Bimolecular Formation of Radicals by Hydrogen Transfer, 13. Uncatalyzed Transfer Hydrogenation of Azulene and the Initiation of the Azulene‐Naphthalene Rearrangement

From the uncatalyzed transfer‐hydrogenation of azulene 1 with the hydrogen donors 9,10‐dihydroanthracene ( 4 ), 7 H ‐benz[ de ]anthracene ( 5 ) and 9,9‐diphenyl‐10,10‐dideuteroanthracene ( 6 ) at 340–375°C in diphenyl ether, two isomeric octahydroazulenes and, in addition, naphthalene and tetralin are obtained. From product studies, kinetics, isotopic labelling experiments and semiempirical MO calculations, a nonchain stepwise radical mechanism is proposed, which is initiated by H‐atom transfer from the donor to azulene. The proposed mechanism for the formation of naphthalene and tetralin via azulene‐naphthalene rearrangement under these conditions is a combination of the Scott mechanism and the Alder “walk” mechanism, which are initiated by the hydrogen‐transfer step.