Why is in the reaction of cyclic unsaturated aldehydes with hydrazine and hydrogen peroxide the aldehyde preserved?


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> **Question**
>  What is the product of this reaction?  
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> ![enter image description here](https://i.stack.imgur.com/DZLKem.png)
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> The answer given:  
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> ![enter image description here](https://i.stack.imgur.com/nwyoMm.png)
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At first, I thought this was a typical Wolff-Kishner reduction, but then I saw the reagents didn't match up (no $\ce{OH-}$). 


How is this getting formed? Shouldn't the $\ce{CHO}$ group be reduced? What am I missing here?




The reaction of $\ce{H2O2}$ with hydrazine forms diazene, $\ce{HN=NH}$. This reacts in a syn fashion to deliver both hydrogens to the same side of the alkene. 


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Actually it's a di-imide which itself gets oxidised to give up nascent hydrogen and reduce the other reactant.
Thus, when you react it with an alkene , hydrogen add on the same side i.e. syn addition. Unlike $\ce{N2H4/KOH}$, it is not strong enough to reduce carbonyl groups. Therefore you get a different product.


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