Intermolecular [2+2] photocycloadditions represent a powerful method for the synthesis of highly strained, four-membered rings. Although this approach is commonly employed for the synthesis of oxetanes and cyclobutanes, the synthesis of azetidines via intermolecular aza Paternò–Büchi reactions remains highly underdeveloped. Here we report a visible-light-mediated intermolecular aza Paternò–Büchi reaction that utilizes the unique triplet state reactivity of oximes, specifically 2-isoxazoline-3-carboxylates. The reactivity of this class of oximes can be harnessed via the triplet energy transfer from a commercially available iridium photocatalyst and allows for [2+2] cycloaddition with a wide range of alkenes. This approach is characterized by its operational simplicity, mild conditions and broad scope, and allows for the synthesis of highly functionalized azetidines from readily available precursors. Importantly, the accessible azetidine products can be readily converted into free, unprotected azetidines, which represents a new approach to access these highly desirable synthetic targets.

分子间[2 + 2]光环加成反应是合成高应变四元环的有力方法。尽管这种方法通常用于合成氧杂环丁烷和环丁烷，但通过分子间氮杂Paternò–Büchi反应合成氮杂环丁烷的方法仍很不完善。在这里，我们报告了可见光介导的分子内氮杂Paternò–Büchi反应，该反应利用了肟的独特三重态反应性，特别是2-异恶唑啉-3-羧酸酯。此类肟的反应活性可通过从可商购的铱光催化剂进行三重态能量转移来利用，并允许与各种烯烃进行[2 + 2]环加成反应。这种方法的特点是操作简单，条件温和且适用范围广，并允许从容易获得的前体合成高度官能化的氮杂环丁烷。重要的是，易接近的氮杂环丁烷产物可以容易地转化为游离的，未保护的氮杂环丁烷，这代表了一种接近这些高度期望的合成靶标的新方法。