Synthesis and [2+4]cycloadditions of two 1-aza-1,3-butadiene-1-carbonitriles

Article abstract of DOI:10.1016/j.tetlet.2009.09.146

3-Methyl- and 3-ethyl-1-aza-1,3-butadiene-1-carbonitriles were synthesized by reaction of the corresponding 3-alkylacroleins with bis(trimethylsilyl)-carbodiimide using titanium tetrachloride as catalyst. They were highly reactive and difficult to purify

Full text of DOI:10.1016/j.tetlet.2009.09.146

Tetrahedron Letters 50 (2009) 7116–7117
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Tetrahedron Letters
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Synthesis and [2+4]cycloadditions of two 1-aza-1,3-butadiene-1-carbonitriles
*
In Seo Kee, H. K. Hall Jr.
Department of Chemistry, The University of Arizona, Tucson, AZ 85721, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2009
Revised 18 September 2009
Accepted 24 September 2009
Available online 29 September 2009
3-Methyl- and 3-ethyl-1-aza-1,3-butadiene-1-carbonitriles were synthesized by reaction of the corre-
sponding 3-alkylacroleins with bis(trimethylsilyl)-carbodiimide using titanium tetrachloride as catalyst.
They were highly reactive and difficult to purify rigorously. Attempted anionic polymerizations gave only
oligomers of molecular weight ꢀ500 Da. These 3-alkyl-azadienecarbonitriles cycloadded to the electron-
rich olefins iso-butyl vinyl ether and p-methoxystyrene to give [2+4] cycloadducts with moderate yields.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
highly viscous oil during removal of the solvent. It could be dis-
tilled under high vacuum. However, synthesis of the unsubstituted
During the investigation of 1-aza-1,3-butadiene compounds
containing N-electron-withdrawing groups as potential mono-
mers,1–3 it was observed 1-phenylsulfonyl-3-methyl-1-aza-1,3-
butadiene was found to be only moderately reactive. When the
more electron-attracting carbethoxy moiety was placed on the
nitrogen, the 3-methyl derivative could again be obtained. This
compound was highly reactive, dimerizing and hydrolyzing readily
making it difficult to purify rigorously. Anionic initiators gave only
oligomers in the range of 2K Da, possibly because of this inability
to rigorously purify the monomer.
In the course of these investigations, we noticed a growing
interest in the use of 1- and 2-azabutadienes for the synthesis of
heterocyclic compounds by cycloaddition reactions.4–12 The azadi-
enes carried either electron-donating aryl substituents or electron-
attracting carboalkoxy groups. Accordingly, they readily cycloadd-
ed to olefins of opposite polarity, varying from tetracyanoethylene
to vinyl ethers. Our 3-methyl-1-carbethoxy-1-azabutadiene also
cycloadded to the electron-rich olefins which accords with the
moderately electron-poor character of the diene. This work ex-
tends our studies on an even more electron-attracting group,
namely the 1-azabutadienecarbonitriles.
monomers from acrolein did not succeed, as with previous mono-
mers, probably owing to Michael addition.
Treatment of monomers 1 and 2 with various anionic initiators
gave only oligomers, the highest molecular weight being ꢀ500 Da.
The highly electron-accepting cyano group makes these monomers
difficult to isolate and to purify extensively, and this may account
for the observed low molecular weights.
The Diels–Alder reaction of 3-methyl-1-azabutadiene-1-carbo-
nitrile with electron-rich olefins was slow at room temperature.
However, complexation of the N-cyano substituent with titanium
tetrachloride accentuated the electron-deficient nature and accel-
erated the [2+4] cycloaddition. The reaction was carried out with
iso-butyl vinyl ether and p-methoxystyrene. There was no ob-
served cycloaddition with the electron-poor benzoquinone. Other
Lewis acids such as boron trifluoride etherate and trimethylsilyl
triflate catalyst gave very low yields of cycloadduct and spontane-
ous homopolymerization occurred. Solvent effects were observed,
with tetrahydrofuran giving the highest yield of cycloadduct, while
solvents such as methylene chloride, toluene, and diethyl ether en-
hanced competitive spontaneous polymerization.
3. Reactions
2. Results
Both 3-substituted-1-azabutadiene-1-carbonitriles 1 and 2 par-
ticipated in inverse [2+4] Diels–Alder reactions (Scheme 2).
The 3-substituted-1-azabutadiene-1-carbonitriles 1 and 2 were
synthesized, respectively, by the condensation of methacrolein or
ethacrolein with bis(trimethylsilyl)-carbodiimide using titanium
tetrachloride with yields of 30–50% (Scheme 1).13 The 1-azabutadi-
ene-1-carbonitrile readily hydrolyzed during attempted purifica-
tion by silica-gel chromatography, and oligomerized to afford a
4. Conclusions
Two 3-alkyl-1-aza-1,3-butadiene-1-carbonitriles were synthe-
sized. They were reactive in oligomerization and cycloadditions,
reacting with electron-rich alkenes to give [2+4] cycloadducts in
moderate yields.
* Corresponding author. Tel.: +1 520 621 6325; fax: +1 520 621 8407.
E-mail address: hkh@u.arizona.edu (H.K. Hall Jr.).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.146

I. S. Kee, H. K. Hall / Tetrahedron Letters 50 (2009) 7116–7117
7117
R
1/2 TiO2
R
Me3SiNCNSiMe3
1/2 TiCl4
N - CN
CHO
2 Me3SiCl
1: R = CH3
2
: R = C2H5
Scheme 1. Synthesis of 1-azabutadiene-1-carbonitriles 1 and 2.
Supplementary data
CN
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.09.146.
Do
R
N
Do
N
CN
R
References and notes
3
: R = CH3. Do = i-BuO-
4: R = C2H5, Do = i-BuO-
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5
: R = CH3, Do = p-CH3C6H4-
6: R = C2H5, Do = p-CH3C6H4-
Scheme 2. cycloaddition of 1-azabutadiene-1-carbonitriles with electron-rich
donor olefins.
Acknowledgments
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We are deeply grateful to the U.S. Army Research Office for gen-
erous support of this work and to Dr. Anne Padias and Dr. Ian W.
Jones of this Department for their assistance in this work.
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