Diradicals 1,4-didehydrobenzene

Other researchers have reported that the cyclization step is believed to be rate determining in the cycloaromatization (Bergman) reaction of aliphatic enediynes." It has been found that the rate-limiting step is hydrogen abstraction by benzannelation. This effect should be attributable to the faster rate of retro-Bergman cyclization from the aromatic ring-condensed 1,4-didehydrobenzene diradicals and/or the slower rate of hydrogen abstraction by them. 

Aza-enediynes are a new class of anti-neoplastic agents that produce a cytotoxic effect through an aza-Bergman cyclization (1) of an enediyne core to a 1,4-didehydrobenzene reaction intermediate. Both Bergman and benzothiazolium salt diradical cyclizations are illustrated in Eq. 1 and Eq. 2, respectively. 

Bergman and coworkers initially studied the cyclization of enediynes more than two decades ago [5, 33], The parent compound of these systems, (Z)-hex-3-ene-l,5-diyne (100) cyclizes at elevated temperatures to afford the 1,4-didehydrobenzene diradical 6 (Fig. 8). 

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