Late-stage trifluoromethylation of the bioactive molecule was successful (Scheme 4). In this study, we focused on the trifluoromethylation of quinoxyfen ( ), an agrochemical. The reaction of 6 afforded 3-trifluoromethyl quinoxyfen 7 in 60% yield (Scheme 4). To reveal the reaction mechanism, the reaction system was monitored using 1 H NMR spectroscopy (Scheme 5A). The reaction of 1a with methylphenylsilane in the presence of tris(pentafluorophenyl)borane (5.0 mol%) at 65 °C for 5 h in CDCl3 resulted in the formation of N-silyl enamine intermediate A, which was observed by 1 H NMR analysis (for more detail, see Supporting Information). 2a was added to the resulting mixture at 0 °C, and the mixture was stirred at 25 °C for 16 h. 1 H and 19 F NMR measurements indicated the formation of 3-trifluoromethylated 1,4-dihydroquinoline B, which might be formed via isomerization of 3-trifluoromethylated 3,4-dihydroquinoline (for details, see Supporting Information). The oxidation of intermediate B with DDQ produced 3a in 22% yield as the sole regioisomer. We then investigated the isolation of the byproduct silyl ether (Scheme 5B). The mixture of 1a with diphenylsilane in the presence of tris(pentafluorophenyl)borane (5.0 mol%) was heated at 65 °C for 5 h in 1,2-DCE. 15 2a was added to the resulting mixture at 0 ºC and stirred at 25 ºC for 16 h. The reaction mixture was purified by column chromatography on silica gel to obtain the corresponding silyl ether C in 38% yield.