Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
Nat Commun. 2014;5:3018. doi: 10.1038/ncomms4018.
Cyclic polyene with six carbon atoms (benzene) is very stable, whereas cyclic polyene with four carbon atoms (cyclobutadiene) is extremely unstable. The electron-withdrawing pentafluorophenyl group of a substituted cyclobutadiene lowers the energy of the lowest unoccupied molecular orbital, greatly increasing its reactivity as a diene in Diels-Alder reactions with acetylene, ethylene and even benzene. Here we show that the reaction with benzene occurs cleanly at the relatively low temperature of 120 °C and results in the formal fragmentation of benzene into C2H2 and C4H4 units, via a unique Diels-Alder/retro-Diels-Alder reaction. This is a new example of the rare case where breaking the C-C bond of benzene is possible with no activation by a transition metal.
六碳环多烯（苯）非常稳定，而四碳环多烯（环丁二烯）则极不稳定。取代环丁二烯的强吸电子五氟苯基降低了最低未占据分子轨道的能量，极大地增加了其作为二烯与乙炔、乙烯甚至苯进行 Diels-Alder 反应的反应活性。在这里，我们表明该反应在相对较低的 120°C 温度下可顺利进行，并通过独特的 Diels-Alder/retro-Diels-Alder 反应将苯形式上断裂成 C2H2 和 C4H4 单元。这是一个新的例子，表明在没有过渡金属活化的情况下，有可能打破苯的 C-C 键，这种情况非常罕见。
