Thermal decomposition of homoquinones

Abstract

Thermolysis of diphenyl- and biphenyl-2,2′-diyl-substituted homobenzoquinones 1 and homonaphthoquinones 2 with substituents X³(= Me, Cl, Br) has been investigated at 100 °C. The biphenyl-2,2′-diylhomoquinones 1 and 2 bearing halogeno substituents were thermolysed to 2-(9-halogenofluoren-9-yl)-1, 4-benzoquinones 8g, h and -1,4-naphthoquinones 19e, f, respectively, via a cyclopropane ring-opening reaction. The diphenylhomoquinones were thermally less labile so that only ones bearing a bromo substituent underwent a cyclopropane ring-opening to afford a different type of product, 2-bromo-3-diphenylmethylene-2, 3-dihydro-1,4-benzoquinones 3c–e and -1,4-naphthoquinone 10, respectively. The product change was attributed to the steric requirement of the intermediary allyl cations. On further heating, compounds 3c–e were converted into xanthylium salts via a 6π electrocyclization, whereas compound 10 was transformed into the coupling dimer via an allyl radical. The different behaviour of the bromo quinones 3c–e and 10 was interpreted on the basis of Frontier Molecular Orbital considerations.