Generation of selenoaldehydes via retro Diels-Alder reaction and their behavior in the reaction with some enophiles

Article abstract of DOI:10.1080/10426500590906283

The [4 + 2] cycloadducts of selenoaldehydes and anthracene regenerate selenoaldehydes in situ quantitatively under neutral conditions via thermal retro Diels-Alder reaction. The reactions of selenoaldehydes generated by this method with 2-silyloxy-1,3-but

Full text of DOI:10.1080/10426500590906283

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Phosphorus, Sulfur, and Silicon
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Generation of Selenoaldehydes
via Retro Diels–Alder Reaction
and Their Behavior in the
Reaction with Some Enophiles
Masahito Segi a , Aojia Zhou a & Mitsunori Honda a
a Department of Applied Science, Graduate School
of Natural Science and Technology, Kanazawa
University , Kanazawa, Japan
Published online: 01 Feb 2007.
To cite this article: Masahito Segi , Aojia Zhou & Mitsunori Honda (2005) Generation
of Selenoaldehydes via Retro Diels–Alder Reaction and Their Behavior in the Reaction
with Some Enophiles, Phosphorus, Sulfur, and Silicon and the Related Elements,
180:3-4, 1045-1049, DOI: 10.1080/10426500590906283
To link to this article: http://dx.doi.org/10.1080/10426500590906283
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Phosphorus, Sulfur, and Silicon, 180:1045–1049, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590906283
Generation of Selenoaldehydes via Retro Diels–Alder
Reaction and Their Behavior in the Reaction with Some
Enophiles
Masahito Segi
Aojia Zhou
Mitsunori Honda
Department of Applied Science, Graduate School of Natural Science
and Technology, Kanazawa University, Kanazawa, Japan
The [4 + 2] cycloadducts of selenoaldehydes and anthracene regenerate selenoalde-
hydes in situ quantitatively under neutral conditions via thermal retro Diels–Alder
reaction. The reactions of selenoaldehydes generated by this method with 2-silyloxy-
1,3-butadiene, 2-methoxyfuran, and 5-ethoxyoxazoles are described.
Keywords 3-Selenazoline; penta-2,4-dienoate; retro Diels–Alder reaction; selenoalde-
hyde; selenacyclohexanone
INTRODUCTION
In recent decades, there has been a growing interest in organoselenium
chemistry.1−3 Some of the compounds bearing a carbon-selenium dou-
ble bond, a so-called selenocarbonyl group, have been paid attention as
building blocks in organic synthesis and as important intermediates
in the synthesis of selenium-containing heterocycles.4−6 Among them
selenoaldehydes generally have been considered to be a highly reac-
tive dienophile for cycloaddition reactions. Several synthetic methods
of these reactive selenoaldehydes have already been reported.7−9 We
have also described an efficient method for the straightforward syn-
thesis of selenoaldehydes and their in situ trapping by a conjugated
diene.10−12 The synthetic procedure involves the reaction of aldehydes
with bis(dimethylaluminum) selenide, (Me2Al)2Se, in which aluminum
plays a key role to replace the carbonyl oxygen with a selenium atom.
Received January 27, 2004; accepted October 5, 2004.
Address correspondence to Masahito Segi, Kanazawa University, Department of Ap-
plied Science, Graduate School of Natural Science and Technology, 2-40-20 Kodatsuno,
Kanazawa 920-8667, Japan. E-mail: segi@t.kanazawa-u.ac.jp
1045

1046
M. Segi et al.
However, the cycloaddition by this method was inapplicable to 1,3-
dienes substituted by oxygen and/or nitrogen atoms because of the co-
ordination of the aluminum to a lone pair of electrons on those atoms.
So, another protocol for selenoaldehyde synthesis was required in or-
der to attain the cycloaddition reaction with a heteroaom-functionalized
1,3-diene. We selected retro Diels–Alder reaction for generation of se-
lenoaldehydes. In this paper we describe the formation of selenoalde-
hydes under neutral conditions via thermal retro Diels–Alder reac-
tion and their reaction behavior with some heteroatom-substituted
enophiles.
RESULTS AND DISCUSSION
The reaction of aromatic and aliphatic aldehydes with (Me2Al)2Se in
the presence of anthracene gave the [4+2] cycloadducts (1) of the cor-
responding selenoaldehydes and anthracene in good yields as a stable
solid in the most cases. These cycloadducts were very stable at room
temperature after purification by column chromatography on silica gel.
The results are summarized in Table I. Heating of a toluene solution
of 1 (R = aromatic) and 2,3-dimethyl-1,3-butadiene at 100◦C for 30 min
gave the cycloadducts of the corresponding selenoaldehydes and 2,3-
dimethyl-1,3-butadiene in excellent yields. This indicates the thermal
retro Diels–Alder reaction of 1 proceeds efficiently under the above con-
ditions to regenerate selenoaldehydes quantitatively. Cycloadducts 1
with an aliphatic substituent were very stable at 100◦C in toluene,
TABLE I [4 + 2] Cycloaddition of Selenoaldehydes with Anthracene
R
Time (h)
Yield (%)a
M.p. (◦C)
Ph
4.0
4.0
4.0
4.0
4.0
4.0
6.0
6.0
85
66
96
73
90
91
60
40
134–135
120–121
142–143
144-145
157–158
116–117
106–108
140–142
p-MeOC6H4
p-CF3C6H4
p-MeSC6H4
p-NCC6H4
p-FC6H4
n-Pr
PhCH2CH2
aIsolated yield.

