Thermolysis of benzoenyne-allenes to form biradicals and subsequent intramolecular trapping with a tetraarylallene to generate two triarylmethyl radical centers

Thermolysis of the benzoenyne-allene 20 in 1,4-cyclohexadiene (1,4-CHD) at 75 degrees C produced the cycloaromatized adduct 23 in 22% yield. The react ion presumably proceeds through a cascade sequence involving an initial Mye rs cyclization reaction to form the biradical 21. The subsequent trapping o f the aryl radical center in 21 with the tetraarylallenic moiety intramolec ularly affords 22, having two stabilized triarylmethyl radical centers. Hyd rogen-atom abstraction from 1,4-CHD by 22, manifesting its radical characte r, then produces 23. Similarly, thermolysis of benzoenyne-allenes 25 in 1,4 -CHD furnished fluoranthenes 27 in essentially quantitative yields. The pre sence of the fused five-membered ring in 20 and 25 is necessary to direct t he initial biradical-forming step toward the Myers cyclization reaction. Wi thout the five-membered ring as in the cases of 31, the C2-C6 cyclization r eaction became the preferred pathway, leading to benzofluorenes 34.