Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors

Abstract

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Synthesis of Tetrasubstituted 1,4-Dicarbonyl (Z)-2,3-Dihaloalkenes via Electrophilic Halogenation of Alkynyl Hydrazones

“…As a continuation of our research interest in exploring the reactivity of unprotected alkynyl hydrazones, we attempted their decomposition in the presence of NBS (2 equiv) and observed α,γ-dibromoallenoates 3a . During the optimization of 3a , we also observed an interesting side product 4a in trace amounts (Table , entry 1).…”

“…Reactions were carried out by adding NBS 2a (2.0 equiv) and a catalyst (20 mol %) to the stirred solution of 1a (0.1 mmol) in the given solvent (2 mL) at 0 °C, and the contents were stirred until the evolution of N 2 ceased. Then, 1.3 equiv of NBS and H 2 O (4 equiv) were added, and the reaction mixture was allowed to stir at the given temperature for 3 h. As a continuation of our research interest in exploring the reactivity of unprotected alkynyl hydrazones, 16 we attempted their decomposition in the presence of NBS (2 equiv) and observed α,γ-dibromoallenoates 3a. 17 During the optimization of 3a, we also observed an interesting side product 4a in trace amounts (Table 1, entry 1).…”

Decomposition of Alkynyl Hydrazones: Synthesis of Allenoates, Dihaloallenoates, and Angularly Fused Tricyclic Azepines

“…10 Importantly, alkynyl hydrazone is a special core that has been explored in alkynyl carbene transfer reactions. 11 Particularly, Nprotected alkynyl hydrazones are good partners for the haloniumion-assisted cyclization reactions that provide the respective halopyrazole derivatives. 12 Herein, we report an unexpected reactivity of unprotected alkynyl hydrazones with halonium ions that captured an electrophile in 2-fold and provided a tetrasubstituted α,γ-dihaloallenoate core (Scheme 1c).…”

“…In continuation of our research interest in unprotected alkynyl hydrazones, 11 we started investigating their effective decompositions for fruitful transformations. First, we attempted their reactivity in the presence of different electrophilic brominating reagents 2a−2d under DABCO catalysis (Table 1).…”

“…Gratifyingly, hydrazones emerged as excellent alternatives to diazo compounds as they are bench-stable, operationally safe, and accessible for numerous synthetic transformations . Importantly, alkynyl hydrazone is a special core that has been explored in alkynyl carbene transfer reactions . Particularly, N-protected alkynyl hydrazones are good partners for the halonium-ion-assisted cyclization reactions that provide the respective halopyrazole derivatives .…”