Zinc halides, cocatalysts

Effective anionic cocatalysts are bifluoiide ions e.g., tiis(dimethyl amino) sul-fonium bifluoride [(Me2N)3S HF2] or azides (TAS N3) and cyanides (TASCN), where TAS = (McjNOaS. Lewis acids, zinc halides, and dialkyl aliunimun chloride can also act as cocatalysts. 

The use of a co-catalyst was crucial to the development of practical hydrocyana-tion. The rate and catalyst lifetime for hydrocyanation of simple alkenes increases dramatically by conducting the reactions in the presence of a Lewis acid. As shown in Table 16.1, the reaction of propene occurs much faster in the presence of aluminum and zinc halides. Lewis acid cocatalysts also promote isomerization and selective additions during some steps of the hydrocyanation of butadiene. This effect is presented later in this section. 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Ethylene carbonate can be prepared by a well-known process, from ethylene oxide and carbon dioxide with a catalyst such as a quaternary ammonium halide at 150-175°C. Addition of a cocatalyst such as zinc chloride to a quaternary ammonium iodide allows milder reaction conditions (50-100°C). ... 

Most commonly, the catalyst component consists of halides or oxyhalides of titanium, vanadium, chromium, molybdenum, or zirconium, and the cocatalyst component often consists of an alkyl, aryl, or hydride of metals such as aluminum, lithium, zinc, tin, cadmium, beryllium, and magnesium. The catalyst systems may be heterogeneous (some titanium-based systems) or soluble (most vanadium-containing species). Perhaps the best known systems are those derived from TiCl4 or TiCls and an aluminum trialkyl. 

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