Carbocation precursors

Because electrophilic attack on benzene is simply another reaction available to a carbocation other carbocation precursors can be used m place of alkyl halides For exam pie alkenes which are converted to carbocations by protonation can be used to alkyl ate benzene... 

Apart from the alkyl halide-Lewis acid combination, two other sources of carbo-cations are often used in Friedel-Crafts reactions. Alcohols can serve as carbocation precursors in strong acids such as sulfuric or phosphoric acid. Alkylation can also be effected by alcohols in combination with BF3 or A1C13.37 Alkenes can serve as alkylating agents when a protic acid, especially H2S04, H3P04, and HF, or a Lewis acid, such as BF3 and A1C13, is used as a catalyst.38... 

The diffusional association constant K s was assumed to be equivalent to the value obtained for benzylic carbocation precursors. The parameters /Caz and ATas were fixed and the other parameters were adjusted to optimize the fit. The data could be fitted to a mechanism in which preassociation was... 

Isopentane and trimethylpentanes (C8 alkanes) are inevitably formed whenever isobutane is alkylated with any alkene. They are often called abnormal products. Formation of isopentane involves the reaction of a primary alkylation product (or its carbocation precursor, e.g., 3) with isobutane. The cation thus formed undergoes alkyl and methyl migration and, eventually, P scission ... 

A diastereoselective ewrfo-cyclization into an oxidatively generated oxocarbenium ion was a key step in a formal synthesis of leucascandrolide A. Exposing 56 to CAN provided cw-tetrahydropyran 57 in high yield and with excellent stereocontrol (Scheme 3.20). This transformation provides further evidence that oxidative electrophile formation is tolerant of several functional groups and can be applied to complex molecule synthesis. The synthetic sequence also utilized a Lewis acid mediated ionization reaction to form an oxocarbenium ion in the presence of the homobenzylic ether (58, 59), illustrating that two carbocation precursors that ionize through chemically orthogonal conditions can be incorporated into the same structure. 

Stop-flow apparatus. The acid concentrations were around 10 M and those of anhydroretinol about ten times smaller. In the first second or so, after the mixing of the reactants the following processes gave rise to the carbocation precursors ... 

SO on, loss of a proton leads to aromatization, completing the substitution reaction to form the new carbon bond in 159. Electrophilic aromatic substitution with alkyl carbocations is called Friedel-Crafts alkylation. It was reported by Friedel and Crafts in 1877. The carbocation precursor can be an alkene, an alcohol, or an alkyl halide. 

Oxidation of (36) at a platinum anode gave the TV-methoxy-A -methyl-co-hydroxyamide (37), which was converted under acidic conditions to the l,3-oxazepin-4-one (38) (Scheme 5). The carbocation precursor of (37) is not converted directly to (38) probably due to anode interaction with the terminal hydroxy group <82ACS(B)165>. 

In order to understand why 21 is the major product, examine the mechanism. Look at the final product and use the knowledge that the reaction of an alkene and HBr proceeds by a carbocation intermediate to look for changes in the reaction pathway. First, ask what carbocation is expected when HBr reacts with 3-methyl-l-pentene. The answer is carbocation 20, which is the secondary carbocation that is formed preferentially to the regioisomeric primary carbocation. The bromine atom in 21 clearly comes from the bromide ion, which must react with a carbocation, so the carbocation precursor to 21 must be 22. In other words, bromide 21 cannot arise from 20, but must resrdt from tertiary carbocation 22. Note that 22 is a tertiary carbocation, whereas 20 is a secondary carbocation. 

Alkylation is accomplished by use of haloalkanes, alcohols, or alkenes any species that can function as a carbocation precursor. The alkylation reaction is accompanied by two significant and limiting side reactions polyalkylation, due to ring activation by the added alkyl groups, and rearrangement of the intermediate carbocation. These lead to diminished yields, and mixtures of products that can be difficult to separate as shown here ... 

Friedel-Crafts Alkylations Using Other Carbocation Precursors... 

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