Acyl halides with alkenes

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

The acylpalladium complex formed from acyl halides undergoes intramolecular alkene insertion. 2,5-Hexadienoyl chloride (894) is converted into phenol in its attempted Rosenmund reduction[759]. The reaction is explained by the oxidative addition, intramolecular alkene insertion to generate 895, and / -elimination. Chloroformate will be a useful compound for the preparation of a, /3-unsaturated esters if its oxidative addition and alkene insertion are possible. An intramolecular version is known, namely homoallylic chloroformates are converted into a-methylene-7-butyrolactones in moderate yields[760]. As another example, the homoallylic chloroformamide 896 is converted into the q-methylene- -butyrolactams 897 and 898[761]. An intermolecular version of alkene insertion into acyl chlorides is known only with bridgehead acid chlorides. Adamantanecarbonyl chloride (899) reacts with acrylonitrile to give the unsaturated ketone 900[762],... 

Enamines 1 are useful intermediates in organic synthesis. Their use for the synthesis of a-substituted aldehydes or ketones 3 by reaction with an electrophilic reactant—e.g. an alkyl halide 2, an acyl halide or an acceptor-substituted alkene—is named after Gilbert Stork. 

Alkenes can be acylated with an acyl halide and a Lewis acid catalyst in what is essentially a Friedel-Crafts reaction at an aliphatic carbon. ° The product can arise by two paths. The initial attack is by the acyl cation RCO (or by the acyl halide free or complexed see 11-14) at the double bond to give a carbocation ... 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

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. 

As with carboxylic acids obtained by palladium hydroxycarbonylation, their derivatives esters, amides, anhydrides and acyl halides are synthesized from alkenes, CO and HX (X = OR, NR2 etc.). The Pd-catalyzed methoxycarbonylation is one of the most studied reactions among this type of catalyzed carbonylations and has been reviewed and included in reports of homogeneous catalysis.625, 26 The methoxycarbonylation has been applied to many different substrates to obtain intermediates in organic syntheses as well as specific products. For instance, the reaction has been applied for methoxycarbonylation of alkynes666 Highly efficient homogeneous Pd cationic catalysts have been reported and the methoxycarbonylation of alkynes has been used to develop economically attractive and environmentally benign process for the production of methyl... 

An indirect method for the hydroformylation of olefins involves formation of the tri-alkylborane (5-12) and treatment of this with carbon monoxide and a reducing agent (see 8-26). Hydroacylation of alkenes has been accomplished, in variable yields, by treatment with an acyl halide and a rhodium complex catalyst, e.g.,587... 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

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