Thermolysis of Benzoenyne-Allenes To Form Biradicals and Subsequent Intramolecular Trapping with a Tetraarylallene To...

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 reaction presumably proceeds through a cascade sequence involving an initial Myers cyclization reaction to form the biradical 21. The subsequent trapping of the aryl radical center in 21 with the tetraarylallenic moiety intramolecularly affords 22, having two stabilized triarylmethyl radical centers. Hydrogen-atom abstraction from 1,4-CHD by 22, manifesting its radical character, then produces 23. Similarly, thermolysis of benzoenyne-allenes 25 in 1,4-CHD furnished fluoranthenes 27 in essentially quantitative yields. The presence of the fused five-membered ring in 20 and 25 is necessary to direct the initial biradical-forming step toward the Myers cyclization reaction. Without the five-membered ring as in the cases of 31, the C2-C6 cyclization reaction became the preferred pathway, leading to benzofluorenes 34.