Tetraarylethenes

Maeda, K., Okamoto, Y., Morlender, N., Haddad, N., Eventova, I., Biali, S. E., and Rappoport, Z. (1995) Does the threshold enantiomerization route of crowded tetraarylethenes involve double bond rotation , J. Am. Chem. Soc. 117, 9686-9689. 

Benzylic electrophiles bearing electron-withdrawing groups at the arene do not always yield the expected products of nucleophilic substitution on treatment with a nucleophile. One important side reaction is the dimerization of these compounds to yield 1,2-diarylethenes (stilbenes). This dimerization does not require such highly activated systems as the example sketched in Scheme 4.28, but can even occur with, for example, 2- or 4-nitrobenzyl chloride [120, 121]. The latter compounds are converted into the corresponding stilbenes by treatment with KOH in ethanol [120]. Di-arylmethyl halides behave similarly and can yield tetraarylethenes on treatment with a base. These reactions presumably proceed via the mechanism sketched in Scheme 4.27, in which the amphiphilic character of the nitro group plays a decisive role (metalated nitroalkanes or 4-nitrobenzyl derivatives can act as nucleophiles and as electrophiles). 

A dramatic solvent effect has been revealed in the carbocyclization of co-alkenylbenzyllithiums, which are formed from the corresponding selenides. Selenobenzophenones undergo coupling to give symmetrical tetraarylethenes on treatment with BuLi. 

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