Acylation of alkenes

Of the acylation of aliphatics, the acylation of alkenes pioneered by Nenitzescu was mainly explored, because the resulting unsaturated ketones are intermediates in synthesis. This acylation of alkenes, however, has its difficulties. First, the product unsaturated ketones, which are reactive compounds themselves, can undergo various further transformations, such as addition, elimination, and isomerization, often resulting in complex product mixtures. The acylation of alkenes, therefore, is less selective and often yields products other than expected by a simple substitution of one of the vinylic hydrogens. 

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  

A1C13 is the strongest and most frequently used Lewis acid catalyst in aliphatic Friedel-Crafts acylations usually applied with acid chlorides, whereas ZnCl2 is 

Alkene acylation may be used in the synthesis of cyclic products by appropriately selecting the alkene and the acid derivative. When a,P-unsaturated acid halides react with cycloalkenes, the intermediate undergoes cyclization to yield octalones and indanones 112 

Acylation with cyclopentenyl- or cyclohexenylacetyl chloride gave three-membered ring systems.113 114 An interesting example is the reaction of ethylene with acid chlorides complexed with excess A1C13 to give the 17 p.y-unsaturated ketones 115 

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The P-halo ketone intermediates formed in the foregoing reactions arise from the capture of carbocationic intermediates by halide of the gegenions. In some cases, solvents such as acetonitrile can act as the competing nucleophilic species. For example, P-amido ketones could be obtained by the acylation of alkenes in acetonitrile (172). 

A variety of reaction conditions have been examined for acylation of alkenes by acyl chlorides. With the use of Lewis acid catalysts, reaction typically occurs... 

A formal asymmetric nucleophilic addition to carbonyl compounds is achieved by Trost and his co-workers in the allylic alkylation of acylals of alkenals. An excellent enantioselectivity is observed in this alkylation. The starting acylals are easily prepared by the Lewis-acid catalyzed addition of acid anhydrides to aldehydes, by use of Trost s ligand 118 (Scheme 13), where various carbon-centered nucleophiles are available (Scheme l4),101,101a-10lc Asymmetric synthesis of some natural products is achieved according to this procedure. 

The acylation of alkenes with trifluoroacetic anhydride having only weak elec-trophilicity was reported to give a,p-unsaturated ketones in 19-49% yields.117 118 The reaction requires the use of the Me2S—BF3 complex as catalyst and takes place only with alkenes that readily form stable carbocations. Alkenes were also electrolyzed in CH2C12 solution of Et4NCl as the electrolyte at —5 to — 10°C in the... 

In the acylation of alkenes with acetyl chloride120 or acetic anhydride120,121 over zeolites (HX, HY, H-mordenite, H-ZSM-5, H-Beta) usually complex product mixtures including isomeric unsaturated ketones, chloroketones, and acetates were obtained. Zeolite HY proved to be the most active catalyst under mild experimental conditions (25-60°C).120,121... 

An interesting example of an indirect acylation of alkenes is the reaction of small olefins (ethylene, propylene, isobutylene) over H-ZSM-5 in the presence of CO at 23°C to give unsaturated ketones.122 In this case the acylium ion is formed in situ from the alkene and CO in the pores of the zeolite at ambient temperature ... 

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. 

Hydro acylation of alkenes was achieved in the presence of Wilkinson s catalyst and microwave irradiation under solvent-free conditions. As an example, benzaldehyde was reacted with dec- 1-ene to give 1-phenylundecan- 1-one in 83%yield within 30 min. Both domestic microwave ovens and single-mode reactors have been used for this reaction. The presence of an amine such as 2-amino-3-picoline or aniline and a carboxylic acid is crucial for the success of the reaction, showing that the formation of an imine plays an important role as an intermediate in the mechanism of this reaction29. 

Friedel-Crafts acylation of alkenes.9 C2H5A1C12 is an effective catalyst for acylation of alkenes with acyl chlorides or anhydrides in CH2C12. The reaction proceeds in higher yield than that previously reported with other Lewis acid catalysts (such as ZnCl2). The reaction provides a useful route to /J,y-enones. 

Acylation of alkenes. Friedel-Crafts acylation (AlCl,) of alkenes suffers from lack of selectivity and low yields. The reaction is markedly improved by use of CHdj-Zn/Cu (3, 255) as catalyst. No cyclopropanation is observed. 

Acylation of alkenes and alkynes. Dienes and alkynes can be acylated under phase-transfer conditions by carbon monoxide and methyl iodide in the presence of -catalytic amounts of Co2(CO)s. The reaction with alkynes results in a 4-hydroxy-2-butenolide the probable mechanism is shown in equation (I). 

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. 

A significant advance in the use of Friedel-Crafts acylation of alkenes to prepare divinyl ketones was the employment of vinylsilanes to control the site of electrophilic substitution. Two groups have developed this approach to cyclopentenone annulation using slightly different strategies. In the method described by Magnus the reagent vinyltrimethylsilane (80) is used primarily as an ethylene equivalent (equation 44). The construction of bicyclic systems followed readily as Nazarov cyclization proceeded under the reaction conditions. Tin(lV) chloride was found to be the most effective promoter of the overall transformation. As expected the position of the double bond is thermodynamically controlled. 

The acylation of alkenes can also be similarly effected with SbFs [32], For example, a,/3-unsaturated carboxylic acid fluorides react with perfluoroalkenes to form a,/3-unsaturated ketones (Eq. 16) [32b]. 

Smith et al. have isolated a series of cyclic carboxonium salts (such as 119-121) by acylation of alkenes. 

In chapter 16 we shall discover that vinyl-copper reagents can be prepared with stereochemical control over double bond geometry and acylated directly with acid chlorides. We shall also meet vinyl silanes and see how they too can be acylated with acid chlorides. In this chapter we shall consider only the acylation of alkenes themselves with acid chlorides, that is the aliphatic Friedel-Crafts reaction.15 The normal Friedel-Crafts reaction 66 combines an aromatic compound with an acid chloride and a Lewis acid to give a cation 67 which loses a proton to give an aryl ketone 68. 

Significant advances resulting from the use of aluminosilicate solids were made during the last few years [3-6] and the first industrial application of zeolites in large scale Friedel-Crafts acylations was reported very recently [7]. However, most of the efforts devoted so far focused on the acylation of aromatic compounds. To the best of our knowledge, recourse to heterogeneous aluminosilicate catalysts for the acylation of alkenes has not yet been reported. Conventional methods for alkene acylation [8] involve the use of Br0nsted or Lewis acids such as sulfuric acid [9], boron trifluoride [10], zinc chloride [11], or... 

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