State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments , Tsinghua University , Beijing 100084 , People's Republic of China.
MOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Department of Chemistry , Tsinghua University , Beijing 100084 , People's Republic of China.
J Am Chem Soc. 2020 Feb 19;142(7):3499-3505. doi: 10.1021/jacs.9b12120. Epub 2020 Feb 11.
The [2 + 2] photocycloaddition of alkenes and carbonyls is of fundamental interest and practical importance, as this process is extensively involved in oxetane-ring constructions. Although individual carbonyl group or alkene moiety has been utilized as photoactive species for oxetane formations upon ultraviolet photoexcitation, direct excitation of the entire noncovalent complex involving alkene and carbonyl substrates to achieve [2 + 2] photocycloadditions is rarely addressed. Herein, complexes with noncovalent interactions between benzophenone and C═C bonds in unsaturated lipids have been successfully characterized, and for the first time a [2 + 2] cycloaddition leading to the formation of oxetanes has been identified under visible-light irradiation. The mechanism of this reaction is distinctly different from the well-studied Paternò-Büchi reaction. The entire complexes characterized as dimeric proton-bonded alkene and carbonyl substrates can be excited under visible light, leading to electron transfer from the alkene moiety in fatty acyls to the carbonyl group within the complex. These results provide new insight into utilizing noncovalent complexes for the synthesis of oxetanes in which the excitation wavelength becomes independent of each individual substrate.
[2 + 2]环加成反应是烯烃和羰基化合物的基本反应，具有重要的理论意义和实际应用价值，因为该过程广泛涉及到氧杂环丁烷环的构建。虽然单独的羰基基团或烯烃部分已被用作紫外光激发下形成氧杂环丁烷的光活性物质，但直接激发涉及烯烃和羰基底物的整个非共价配合物以实现[2 + 2]环加成反应的情况很少被涉及。本文中，成功地对苯甲酮和不饱和脂质中的 C═C 键之间的非共价相互作用的复合物进行了表征，并且首次在可见光照射下鉴定出导致氧杂环丁烷形成的[2 + 2]环加成反应。该反应的机理明显不同于研究得很好的 Paternò-Büchi 反应。作为二聚质子键合的烯烃和羰基底物的整个复合物可以在可见光下被激发，导致电子从脂肪酸酰基中的烯烃部分转移到复合物中的羰基基团。这些结果为利用非共价复合物合成氧杂环丁烷提供了新的见解，其中激发波长变得与每个单独的底物无关。
