1.3- Dienes Friedel-Crafts reaction

Conjugated dienes, upon complexation with metal carbonyl complexes, are activated for Friedel-Crafts acylation reaction at the akyhc position. Such reactions are increasingly being used in the stereoselective synthesis of acylated dienes. Friedel-Crafts acetylation of... 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

For the preparation of unsymmetrical ferrocenes, two ways by which only mono-substituted derivatives are produced have been suggested. One route starts from iron tetracarbonyl and a substituted cyclopenta-diene the other from monocyclopentadienyl iron dicarbonyl bromide, which on treatment with a substituted cyclopentadienyl lithium is finally converted into the corresponding mono-substituted ferrocene. Experience shows the first method to be more suitable for the preparation of aryl, and the latter method for the preparation of alkyl, derivatives (57). Corresponding work already carried out on substitution in Ru(C5H6)2 and Os CsH5)2 has also been fruitful. It is found that in the Friedel-Crafts reaction with acetyl or benzoyl chloride there is a distinct predominance of mono- over disubstitution as the atomic weight of the central atom increases (47, 72). 

In our own work (ref.39) the methyl, ethyl and benzyl esters of coccinellic anhydride methyl ether were prepared by the Dlels-Alder reaction of 2-chloro and2-bromomaleicanhydride with 3-alkoxy carbonyl-2,4-bistrimethylsiloxypenta-1,3-diene (R = Me,Et,Bn) rather than by the lengthy procedure used formerly (ref. 214). Nevertheless the Friedel-Crafts reaction of these esters with 1,4-dihydroxybenzene to afford finally 6-deoxykermesic add could not be effected. 

An intramolecular Friedel-Crafts reaction of a diene and an electron-rich aromatic moiety, promoted by BF3-Ft20, was used in the late stage construction of cyclohexyl ring in an approach to the marine natural product pseudopterosin aglycone. In a somewhat related transformation, it has been demonstrated that aromatics can be converted into the corresponding diarylmethane derivative on exposure to a-methoxyacetic acid in the presence of trifluoroacetic anhydride (TFAA) and BF3-Ft20 (eq 49). ... 

The presence of one or more molar equivalents of AlGlg can bring about a remarkable acceleration of diene syntheses.—E Anthracene allowed to react for 5 min. with an equimolar amount of dimethyl fumarate in the presence of 2 molar equivalents of AlClg frans-dimethyl ester adduct. Y ca. 100%.— Reaction in the absence of AlGlg when carried out in boiling dioxane requires 2-3 days and affords a less pure product. Limitations are imposed by the instability of some dienes and dienophiles under the reaction conditions and by the preferential occurrence of Friedel-Crafts reactions in some cases. F. e. s. P. Yates and P. Eaton, Am. Soc. 82, 4436 (1960) also with other Friedel-Grafts type catalysts cf. G. I. Fray and R. Robinson, Am. Soc. 83, 249 (1961). 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

It was reported that Sc(OTf)3 catalyzes Diels-Alder reactions in [bmim][X] (X = BF4, SbFg or OTf), in this case at much lower catalyst loadings (0.2 m%). In contrast to the Friedel-Crafts alkylation (see below) the product did not form a separate phase and was recovered by extraction with ether. It was shown, however, that the ionic liquid containing the catalyst could be recycled eleven times without loss of activity, Scheme 4. Furthermore, improved endojexo selectivities were observed with cyclic dienes. 

Friedel Crafts acetylation of butadiene complex 56 proceeds smoothly to give a mixture of 1-acetyldienes 58 and 59 via the cationic 7r-allyl complex 57 [16]. Intramolecular Friedel-Crafts acylation with the acid chloride of the diene complex 60, promoted by deactivated AICI3 at 0 °C, gave the cyclopentanones. The (Z)-dienone complex 61 was the major product and the ( )-dienone 62 the minor one [17]. Acetylation of the 1,3-cyclohexadiene phosphine complex 63 proceeded easily at —78°C to give the rearranged complex 65 in 85% yield. Without phosphine coordination, poor results were obtained [18,19], In this reaction, the acetyl group at first coordinates to Fe, and attacks at the terminal carbon of the diene from the same... 

A complex of SbCls and benzyltriethylammonium chloride, which is insensitive to air and moisture, catalyzes the Friedel-Crafts acylation of arenes with acyl chlorides and sulfonyl chlorides (Equation (36)).69 The Diels-Alder reaction of toluquinone with 1,3-dienes is catalyzed by SbCl5 with high regioselectivity (Equation (37)).70... 

A number of the reactions discussed above were used in a synthetic sequence. Friedel Crafts acylation of (98) gives (99) after isomerization of the diene geometry using sodium methoxide. Bromination of aTMS-enol ether, formed from (99), with DBA was nonstereospecific however, the isomers were readily separated (Scheme 159). The pure a-bromoketone is stereospecificaUy methylated and transformed into an epoxide. Lewis acid mediated isomerization of the epoxide affords an optically pure ketone (100). 

Iron tricarbonyl forms exceptionally stable complexes with 1,3-dienes. The complexes are uncharged, readily soluble species, chromatographable and, for the simpler versions, distillable. They are formed by direct reaction of the 1,3-diene with Fe(CO)5, Fc2(CO)9, or Fe3(CO)i2. These iron diene complexes are known to be reactive toward electrophiles, undergoing the analogous reaction to electrophilic aromatic substitution under Friedel-Crafts conditions. However, it is clear that the metal-ligand unit increases the polarizibility of the diene unit, and, with a sufficiently reactive nucleophile, can provide a sink for electron density. How reactive does the nucleophile need to be The other important selectivity question for 1,3-dienes concerns the regioselectivity. 

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