Department of Chemistry, University of Cologne, Greinstr. 4, 50939 Köln, Germany.
Molecules. 2021 Jan 26;26(3):642. doi: 10.3390/molecules26030642.
The α-methylated chalcones - behave as P-type photochromic substances with photo-stationary states (PSS) as high as 15:85 when irradiated at 350 nm. These compounds are easily accessible in pure E-configuration by aldol condensation or by oxidative coupling/elimination. The α-methyl groups make these compounds potentially reactive with singlet oxygen following the gem-rule that predicts O regioselectivity. Even after long irradiations times in the presence of the singlet oxygen sensitizer tetraphenylporphyrin (TPP) and oxygen, however, no oxygenation products were detected. Under these conditions, all substrates were converted into 9:1 E/Z-mixtures despite the use of low-energy light that does not allow direct or sensitized excitation of the substrates . Additionally, chalcone reduced the singlet oxygen reactivity of the tiglic ketone by about a factor of two, indicating substantial physical quenching of singlet oxygen by the α-methylated chalcones -. Thus, a singlet oxygen-induced E/Z-isomerization involving 1,2-dioxatetra-methylene biradicals that leads to triplet oxygen and thermodynamic E/Z-mixtures is postulated and supported by quantum chemical (DFT)-calculations.
α-甲基化查耳酮在 350nm 光照下表现出 P 型光致变色性质，光稳定态（PSS）高达 15:85。这些化合物可以通过羟醛缩合或氧化偶联/消除，以纯 E-构型轻易获得。α-甲基使这些化合物具有潜在的反应活性，可以与单线态氧发生反应，符合预测 O 区域选择性的gem规则。然而，即使在存在单线态氧敏化剂四苯基卟啉（TPP）和氧气的情况下进行长时间光照，也未检测到氧合产物。在这些条件下，尽管使用了不会直接或敏化激发底物的低能量光，但所有底物都转化为 9:1 E/Z 混合物。此外，查尔酮将 tiglic 酮的单线态氧反应性降低了约两倍，表明 α-甲基化查耳酮通过物理猝灭显著消耗了单线态氧。因此，提出并通过量子化学（DFT）计算支持了一种涉及 1,2-二氧杂四亚甲基双自由基的单线态氧诱导的 E/Z-异构化，导致三重态氧和热力学 E/Z-混合物。
