Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions: A Mechanistic Comparison between Representative Coumarin and Enone Substrates
作者:Richard Brimioulle、Andreas Bauer、Thorsten Bach
DOI:10.1021/jacs.5b01740
日期:2015.4.22
substitution), the stereochemical outcome of the [2 + 2] photocycloaddition was investigated. The mechanistic course of the uncatalyzed coumarin reactions was found to be a singlet pathway, whereas Lewis acid-catalyzed reactions proceeded with higher reaction rates in the triplet manifold. Contrary to that, the dihydropyridones underwent a fast triplet reaction in the absence of the Lewis acid. In the presence
在路易斯酸不存在和存在的情况下,研究了三个 4-(alk-4-enyl) 香豆素和三个 1-(alk-4-enoyl)-2,3-二氢吡啶酮的分子内 [2 + 2] 光环加成反应。照射波长 λ = 366 nm)。收集了具有戊-4-烯基和戊-4-烯酰基链的各个母体化合物的光谱和动力学数据。对于在末端烯烃碳原子(hex-4-enyl 和 hex-4-enoyl 取代)的顺式或反式位置具有甲基的底物,研究了 [2 + 2] 光环加成的立体化学结果。发现未催化的香豆素反应的机理过程是单线态途径,而路易斯酸催化的反应在三线态歧管中以更高的反应速率进行。与此相反,二氢吡啶酮在没有路易斯酸的情况下经历了快速的三线态反应。在手性路易斯酸的存在下,反应减慢,但由于路易斯酸/二氢吡啶酮复合物在 λ = 366 nm 处的高消光系数,仍然导致高对映选择性。