Dehalogenation heterogeneous

Br and Cl are much less effective at inducing 1,2-dehalogenation than I , but metals—particularly Zn—have long been used. Reaction takes place heterogeneously at the surface of the metal, the solvent... 

Electrocatalytic dehalogenation of organohalides has also been performed in heterogeneous conditions, at a graphite electrode coated by a poly(acrylic acid) film grafted with the nickel(II) tetra-azamacrocyclic complex (20).292... 

One of the earliest reported preparations of the requisite glycosidation precursor 5-deoxy-l,2,3-tri-0-acetyl-p-D-ribofuranoside (17) was published by Kissman and Baker in 1957.23 D-Ribose was heated at reflux in a methanol/acetone mixture in the presence of concentrated HCI to provide methyl 2,3-O-isopropylidene-D-ribofuranosidc (21), which was in turn converted to the corresponding 5-O-mesyl ribofuranoside 22 with methanesulfonyl chloride in pyridine in 63% yield. The sulfonate moiety of 22 was then displaced with sodium iodide in refluxing DMF to provide 5-deoxy-5-iodo derivative 23 in 76% yield on a multigram scale. Reductive dehalogenation of 23 was accomplished under heterogeneous catalytic hydrogenation conditions to provide the reduced 2,3-0-protected intermediate 24 in 56% yield, which was subjected to hydrolysis conditions in... 

Studies have also been conducted on electrode processes where homogeneous chemical reactions are coupled to heterogeneous electron transfer(s). In this example, the reductive dehalogenations of 3- and 4-bromobenzophenone and of o-bromonitrobenzene (denoted as ArBr) dissolved in N,N-dimethylformamide solution were studied (Compton et al., 1996b). The one-electron reductions of these compounds result in the formation of the corresponding chemically reactive radical anions as shown in (96), where HS denotes the solvent/supporting electrolyte system. 

The optimization of catalyst and reaction parameters allowed development of a high performance catalyst for the reaction of bromoarenes with olefins whose activity exceeds that of any heterogeneous palladium catalysts reported in the literature by at least one order of magnitude (Table 6.) With the Pd/C catalysts used in this study no dehalogenation of bromoarenes occurred. This is in contrast to Arai et al. (15) where up to 80 % of benzene formation by dehalogenation was observed. Bromobenzene can be converted to 90 % within 2 hours and catalyst concentrations of only 0.005 mol% palladium. Turnover numbers (TON) up to... 

Heterogeneous dehalogenations of vicinal dihalides can be achieved smoothly with zinc in protic solvents or magnesium, lithium or sodium in ethereal solution. In general the elimination occurs at the surface of the metal and the reactivity of this surface is maintained by the presence of a solvent which redissolves any produced olefin and any metallic salt. However, in one case, catalysis by the zinc dihalide deposited on the metal surface during the elimination has been reported . ... 

It would be instructive to apply electron spin resonance spectroscopy in these various metal cation oxidations. Certainly the homogeneous nature of these reactions offers greater possibilities to the kineticist than is afforded by the heterogeneous nature of the metal-catalysed eliminations. Considering the recent discoveries of the synthetic utility of thallium in organic synthesis " , thallous salts might prove to be extremely useful catalysts for dehalogenations. as under the reaction conditions the less stable olefins are rapidly isomerised. 

These reactions are usually heterogeneous, hence no meaningful data on their mechanisms have yet been obtained. It appears, however, that the reactions illustrated in Fig. 2 which occur under reducing conditions and where the metal carbonyls function as dehalogenating agents, probably go by paths as outlined in Fig. 3. In this scheme two main steps are envisioned,... 

Zinc serves as a reducing agent in zinc/acetic acid dehalogenation. The reaction is heterogeneous (part solid and part liquid), with the actual reduction taking place at the surface of the metallic zinc. Zinc is oxidized from the 0 oxidation state to the +2 oxidation state, forming ZnBr2-... 

The dehalogenation of aryl halides can be carried out in stoichiometric or catalytic reactions in the presence of bases. Reducing agents as LiAlH or NaBH are used in the stoichiometric reactions [2]. The reaction can be carried out in the presence of homogeneous and heterogeneous catalyst under condition of transfer hydrogenation... 

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