Benzylic C(sp³)–H Bonds Play the Dual Role of Starting Material and Oxidation Inhibitor for Hydrazides in the Electrochemical Synthesis of Hydrazones

Abstract

The electrooxidation of benzylic C(sp 3 )−H bonds to produce hydrazones as an alternate for conventional pathways has an enormous dignity. Under the aegis of electricity, instead of hazardous metal catalysts and external oxidants, we unveil an electrochemical process for electrooxidation of various benzylic C(sp 3 )−H bonds in aqueous media in all pH ranges that subsequently produce hydrazones with further reactions. This electrooxidative reaction strategy provides an acceptable condition for synthesizing hydrazones with various functional groups in good efficiency and amenable to gram-scale synthesis. The electrochemical oxidation condition proves an excellent level of compatibility with super cheap electrolyte NaCl for the oxidation of benzylic C(sp 3 )−H position despite the highly oxidizable hydrazide group remaining intact in the reaction.

Synthesis of Acyl Hydrazides and Hydrazones from Activated Amides

“…31, 145.97, 138.32, 133.18, 131.92, 129.79, 129.54, 128.51, 127.67, 125.73 (q, J = 3.9 Hz), 124.08 (q, J = 272.2 Hz). 11 According to the general procedure, 4-chlorobenzaldehyde (281 mg, 2.0 mmol) gave 4f (160 mg, 0.62 mmol, 62% yield, white solid) after column chromatography (n-hexane/EtOAc, 4:1); mp 166-167 °C. FTIR (neat): 1673, 3439 cm -1 .…”

“…(E)-N′-(4-Methylbenzylidene)benzohydrazide (4a) 11 According to the general procedure, 4-methyl-benzaldehyde (240 mg, 2.0 mmol) gave 4a (190 mg, 0.80 mmol, 80% yield, white solid) after column chromatography (n-hexane/EtOAc, 4:1); mp 188-190 °C. FTIR (neat): 1674, 3440 cm -1 .…”

Co(II) Acetate‐Assisted Direct Synthesis of Acyl Hydrazones from Acyl Hydrazides under Mild Conditions

“…Ding et al reported a photocatalytic pathway using methylene blue and blue LED light to synthesize acyl hydrazones from acyl hydrazide and styrene (Scheme 1b). [16] Contemporary methodologies also use alcohols as a source of aldehydes, generated in situ, which then couple with acyl hydrazides when catalyzed by electrochemical means [17] or metal complexes like Fe/Ru complex [18] (Scheme 1c). All of these traditional and contemporary ways of acyl hydrazone synthesis rely on two reacting components, which often require special precautions, inert atmosphere, or prior preparation of expensive catalysts or special reaction set-up (in case of photocatalysis).…”