Active hydrogen compounds dimerization

Only few studies of the stereochemistry of anodic oxidative dimerization of active hydrogen compounds (carbon acids) and their salts (carbanions) have been reported. 

The nitrosation of aliphatic carbon atoms, particularly of carbon atoms activated by adjacent carbonyl, carboxyl, nitrile, or nitro groups, has been reviewed in great detail [2]. Judging from this review, with few exceptions, nitrosation of active methylene compounds leads to the formation of oximes (unfortunately termed isonitroso compounds in the older literature). The few exceptional cases cited in which true nitroso compounds (or their dimers) were formed involved tertiary carbon atoms in which no hydrogen atoms were available to permit tautomerism to the oxime or involved a reaction which was carried out under neither acidic nor basic conditions. 

Further results on asymmetric hydrogenations of activated carbonyl compounds catalyzed by bis(dimethylglyoximato) cobalt (Il)-chiral amine complexes have been reported (55,56). Some chiral reductive dimerizations were observed (55). 

TMS production involves one specific functional group (-OH, -COOH, =NH, -NH2, or -SH), which loses an activated hydrogen and is replaced by a trimethylsilyl group (Proestos et ah, 2006). To achieve silylation, some authors have used BSTFA (N,0-hA(trimethyl-silyl)trifluoroacetamide) and TMCS (trimethylchlorosilane) successfully in several matrices (e.g. aromatic plants, cranberry fixiit) (Zuo et ah, 2002 Proestos et ah, 2006). Using silylated derivatives is advantageous for several reasons phenols and carboxylic acids are prone to silylation, these compounds can be derivatized in the same part of the process, and the minor products do not impede analysis and are well documented (Little, 1999 Stalikas, 2008). A two-step methylation procedure was used to analyze catechins and tannins in plant extracts. The first step used trimethylsilyl diazomethane (TMS-diazomethane) to pre-methylate the sample, and the second step used thermally assisted hydrolysis and methylation (THM). The pre-methylation step with TMS-diazomethane stabilized the dimer molecule m/z 540) by minimizing isomerization and reducing reactivity. (Shadkami et ah, 2009). 

Compounds containing active hydrogen atoms (pK = 11-25) could be brominated when DBU was added dropwise into a mixture of substrate (phenylacetylene, diethyl butylmalonate, indene, or fluorene) and BrCClj in benzene (78CL73). Compounds with lower acidity (pK 29) exhibited no reaction. With more acidic derivatives (diethyl malonate and benzyl cyanide), oxidative dimerization also occurred. Oxidative dimerization also took place when BrCClj was added dropwise into a solution of active methylene compound and DBU in benzene. The ratio of the reaction products depended on the ratio of DBU and BrCCIj. 

In the dimerization reaction of butadiene catalyzed by palladium complexes, nucleophiles (YH), such as amines, alcohols, phenols, carboxylic acids 41 4S>, and active methylene compounds 46) are introduced. This reaction can be explained by the attack of these nucleophiles on the jr-allylic complexes formed as intermediates-in the reactions. Takahashi, Shibano, and Hagihara confirmed by using deuterium that the hydrogen of YH migrates to C6 of the dimeric product, probably via the oxidative addition reactions of YH to the palladium species 42). 

In the presence of added ligand (Pd L= 1 1), 1,3, 7-octatriene isomers are obtained (Fig. 12.8). The conformation of the C chain in the intermediate (C) has been determined in solution by n.m.r. spectroscopy and also in the solid state (L = Me3 ) by X-ray diffraction. While the analogous nickel intermediate rearranges at room temperature, leading to cyclic products, the palladium compound does not. If the dimerization is carried out in the presence of active hydrogen... 

Remains bonded to the metal Specifically cyclo-tetra-, -penta-, and -hexa-merization Reduction and dimerization take place in the presence of active-hydrogen-containing compounds. 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

Phenols (p-cresol, guaiacol, pyrogallol, catechol) and aromatic amines (aniline, p-tolidine, o-phenyldiamine, o-dianisidine) are typical substrates for peroxidases [90 -109]. These compounds are oxidized by hydrogen peroxide or hydroperoxides under peroxidase catalysis to generate radicals, which after diffusion from the active center of the enzyme react with further aromatic substrates to form dimeric, oligomeric or polymeric products. 

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