Iterative Dual-Metal and Energy Transfer Catalysis Enables Stereodivergence in Alkyne Difunctionalization: Carboboration as Case Study

Stereochemically defined tetrasubstituted olefins are widespread structural elements of organic molecules and key intermediates in organic synthesis. However, flexible methods enabling stereodivergent access to E and Z isomers of fully substituted alkenes from a common precursor represent a significant challenge and are actively sought after in catalysis, especially those amenable to complex multifunctional molecules. Herein, we demonstrate that iterative dual-metal and energy transfer catalysis
constitutes a unique platform for achieving stereodivergence in the difunctionalization of internal alkynes. The utility of this approach is showcased by the stereodivergent synthesis of both stereoisomers of tetrasubstituted β-boryl acrylates from internal alkynoates with excellent stereocontrol via sequential carboboration and photoisomerization. The reluctance of electron-deficient internal alkynes to undergo catalytic carboboration has been overcome through cooperative Cu/Pd-catalysis, whereas an Ir complex was identified as a versatile sensitizer that is able to photoisomerize the resulting sterically crowded alkenes. Mechanistic studies by
means of quantum-chemical calculations, quenching experiments, and transient absorption spectroscopy have been applied to unveil the mechanism of both steps.