standing of the mixture
for 24 h at 25 °C resulted in the formation of b-1,3,5-
triselenane 6a (d = 5.59 ppm) as main component
besides diselenide 4 (d = 3.06, 3.17 ppm) and a trace
amount of benzaldehyde (7a) along with the formation
of CDCl3-insoluble products. On the other hand, little
information was obtained from the 13C NMR or 77Se
NMR monitoring of the reaction due to the broadening
and complication of the signals. It is noteworthy that no
signal assigned to the intermediates, such as acylselo-
nium ion A or selenoaldehyde 3a, was observed at all
throughout the NMR monitoring experiments.
1
Treating a CH2Cl2, a CHCl3, or a benzene solution of
1a–f with TsOH or a hard Lewis acid, such as BF3ÆOEt2,
resulted in the recovery of 1 in all cases. On the other
hand, treatment of 1a–d with SnCl4 (2.0 mol amt.) at
rt under an Ar atmosphere afforded b-1,3,5-triselenanes
6a–d, the trimers of selenoaldehydes 3a–d, along with
the recovery of 1. The yields of 6 were efficiently im-
proved by the use of 3.0–5.0 mol amt. of SnCl4, and in
such cases neither the stereoisomers of 6a–d nor any
other byproducts, except for trace amounts of 4 and
aldehydes 7, were found in the crude products. In con-
trast, 1e and 1f were not reactive toward SnCl4 under
the similar reaction conditions. Interestingly, a similar
treatment of 1a with SnCl4 under an aerobic atmosphere
just gave benzaldehyde (7a) as main products besides 4.
All the results are given in Table 1.
Trapping of selenoaldehydes 3 was efficiently carried out
by treating a benzene solution of 1 with SnCl4 in the
presence of 2,3-dimethyl-1,3-butadiene (8) to afford the
[4+2] cycloadducts 9 as shown in Scheme 1. Further-
more, when a CH2Cl2 solution of 1a was treated with
SnCl4 in the presence of allyltrimethylsilane (10) at
À70 °C, compound 11a was obtained in 24% yield be-
sides 4 (5%) and the recovery of 1a (53%). The yield
of 11a was lowered to 10% by carrying out the same
reaction at 0 °C, by which an inseparable mixture of
diselenide 12a and monoselenide 13a (12a:13a = 10:1,
approximately), N,N-dimethyl-3-butenamide (14,8 40%,
allylation product of acyl cation C), and 4 (11%) were
obtained besides the recovery of 1a (11%). All the results
of the trapping experiments using allyltrimethylsilane
(10) are given in Table 2.
When a CDCl3 solution of 1a was treated with SnCl4
(0.5 mol amt.) in an NMR tube at 25 °C, the signals
of the 1H NMR spectra of the reaction mixture revealed
a slight downfield shift with the complete retaining of
their original symmetrical spectral pattern involving a
singlet methine proton and a pair of sharp singlet signals
assigned to the N,N-dimethylcarbamoyl groups. Further
addition of SnCl4 (1.5 mol amt.) to the mixture also re-
sulted in larger downfield shift of these signals
(d = 3.12 ppm (Dd = +0.29 ppm) and 3.44 ppm (Dd =
+0.48 ppm) assigned to the N,N-dimethylcarbamoyl
group, and d = 7.00 ppm (broadening, Dd = +0.89 ppm)
assigned to the methine proton) with retaining the spec-
It is noteworthy that the independent reaction of sele-
nocarbamate 11a with SnCl4 in the presence of allyltri-
methylsilane (10) at 0 °C only gave the recovery of 11a
Table 1. SnCl4-induced conversion of 1 into b-1,3,5-triselenanes 6
R
Se
R
O
O
R
O
Lewis acid
R.T.
+
Se
Se
+
4
R
H
Me2N
Se Se NMe2
R
6
1
7
Substrate/1, R
Lewis acid (mol amt.)
Solvent
Time/h
Yield/%
6
7
4
Recov.
C6H5 (1a)
C6H5 (1a)
BF3ÆOEt2 (2.0)
SnCl4 (3.0)
SnCl4 (5.0)
SnCl4 (5.0)
SnCl4 (3.0)
CH2Cl2
CH2Cl2
Benzene
Benzene
CH2Cl2
1
1
0
0
0
Quant.
16
56 (6a)a
41 (6b)
43 (6c)a
15 (6d)a
Trace (7a)
Trace (7b)
Trace (7c)
0b
Trace
Trace
Trace
Trace
m-ClC6H4 (1b)
p-ClC6H4 (1c)
CH3 (1d)
1
Trace
Trace
5
6
24
a Refs. 1k,p, and 7.
b Acetaldehyde (7d) was not found or detected at all in the crude mixture maybe due to evaporation during the workup procedure.
O
R
O
Se
9a (R=C6H5, 51%)
9b (R=m-ClC6H4, 25%)
9c (R=p-ClC6H4, 51%)
8 (10 mol amt.)
Me2N
Se Se NMe2
R
SnCl4 (5.0 mol amt.)
Benzene, R.T. 5-6 h
1
9
Scheme 1. Trapping of selenoaldehydes 3 using 2,3-dimethyl-1,3-butadiene (8).

K. Shimada et al. / Tetrahedron Letters 46 (2005) 3775–3778
3777
Table 2. Reaction of bis(N,N-dimethylcarbamoylseleno)phenylmethane (1a) with SnCl4 in the presence of allyltrimethylsilane (10)
SiMe3
O
R
R
O
R
10
Se
+
Me2N
Se
11a
+
Me2N
+
1a
Se
2
SnCl4
CH2Cl2
2
14
12a
13a
SnCl4 (mol amt.)
10 (mol amt.)
Temp/°C
Time/h
Yield/%
14
11a
12a
13a
4
Recov.
2.0
2.0
5.0
5.0
À70
2.5
5
24
10
0
20a
0
2a
0
40
5
11
53
11
0
a Yields of diselenide 12a and monoselenide 13a were estimated approximately from the integration of the 1H NMR spectrum of the mixture.
11
12 + 13
SiMe3
SiMe3
Me2N
SnCl4
O
Se
SnCl4
[ O ]
1
4
6
Se
R
R
H
SnCl4
Me N
•
O
-
3
A
2
C
O
SiMe3
14
Me2N
Se
•SnCl4
B
9
Scheme 2. Plausible stepwise pathway for the generation of selenoaldehydes 3 through the reaction of bis(N,N-dimethylcarbamoylseleno)methanes 1
with SnCl4.
and the treatment of 6a with SnCl4 in the presence of 10
under a similar condition just gave a mixture of 12a and
13a with a similar 12a:13a ratio to that obtained
through the reaction of 1 with SnCl4. Both 11a and 14
were regarded as the allylation products of acylselonium
ion A, generated through SnCl4-induced removal of
N,N-dimethylselenocarbamate ion from 1, and N,N-
dimethylcarbamoyl cation (C), respectively, and these
results supported the stepwise pathway of conversion
of 1 into selenoaldehydes 3 involving the formation of
A9 at low temperature and the subsequent removal of
N,N-dimethylcarbamoyl cation (C) from A at higher
temperature as shown in Scheme 2. The formation of
4 could also be explained by aerobic oxidation of N,N-
dimethylselenocarbamate ion–SnCl4 complex B during
standing and the usual workup procedure.
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1
with
SnCl4 as well as the evidences of stepwise fragmentation
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.03.153.

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