Benzyne Diels-Alder reaction with anthracene

Aromatic compounds can also behave as dienes.858 Benzene is very unreactive toward dienophiles very few dienophiles (one of them is benzyne) have been reported to give Diels-Alder adducts with it.859 Naphthalene and phenanthrene are also quite resistant, though naphthalene has given Diels-Alder addition at high pressures.860 However, anthracene and other compounds with at least three linear benzene rings give Diels-Alder reactions readily. The interesting compound triptycene can be prepared by a Diels-Alder reaction between benzyne and anthracene 861... 

The dienic character of the central ring is illustrated by the reaction of anthracene with dienophiles in Diels-Alder reactions. For example, c -butenedioic anhydride (maleic anhydride) reacts readily when benzyne is generated in the presence of anthracene, triptycene (19) is produced. 

Benzyne may not look like a good dienophile but it is an unstable electrophilic molecule so it must have a low-energy LUMO (ti of the triple bond). If benzyne is generated in the presence of a diene, efficient Diels-Alder reactions take place. Anthracene gives a specially interesting product with a symmetrical cage structure. 

Benzyne is very reactive as a dienophile in the Diels-Alder reaction. The central ring of anthracene reacts like a diene because the adduct still has the resonance stabilization of two benzene rings. A very symmetrical product, with only four types of carbons, results. 

The high reactivity of benzynes as dienophiles in Diels-Alder reactions has also been observed in reactions with aromatic hydrocarbons, a class of compounds usually considered as inert as dienes. In a two-step synthesis of triptycene di- and tetracarboxylic acids, arynes generated in situ from anthranUic adds 158 cydized with anthracenes 159 affording di- or tetramethyltriptycenes 160 in 41-69% yields (Scheme 12.47) [85], Acids 161 were obtained after subsequent oxidation of the cycloadducts 160 with potassium permanganate. 

A series of alkyl-substituted anthracenes (14,18, and 24) were prepared from Diels-Alder reactions of bis(benzyne) equivalent 4 or dehydronaphthalene equivalent 8 with alkyl-substituted furans (11 or 27). The structural features of the crystals were characterized by X-ray crystallographic analysis. Depending on the length, position, and number of alkyl side chains, the molecular structures (alkyl conformations) and packing arrangements of the alkyl-substimted anthracenes changed drastically. Consequently, such modifications led to diffaent fluorescait properties for the anthracenes in the solid-state. 

Triptycene is the Diels-Alder cycloaddition product from the reaction of benzyne with anthracene (compound A). Although anthracene is aromatic, it is able to undergo cycloaddition at the center ring with a dienophile because the adduct retains the stabilization energy of two benzene rings. 

Three rotameric forms of (26) have been isolated at room temperature. In fact Diels Alder addition of dimethylacetylene dicarboxylate to 2,3-dichloro-9-(l,l-dimethyl-2-phenylethyl)anthracene produces only the ap-rotamer (26). The other rotamers can be isolated by thermal equilibrium at 150 °C and subsequent chromatographic separation. Reaction of benzyne with the above anthracene yields only the d,/-triptycene derivative, and 4,5-dichlorobenzyne addition to 9-(l,l-dimethyl-2-phenylethyl)-... 

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