Generation and Reaction of Selenoaldehydes
1047
TABLE II Reaction of Selenoaldehydes with 2-Trimethylsilyloxy-
1,3-butadiene
R
Temp. (◦C)
Time (h)
Total yield (%)a
“3-selena” : “4-selena”b
p-NCC6H4
p-CF3C6H4
p-CF3C6H4
p-FC6H4
p-MeSC6H4
Ph
110
110
160
110
110
110
110
160
5.0
5.0
3.0
5.0
5.0
5.0
4.0
5.0
89
83
93
88
89
87
80
85
59:41
51:49
50:50
42:58
42:58
40:60
36:64
23:77
p-MeOC6H4
n-Pr
aIsolated yield.
bDetermined by 1H NMR.
but the heating at 160◦C resulted in the regeneration of aliphatic se-
lenoaldehydes in situ effectively. Thus, 1 can serve as convenient and
clean precursors of selenoaldehydes.13,14
Cycloadducts 1 were heated with 2-trimethylsilyloxy-1,3-butadiene
at 110◦C or 160◦C, followed by addition of trifluoroacetic acid to afford
a mixture of 3- and 4-selenacyclohexanones in good yields.13 The re-
sults are shown in Table II. The regiochemistry in this cycloaddition
reaction was influenced by the electronic character of a substituent R
on selenoaldehydes. p-Cyanophenyl substituted selenoaldehyde gave
3-selena adduct as the slightly preferential product, while that with
a p-methoxyphenyl substituent formed 4-selena adduct preferentially.
Selenoaldehyde with n-propyl group as a donor substituent exhibited
the regiochemical result with preference of 4-selena adduct, though se-
lectivity was not high.
The reaction of aromatic and aliphatic selenoaldehydes, generated
via thermal retro Diels–Alder reaction from 1, with 2-methoxyfuran
gave methyl penta-2,4-dienoates in high yields along with the depo-
sition of elemental selenium as shown in Scheme 1. We have already
reported the above reaction with detail results containing mechanistic
considerations.15
5-Alkoxyoxazoles correspond to the compounds replaced by the C4
atom of the above 2-methoxyfuran with a nitrogen atom, and also
are cyclic 2-azadienes. Thermally generated selenoaldehydes from

1048
M. Segi et al.
SCHEME 1 Reaction of selenoaldehydes with 2-methoxyfuran.
1 reacted with 5-ethoxyoxazoles to afford 1:1 adducts having 3-
selenazoline structure as a mixture of diastereomers in good yields.
The Diels–Alder-type adducts were not detected. The results are sum-
marized in Table III. The yields in the reaction at 160◦C generally
were higher than those at 90◦C. A major isomers in the reaction of
selenoaldehydes with 5-ethoxy-4-methyl-2-phenyloxazole was assigned
to trans, which refers to the relationship between the phenyl and the
ester groups, on the basis of NOE measurement.
The reaction mechanism for the formation of 3-selenazoline products
has not been established, but the most likely pathway, at the present
stage, may be as follows. The Diels–Alder reaction of selenoaldehyde
with 5-ethoxyoxazole affords two regioisomeric [4 + 2] cycloadducts that
would be at equilibrium with each other under thermal conditions. The
isomer having an orthoester structure would be expected readily to un-
dergo C Se bond cleavage selectively to give the betaine intermediate,
TABLE III Reaction of Selenoaldehydes with 5-Ethoxyoxazoles
R
Rꢀ
Temp. (◦C)
Time (h)
Yield (%)a
trans : cisb
Ph
Ph
Me
Me
Me
Me
Me
Me
H
90
160
160
160
160
160
160
160
160
4.0
2.0
2.0
2.0
3.0
3.0
2.0
2.0
3.0
65
95
91
88
96
67
95
88
91
85:15
69:31
70:30
78:22
69:31
67:33
71:29
69:31
80:20
p-MeOC6H4
p-NCC6H4
PhCH2CH2
n-Pr
p-NCC6H4
p-MeOC6H4
PhCH2CH2
H
H
aIsolated yield.
bDetermined by 1H NMR.

Generation and Reaction of Selenoaldehydes
1049
followed by the ring opening and the recyclization to yield an isomeric
mixture of 3-selenazoline compound. The similar reaction of thioalde-
hydes with 5-alkoxyoxazole already has been reported through both
experimental16 and theoretical17 studies, in which the reaction mecha-
nism involves the Diels–Alder reaction as the first step.
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