Contra‐Thermodynamic, Photocatalytic E → Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two‐Dimensional Chemical Space

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z ‐α‐substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp 2 )−B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E ‐α‐substituted styrenyl BPins to generate the corresponding Z ‐isomers with high fidelity. Preliminary validation of this contra‐thermodynamic E → Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z ‐configured trisubstituted alkenes, as well as in a concise synthesis of the anti‐tumor agent Combretastatin A4.