Boron-enabled geometric isomerization of alkenes via selective energy-transfer catalysis

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Science  17 Jul 2020:
Vol. 369, Issue 6501, pp. 302-306
DOI: 10.1126/science.abb7235

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An E-Z boron swivel

Compounds with carbon-carbon double bonds can form two distinct isomers, depending on whether the heaviest substituents on both carbons lie on the same side (labeled Z) or diagonally across from each other (labeled E). Molloy et al. present a convenient method to reorient double bonds that bear boron and carbonyl substituents. When they are diagonally opposed, both substituents stay in plane, and the double bond is easily swiveled by photosensitization. However, once on the same side as the carbonyl, the boron rotates out of plane and further sensitization is inhibited.

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Isomerization-based strategies to enable the stereodivergent construction of complex polyenes from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low–molecular weight precursors. In this work, we report a general ambiphilic C3 scaffold that can be isomerized and bidirectionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-borylacrylates is contingent on the participation of the boron p orbital in the substrate chromophore. Rotation of the C(sp2)–B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.

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