Organic halides Direct Process Reaction

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

The addition of H—X, where X = F, Cl, Br and I, to alkenes, dienes and alkynes has been extensively studied from both mechanistic1-3 and synthetic standpoints. While it is one of the earliest methods employed for the synthesis of organic halides, it can often lead to mixtures of products, and the direct conversion of the corresponding alcohols is more commonly used nowadays. The early literature abounds with examples of this reaction in which either mixtures of products are formed, or the products are not well characterized. Few major advances have been reported in more recent times which overcome the synthetic disadvantages of this direct process. It can nevertheless be the method of choice for the synthesis of certain substrates. 

Some electron-transfer reactions have been suggested to take place although their rate constants are too slow to be measured directly. A typical example is the reduction of organic halides by substituted alkyl radicals. Halide ions are formed in a process such as (62). In the... 

A general process that involves direct and efficient reaction of fluorspar with organic halides would be very desirable but, so far, this has not been realised, except through generation in situ of hydrogen fluoride [60]. 

Radical reactions can often be rationalized on the basis of frontier orbital considerations for intermediate radical species, the reactivity and stereochemistry of which can certainly be regulated with Lewis acid additives [21-23]. The first appearance of Lewis acids in radical reactions was in polymerization reactions resulting in alternation of copolymers different from that obtained without Lewis acids [24-26]. This concept, Lewis acid-directed radical reactions, has been applied to reductions and alkylations of organic halides or olefins, and has resulted in highly stereospecific processes. 

The order of reactivity of organic halides is I > Br > Cl > F. Although organic iodides are the most reactive, they produce more Wurtz coupling products through the radical oxidative addition process. Thus, it is usually advisable to use the chlorides or bromides, except in the case of aromatic iodides and methyl iodide. 1-Alkenyl Grignard compounds are obtained by the direct reaction of magnesium and vinylic halides in THF [6], but the reaction is not completely stereospecific and the retention of the stereochemistry of haloalkenes varies from 60-90% [7]. The difficulty associated... 

Silicon tetraalkyl and tetraaryl derivatives (R4Si), as well as alkyl or aryl silicon halides (R SiCl4 , n= 1-3) can be prepared by reaction types 19.39-19.43. Note that variation in stoichiometry provides flexibility in synthesis, although the product speciflcity may be influenced by steric requirements of the organic substituents. Reaction 19.39 is used industrially (the Rochow or Direct process). 

Palladium-catalyzed carbonylation of organic halides to form esters, amides, ketones, and aldehydes (Equation 17.58) has been studied and reviewed extensively. " These reactions are closely related to the palladium-catalyzed cross-coupling processes presented in Chapter 19. However, the addition of CO to ttiese processes generates organic carbonyl compounds, rather than products from direct cross coupling. 

The direct reaction of organic halide with zinc, shown in eq. (5.1), proceeds with active zinc in the absence of the metal catalyst. Two processes for the activation of zinc are exploited [14-17]. The first process is the reduction of zinc halide with organoalkali metal compounds. For example, ZnCl2 is reduced by naphthalene-lithium. The second process is the irradiation by ultrasound [16,17]. 

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