一些环加成反应由热激活,而另一些则由光引发。 例如,两个乙烯分子之间的[2+2]环加成仅在光存在下发生。 它在光化学允许的,而热学禁止的。
热诱导的[2 + 2]环加成是对称性禁止的。 这是因为一个乙烯分子的基态 HOMO 和另一个乙烯分子的 LUMO 异相,从而阻止了一致的同面-同面重叠。
乙烯分子之一吸收紫外光促进电子从基态 HOMO 到达 LUMO。 激发态 HOMO 现在具有所需的对称性,可以通过对称性允许的协同途径与另一个乙烯的 LUMO 相互作用。
来自章节 16:
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