Hydrogen-transfer agent

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

The system R-SH/RS-RS thus acts as a hydrogen transfer agent. 

Relevant complexes have been isolated and fully characterized (4-10, 13-lb). The resulting arylpalladium complex 8 is able to react with various compounds such as terminal alkenes, alkynes, aryl boronic acids or hydrogen-transfer agents to give an organic molecule and palladium(O) (3, 17, 18). 

Fundamental studies of coal liquefaction have shown that the structure of solvent molecules can determine the nature of liquid yields that result at any particular set of reaction conditions. One approach to understanding coal liquefaction chemistry is to use well-defined solvents or to study reactions of solvents with pure compounds which may represent bond-types that are likely present in coal [1,2]. It is postulated that one of the major routes in coal liquefaction is initiation by thermal activation to form free radicals which abstract hydrogen from any readily available source. The solvent may, therefore, function as a direct source of hydrogen (donor), indirect source of hydrogen (hydrogen-transfer agent), or may directly react with the coal (adduction). The actual role of solvent thus becomes a significant parameter. 

The reduction of pyrylium salts to monomeric 2H- or 4//-pyrans has been accomplished by complex hydrides, hydrogen-transfer agents, electrochem-ically, and by catalytic hydrogenation. 

Reductions of pyrylium salts with organic hydrogen-transfer agents, interesting as models of biological redox systems, exhibit regioselectivity... 

Although the synthetic usefulness of such reactions has not yet been widely recognized, it should be possible to carry out hydroperfluoroalkylation of olefins and alkynes very efficiently by the strategic use of an appropriate homolytic hydrogen-transfer agent. The rates of hydrogen transfer for many such agents... 

Aromatic amines and some other compounds are also effective hydrogen transfer agents and will give excellent molecular stability in most cases when used in place of the hindered phenolics. However, these compounds are rapidly oxidized to form color bodies in the polymer and may be toxic compared with the phenolics. Hence, aromatic amines are not widely used in polypropylene stabilizer systems. 

Heat Stability. Polymeric phenolic phosphites seem to be efficient hydrogen transfer agents which provide excellent heat stability when used in conjunction with thiodipropionates. Presumably, they scavenge free radicals and form resonance-stabilized phenoxy radicals in much the manner as simple hindered phenolics (see reaction scheme on p. 214). 

Chain transfer to active hydrogen transfer agent ... 

The selective and rapid reduction of nitro compounds is an active area of research, particularly when other potentially reducible moieties are present in the molecules. Anhydrous ammonium formate has been developed as a catalytic hydrogen transfer agent for selectively reducing nitro groups in the presence of acid, ester, amide, halogen and nitrile groups (equation 13). ... 

In cases where hydrogen atom transfer gives primarily reduced products, BusSn—SnBus under photochemical generates the radical which can cyclize (see 15-46), but a halogen atom transfer agent, such as iodoethane, is used rather than a hydrogen-transfer agent, so the final product is an alkyl iodide. 

Another type of free-radical reaction was explored starting from a long-chain alkyl iodide for the synthesis of a macrolactone (eq. (15)). The reaction utilized (MesSiliSiH as a hydrogen transfer agent [54, 55]. 2,2 -Azoisobutyronitrile (AIBN) was used as a radical initiator. Furthermore, a CO pressure of 30 bar was required for the alkyl radical to trap CO. The resultant acyl radical reacted with the activated double bond to give desired product. 

TH is an indirect form of hydrogenation that can be catalyzed by transition metal complexes.29 Equation 12.11 outlines the concept where 2-propanol is the hydrogen transfer agent, which means that H2 does not have to be present. Two hydrogens are transferred from 2-propanol, and the by-product is acetone. 

The phosphine and arsine hydrides are obtained from the corresponding halides (the c/s-isomer is usually most reactive) by the action of a variety of hydrogen-transfer agents such as KOH in ethanol, H2 at 50 atm/95°, LiAlH4 in THF or, most conveniently, 90% aqueous hydrazine, e.g., c 5-PtBr2(Et3P)2—> /ra/u-PtHBr(Et3P)2... 

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