Reduction mechanism, catalytic hydrogenolysis

Diphenylmethane catalytic hydrogenolysis kinetics, 29 241-243 reduction mechanism, 29 267 cyclization, 30 65 dehydrocyclization, 28 318 [(Diphenylphosphino)alkyl]phosphonates, 42 479... 

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

The past decade has also seen the publication of many articles on the catalytic reduction of oxiranes. An account of the mechanism and stereochemistry of the hydrogenolysis is presented in various reviews and papers discussed in the next section. A review that appeared recently on the reduction of oxiranes to alcohols covers all of these methods. 

The limit of stability of the crystal framework at different extents of Ni ion exchange of type A molecular sieve is shown by means of electron microscopy, differential thermal analysis, and x-ray diffraction. The data obtained from catalytic studies are in accord with the results of physical methods, showing preservation of the molecular sieve properties after reduction of the Ni ions. Metallic Ni aggregates on the external surface of the zeolite. In the dehydrogenation of cyclohexane and the hydrogenolysis of n-hexane, type A molecular sieve shows the properties of metallic Ni on an inert support. When NiNaA is mixed mechanically with CaY, a typical bifunctional catalyst is obtained. 

The mechanisms for the model substrates BT (a) and DBT (b) involve the steps of C-S insertion, hydrogenation of the C-S inserted thiophene to the corresponding thiolate, base-assisted reductive elimination of the thiol (rds) to complete the cycle (in the catalytic reactions carried out in the absence of base, the displacement of the thiol by the substrate occurs thermally [10 b, c]). The addition of a strong base to the catalytic mixtures results in a remarkable rate enhancement for example, the TOP relative to the hydrogenolysis of BT to 2-ethylthiophenol catalyzed by [RhH(TRIPHOS)] increases from 12 to 40 by simply adding an excess of KOBu to the catalytic mixture [10 b, c]. 

Hydrogenolysis catalyzed by Pd/C is widely used to convert benzylic ethers ArCIP-OR to ArCH3 and ROH. The reaction is often accelerated by acid. The simplest possibility for a catalytic cycle is oxidative addition of both H2 and Bn-OR to Pd(0) to give a Pd(IV) complex, followed by reductive elimination of both Bn-H and H-OR to regenerate Pd(0). This mechanism seems very unlikely because the Pd(IV) oxidation state is high in energy. 

Selective catalytic hydrogenation of aromatic nitro compounds finds many applications in fine and specialty chemical industries (1). This class of hydrogenation reactions has been studied extensively using various solvents, catalysts and under various reaction conditions (1). The hydrogenation reaction has been found to follow mainly a mechanism that was delineated by Haber in 1898 from his study of electrochemical reduction of nitrobenzene (2). The mechanism, consisting of two types of reaction pathways, is schematically described in Fig. 1. The first pathway is a monomeric one that proceeds in three consecutive steps (a) hydrogenolysis of one of the N-O bonds in the nitro group to produce the nitroso intermediate ... 

Catalytic hydrogenations of aromatic nitro compounds with a stable hydroxylamine intermediate often have two different kinetic phases hydrogen uptake is rapid up to ca 60 %, then distinctly slower in the second phase. This means that reduction of the hydroxylamine to the aniline, formally a hydrogenolysis, is difficult in these cases. In the presence of the promoters discussed in Section 8.5.4.3, the second phase is less pronounced or disappears. This suggests a mechanism which could be called catalytic by-pass (see Figure 4). Experiments in the absence of hydrogen indicated that the vanadium promoters catalyze the disproportionation to give aniline and the nitroso intermediates that re-enter the catalytic cycle. As a consequence, the hydroxylamine does not accumulate and aniline formation is accelerated. 

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