Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58108, United States.
J Am Chem Soc. 2014 Jun 18;136(24):8729-37. doi: 10.1021/ja5034638. Epub 2014 Jun 9.
Atropisomeric maleimides were synthesized and employed for stereospecific [2 + 2] photocycloaddition. Efficient reaction was observed under direct irradiation, triplet-sensitized UV irradiation, and non-metal catalyzed visible-light irradiation, leading to two regioisomeric (exo/endo) photoproducts with complete chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity (ee > 98%) and diastereoselectivity (dr > 99:1%) were observed under the employed reaction conditions and were largely dependent on the substituent on the maleimide double bond but minimally affected by the substituents on the alkenyl tether. On the basis of detailed photophysical studies, the triplet energies of the maleimides were estimated. The triplet lifetimes appeared to be relatively short at room temperature as a result of fast [2 + 2] photocycloaddition. For the visible-light mediated reaction, triplet energy transfer occurred with a rate constant close to the diffusion-limited value. The mechanism was established by generation of singlet oxygen from the excited maleimides. The high selectivity in the photoproduct upon reaction from the triplet excited state was rationalized on the basis of conformational factors as well as the type of diradical intermediate that was preferred during the photoreaction.
对映异构的马来酰亚胺被合成并用于立体特异性 [2 + 2] 光环加成反应。在直接照射、三重态敏化的 UV 照射和无金属催化的可见光照射下，观察到高效反应，导致两种区域异构体（外/内）光产物具有完全的化学选择性（排他性 [2 + 2] 光产物）。在采用的反应条件下，观察到高对映选择性（ee > 98%）和非对映选择性（dr > 99:1%），并且对马来酰亚胺双键上的取代基高度依赖，但对烯丙基连接基上的取代基影响很小。基于详细的光物理研究，估计了马来酰亚胺的三重态能量。三重态寿命在室温下似乎相对较短，这是由于快速的 [2 + 2] 光环加成。对于可见光介导的反应，三重态能量转移以接近扩散限制的值发生。通过从激发的马来酰亚胺中生成单线态氧来建立该反应机制。根据构象因素以及光反应中优先的自由基中间体类型，对三重态激发态反应中光产物的高选择性进行了合理化。
