Halides reduction reactions

Various options for the production of tantalum from pure tantalum compounds are summarized in Figure 4.26. The oxide and halide reduction reactions described above have been carried out in basically different types of reactors, using processes that are characteristically different because the forms and the physico-chemical nature of the feed materials are different. A relatively recent development with regard to process metallurgical equip-... 

Allyl halides, reduction reactions, 31 Aluminum chloride reagent/catalyst alkyl halide reduction, 30-31 secondary alkyl alcohol reduction, 14-15... 

The reducing power of this system is enhanced by the addition of acid. In the presence of 0.15 N sulfuric acid, aldehydes are rapidly reduced. If the concentration of acid is increased to 1.5 N, ketones are rapidly reduced. The addition of acid appears to have no influence on the halide reduction reaction. In Table 12.3 are recorded several applications of tetrabutylammonium cyanoborohydride as a reducing agent. 

The reaction of an alkyl halide with lithium is an oxidation-reduction reaction. Group I metals are powerful reducing agents. 

Reduction is a chemical reaction in which an element gains an electron, in other words, when the oxidation state is lowered. Reduction reactions are widely used, particularly the hydrogen reduction of the halides, as shown in the following examples ... 

Hydrogen reduction has a major advantage in that the reaction generally takes place at lower temperature than the equivalent decomposition reaction. It is used extensively in the deposition of transition metals from their halides, particularly the metals of Groups Va, (vanadium, niobium, and tantalum) and Via (chromium, molybdenum, and tungsten). The halide reduction of Group IVa metals (titanium, zirconium, and hafnium) is more difficult because their halides are more stable. 

A thio-substituted, quaternary ammonium salt can be synthesized by the Michael addition of an alkyl thiol to acrylamide in the presence of benzyl trimethyl ammonium hydroxide as a catalyst [793-795]. The reaction leads to the crystallization of the adducts in essentially quantitative yield. Reduction of the amides by lithium aluminum hydride in tetrahydrofuran solution produces the desired amines, which are converted to desired halide by reaction of the methyl iodide with the amines. The inhibitor is useful in controlling corrosion such as that caused by CO2 and H2S. 

Organocadmium compounds can be prepared from Grignard reagents or organo-lithium compounds by reaction with Cd(II) salts.180 They can also be prepared directly from alkyl, benzyl, and aryl halides by reaction with highly reactive cadmium metal generated by reduction of Cd(II) salts.181... 

Aryl aldehydes, alkane reduction, 71-72 Aryldiazonium salts, reduction of, 104 Aryl halides and triflates, reduction reaction, 32 Aryl ketone, allylation,... 

Although the reaction with thiosulfate and with iodide ions may be a mere reduction of the halide, the reaction with sodium benzoate would appear to be a radical dissociation induced by the attack of a negative ion. The fate of the benzoate ion is unknown. Tris-( -nitrophenyl)-methyl benzoate is a stable substance which does not dissociate into radicals.23... 

A variety of alkyl halides have been reduced at room temperature, including benzyl halides, primary, secondary and tertiary alkyl halides. The reaction times depend on the halide, and vary between 20min (benzyl bromide, 0.5 mol% 28) up to several days (iodopentane, fluoropentane). The reactivity of alkyl halides decreases in the order R—Br > R—Cl > R—1 when reductions are performed in separate flasks. Several mechanistic details of the reaction have been uncovered by in situ monitoring of the reaction by NMR spectroscopy. The precatalyst 28 appeared to be activated by a rapid reduction of the coordinated acetone to PrO—SiEt3 and concomitant coordination of an alkyl halide (H Scheme 12.11). This complex represents a resting state that is in equilibrium with a o-silane... 

Excellent quality metal, comparable to that from the halide reduction, can be prepared by this technique. A big advantage is that no neutrons are present from (a,ra) reactions on fluorine nuclei, in marked contrast to the case with actinide fluorides. 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

Compounds of the formulas Re(CH3)6, ReO(CH3)4, Li e CH ] [60975-25-9], Re02(CH3)3 [56090-011-8], and Re03CH3 [70197-13-6] have been prepared. The first two compounds were obtained from reaction of rhenium halides or oxyhalides and methyllithium the last three were formed from the species by oxidation or reduction reactions. The use of these hydride and alkyl complexes as catalysts is under investigation. 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

---