Cyclopentadienate addition to maleic anhydride

At the other extreme are cycloadditions (Chapter 35) such as the Diels-Alder reaction we examined a few pages back. Not only do two reagents become one product but a veiy precise orientation is required in the transition state usually meaning a large negative AS. Diels-Alder reactions usually have AS of about -120 to -160 J mol-1 K-1. The classic cyclopentadiene addition to maleic anhydride has AS = -144 J moT K-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. 

An other correlation has been found between log endolexo) in the cyclopentadiene addition to ring-substituted rrun.v-cinnamic acids and Hammett-type substituent constants the low absolute value of the slope (—0.30)"" indicates very similar substituent effects on the two reaction paths. For the reaction of 2-substituted anthracenes with maleic anhydride" the influence of substituents on log (synlanti) is even weaker"". 

A less-common activation parameter, the volume of activation (AF ), has been determined for a few Diels-Alder reactions carried out under pressure in liquid phase. The processes are cyclopentadiene dimerisation , isoprene dimerisation , addition of 2,3-dimethylbutadiene to butyl acrylate , addition of cyclopentadiene to dimethyl acetylene dicarboxylate , and addition of maleic anhydride to 1,3-cyclohexadiene, /rans-l-methoxybutadiene and isoprene . Activation volumes are negative, i.e. the reacting systems contract on passing from the initial to the transition state. In some cases the transition state appears to be even smaller than the adduct, independently of the solvent . Some of these experimental results gave rise to controversial interpretations however, the most recent ones favour a concerted four-center mechanism for the reaction. 

To illustrate this aspect of stereoselectivity, the addition of maleic anhydride to cyclopentadiene gave almost exclusively the endo product 77 (3.66). The thermodynamically more stable exo compound is formed in yields of less than 2%. 

A second class of monomers was prepared in which carboxylate functional groups had a well-defined stereochemical relationship to each other and hence to the polymer repeat unit produced on ROMP. These were the set of endo,endo-, exo,exo- and endo,exo-5,6-dimethylcarboxylates of norbomene obtained via the Diels-Alder reactions of the dimethylesters of maleic and fumaric acids with cyclopentadiene and via addition of maleic anhydride followed by esterification this set of monomers leads to polymers in which the functional groups are close to the polymer backbone. 

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). 

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... 

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]. 

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. 

There is a further feature of the Diels-Alder reaction that concerns the stereochemical orientation of the addends. In the addition of cf.v-butenedioic anhydride (maleic anhydride) to cyclopentadiene there are two possible ways that the diene and the dienophile could come together to produce different products. These are shown in Equations 13-3 and 13-4 ... 

An irreversible tt-ct rearrangement has been observed to occur with the treatment of titanocene dicloride (8) with dimethyl sulfoxide (17). The o complex (9) has been isolated, however, treatment of the reaction mixture with benzene produces the titanium complex of dimethyl sulfoxide (10), and treatment of the reaction mixture with maleic anhydride produces the cyclopentadiene addition product (11). Monitoring the reaction mixture by proton NMR spectroscopy demonstrates the ir-a change in the bonding of the cyclopentadienyl ligands. 

The Diels-Alder reaction between a diene and a dienophile is one of the major synthetic reactions that are used for the formation of C C bonds. Its usefulness lies in the mild conditions and the predictable regio- and stereospecifidty of the process. For most purposes the thermal reaction requires the presence of an electron-withdrawing substituent on the alkene. The reaction is exemplified by the addition of the alkene of maleic anhydride as the dienophile to the diene of cyclopentadiene (Scheme 3.19). 

Alder s endo rule specifies a preference for endo (C) over exo (D) addition. However, this rule appears to be strictly applicable only to the addition of cyclic dienophiles (e.g. maleic anhydride, p-qui-nones) to cyclic dienes (e.g. cyclopentadienes). 

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. ... 

Diels-Alder reaction. An important organic reaction for the synthesis of 6-membered rings discovered in 1928. It involves the addition of an ethyl-enic double bond to a conjugated diene, i.e., a compound containing two double bonds separated by one single bond, as in 1,3-butadiene (CH2=CH-CH=CH2) or cyclopentadiene. The ease of addition of the ethy lenic compound is greatly enhanced by adjacent carbonyl groups hence maleic anhydride reacts quantitatively with hexachloro-cyclopentadiene to form chlorendic anhydride. 

The Diels-Alder reaction (diene synthesis) is the addition of compounds containing double or triple bonds (dienophiles) to the 1,4 positions of conjugated dienes with the formation of six-membered hydroaromatic rings. Hydrocarbons most often used in the reaction are 1,3-butadiene, cyclopentadiene, and isoprene, and dienophiles used include maleic anhydride, acrolein, and acrylic acid. The literature on this process is thoroughly reviewed by Alder (1), Kloetzel (59), Holmes (48), and Norton (82). 

The preference for the endo mode of addition is not restricted to cyclic dienes such as cyclopentadiene. By using deuterium labels it has been shown that in the addition of 1,3-butadiene and maleic anhydride, 85% of the product arises from the endo TS. ... 

Ring compounds containing carbon bridges are also formed by addition of, e.g., maleic anhydride to alicyclic dienes such as cyclopentadiene and cyclo-heptadiene of the two possible stereoisomers it is always the erafo-compound that is produced (Woodward-Hoffmann rules38). Such reactions, however, fail when the product would break Bredt s rule. 

Most of the classical dienes are hydrocarbons, like cyclopentadiene. butadiene, anthracene, 9,10-dimethylanthracene, isoprene, 2,3-dimethylbutadiene. For dienes of this kind it is generally true that electrophilic dienophiles are the most reactive, and actually acrylic derivatives, maleic anhydride, p-benzo-quinone and similar compounds have found a large use as dienophiles both for preparative purposes and for kinetic studies. The latter demonstrated quantitatively the importance of electronegative groups on the dienophiles, and conversely of electron-releasing substituents on the diene, in order to accelerate such type of Diels-Alder reaction. It was also realised later that Diels-Alder additions with inverse electron demand , that is between electrophilic dienes and nucleophilic dienophiles, do occur . ... 

Photolysis of methyl diazoacetate in benzene gives all four isomers of methoxy-carbonylcycloheptatriene, of which only one reacts with maleic anhydride at room temperature, giving (302). Details have appeared of the addition of methyl coumalate (303) to butadienes to give (304) and (305). The readily accessible pyran (306) adds stereospecifically to cyclopentadiene at room temperature and to butadienes at 120 °C to give synthetically useful adducts (307) and (308). ... 

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