Hexaphenylethane The Modem Period

There are three aspects to what we can call the modem hexaphenylethane period the demonstration in 1968 that hexapheitylethane did not exist, the preparation in 1978 of the first genuine (unbridged) hexapheitylethane derivative, and the interplay at about this time of reliable computational and experimental methods of probing the stmcture of hexaphei rlethane and related compounds. 

At the time too (ca. 1906) it was well-known that methylenecyclohexadiene ( quinoid ) systems aromatize readily, and that allylic halogens, as in 3, are veiy reactive, in contrast to aromatic halogen. Nevertheless, the Jacobsen stmcture 3 was to disappear into obscnrity until 1968 [16]. 

The preparation in 1978 of the first genuine (unbridged) hexaphenylethane. With the revelation in 1968 that the dimer in equilibrium with tripheitylmethyl was the methylenecyclohexadiene 3, the hexaphenylethane problem, believed for decades to have been solved, arose a in in a sharply focused form synthesis. Around the time of the stmcture disproof of the traditional hexaphenylethane, derivatives of this compound were known, bnt in all of these steric ortho-type repulsions (Fig. 8.1) were redneed or eliminated by ortho bonding, as infor example 6 [22] and 

7 [23]. In these molecules nonbonded interactions across the central CC bond have been reduced by removing H atoms and pinning back aryl groups with zero-caibon bridges. 

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