An organocatalytic [2+2] cycloaddition reaction of norbornadienes (NBDs) using catalytic amount of TEMPO was reported. Single crystal X-ray diffraction of the product revealed its detailed multicyclic structure containing a 4-membered ring, formed in intermolecular reaction. Addition of AIBN to the current catalytic system improved the product yield. Quantitative reaction of the NBD and TEMPO gave a 2:2 adduct of NBD and TEMPO, which was confirmed by HR-MS. This catalytic [2+2] addition of NBDs has great advantage in selective intermolecular coupling in comparison with [2+2] photocycloaddition.

中文翻译：

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稳定的有机自由基化合物对降冰片二烯的有机催化分子间[2 + 2]环加成反应

报道了使用催化量的TEMPO进行降冰片二烯（NBD）的有机催化[2 + 2]环加成反应。该产物的单晶X射线衍射揭示了其详细的多环结构，该多环结构包含在分子间反应中形成的4元环。在当前的催化体系中添加AIBN可以提高产品收率。NBD和TEMPO的定量反应产生了NBD和TEMPO的2：2加合物，这已通过HR-MS证实。与[2 + 2]光环加成相比，这种NBD的催化[2 + 2]加成在选择性分子间偶联方面具有很大优势。