Vapor-phase synthesis of 1,2-dihydro-2,2,4-trimethylquinolines from anilines and acetone over group 5–7 metal halide clusters as catalysts

Reaction of aniline with acetone to form 1,2-dihydro-2,2,4-trimethylquinoline (1) was studied in a gas-phase reaction over halide clusters as solid acid catalysts. After activation of a niobium halide cluster, [(Nb6Cl12)Cl2(H2O)4]·4H2O (2), which has an octahedral metal framework, at an elevated temperature in a hydrogen stream for 1 h, reaction was initiated by introduction of stoichiometric amounts of aniline and acetone at the activation temperature. The catalysis to yield 1 became evident above 200 °C. Both the catalytic activity of 2 and the selectivity for 1 increased with increasing temperature, having a local maximum at 300 °C. The selectivity for 1 was 76% with 34% conversion at 450 °C. Reactions of o-, m-, and p-toluidines with acetone also produced the corresponding quinolines. The chloride clusters of tantalum with the same metal framework and rhenium with a triangular metal framework also catalyzed the condensation. Thus, a halide cluster can be a substitute for liquid acid, particularly at high temperatures.