Hydrogenolysis of C-S bonds

Desulfurization of organic compounds over Ra-Ni is a well-known procedure.364-369 Ra-Ni exists in different forms (W1 to W8), differing in the preparation procedure and sometimes symbolized as Ni(H). Ra-Ni is sometimes called a catalyst however, it is used usually in large excess and the reaction is stoichiometric. Ra-Ni can desulfurize every C—S bond containing compound but it can also hydrogenate a lot of other functional groups. Unwanted side reactions are sometimes suppressed by using deactivated Ra-Ni. 

At the adsorption of benzenethiol on Ni(100), benzene and hydrogen are the only gas-phase products.370 Methanethiol adsorption has also been studied 

The initial step of the adsorption of thiols on a Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon adsorption of thiophenol at 120 K on a clean Mo(110) surface.381 The thiolate intermediate subsequently undergoes competing C-S bond hydrogenolysis to form benzene, or C-S and C-H bond scission to form surface benzyne. The adsorption of thiophenol was also studied on a sulfur-covered Mo surface382 Phenyl disulfide is formed via S-H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

Sulfides are hydrogenolyzed usually by Ra-Ni.383-391 Phenyl and methylthio groups were removed by Ra-Ni (10 mmol Ni/mmol) refluxing in ethanol for 2 hours.392 A thio enol ether was desulfurized with Ra-Ni (100 mmol Ni/mmol) in MeOH at room temperature for 30 minutes.393 During the synthesis of 2-deoxy-p-gl y cos ides the removal of the arylthio group was achieved using Ra-Ni with no problem.394 

A hydroxysulfoxide was desulfurized with Ra-Ni in MeOH for 30 minutes.400 Treatment with hydrogen in the presence of catalytic amounts of Ra-Ni in EtOH caused the desulfurization of a 2,6-anhydro-2-thio sugar, the reduction of an N-oxide, and the removal of benzyl and carbomethoxy groups at the 

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The reactions of the adsorbates hydrogenation of unsaturated bonds and hydrogenolysis of C-S bonds... 

Once an organosulfur compound and hydrogen have been chemisorbed on a catalytic surface they must interact with each other in order for desulfurization to be completed therefore, all the major pathways that need to be considered involve hydrogenation of C=C bonds and/or hydrogenolysis of C-S bonds. The intimate details of how such elementary steps occur, and the way in which they are interlinked to form a catalytic cycle are not easy to establish on real operating systems, because of the intrinsic complexity of the catalyst and of the intervening reaction schemes nevertheless, a number of very reasonable mechanistic proposals have been advanced by several research groups on the basis of kinetic studies, detection of intermediates, deuteration experiments, and other appropriate techniques, and the literature on this point is ample. 

Hydrogenolysis of C-S bonds is followed by hydrogenation. Petroleum fractions are desulfurized for many reasons to protect catalysts, to improve product quality, and to prevent environmental pollution. Feedstocks vary from light naphthas to heavy residua and require reactors of increasing... 

Hydrogenolysis of C—S bonds of sulfones can be realized by H2 in the presence of a Raney nickel catalyst, by... 

Pd and Ni are the most widely used catalysts for the hydrogenolysis of C-O bonds. The rates of hydrogenolysis increase in the order OH < OR << OAr < OCOR. This order corresponds to the leaving-group properties of these moieties, which is characteristic of S -type reactions. The stereochemistry of the reaction depends mainly on the metal. Inversion of the configuration of carbon is characteristic of Pd whereas retention is usually observed with Ni [2,11]. 

Elevated temperature is usually needed for the hydrogenolysis of C-O bonds in alcohols [12]. Hydrogenolysis of primary alcohols can be deseribed as an 8 2-like reaction involving hydride attack on the carbon, whereas transformation of a tertiary alcohol is closer to an S jl-type reaetion [2,13]. Tertiary alcohols can be hydrogenolyzed much faster than secondary. Alcohols are readily converted to the corresponding 0-alkylisoureas. Hydrogenolysis of these compounds leads to hydrocarbons (Scheme 1) [14]. 

Palladium catalysts, usually Pearlman s catalyst [42] and Pd/C are used for the hydrogenolysis of benzyl-nitrogen bonds. Acetic acid, ethyl acetate, ethanol, or methanol are frequently used as solvents. Sometimes a small amount of a strong acid is added to the reaction mixture [54-56]. The bond rupture needs the adsorption of carbon [57]. The hydrogenolysis of C-N bonds occurs with inversion on both Pd and Ni [2,58]. 

M0O3/AI2O3 (hydrotreating, Ni, Co increased hydrogenolysis of C-S and C-N bonds... 

Catalytic Hydrogenolysis of Carbon-Huonne Bonds it-Bond Participation Mechamsm Hudlicky, M J Fluorine Chem 44, 345-359 58 n o C) S ... 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

Carbonyl groups can be converted to methylene groups by desulfurization of thioketals. The cyclic thioketal from ethanedithiol is commonly used. Reaction with excess Raney nickel causes hydrogenolysis of both C—S bonds. 

The elimination of the chiral auxiliary from hydroxy sulfoxides by hydrogenolysis of the C-S bond with Ra-Ni did not give the expected desulfurized... 

The general reaction occurring in hydrodesulfurization has been described in Section 2.1.1. The most studied model compound is DBT. The reactivity towards hydrogenation of the phenyl substituents already mentioned (Section 2.1.1) is also observed in the hydroprocessing of sulfur compounds. The reactivity towards hydrogenolysis of the C-S bond masks the effects associated to aromatics hydrogenation. The DBT reaction network is sketched in Fig. 8 the pseudo-first-order reaction constants measured by Houalla [68] have been included. 

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