Acylium ions, with alkenes

Reactions with Acylium Ions. Alkenes react with acyl halides or acid anhydrides in the presence of a Lewis acid catalyst to give (3,y-unsaturated ketones. The reactions generally work better with cyclic than acyclic alkenes. 

Both the reactions are essentially the additions of iodine carboxylate (formed in situ) to an alkene, i.e., the reaction of an alkene with iodine and silver salt. The Prevost procedure employs iodine and silver carboxylate under dry conditions. The initially formed transiodocarboxylate (b) from a cyclic iodonium ion (a) undergoes internal displacement to a common intermediate acylium ion (c). The formation of the diester (d) with retention of configuration provides an example of neighbouring group participation. The diester on subsequent hydrolysis gives a trans-glycol. 

Under nonequilibrium conditions the p,y-unsaturated ketone tends to predominate, which may be transformed to the more stable conjugated isomer. The intermediacy of 15 may allow further rearrangements to occur. However, acylation with preformed acylium ions, such as acetyl hexachloroantimonate, and in the presence of a base to prevent isomerization, affords exclusive formation of P,y-unsaturated ketones.109 This led to the suggestion of an ene reaction between the acylium ion and the alkene as a possible mechanistic pathway ... 

The Friedel-Crafts acylation of alkanes requires hydride abstraction, which can be induced by the acylium ion itself, to form the corresponding carbocation. This may undergo carbocationic rearrangements prior to a proton loss to form an alkene, which then reacts with the acylating agent. Similar to the acylation of alkenes, the product is an unsaturated ketone. The reaction is limited to alkanes that are prone to undergo hydride transfer. 

Friedel-Crafts acylation of alkenes (Daizens-Nenitzescu reaction ) with unsaturated acylium ions generated from acid halides and Lewis acids constitutes a general synthesis of divinyl ketones. 

Competition studies reported by Kuwajima, " which also complement the results of Nakai," illustrate the limitations of the 3-effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone proiduct (8c) arising fr the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary 3-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary carbocations appear to be more stable than secondary 3-silyl cari ations, as judged by the formation of compound (te). 

The 77 electrons in a C=C 77 bond can react with a Lewis acidic electrophile to give a carbocation. The simplest example is the reaction of an alkene with H+. Note that one of the C atoms of the 77 bond forms the bond to H+ using the electrons of the 77 bond, whereas the other C becomes electron-deficient and gains a formal positive charge. Other cationic electrophiles (carbocations, acylium ions, and Br+) can react with C=C 77 bonds too. 

New C—C bonds to arenes can be made by Friedel-Crafts reactions. Friedel-Crafts alkylations are traditionally executed with an alkyl chloride and catalytic AICI3 or an alkene and a strong Brpnsted or Lewis acid the key electrophilic species is a carbocation. Friedel-Crafts acylations are usually executed with an acyl chloride and an excess of AICI3 the key electrophilic species is an acylium ion (RC=0+). In the Bischler-Napieralski reaction, intramolecular attack on a nitrilium ion (RC=NR) occurs. 

Unsaturated acylium ions generated from alkenic acids or anhydrides react with alkenes to produce cy-clopentenones (equation with cycloheptene the major products arise from ring contraction. 

The acylation of alkanes has also been known for a long time, but for synthetic purposes is limited to simple substrates. The initial step is hydride abstraction by an acylium ion, a process well established in the presence of a powerful Lewis acid, most commonly an aluminum halide, or strong protic acid. The carbocation so formed can then undergo elimination, possibly after hydride or alkyl migration, to give an alkene which is then acylated. In the presence of excess alkane, saturated ketones are formed by a further intermolecular hydride transfer, whereas with an excess of acyl halide, the product is the (conjugated) unsaturated ketone. -" The synthetic potential is obviously likely to be limited to simple substrates. 

The first stage is an aliphatic Friedel-Crafts reaction with an acylium ion attacking the alkene. 

The mechanism involves carbocation formation via protonation of the alkene (87,89) (eq. 62) followed by its reaction with carbon monoxide to form acyl cation (acylium ion) 23 (eq. 63). In the final step (eq. 64), 23 is quenched with water to 3ueld the product a-methyl carboxylic acid. 

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