Lewis acids, reaction with alkyl halides

Therefore, it must be concluded that earlier attempts to prove the existence of stable, well-defined alkyl cations were unsuccessful in experiments using sulfuric acid solutions and inconclusive in the interaction of alkyl halides with Lewis acid halides. Proton elimination reactions or dialkyl halonium ion formation may have affected the early conductivity studies. 

Although alkylation with these anions is usually a high-yielding reaction, there are several possible side-reactions that must be eliminated by careful choice of reaction conditions494 98. These side-reactions include O-alkylation and multiple C-alkylations. The yields of alkylation reactions with anions is often significantly reduced if the halide is unable to react via a S -type process. This may be alleviated to some extent by the use of trimethylsilyl enol ethers using Lewis acid catalysis499 503. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

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. 

This article describes further progress in the chemistry of 7r-allylnickel compounds. First, preparative methods for 7r-allylnickel halides, alkoxides, amides, and alkyls are described. Next, some chemical properties of these compounds—e.g., a recently observed disproportionation reaction—are discussed. Then, the use of 7r-allylnickel halides as homogeneous catalysts is discussed. Whereas bis (7r-allyl) nickel is a catalyst for butadiene cyclotrimerization, 7r-allylnickel halides combined with Lewis acids, such... 

One of the more commonly used organotin reagents in both free-radical reactions and Lewis acid-mediated reactions is allyltributyltin [3, 7]. This reagent permits the construction of new carbon-carbon bonds from free radical precursors such as alkyl halides however, reactions with a-ketocydopropanes were poorly understood. Tin(IV) enolates generated from a-ketocyclopropane 44 and allyltributyltin undergo both radical allylation and electrophilic quench as shown in Scheme 11, forming O-stannyl ketyl 45 with allyltributyltin and subsequent scission of the cyclopropane... 

Alkyl halides react with Lewis acids to form tertiary, secondary, or primary carbocations. The tertiary cation (160, r1=r2=r3= alkyl) is sufficiently stable that good yields of alkylated products are obtained upon reaction with benzene. Formation of the primary cation (160, R1=r2=H, R = alkyl) is energetically more difficult and is, of course, subject to rearrangement primary alkyl derivatives are rarely observed under these conditions. Secondary cations (160, R =H, r2=R = alkyl) are readily formed but are also subject to rearrangement. The extent of rearrangement is dependent on the reaction conditions and the nature of the... 

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