Halides, alkyl, addition catalysts

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

Acrylonitrile, polymerization, 120 Activity of phase-transfer catalysts Sjj2 reactions, 170-175 weak-nucleophile Sj.Ar reactions, 175-182 Acyltetracarbonyl cobalt compound, cleavage in the carboxyalkylation of alkyl halides, 150 Addition reactions, Michael, catalytic asymmetric, 69,70f... 

The first kinetic study of the Friedel-Crafts alkylation reaction was repotted by Brown and Grayson in 1953. This involved the reactions of some substituted benzyl halides with aromatic compounds in the presence of AlClj-PhNOa catalyst. The transition state for the rate-determining step (attack of the aromatic component on a polar alkyl halide-AlCb addition compound) was depicted as a o -complex. 

Relative reactivity data show that the order of reactivity for alkyl halides is tert>sec>pri>methyl This order can be accommodated by a carbonium-ion mechanism or by a process involving a complex of the halide and the catalyst. In either case, the heterolysis of the C-halide bond will be facilitated by additional alkyl groups. 

Halide-promoted heterogeneous catalysts for carbonylation are analogues to homogeneous carbonylation catalyzed by metal carbonyls, that is, the Reppe reaction (11). The first step of the Reppe reaction involves the oxidative addition of alkyl halide promoter to carbonyl metal, for example, Rh(I) complex (Fig. 1). This step is followed by methyl migration, bonding of carbon monoxide to give a coor-dinatively saturated Rh(III) complex, and subsequent decomposition of this complex in the presence of methanol to yield a carbonylated product and regenerate the promoter and the catalyst. 

A great range of Ziegler-Natta catalysts has been developed, and all consist of at least two components. Usually, a trialkylaluminum compound is involved, but alkyl aluminum halides, alkyl sodiums, and dialkyl zincs have also been used. The second component is typified by TiCh or TiCls, although other transition metal compounds have been used. Other additives (a third component) have included NaF, amines, and HMPA. These catalysts are not especially well-defined or well characterized chemical entities. They are empirically obtained mixtures that produce desirable results. We return to this issue below. 

Triazoks were synthesized from simple starting materials without the need for additional catalyst. Organic azides, generated in situ from alkyl halides and sodium azide, were reacted with acetylenes using the copper-catalyzed Huisgen 1,3-dipolar cydoaddition. 

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

Esters are usually prepared from carboxylic acids by the methods already discussed. Thus, carboxylic acids are converted directly into esters by SK2 reaction of a carboxyfate ion with a primary alkyl halide or by Fischer esterification of a carboxylic acid with an alcohol in the presence of a mineral acid catalyst. In addition, acid chlorides are converted into esters by treatment with an alcohol in the presence of base (Section 21.4). 

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. 

Thiolates, generated in situ by the action of ammonium tetra-thiomolybdate on alkyl halides, thiocyanates, and disulfides, undergo conjugate addition to a, (1-unsaturatcd esters, nitriles, and ketones in water under neutral conditions (Eq. 10. II).29 Conjugate addition of thiols was also carried out in a hydrophobic ionic liquid [bmim]PF6/water-solvent system (2 1) in the absence of any acid catalyst to afford the corresponding Michael adducts in high to quantitative yields with excellent 1,4-selectivity under mild and neutral conditions (Eq. 10.12). The use of ionic liquids helps to avoid the use of either acid or base catalysts... 

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