Department of Chemistry, Ochanomizu University 2-1-1 Otsuka, Bunkyo-ku, Tokyo, 112-8610, Japan.
Chem Asian J. 2020 Mar 2;15(5):581-584. doi: 10.1002/asia.201901707. Epub 2020 Feb 12.
Irradiation of HX (X=CF SO or CF CO ) salts of 1-aryl-4-pyridylbutadienes 1 a-1 c in the solid-state afforded syn head-to-tail dimers in good yields among a number of possible dimers, whereas irradiation of the neutral substrates gave a complex mixture or no products. A comparison of the X-ray crystal structures of the neutral compounds and the HX salts clarified that their orientation modes are head-to-head and head-to-tail, respectively. Moreover, while the distances between the two neighboring double bonds of the neutral compounds are relatively far apart from each other, those of HX salts are close together, satisfying Schmidt's requirement. These findings suggested that cation-π interactions between the pyridinium and aromatic rings are effective for the preorientation of the HX salts of substrates, leading to photodimers in high regio- and stereoselectivities.
HX（X=CF SO 或 CF CO ）盐的 1-芳基-4-吡啶基丁二烯 1a-1c 的固态辐照在许多可能的二聚体中以高收率得到顺式头对头二聚体，而中性底物的辐照得到复杂混合物或无产物。中性化合物和 HX 盐的 X 射线晶体结构的比较表明，它们的取向模式分别为头对头和头对尾。此外，虽然中性化合物的两个相邻双键之间的距离彼此相对较远，但 HX 盐的距离很近，满足施密特的要求。这些发现表明，吡啶鎓和芳环之间的阳离子-π 相互作用有利于底物的 HX 盐的预定向，从而以高区域和立体选择性得到光二聚体。
