Hydrazone anions

The mechanism of the reaction may involve the formation of an anion by the base B, followed by the shift of hydrogen on the hydrazone anion with simultaneous loss of nitrogen to yield a carbanion ... 

Base abstracts a weakly acidic N-H proton, yielding a hydrazone anion. This anion has a resonance form that places the negative charge on carbon and the double bond between nitrogens. 

Protonation of the hydrazone anion takes place on carbon to yield a neutral intermediate. 

Scheme 1.11 provides some examples of alkylation of imine and hydrazone anions. Entries 1 and 2 involve alkylation of anions derived from IV-alkylimines. In Entry 1, two successive alkyl groups are added. In Entry 2, complete regioselectivity... 

Hydrazone anions are more reactive than the corresponding ketone or aldehyde enolate. 

Hydrazone anion 957 induced ring opening of the enantiopure 4-substituted oxetan-2-ones 958 followed by cycliza-tion/dehydroamination of the resulting P-ketohydrazones 959 affords dihydropyran-4-ones 960 in good to excellent yield and enantioselectivity (Scheme 256, Table 46) <20020L1823>. 

Vinylmagnesium bromide reacts as a Michael acceptor with a zincated hydrazone, yielding a 1,1-bimetallic species, which in some cases can be sequentially trapped with two different electrophiles.246 2463 The reaction proceeds via metalla-aza-Claisen rearrangement. The dimethyl hydrazone anion behaves as an azaallylic system (Scheme 81).247 Vinylphosphonium salts add Grignard reagents, forming alkylphosphonium ylides. These ylides... 

Since the hydrazone anion is stable in the absenee of a proton donor, the overall reaction must depend on the presenee of an aleohol molecule in the vieinity of the hydrazone anion. This is in agreement with the observation that rate eonstants obtained in ROH/CH3SOCH3 mixtures, as a funetion of dimethyl sulfoxide eoneentration, exhibit a maximum at low alcohol concentration [330]. The rate-determining step of this reaction appears to be a eoncerted proton-transfer from the protie solvent to the hydrazone anion and a proton abstraetion from the hydrazone anion by the basie solvent [329]. 

Oxygenation of the dianion of a ketoxime - by MoOPH (c/. Section 2.3.2.1.2) provides a-l droxy ketones after hydrolysis, albeit in relatively low yield. The equivalent process for hydrazcmes was unsuc-essful, although it appears that this was a problem associated with manipulation of the products rather than an intrinsic failure of reaction. Indeed subsequent work with hydrazone anions has shown that the process is viable. 

Alkylations of hydrazone anions derived from aldehydes and ketones are closely related to imine anion alkylations. Lithiated hydrazones are particularly powerful nucleophiles for alkylation reactions. The alkylated hydrazones may be cleaved with periodate, as shown below.The choice of solvent and base is critical to the success of hydrazone alkylations hexane-free medium is essential for complete metalation. °... 

Alkenylithium reagents derived from symmetrical acyclic ketone hydrazones possess the /ran -configuration, which is consistent with a jyn-deprotonation of the hydrazone anion in a conformation that places the alkyl group R anti to the hydrazone moiety during dianion formation. The presence of an a-branch such as an isopropyl group, however, diminishes the stereoselectivity. 

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