Halogenation hydrazone derivatives

Hydroxybenzaldehyde has extensive use as an intermediate in the synthesis of a variety of agricultural chemicals. Halogenation of Nhydroxybenzaldehyde, followed by conversion to the oxime, and subsequent dehydration results in the formation of 3,5-dihalo-4-hydroxybenzonitrile (2). Both the dibromo- and dhodo-compounds are commercially important contact herbicides, hromoxynil [1689-84-5] (2) where X = Br, and ioxynil [1689-83-4]( where X = I respectively (74). Several hydrazone derivatives have also been shown to be active herbicides (70). 

In the latest development of his elegant work with hydrazone derivatives, Andrew Myers of Harvard reports (J. Am. Chem. Soc. 2004,126, 5436) that Sc(OTf), catalyzes the addition of l,2-bis(r-butyldimethylsilyl)hydrazine, to aldehydes and ketones to form the t-butyldimethylsilylhydrazones. Addition of tBuOH/tBuOK in DMSO to the crude hydrazone effects low temperature Wolff-Kishner reduction. Alternatively, halogenation of ketone hydrazones can lead to vinyl halides such as 11, or the 1,1-dihalo derivatives, depending on conditions. Halogenation of aldehyde hydrazones provides the 1,1-dihalo derivatives such as 13. 

Furthermore, direct halogenation sometimes leads to substitution in the C-residue of the aldehyde hydrazone. For example, hydrazonoyl bromide 28c was obtained by direct bromination of hydrazone 58a or 58b (75CJC1484). Electron-withdrawing substituents in the N-aryl moiety of the arylhydrazone derivatives of heterocyclic aldehydes inhibit ring bromi-... 

Acyl-substituted imidazoles have distinctive UV and IR spectra, and can exist as hydrates in solution, e.g. imidazole-2-carbaldehyde. The aldehyde group is sufficiently electron withdrawing to assist nucleophilic displacement of an adjacent halogen atom. The normal aldehyde derivatives such as oximes, acetals and hydrazones can be formed, and the kinetics of oxime formation with the 4-carbaldehyde have been studied. The tautomeric ratio (zwitterion uncharged aldehyde form) of 1-substituted imidazole- and benzimidazole-2-aldoximes rises in parallel with an increase in basicity of the parent molecule (73CHE1074). These compounds have been shown by NMR studies to adopt the syn configuration. With hydrazine hydrate, imidazole-4,5-dicarbaldehydes give imidazo[4,5-d]pyridazines (208 Scheme 110). 

Scott and others have shown that the hydrazones of many azoles undergo ring closure to fused triazoles [Eq. 20)] either by oxidation (R = H) or by displacement of halogen (R = Br). Thus triazolotriazoles (180) were obtained by lead tetraacetate oxidation of the corresponding s-triazolehydrazone, " or by displacement of bromine from the appropriate bromo derivative. s-Triazolo[4,3-rf]tetrazoles (181) were prepared by oxidation or displacement of bromine " from the appropriate derivative, and similar methods were used to prepare s-triazolobenzothiazoles (182) s-triazolooxadiazoles (183), and pyrazolo[3,2-c]-s-triazoles. ... 

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