Regioselectivity of the Reaction between β‑Enamino Diketones and Methylhydrazine Explained

TL;DR

This paper explains how different reaction pathways and electronic effects determine the product distribution in the reaction between β-enamino diketones and methylhydrazine.

Contribution

The study provides a detailed quantum chemical explanation of the regioselectivity and mechanism in this organic reaction.

Findings

The reaction mechanism branches into multiple pathways based on nitrogen attack and electronic effects.

Three of four possible regioisomers are observed experimentally due to cyclization and proton transfer.

Proton transfer catalysts significantly influence the reaction in different solvents like acetonitrile.

Abstract

The complete mechanism
for the reaction of β-enamino diketone
(BED) with methylhydrazine in H2O/MeOH and acetonitrile
is elucidated via state-of-the-art quantum chemical calculations.
Our results show that the mechanism branches into multiple pathways
with distinct energetic profiles, leading to a product distribution
governed by a delicate balance of electronic effects. The initial
branching point is determined by which the nitrogen atom of the asymmetric
methylhydrazine (NH2 or NHMe) attacks the BED β-carbon. Following the elimination of HNMe2, a cyclization
step occurs, where the remaining methylhydrazine nitrogen attacks
one of two distinct carbonyl carbons, leading to a product distribution,
where three of four possible regioisomers are observed experimentally.
The activation energy for this cyclization is influenced by the electronic
properties of the substituents on both the carbonyl carbon (methoxycarbonyl
or chlorophenyl) and the nucleophilic nitrogen (H or Me). Critically,
this cyclization is contingent on a proton transfer from nitrogen
to the carbonylic oxygen, promoted by a proton transfer catalyst (PTC).
In H2O/MeOH, the protic solvent molecules act catalytically,
whereas in acetonitrile, an acidic nitrogen center, as in methylhydrazine
or dimethylamine, is required. The importance of the proton transfer
catalyst is confirmed by experimentally determined product ratios
of the reaction in acetonitrile with catalytic acetic acid.

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