The C—B bond is a critical lynchpin in organic synthesis, enabling a vector for the exploration of chemical space. Perhaps the most pertinent manipulation of organoborons is the Suzuki-Miyaura reaction which has reshaped the pharmaceutical landscape with greater than 40% of all C—C bond-forming reactions in medicinal chemistry being attributed to this process. A notable highlight of this venerable transformation is the stereospecifity observed in cross-coupling reactions and, as a result, design strategies to access stereodefined olefinic organoborons is an important synthetic challenge. Recent, advances in hydroboration and cuproboration of alkynes has enabled direct access to E-isomers of densely functionalized styrenyl organoboron species. Despite this, a direct method to the corresponding Z-isomer is conspicuously absent from the synthetic organic chemists repertoire. A recent renaissance in the use of photosensitizers in energy transfer protocols has enabled the generation of high energy intermediates in solution. This phenomenon has been capitalized upon to facilitate the rapid contra-thermodynamic E→Z isomerization of substituted styrenes. We rationalized that by exploiting the polarity of the C(sp2)—B to activate the neighbouring π-system a mild, traceless photocatalytic isomerization, via a high energy species, could be enabled to generate synthetically useful Z-isomers with high fidelity. Herein we present the contra thermodynamic, photocatalytic E to Z isomerization of alkenyl organoborons enabled by a photosensitizer. Triplet energy sensitization and the origin of stereoselectivity is firmly established by mechanistic studies, and the utility of the Z-isomers is suitably demonstrated in the rapid synthesis of complex molecules.

Contra thermodynamic, photocatalytic E to Z isomerization of alkenyl organoborons: Vectors to facilitate exploration of two dimensional chemical space

2019 Fall National Meeting

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