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When acrolein is heated with hydrazine we get dihydropyrazole. But I can't see how. First I think hydrazine should add on acrolein to form corresponding hydrazone, but what will happen next? Also is my first step correct?

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  • $\begingroup$ If your first step is correct is the resulting hydrazone a Michael acceptor? $\endgroup$
    – Waylander
    Jan 16, 2020 at 8:39
  • $\begingroup$ Count the number of atoms on the chain in the resulting hydrazine, starting from the carbonyl carbon as 1. You'll see the last nitrogen is 5. This is a setup for a nice 5-membered ring. So, the next step after hydrazine formation is dehydration to give dihydropyrazole. $\endgroup$
    – joehua
    Jan 16, 2020 at 8:51

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Your suggestion of resulting corresponding hydrazone by the reaction between acrolein and hydrazine is correct. It is normal hydrazine-hydrozone reaction you perform in idetifocation of aldehyde and ketone carbonyl groups. Usually, E-isomer is preferred (Ref.1). E- to Z-isomerization can be occurred in presence of trace amount of acid (Ref.1). I expect that may be the case during constant heating. As a result, Z-isomer can undergo intramolecular Michael addition as depicted in following mechanism to give dihydropyrazole:

Dihydropyrazole


References:

  1. Shigehisa Uchiyama, Masanori Ando, Shohei Aoyagagi, "Isomerization of aldehyde-2,4-dinitrophenylhydrazone derivativesand validation of high-performance liquid chromatographic analysis," Journal of Chromatography A 2003, 996(1-2), 95–102 (https://doi.org/10.1016/S0021-9673(03)00542-9).
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  • $\begingroup$ The reference you cite did not deal with acrolein but rather the hydrazones of saturated, aliphatic aldehydes. While the cyclization you propose is a 5-endo-trig cyclization (Baldwin's Rules), it is not precluded since there is no competitive cyclization, i.e., 5-exo-trig. However, conjugate addition of hydrazine to the acrolein hydrazone would permit a preferred 5-exo-trig cyclization with regeneration of hydrazine by elimination. $\endgroup$
    – user55119
    Jan 17, 2020 at 19:43
  • $\begingroup$ @user55119: To my knowledge, hydrozone formation is preferred over Michael addition with hydrazene. OP is a novice to organic chemistry so I don't want to confused him. I just gave a reasonable mechanism for formation of dihydropyrazole. Also, Baldwin's rule do not apply to all cyclizations. There should be sme exceptions. Don't you think? :-) $\endgroup$ Jan 17, 2020 at 21:37
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    $\begingroup$ I did account for exceptions. That is why I said " it is not precluded since there is no competitive cyclization". Baldwin's Rules apply to competitive processes. Just another possibility. $\endgroup$
    – user55119
    Jan 17, 2020 at 21:42

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