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 investig ated in the temperature range between 600 and 800 degrees C and in the presence of different carrier gases (N-2, N-2/toluene, toluene, H-2). By a detailed analysis (GC, GC/MS, GC/FTIR) all volatile reaction pro ducts were identified and quantitatively determined. Besides several f ragmentation products (substituted benzenes C-7-C-10) and some carbon- rich deposits the main products are benzene (4), naphthalene (5) as we ll as 1- (2) and 2-phenylnaphthalene (3). Under similar conditions 4-( 4-fluorophenyl)-1-phenyl-1-buten-8-yne (1a) furnished mainly fluoroben zene (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 processe s. The cycloisomerization of 1/1a presumably starts with previously fo rmed vinyl radicals, which might initiate a multi-step reaction cascad e involving a 1,x-H shift and homoallyl reactions. However, if the rea ction system is poor in H atoms (or other chain carrier radicals), a c ycloisomerization path via vinylidene carbene species and their subseq uent stabilization by intramolecular 1,6-C,H insertion into 3/3a obvio usly dominates the reaction events. It is concluded that the 1,2-dihyd ro-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 onl y 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.