Cyclopentadiene/maleic anhydride adduct

All of the isomerization data shown above is consistent with the normal electrocyclic reaction chemistry expected for such substrates (4). That such fused norbornenyl systems undergo exo/endo isomerization via Diels Alder/retro Diels Alder reactions has been explicitly proven for simple cyclopentadiene-maleic anhydride adducts (5) and... 

Two routes to substituted butenolides involve modifications of earlier procedures based on retro-Diels-Alder reactions. 2-Substituted butenolides (46) can be obtained by alkylations of (45) followed by thermal degradation (Scheme 14) (45) is prepared from the cyclopentadiene-maleic anhydride adduct (44). 4,4-Dialkylbutenolides can be obtained from the Diels-Alder adduct of furan and maleic anhydride by sequential methanolysis, Grignard reaction, and... 

In contrast, the radical catalyzed homopolymerization of the cyclopentadiene-maleic anhydride (CPD-MAH) Diels-Alder adduct yields a saturated homopolymer at temperatures as high as 220 C, while retrograde dissociation occurs at even higher temperatures. Nevertheless, the copolymerization of monomeric cyclopentadiene and maleic anhydride yields a saturated 1 2 copolymer (12-15). 

Maleic anhydride adducts of anthracene and cyclopentadiene (Fig. 4) were used in the synthesis of UPRs and apphed directly during the polycondensation reaction of maleic anhydride with ethylene glycol or diethylene glycol (Table 7) [17]. 

Fig. 4 Synthesis of maleic anhydride adducts of cyclopentadiene and anthracene. Reprinted from (1995) Polimery 40 624 [17] with permission...

Consider Diels-Alder reaction of cyclopentadiene and maleic anhydride, leading to endo or exo adducts. 

The exo addition mode is expected to be preferred because it suffers fewer steric repulsive interactions than the endo approach however, the endo adduct is usually the major product because of stabilizing secondary orbital interactions in the transition state (Scheme 1.10). The endo preference is known as Alder s rule. A typical example is the reaction of cyclopentadiene with maleic anhydride which, at room temperature, gives the endo adduct which is then converted at... 

The thermal reaction of cyclopentadiene (1) with maleic anhydride gives 98 % kinetically favoured endo adduct, unless the mixture is heated for a long time [44]. Under photolysis conditions and in the presence of triethylamine in dry ethanol, a reversed selectivity was found [45] (Scheme 4.13). 

The photo-induced exo selectivity was observed in other classic Diels-Alder reactions. Data relating to some exo adducts obtained by reacting cyclopentadiene or cyclohexadiene with 2-methyl-1,4-benzoquinone, 5-hydroxynaphtho-quinone, 4-cyclopentene-l,3-dione and maleic anhydride are given in Scheme 4.13. The presence and amount of EtsN plays a decisive role in reversing the endo selectivity. The possibility that the prevalence of exo adduct is due to isomerization of endo adduct under photolytic conditions was rejected by control experiments, at least for less reactive dienophiles. 

Some more examples of endo addition (1) Addition of maleic anhydride to cyclopentadiene. Here also two orientations are possible and the endo product predominates. Sometimes the exo adducts are completely excluded. 

Solid-state catalytic hydrogenations in the absence of any solvent can be easily and quantitatively performed in ball-mills that allow for evaporation and filling with gases. For example, the Diels-Alder adduct of maleic anhydride and cyclopentadiene 2 hydrogenates readily upon milling with palladium on charcoal in an atmosphere of hydrogen in 200-g batches to give 99, which is easily sublimed off from the catalyst [22] (Scheme 10). 

Alder s endo rule applies not only to cyclic dienes like cyclopentadiene and to disubstituted dienophiles like maleic anhydride, but also to open chain dienes and to mo no-substituted dienophiles diphenylbutadiene and acrylic acid, for example, react by way of an endo transition structure 2.113 to give largely (9 1) the adduct 2,114 with all the substituents on the cyclohexene ring cis, and equilibration again leads to the minor isomer 2.115 with the carboxyl group trans to the two phenyl groups. 

