Addition and cyclization reactions in the thermal conversion of hydrocarbons with enyne structure, 3 A mechanistic study of the thermal cycloisomerization of 1,4‐diphenyl‐1‐buten‐3‐yne

The gas‐phase pyrolysis of 1,4‐diphenyl‐1‐buten‐3‐yne (1) was investigated in the temperature range between 600 and 800°C and in the presence of different carrier gases (N2, N2/toluene, H2). By a detailed analysis (GC, GC/MS, GC/FTIR) all volatile reaction products were identified and quantitatively determined. Besides several fragmentation products (substituted benzenes C7–C10) and some carbon‐rich deposits the main products are benzene (4), naphthalene (5) as well as 1‐ (2) and 2‐phenylnaphthalene (3). Under similar conditions 4‐(4‐fluorophenyl)‐1‐phenyl‐1‐buten‐3‐yne (1a) furnished mainly fluorobenzene (4a), naphthalene (5), 1‐ (2a) and 2‐(4‐fluorophenyl)naphthalene (3a). – If sufficient H atoms are available in the reactive gas phase the conversion of 1/1a is predominantly controlled by radical processes. The cycloisomerization of 1/1a presumably starts with previously formed vinyl radicals, which might initiate a multi‐step reaction cascade involving a 1,x‐H shift and homoallyl reactions. However, if the reaction system is poor in H atoms (or other chain carrier radicals), a cycloisomerization path via vinyl‐idene carbene species and their subsequent stabilization by intramolecular 1,6‐C,H insertion into 3/3a obviously dominates the reaction events. It is concluded that the 1,2‐dihydro‐2‐phenyl‐1‐naphthyl radicals 19/19a and the 1,2‐dihydro‐1‐phenyl‐2‐naphthyl radicals 21/21a are important intermediates which are not only precursors of the aromatics 2/2a and 3/3a but also thought to be key intermediates for the most important degradation products 4/4a and 5.