Cyclopentadienone Diels Alder reactions

Polyphenyls. Another means of carrying the cyclopentadienone Diels-Alder reaction to the monoadduct stage (III, Reaction 6) is to use an acetylenic dienophile and obtain a product which is no longer a diene but an aromatic. The reaction of tetraphenylcyclopentadienone (V) with diphenylacetylene (XII) affords hexaphenylbenzene (XIV) by loss of carbon monoxide from the intermediate adduct XIII (Reaction 10). In certain cases the intermediate adduct XIII can be isolated. 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. 

Diels-Alder reactions of cyclopentadienones with alkynes... 

Diels-Alder reaction of 22 and cyclopentadienones 24. The second step is an oxidative cyclodehydrogenation to yield HBCs 27. 

Glycine acts as an acid-base catalyst in this reaction. C8 and Cl 1 are very acidic, and once deprotonated they are very nucleophilic, so they can attack C2 and C3 in an aldol reaction. Dehydration gives a key cyclopentadienone intermediate. (The mechanism of these steps is not written out below.) Cyclopentadienones are antiaromatic, so they are very prone to undergo Diels-Alder reactions. Such a reaction occurs here with norbomadiene. A retro-Diels-Alder reaction followed by a [4 + 1] retrocycloaddition affords the product. 

A rather novel application of the Diels-Alder reaction is the synthesis of dendrimers. Mullen and coworkers116 made cyclopentadienone 161 react with SA AA -tetraethynylbi-phenyl 162. This afforded, after extrusion of carbon monoxide, a first generation dendrimer 163 containing 22 phenyl rings (equation 45). Cyclopentadienone 161 reacted only as a diene, since the bulky triisopropyl groups prevented the ethynyl functions from reacting. 

We have decorated different generations of polyphenylene dendrimers based on a biphenyl core with up to 16 perylenemonoimide chromophores at the periphery [67]. This has been achieved via the Diels-Alder reaction of a perylenemonoimide functionalized cyclopentadienone as a terminating reagent with the ethynyl precursor dendrimers. A strongly emitting nanoparticle is thus obtained. 

A further method for the synthesis of o-diaroylbenzenes has been described by Ried and Bdnninghausen. Tetracyclone (145), a potent diene for Diels-Alder reactions, together with dibenzoylacetylene (146) gives 147. This method has proved of value in a number of other cases and is also applicable when the cyclopentadienone derivative is dimeric ° as in the case of 2,5-dialkyl-3,4-diarylcyclopentadienones. °... 

This double Claisen-Schmidt reaction takes place under the influence of ethano-lic potassium hydroxide (Expt 7.7) and presumably proceeds in the stepwise manner (cf. formulation in Section 6.12.2, p. 1032). The four aryl groups in tetra-cyclopentadienone effectively stabilise the cyclopentadienone system, which otherwise has only a transient existence and readily undergoes dimerisation by way of a diene-dienophile interaction (Diels-Alder reaction, Section 7.6). The use of tetracyclone as a dienophile for the preparation of 3,4,5,6-tetraphenyl-dihydrophthalic anhydride is noted on p. 1121. 

Convergent synthesis (Fig. 4.18) also uses cyclopentadienone for construction of the dendrons. However, in this case two molecules of the diene are added via Diels-Alder reaction to 4,4 -diethynylbenzil. The resulting second-generation den-dron is reacted with dibenzylacetone in a twofold Knoevenagel reaction. The newly functionalised cyclopentadienone dendron obtained again undergoes (fourfold) Diels-Alder addition to the tetraethynylated tetraphenylmethane core unit [34a]. 

Initial work based on the Diels-Alder reaction provided access to linear polybenzenes via copolymerisation (Fig. 4.22) [40]. Diethynylbenzene acts as dieno-phile, and bis(cyclopentadienone) as diene. 

Some molecules are studied for their theoretical interest one being cyclopentadienone 16. But it turns out that this dimerises instantly by a Diels-Alder reaction and cannot be studied. The simplest cyclopentadienone that can be made is the tetraphenyl compound 17. Aldol disconnection gives 18 but we can now do a second aldol disconnection to reveal the two symmetrical starting materials 19 and 20. 

