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 (N 2 , N 2 /toluene, H 2 ). By a detailed analysis (GC, GC/MS, GC/FTIR) all volatile reaction products were identified and quantitatively determined. Besides several fragmentation products (substituted benzenes C 7 –C 10 ) 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 .