Carbene reactions, XX. Structural Aspects Responsible for the Thermal Extrusion of Carbenes from Norbornadienes

Abstract
The notion is put forward that norbornadienes having weak donor substituents at C‐7 should suffer homolysis of the C‐1/C‐7 bond on heating, leading via the diradical 6 to either cycloheptatrienes or benzylic compounds. In contrast, norbornadienes having strong donor systems at C‐7 should suffer heterolysis of the C‐1/C‐7 bond, whereupon the resulting zwitterion 7 collapses directly to carbenes in a singlet ground state.