Hydrazone Groups

Inspired by the work of Burk and Feaster ) we attempted to use (2-pyridyl)hydrazine (4.36) as a coordinating auxiliary (Scheme 4.10). Hydrazines generally react effidently with ketones and aldehydes. Hence, if satisfactory activation of the dienophile can be achieved through coordination of a Lewis acid to the (2-pyridyl)hydrazone moiety in water. Lewis-add catalysis of a large class of ketone- and aldehyde-activated dienophiles is antidpated Subsequent conversion of the hydrazone group into an amine functionality has been reported previously by Burk and Feaster ... 

The mechanism of the reaction can be interpreted as involving a mesomeric effect of the 2-hydrazone group that leads to a negative charge on the 5-carbon (Scheme 46). 

Lead tetraacetate has been used to convert phenols, with a hydrazone group in the ortho position, to carboxylic esters, for example. 

The hydrazone group is hydrolyzed (16-2) during the course of the reaction. Yields are high. Aryl iodides are converted to unsymmetrical diaryl ketones on treatment with aryImercury halides and nickel carbonyl ArH-Ar HgX-l-Ni(CO)4 ArCOAr... 

Photo-oxidation is the main photofading pathway for the majority of dyes.18 Oxygen, particularly singlet oxygen, and radicals such as hydroxyl and hydroperoxyl, are believed to be the major electrophilic species responsible for photodegradation. Attack normally occurs at or around the azo (or hydrazone) group and plausible mechanistic pathways for the photo-oxidation of hydro-xyazo dyes have been proposed.18,19... 

Figure 18.15 NHS-chromogenic-PEG3-biotin contains an amine-reactive NHS ester that can be used to label biomolecules through an amide linkage. The chromogenic bis-aryl hydrazone group within the spacer arm of the reagent allows the degree of biotinylation to be quantified by measuring its absorbance at 354 nm. The compound also contains a hydrophilic PEG spacer, which provides greater water solubility.

Oxidation of ketone phenylhydrazones generates a radical-cation centre on the nitrogen atom adjacent to the benzene ring. The radical-cation is delocalised by both the hydrazone group and the phenyl ring. Reactions of 1,3,5-triphenyl-A -pyazolines illustrate the properties of these radical-cations. Two one-electron waves are seen at a rotating disc electrode in acetonitrile and for 1,3.5-triphenyl-pyrazoline, Ey. = 0.82 and 1.68 V vs. see [33]. The delocalised radical-cation is... 

Alkylation of 83 (R = OH) takes a variety of paths including alkylation of the NOH to give 1-alkoxyisatins, alkylation of the NOH and the carbonyl group to give 1,2-dialkoxyindoles, alkylation at the hydrazone group, and ring expansion to 2-quinolones.301 The course of the reaction depends upon the nature of the substituent in the side chain. 

Timmen and Dimick (1972) characterized the major hydroxy compounds in milk lipids by first isolating the compounds as their pyruvic ester-2,.6-dinitrophenylhydrazones. Concentrations as weight percent of the compounds from bovine herd milk lipids were 1,2-DGs 1.43, hydroxyacylglycerols 0.61, and sterols 0.35. Lipolysis tripled the DG content. The usual milk fatty acids were observed, except that the DGs lacked 4 0 and 6 00, again indicating that these lipids were in part intermediates in milk lipid biosynthesis. With the large hydrazone group... 

The co-ordination chemistry of macrocyclic ligands containing imine or hydrazone groups has been widely studied and, as expected, the presence of the imine functionality in the ring confers unusual redox properties to the complexes. 

Radical ring closure of an sp3 carbon-centered radical to a hydrazone group in compounds (174) can be carried out via 5-exo-trig manner. The silicon connection (Si-tethered) is removed by oxidative treatment with H202 to generate 2-amino-1,3-diols (175) (eq. 3.66) [189-191]. 

Hydrazone groups can also function as a directing group. In this case, both RuH2(CO)(PPh3)3 and Ru3(CO)12 exhibit catalytic activity (Eq. 20) [22]. 

The 1,3,4-thiadiazole ring system, with three heteroatoms, does not exhibit tautomerism in its fully conjugated form. However, when certain substituents are present, tautomerism is possible. l,3,4-Thiadiazolin-2-ones (39 X = 0) and -2-thiones (39 X = S) exist in the oxo and thione forms, respectively, as shown by spectroscopic and LCAO-MO calculations. 2-Amino-l,3,4-thiadiazoles exist in the amino form in solution and in the solid state the Kt value is 10s as shown by basicity measurements. UV spectroscopy and LCAO-MO calculations show that the amino tautomer is also the main species when there is an alkoxy group in the 5-position (40 R1 = alkoxy, R2 = NH2), or if the exocyclic nitrogen atom is part of a hydrazone group (40 R2 = NHN=CR2). 

Base-catalyzed ring closure of the isothiocyanate derivative (127) gives the pyrazolo-[l,5-a][l,3,5]triazine (128 Scheme 41) <74JHC199>. The sulfur may be removed with Raney nickel in ammonium hydroxide. Another example of this versatile route is the formation of (130) in Scheme 42. However, the starting material (129) also has a hydrazone group... 

Now, everyone with a rudimentrary background on metal hydride reductions knows that a formal addition of the elements M-H to the C=0 bond takes place at a first stage. There is no reason to believe that anything different occurs to a close relative of carbonyls such as the hydrazone group upon exposure to boron hydride. On that account, the postulation of structure IV is as reasonable as the metal alkoxides that have always been proposed for analogous reductions of aldehydes and ketones (see Scheme 8.1). 

The bond distances in the hydrazone group indicated a considerable TT-electron delocalization in the system because of the contribution of resonance structure, which are energetically preferred to bicyclic system 59. It was then concluded that a disruption of the furanoid ring follows the introduction of a hydrazone residue at C-2, which induces a rr-electron delocalization in the system. This was due to the sp2 hybridization at C-3, which precludes the necessary fourth valence for ring formation (82MI9). 

The UV spectrum of 97 shows an absorption maximum at 365 nm. This is considerably lower than that of the parent bis(hydrazone) 65c, which appeared at 445 nm. This could be attributed to the disappearance of conjugation with the C-3 hydrazone group in compound 97. 

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