Recently, the intriguing IEDA adducts 48 of 1-germa-2,3,4,5-tetraphenyl-1,1-dimethyl-2,4-cyclopentadiene (47) with N-methylmaleimide and maleic anhydride were prepared by high-pressure reactions (Scheme 15) [43]. 

In a classic Diels-Alder reaction, cyclopentadiene 24 combines with maleic anhydride 2 with complete stereospecificity to give either 25 or 26 so the two Hs on 2 remain cis to each other in both 25 and 26. These are called endo- and exo- adducts. This refers to the relationship between the alkene on the diene side and the carbonyl groups from the dienophile. These are much closer in the entio-adduct 25. The result is easy to see when both reagents are cyclic. 

The dicyclopentadiene that is formed in the dimerization of cyclopentadiene has the ring of the dienophile in an endo orientation to the cyclopentadiene ring that acts as the diene. Usually, substituents on the dienophile are found to be endo in the adduct if the substituents contain pi bonds. Another example is provided by the reaction of cyclopentadiene and maleic anhydride illustrated in the following equation ... 

The overall Diels-Alder reaction between maleic anhydride and cyclopentadiene gives Diels-Alder adduct 8.5 with endo conformation. On heating at 190°C, the endo conformation adduct 8.5 adopts the thermodynamically more stable exo-adduct 8.6. 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

Cyclopentadiene is obtained from the light oil from coal tar distillation but exists as the stable dimer, dicyclopentadiene, which is the Diels-Alder adduct from two molecules of the diene. Thus, generation of cyclopentadiene by pyrolysis of the dimer represents a reverse Diels-Alder reaction. See Figs. 1 and 2 for nmr and infrared spectra of dicyclopentadiene. In the Diels-Alder addition of cyclopentadiene and maleic anhydride the two molecules approach each other in the orientation shown in the drawing above, as this orientation provides maximal overlap of ir-bonds of the two reactants and favors formation of an initial ir-complex and then the final e do-product. Dicyclopentadiene also has the endo-configuration. 

The colorless zinc compound, Zn(CisH6)2, which sublimes at 160° under partial decomposition, is obtained in small yield from zinc chloride and cyclopentadienyl sodium in diethyl ether however, the less stable cadmium compound decomposes, with separation of cadmium, under these conditions (55). The mercury compound, Hg(CsH5)2, is produced in 20% yield by the action of the sodium derivative on mercuric chloride in tetrahydrofuran (215). The action of cyclopentadiene on the complex K2(HgI ) in aqueous alkaline solution results in the precipitation of a mixture of CsHsHgl and Hg(CsH6)2, from which the latter compound may be obtained in good yield by extraction with a mixture of tetrahydrofuran and petroleum ether (62). It forms pale yellow crystals which begin to decompose at about 60° and which melt at 83-85°. The compound is readily soluble in most solvents it decomposes slowly even when kept in the dark at room temperature it is insoluble in water and reacts with neither water nor bases. On the other hand, decomposition occurs in dilute hydrochloric acid. It converts ferric chloride to ferrocene quantitatively, and it yields an adduct with maleic anhydride (215). 

The experimental observation is that maleic anhydride and cyclopentadiene give the endo adduct (174) faster than they give the exo adduct (176), even though the latter is thermodynamically the more stable. This is a general observation the transition state for most Diels-Alder reactions must be like 173 rather than like 175. In order to account for this, we examine the interaction of those parts of the frontier orbitals which are not directly involved in forming... 

Diets-Alder reaction,2 This substance exhibits high dienophilic reactivity comparable to that of maleic anhydride. It reacts with 5-substituted cyclopentadienes to form adducts in which the 7-substituent is exclusively anti to the bridge bearing the chloro and chloroformyl groups. Thus it reacts with 5-benzyloxymethylcyclopentadiene (1) to give the adduct (2) as a 2 1 mixture of exo- and endo-acid chlorides in about 99%... 

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