Dienes, conjugated acylation

Conjugated dienes can be acylated by treatment with acyl- or alkylcobalt tetracarbonyls, followed by base-catalyzed cleavage of the resulting jt-allyl carbonyl derivatives. The reaction is very general. With unsymmetrical dienes, the acyl... 

The second possibly useful reaction in this group is the tetracarbonylcobalt anion-catalyzed conversion of alkyl halides with a base, CO, and conjugated dienes into acylated dienes (24). In this reaction the alkylcobalt intermediate... 

Dienes are acylated under the same conditions at the least substituted carbon atom to give exclusively an (E)-conjugated enone. ... 

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... 

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. 

Introduction of an additional methyl group on the donor atom of TMM moiety gives a low 33% yield of the perhydroindans (49, X=H2) and (50, X=H2) with substantial production of the diene by-products [24]. However, it is still remarkable that the reaction works at all since the corresponding intermolecular cycloaddition failed. Incorporation of a carbonyl moiety adjacent to the donor carbon atom doubles the yield of the cycloadducts to 66% (Scheme 2.15). This so-called acyl effect works by making the donor carbon of the TMM unit "softer," thus facilitating the initial step of the conjugate addition, as well as inhibiting base-induced side reactions [22]. 

The success of the above alkylation and acylations, without obtaining ring-opening products , extends the usefulness of this method particularly when the anion 323 is being used in a regio- and stereo-specific manner . Thus, the combination of direct alkylation and thermal extrusion of sulfur dioxide provides an ideal route for the preparation of terminally substituted conjugated dienes. 

Some pheromone components are dienes and these can be produced by either the action of two desaturases or one desaturase and isomerization around the double bond. Some dienes with a 6,9-double bond configuration are produced using linoleic acid. Desaturases that utilize monounsaturated acyl-CoA substrates include A5 [39], A9 [36,40], All [41],A12 [42],andA13 [43].These can act sequentially to produce the diene [41,42] or conjugated dienes could be produced by the action of one desaturase followed by isomerization [44-47]. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

The first example of an acyclic S—S dication (50) has been prepared by acylation of DMSO with trifuoroacetyl anhydride followed by reaction with Mc2S (Scheme 9). The S—S dication (50) has been shown to add across a double bond in an anti fashion (50) + (51) (52). Conjugated dienes undergo 1,4-addition (53) -> (54). ... 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

If the conjugated diene group is in the acyl chain of the acylcobalt carbonyl, then cyclization is possible. Thus, sorbylcobalt tricarbonyl triphenylphosphine on heating to 80°C., cyclizes to 2-methyl-7r-cyclopentenonylcobalt dicarbonyl triphenylphosphine (41). 

One of the simplest methods for preparation of an a,/J-unsaturated acyl silane is by hydrolysis of a 1-alkoxy-l-trimethylsilylbutadiene, the conjugated dienol ether of a,fi-unsaturated acyl silane, prepared by deprotonation and alkylation of the 1-alkoxydiene (Scheme 42)11. This method is generally limited in application to simple substrates, presumably due to the complexity of preparation of more highly functionalized 1-alkoxy dienes. 

An unusual synthesis of acyldienes from conjugated dienes, carbon monoxide, and alkyl or acyl halides using cobalt carbonylate anion as a catalyst should be mentioned here (57). The reaction apparently involves the addition of an acylcobalt carbonyl to a conjugated diene to produce a l-acylmethyl-7r-allylcobalt tricarbonyl, followed by elimination of cobalt hydrocarbonyl in the presence of base. The reaction can thus be made catalytic. Since the reaction was discussed in detail in the recent review by Heck (59), it will not be pursued further here. 

The Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

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