Diborons are a paradox: despite featuring a highly electron deficient boron-boron bond, they are powerful reducers. The steric and electronic properties of their substituents have an enormous influence on the reactivity of the boron-boron bond, offering the potential to control diastereoselectivity, regioselectivity, chemoselectivity, and absolute stereochemistry of diboron-mediated reactions. Recently, N-borylpyridinyl radicals were reported to form via homolytic cleavage of dicoordinate bis-pyridine-diboron complexes. These species reductively dearomatize pyrazines, aromatize p-quinones, and deoxygenate sulfoxides. Additionally, they undergo dimerization reactions that suffer from poor regioselectivity and diastereoselectivity. However, more recently, dicoordinate bis-isoquinoline-diboron complexes were reported to undergo highly diastereoselective carbon-carbon bond forming [3,3]-sigmatropic rearrangements. Impressively, this work was expanded to include enantioselective reductive imine couplings. However, these reactions can only selectively access thermodynamic diastereomers, and a preference for kinetic diastereomers has never been reported. This limitation extends to all reductive imine coupling reactions that use different reagents, conditions, or mechanisms. We sought to develop reactions that would overcome this limitation, and to understand the factors that influence diastereoselectivity in diboron-mediated reductive coupling reactions of imines. We found that varying the steric and electronic properties of the diolate ligands on the diboron, and the solvent, enabled the stereodivergent synthesis of all diastereomers of 2,3-diaryltetrahydroquinoxalines. Additionally, we developed unique conditions for the activation of tetraalkoxydiborons, resulting in greatly increased reactivity and kinetic diastereoselectivity. We then explored the mechanism of these reactions computationally to aid in our understanding of how to proceed with further optimization. Finally, we developed a simple, fast protocol for the synthesis of tetraalkoxydiborons and diboron diolates to enable us to rapidly explore the effects of the structure of the diboron on reactivity and selectivity.
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