Cycloadditions are useful transformations in organic synthesis. Light-induced [2+2]-cycloadditions, for example, can be used to prepare substituted cyclobutanes. The direct photoexcitation of the substrates, however, often needs short-wavelength light, which can cause unwanted secondary reactions. Photosensitized reactions can avoid this issue and allow the use of light with longer wavelengths, but often require activated alkenes as reaction partners.

M. Kevin Brown, Indiana University, Bloomington, USA, and colleagues have developed a method to achieve photosensitized [2+2] cycloadditions using temporary ring constraint. The team used dioxaboroles (example pictured) with easily hydrolyzed O–B bonds for cycloadditions with a wide variety of alkenes, giving products in which the boracycle can be simply cleaved to obtain the desired cyclobutanes. They reacted the dioxaboroles with both activated and unactivated alkenes in the presence of fac-Ir(dFppy)₃ (dFppy = 3,5-difluoro-2-(2-pyridinyl)phenyl) as a photosensitizer under a 450 nm LED light, using dichloromethane (DCM) as the solvent.

The desired products were obtained in moderate to high yields. After cleavage of the boracycle, the reaction either gave cyclobutyl diols or led to 1,4-dicarbonyl compounds which can serve as useful intermediates, after glycol cleavage.  The team proposes that this strategy based on temporary ring constraint could be useful for other photosensitized cycloadditions.

• Photosensitized [2 + 2]-Cycloadditions of Dioxaborole: Reactivity Enabled by Boron Ring Constraint Strategy,
  Yanyao Liu, M. Kevin Brown,
  J. Am. Chem. Soc. 2023.
  https://doi.org/10.1021/jacs.3c08105