Hydrogenolysis reaction with

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

An important conclusion from this work is that the measured CO dissociation rates are fast compared to the overall rates of those reactions that are believed to be initiated by CO loss from the corresponding carbene complex, lending strong support to the notion that CO loss is the crucial first step. Those processes include the above-mentioned hydrogenolysis, reaction with alkenes and vinyl esters, substitution of CO by phosphines, the Dotz reactions, and others. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

The first, and still widely used, polymer-supported ester is formed from an amino acid and a chloromethylated copolymer of styrene-divinylbenzene. Originally it was cleaved by basic hydrolysis (2 N NaOH, FtOH, 25°, 1 h). Subsequently, it has been cleaved by hydrogenolysis (H2/Pd-C, DMF, 40°, 60 psi, 24 h, 71% yield), and by HF, which concurrently removes many amine protective groups. Monoesterification of a symmetrical dicarboxylic acid chloride can be effected by reaction with a hydroxymethyl copolymer of styrene-divinylbenzene to give an ester a mono salt of a diacid was converted into a dibenzyl polymer. ... 

Another group of reactions with the predominant cleavage of the ring comprises catalytic hydrogenation of isoxazole derivatives and has been investigated only recently. The most commonly used catalyst has been Raney nickel,but use has sometimes been made of platinum catalysts. Hydrogenolysis of the 0—N bond (172—>173) occurs in isoxazole, its homologs,and their functional derivatives, for example, isoxazole carboxylic acids- and 5-aminoisoxazoles. ... 

Alcohols are the most frequently formed products of ester hydrogenolysis. The hydrogenation of esters to alcohols is a reversible reaction with alcohol formation favored at high pressure, ester at low pressure (/). Copper chromite is usually the catalyst of choice. Details for the preparation of this catalyst (/7) and a detailed procedure for hydrogenation of ethyl adipate to hexamethylene glycol (/[Pg.80]

Aziridines, like oxiranes, undergo hydrogenolysis easily with or without inversion of configuration, depending on the catalyst, reaction parameters, and various additives 65aJ08). For example, hydrogenolysis of 2-methyl-2-phenylaziridine in ethanol occurs mainly with inversion over palladium but with retention over platinum, Raney nickel, or Raney cobalt. Benzene solvent or alkali favor retention over palladium as well. 

Hydrogenolysis (Section 26.7) Cleavage of a bond by reaction with hydrogen. Benzylic ethers and esters, for instance, are cleaved by hydrogenolysis. 

On the other hand, syn-carboxylic acids are obtained from a deprotonation of the /5-silyl ester, giving the (E)-enolate, followed by reaction with different aldehydes and subsequent hydrogenolysis. No diastereomers of the aldol product are detected720. 

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

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. 

As seen in the retro-synthetic Scheme 5.3, intermediate 15 is useful for both routes. The choice of benzyl protection group was made based on the robust stability of benzyl phenol ethers toward most reactions and several possible avenues to remove it, although it was reported from Medicinal Chemistry that benzyl group removal via hydrogenolysis posed challenges in this compound. The choice of iodide substitution was born out of the known high reactivity of iodides in the Ullmann-type coupling reaction with alcohols and the robust stability of aryl iodides in many other common reactions. 

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