Flipping from E- to Z-alkenes

In their search for new medicines, chemists often choose the trusty Suzuki-Miyaura reaction to construct essential carbon-carbon bonds. This reliable reaction connects alkyl halides with various classes of organoborane compounds, including alkenyl organoboranes, which can be further classified by alkene configuration, either cis (Z) or trans (E). E-alkenylboron compounds are faster and simpler to make than their thermodynamically unfavored twin, the Z-configured compounds, and thus have received more attention as precursors for the Suzuki-Miyaura reaction. Now, using photoredox catalysis, researchers led by University of Münster’s Ryan Gilmour and University of St. Andrews’ Allan Watson are offering Z-alkenes a turn in the spotlight (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201800286). Starting with the more accessible E-styrenyl organoboranes, the team employs an iridium catalyst and visible light to generate an excited state intermediate that may be swayed to collapse into either alkene conformation. An intentionally situated methyl substituent in the compound exerts the strain necessary to convince the molecule’s boron pinacol group to flip to the Z configuration. Though the reaction is currently limited to styrenyl boranes, Gilmour says it’s a good first step to providing access to Z-alkenyl organoboranes, which represent an underexplored area of chemical space for Suzuki-Miyaura cross-couplings.