Hydrazone

Hydrazones are a class of organic compounds with the structure R¹R²C=N−NH₂.^[1] They are related to ketones and aldehydes by the replacement of the oxygen =O with the =N−NH₂ functional group. They are formed usually by the action of hydrazine on ketones or aldehydes.^[2][3]

Hydrazine, organohydrazines, and 1,1-diorganohydrazines react with aldehydes and ketones to give hydrazones.

Phenylhydrazine reacts with reducing sugars to form hydrazones known as osazones, which was developed by German chemist Emil Fischer as a test to differentiate monosaccharides.^[4][5] Hydrazones having 1,3-diketomoiety are also known in literature.^[6]

Hydrazones are the basis for various analyses of ketones and aldehydes. For example, dinitrophenylhydrazine coated onto a silica sorbent is the basis of an adsorption cartridge. The hydrazones are then eluted and analyzed by high-performance liquid chromatography (HPLC) using a UV detector.^{[citation needed]}

The compound carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (abbreviated as FCCP) is used to uncouple ATP synthesis and reduction of oxygen in oxidative phosphorylation in molecular biology.

Hydrazones are the basis of bioconjugation strategies.^[8][9] Hydrazone-based coupling methods are used in medical biotechnology to couple drugs to targeted antibodies (see ADC), e.g. antibodies against a certain type of cancer cell. The hydrazone-based bond is stable at neutral pH (in the blood), but is rapidly destroyed in the acidic environment of lysosomes of the cell. The drug is thereby released in the cell, where it exerts its function.^[10]

Hydrazones are susceptible to hydrolysis:

R₂C=N−NR'₂ + H₂O → R₂C=O + H₂N−NR'₂

Alkyl hydrazones are 10²- to 10³-fold more sensitive to hydrolysis than analogous oximes.^[11]

When derived from hydrazine itself, hydrazones condense with a second equivalent of a carbonyl to give azines:^[12]

R₂C=N−NH₂ + R₂C=O → R₂C=N−N=CR₂ + H₂O

Hydrazones are intermediates in the Wolff–Kishner reduction.

Hydrazones are reactants in hydrazone iodination, the Shapiro reaction, and the Bamford–Stevens reaction to vinyl compounds. Hydrazones can also be synthesized by the Japp–Klingemann reaction via β-keto acids or β-keto-esters and aryl diazonium salts. Hydrazones are converted to azines when used in the preparation of 3,5-disubstituted 1H-pyrazoles,^[13] a reaction also well known using hydrazine hydrate.^[14][15] With a transition metal catalyst, hydrazones can serve as organometallic reagent surrogates to react with various electrophiles.^[16]

In N,N-dialkylhydrazones^[17] the C=N bond can be hydrolysed, oxidised and reduced, the N–N bond can be reduced to the free amine. The carbon atom of the C=N bond can react with organometallic nucleophiles. The alpha-hydrogen atom is more acidic by 10 orders of magnitude compared to the ketone and therefore more nucleophilic. Deprotonation with for instance lithium diisopropylamide (LDA) gives an azaenolate which can be alkylated by alkyl halides.^[18] The hydrazines SAMP and RAMP function as chiral auxiliary.^[19][20]

Several methods are known to recover carbonyl compounds from N,N-dialkylhydrazones.^[21] Procedures include oxidative, hydrolytic or reductive cleavage conditions and can be compatible with a wide range of functional groups.

• Hydrazones
• 
  Benzophenone hydrazone, an illustrative hydrazone
• 
  Carbonyl cyanide m-chlorophenyl hydrazone
• 
  Gyromitrin (acetaldehyde methylformylhydrazone), a toxin
• 
  Dihydralazine, an antihypertensive drug
• 
  X-ray structure of DNP-derived hydrazone of benzophenone. Selected parameters: C=N, 128 pm; N-N, 138 pm, N-N-C(Ar), 119 pm^[22]

• Azo compound
• Imine
• Nitrosamine
• Hydrogenation of carbon–nitrogen double bonds