Maleic anhydride-cyclopentadiene

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... 

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). 

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... 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Diels-Alder Reactions. The important dimerization between 1,3-dienes and a wide variety of dienoplules to produce cyclohexene derivatives was discovered in 1928 by Otto Diels and Kurt Alder. In 1950 they won the Nobel prize for their pioneering work. Butadiene has to be in the j -cis form in order to participate in these concerted reactions. Typical examples of reaction products from the reaction between butadiene and maleic anhydride (1), or cyclopentadiene (2), or itself (3), are <7 -1,2,3,6-tetrahydrophthaHc anhydride [27813-21 -4] 5-vinyl-2-norbomene [3048-64-4], and 4-vinyl-1-cyclohexene [100-40-3], respectively. 

In a study of the relative rates of addition of 20 dienophiles to cyclopentadiene, TCNE was at the head of the Hst, eg, it added 7700 times as rapidly as maleic anhydride (15). Reaction with most 1,3-dienes takes place rapidly and in high yield at room temperature. TCNE has often been used to characterize 1,3-dienes, including those that are unstable and difficult to isolate (16). 

Cyclopentadiene contains conjugated double bonds and an active methylene group and can thus undergo a Diels-Alder diene addition reaction with almost any unsaturated compound, eg, olefins, acetylene, maleic anhydride, etc. The number of its derivatives is extensive only the reactions and derivatives considered most important are discussed. 

A number of special purpose resins are available which employ somewhat unusual acids and diluents. A resin of improved heat resistance is obtained by using Nadic anhydride, the Diels-Alder reaction product of cyclopentadiene and maleic anhydride Figure 25.5). 

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

Pulverized maleic anhydride (6 g, 0.061 mole) is dissolved in 20 ml of ethyl acetate by gentle heating on a steam bath. Petroleum ether (20 ml) is added slowly to the solution, which is then cooled in an ice bath. To the cold solution is added 4.8 g (6 ml, 0.073 mole) of cyclopentadiene, and the resulting solution is swirled until the exothermic reaction subsides and the product separates. Recrystallization may be carried out in the reaction solvent by heating until dissolution occurs (steam bath) followed by slow cooling. The product has mp 164-165°, yield about 80%. 

Endo products result from Diels-Alder reactions because the amount of orbital overlap between diene and dienophile is greater when the reactants lie directly on top of one another so that the electron-withdratving substituent on the dienophile is underneath the diene. In the reaction of 1,3-cyclopentadiene with maleic anhydride, for instance, the following result is obtained ... 

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. 

The frontier orbital interactions at other than reaction sites can determine the selectivity [14]. The interaction between the HOMO of cyclopentadiene and the LUMO of maleic anhydride is illustrated in Scheme 26. The HOMO of cyclopentadiene has the same phase property as butadiene (Scheme 18). The LUMO of maleic anhydride is an in-phase combined orbital of and transition state for the... 

Fallis and coworkers studied Jt-facial selectivity in the reactions of series of 5-substituted 1,2,3,4,5-pentamethylcyclopentadienes Cp -X. They reported that the diene 5 (Cp -X X = SCH3) with maleic anhydride proceeded more slowly than that of the 5-oxygen substituted cyclopentadienes 6 and 7 (Cp -X X = OH, OCH3), where the HOMO of the diene 5 lies higher than those of 6 and 7 [7, 8] (Table 2). These results seemed to suggest that in the case of the reaction of 5 the NHOMO considerably contributed to the reactions. 

Scheme 19 Secondary orbital interaction in Diels-Alder reaction of cyclopentadiene with maleic anhydride...

Ohwada extends his theory, unsymmetrization of n orbitals, to Orbital Phase Environment including the secondary orbital interaction (Chapter Orbital Phase Environments and Stereoselectivities by Ohwada in this volume). The reactions between the cyclopentadienes bearing spiro conjugation with benzofluorene systems with maleic anhydride exemplified the importance of the phase environment. The reactions proceed avoiding the out-of-phase interaction between dienophile LUMO and the HOMO at the aromatic rings. The diene 34 with benzo[b]fluorene favored syn addition with respect to the naphtalene ring, whereas the diene 35 with benzo[c]fluorene showed the reverse anti preference (Scheme 22) [28]. 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

Recently Sergeev et al. 90 91> have developed a low temperature condensation method for the formation of inclusion compounds of thiourea with reactive and volatile guests, avoiding the use of solvents. The two guests in the joint inclusion compound of thiourea with 1,3-cyclopentadiene and maleic anhydride underwent Diels-Alder addition at 170 K. These two substances do not react at this low temperature unless they are present in the thiourea complex the usual endo isomer of the product is formed. Apart from copolymerisation reactions this appears to be the first use of the thiourea canal to study reactions between different materials. 

Another stereochemical point of significance is that in some Diels-Alder reactions there is the possibility of two alternative modes of addition, the exo (30) and the endo (31), e.g. with cyclopentadiene (32), and maleic anhydride (33) as dienophile ... 

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

In the reaction of cyclopentadiene with maleic-D2 anhydride206 an inverse experimental KIE of 8% (KIE = 0.92) was found at 298 K. The reaction between butadiene-D4, D2C=CHCH=CD2 and maleic anhydride gave a large inverse D4-KIE of 0.76. The two reactions between anthracene and maleic anhydride presented below also favour the concerted rather than the stepwise mechanism which requires 3-6% KIE in the normal direction (i.e. >1). 

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. 

The interplay of crystal packing, migrational aptitudes, and solid-state reactivity maybe demonstrated with the reactivities of the Diels-Alder cycloadducts of maleic anhydride and cyclopentadiene (1 and 2) or cyclohexadiene (3). 

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