CHEMISTRY OF CATALYTIC DEHYDROGENATIVE OLIGOMERIZATION OF TETRAHYDROQUINOLINE AND STRUCTURAL CHARACTERIZATION OF NONSUBSTITUTED QUINOLINE OLIGOMERS

A new catalytic dehydrogenative polycondensation (CDHP) of 1,2,3,4-tetrahydroquinoline (THQ) using transition metal sulfides or tris- or bis(omicron-aminobenzenethiolate) complexes of groups VIB, VIIB, and VIII transition metals as catalyst precursors is described. The method provides a direct route to the synthesis of nonsubstituted quinoline oligomers, which have not been prepared by known methods. The effective heterogeneous CDHP catalysts were generated in situ in reaction media from precursors by a preheating treatment at 180-210-degrees-C. These active catalysis were found to consist of metal and sulfur stoichiometric composition approaching MoS2, ReS2, RuS, RhS2, PtS, and PdS. We observed a periodic trend on the CDHP activities of transition metal sulfides, showing a maximum oligomer yield (97%) with a ruthenium sulfide catalyst. The structure of quinoline oligomers was elucidated on the basis of X-ray crystallographic studies and various spectroscopic data including the 2D COSY H-1 NMR spectra. The full structural characterization of two quinoline dimers and a trimer, which were successfully isolated from the bulk product, resolved the puzzle of carbon positions at the ring conjunction between quinoline moieties in oligomers. The first dimer of 2,3'-biquinoline crystallized in a space group of P2(1/a) (No. 14) with the monoclinic cell dimensions a = 12.492 (2) angstrom, b = 5.3821 (9) angstrom, c = 19.253 (3) angstrom, and beta = 100.20 (1)degrees, showing a quinoline ring conjunction at carbons C2 and C3'. The second dimer of 2,6'-biquinoline crystallized in a space group of Pa (No. 7) with the monoclinic cell dimensions a = 8.1687 (6) angstrom, b = 5.9748 (5) angstrom, c = 13.126 (1) angstrom, and beta = 95.032 (7)degrees, showing a quinoline ring conjunction at carbons C2 and C6'. The structure of one quinoline trimer was found to contain two quinoline ring conjunctions at carbons C2-C6' and C2'-C3''. On the basis of results from the structural and kinetic studies, a hypothetical CDHP reaction mechanism was proposed. That leads to two major oligomeric quinoline products 1a and 1b differing in only the orientation of the head unit in structure. In the CDHP chemistry, we conclude that a delicate balance between dehydrogenation and polymerization activities of catalyst is required to optimize the yield and the molecular weight of resulting products. We also observed a coexistence of hydrogenation and dehydrogenation activity, probably at different surface sites of transition metal sulfide.