Benzadiynes

The procedure given here is a modification of the first synthesis of the tetrahydroanthracene diepoxides.5 7 These compounds have also been prepared from another 1,4-benzadiyne equivalent, namely 1,5-diamino-1,5-dihydrobenzo[1,2-d 4,5-d ]bistriazole which, although it has certain advantages,12 is not as available as 1,2,4,5-tetrabromobenzene. 

Khalil Shahlai, Samuel Osafo Acquaah, and Harold Hart 201 USE OF 1,2,4,5-TETRABROMO-BENZENE AS A 1,4-BENZADIYNE EQUIVALENT anti- AND syn-1,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES... 

Originally, pyromellitic and mellophanic dianhydrides were thought to go to benzadiyne in possibly a concerted rather than stepwise fashion, because no products retaining an anhydride function could be found. In the reactions with halogenated benzenes, however, such products do appear evidently, at least in some environments, these dianhydrides first decompose to benzynedicarboxylic anhydride, followed by further loss of C02 and CO. 

Pyromellitic and mellophanic dianhydrides gave similar product distributions, derived from presumed benzadiynes. These arynes evi-... 

These reactions possibly took place in a step-wise, rather than a concerted fashion, although the data tend to favor the formation of benzadiyne. Under the same conditions in which an appreciable amount of phthalic anhydride could be recovered, pyromellitic dianhydride gave no products that still retained an anhydride group. 

Bistriazole (508) served as a formal 1,4-benzadiyne equivalent in a sequence of two Diels-Alder additions to 2-methoxycarbonylfuran (509), which yielded (511) as the sole regio- and (tentatively assigned) stereo-isomer (Scheme 119). It appears, therefore, that the C02Me group, in the transient monoadduct (510), controls the orientation in the second step. 

Cycloaddition of benzyne (478) to (the less aromatic) anthracene provides a well-known approach to triptycene (524). More recently, this type of reaction was extended to elegant syntheses of iptycenes, e.g. (526), using 1,2,4,5-tetrabromobenzene (525) as a 1,4-benzadiyne equivalent (Scheme 123). ... 

Theoretical calculations show that 1,3- and 1,4-benzadiyne 433 and 434 are each considerably higher in energy than benzyne, though it is not clear which of the two diynes is the more stable ". Furthermore, their isomerization to 1,3,5-hexatriyne is predicted to be highly exothermic " . Consequently, even though C6H2 ions are the dominant ions in the mass spectra of all three tetrabromobenzenes, it seems unlikely that these diynes are involved in the solution chemistry of various precursors. Instead, it is more likely that two... 

Anthracenes react to give iptycenes 448 all of these reactions, the 1,2,4,5-tetrahalobenzene acts as the synthetic equivalent of 1,4-benzadiyne 434. 

By using a 1,2,3,4-tetrahalobenzene in which metal-halogen exchange occurs preferentially at the 1 and 4 halogens, one has a synthetic equivalent of 1,3-benzadiyne 433 . For example, the highly strained permethylphenanthrene 454 can be synthesized in this way by a double cycloaddition to 452. Initial exchange occurs primarily at the iodine atoms of 452. 

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