Conjugated diynes can function as dienophiles in Diels-Alder reactions, and adducts have been obtained in which one or both of the triple bonds participates. Most of the studies have involved the use of substituted cyclopentadienones, e.g. tetraphenylcyclopentadienone ( tetracyclone ), which form adducts that undergo decarbonylation to give aromatic hydrocarbons. Butadiyne itself reacts with tetracyclone to give hexaphenylquaterphenyl (221) while diphenylbutadiyne gives the mono(222)- and di(223)-adduct . Bis(4-biphenylyl)butadiyne gives the monoadduct 224 . 

Cyclopentadienone is an elusive compound that has been sought for many years but with little success. Molecular orbital calculations predict that it should be highly reactive, and so it is it exists only as the dimer. The tetraphenyl derivative of this compound is to be synthesized in this experiment. This derivative is stable, and reacts readily with dienophiles. It is used not only for the synthesis of highly aromatic, highly arylated compounds, but also for examination of the mechanism of the Diels-Alder reaction itself. Tetraphenylcyclopentadienone has been carefully studied by means of molecular orbital methods in attempts to understand its unusual reactivity, color, and dipole moment. In Chapter 48 this highly reactive molecule is used to trap the fleeting benzyne to form tetraphenylnaphthalene. Indeed, this reaction constitutes evidence that benzyne does exist. 

Cyclopentadienone equivalents, such as 4-acetoxy-2-cyclopenten-l-one, below, may serve as conjunctive reagents for tandem Diels-Alder reactions providing polycyclic adducts." ... 

Cyclopentadienones are very prone to do Diels-Alder reactions because of their enforced s-cis conformation and their antiaromaticity. The Diels-Alder reaction of a substituted cyclopentadienone and an alkyne is followed immediately by a [4 + 1] retro-cycloaddition to generate CO and an aromatic compound. 

Recently, Mullen et al. have reported a convergent approach134 to poly(phenylene) dendrimers (Scheme 5) similar to the divergent [4 + 2] cycloaddition route they had previously developed.135 The process was initiated by a Diels—Alder reaction between tetra-substituted cyclopentadienone 61 and dialkynyl mono-... 

Son et al. reported that the cobalt-catalyzed [2 + 2 + 1]-cycloaddition of the diyne 102 with CO, followed by Diels—Alder reaction of the resulting product with 2,3-dimethyl-1,3-butadiene, gave the tricyclic heterocycle 107 in a good yield (Scheme 37).115 The Pauson-Khand reaction of 69, followed by the [2 + 2 + 2]-cycloaddition of the resulting bi-cyclopentadienone with the cobaltacyclopentadiene... 

Ar)122 participate with increasing selectivity as 4it components in [4 + 2] cycloadditions, and much of this work has been reviewed.5-71-113 In selected instances, the expected bicyclodiazine products of the normal Diels-Alder reaction of cyclopentadienes and stable cyclopentadienones with 2ir participation of azodicarboxylates and diacyldiimides may rearrange to the corresponding 1,3,4-oxadiazines, the formal products of Diels-Alder 4ir participation of the azodicarboxylate or diacyldiimides with the dienes [Eq. (48)].123... 

Diels-Alder reactions. Unlike cyclopentadienone, this substance is stable as the monomer, probably because of the bulky substitutents. However, this dienone undergoes Diels-Alder reactions with alkenes, sometimes at room temperature or below. In reactions with 1,3-dienes, it functions as the diene rather than the dienophile, probably because of steric effects. 

Kinetic data on addition reactions, and their implications as to the reaction mechanism, have been reviewed in the recent past this is particularly true for the Diels-Alder reaction - - and its reverse reaction , and for 1,3-cyclo-additions . As well, kinetics and mechanism have been reviewed for particular cases, like Diels-Alder reactions of cyanogen and nitroso derivatives as dienophiles, and of cyclopentadienones as dienes. Other discussions of mechanism, of the Diels-Alder reaction and of 1,2-cycloadditions - , are also worth noting. Another particular case is that of 1,3-cycloadditions of nitrile oxides ", ... 

The Diels-Alder reaction has been used to form many polymeric materials. One such material, for instance, forms from a reaction of diethynylbenzene with cyclopentadienone. The products, phenylated polyphenylenes, reach molecular weights Mn up to 40,000 ... 

Cyclopentadienone derivatives condense by the Diels-Alder reaction in homopolycycloaddi-... 

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