Vinylacidic Acid in the Reaction of Aza-Michael with 1-Ethylpyrazole
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Vinylacidic Acid in the Reaction of Aza-Michael with 1-Ethylpyrazole H. N. Khachatryana,*, A. G. Shahkhatunib, A. M. Arzumanyana, G. G. Danagulyana,c, and H. S. Attaryana,c a
Scientific Technological Center of Organic and Pharmaceutical Chemistry, National Academy of Science of Armenia, Yerevan, 0014 Armenia b Molecular Structure Research Center, Scientific Technological Center of Organic and Pharmaceutical Chemistry, National Academy of Science of Armenia, Yerevan, 0014 Armenia c Russian-Armenian (Slavonic) University, Yerevan, 0051 Armenia *e-mail: [email protected] Received March 5, 2020; revised March 14, 2020; accepted March 19, 2020
Abstract—Commercial vinylacetic acid is a mixture of isomers of but-3-enoic and but-2-enoic acids. It was shown that but-3-enoic acid undergoes isomerization in the presence of a catalytic amount of 1-ethylpyrazole. The resulting Z- and E-isomers of but-2-enoic acid enter the aza-Michael reaction with pyrazole. The 1H NMR analysis showed that by the end of the experiment the ratio of unreacted Z- and E-isomers of but-2-enoic acid in the reaction mixture decreased by half (to 3 : 1), which pointed to a higher reactivity of the Z-isomer. Keywords: vinylacetic acid, isomerization, ethylpyrazole, aza-Michael reaction, Z/E-isomers
DOI: 10.1134/S1070428020080229 We have shown in [1] that electrophilic alkenes, in which the double bond is isolated by a methylene group, allyl cyanides in particular, should undergo isomerization of the double bond before aza-Michael reaction with pyrazoles. Such isomerization readily occurs in the presence of basic catalysts or at increased temperatures, as well as an appropriate basicity of the nucleophiles [1, 2]. The present work is similar to that our previous work [1], except that we now focused on vinylacetic acid (1) instead of allyl cyanide. Unlike the latter, vinyl acetic acid we dealt with in the present work was, according to 1H NMR, was a mixture of isomeric vinylacetic acid (but-3-enoic acid, 1) and but-2-enoic acid (2).
According to the 1H NMR data, the initial 1 : 2 ratio was 19 : 1, respectively (Scheme 1). In view of the fact that commetcial acetic acid is generally a mixture of isomers 1 and 2 [3], we studied their properties under heating and in the presence of a catalytic amount of 1-ethylpyrazole (3). We expected that in the presence of pyrazole 3 (as a base) compound 1 would convert into compound 2 (Scheme 2). The dependence of the degree of isomerization of vinylacetic acid (1) from time (at 130°C) was studied by 1H NMR spectroscopy (Fig. 1). As seen from the Fig. 1, the degree of conversion of compound 1 into compound 2 was about 63% within 2 h and about 85% within 4 h. After 4 h, the ratio of compounds 1 and 2 no longer changes.
Scheme 1.
Scheme 2.
O
O
OH
OH 1
2
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VINYLACIDIC ACID IN THE REACTION OF AZA-MICHAEL WITH 1-ETHYLPYRAZOLE
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By the end of the process, compound 2 was a mixture of the Z- and E-isomers in a 6 : 1 ratio (Scheme 3). To obtain further evidence for our results, we have studi
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