Thermolysis of phenyl-substituted 1,2-dihydronaphthalenes. Evidence for diphenylbutadienes as intermediates

Abstract

The thermal rearrangements of the four phenyl-substituted 1,2-dihydronaphthalenes (15), (16), (19), and (20) have been studied by flash vacuum pyrolysis (FVP).

By using the deuteriated starting compounds [3,4-²H₂]-(15) and [3,4-²H₂]-(16), it has been established that 1- and 4-phenyl-1,2-dihydronaphthalene (15) and (19) and 2- and 3-phenyl-1,2-dihydronaphthalene (16) and (20) are interconverted via the intermediates 1- and 2-phenyl-2,3-dihydronaphthalene (17) and (18), respectively, through two consecutive, sigmatropic 1,5-hydrogen shifts. In both processes partial oxidation to the corresponding phenylnaphthalenes (21) and (22) takes place. The deuterium distribution in the pyrolysis products suggests that in the hot zone diphenylbutadienes are formed, which are reconverted into phenyldihydronaphthalenes upon reaching the cold receiving flask. By FVP of 4-(p-tolyl)-1,2-dihydronaphthalene (34), 1 -phenyl-1 -(p-tolyl) butadiene (39), and 1 -phenyl-4-(p-tolyl) butadiene (45) the latter type of interconversion could be confirmed.