The formation of highly enantioenriched boronic esters through both stoichiometric and catalytic methods has received much attention over the past decade. Accordingly, the transformations of the boronic ester moiety into other functional groups is of considerable interest in synthesis. Specifically, transformations which retain the high enantioenrichment of the starting boronic ester, either through a stereoretentive or a stereoinvertive pathway, lead to the formation of new C-C, C-O, C-N, C-X, or C-H bonds at stereogenic centres. This feature article summarises the current state of the art in stereospecific transformations of both secondary and tertiary boronic esters into other functionalities and groups, whilst considering critically the transformations that are currently unattainable and would represent future advances to the field.
The oxidative addition of organic electrophiles into
electrochemically
generated Co(I) complexes has been widely utilized as a strategy to
produce carbon-centered radicals when cobalt is ligated by a polydentate
ligand. Changing to a bidentate ligand provides the opportunity to
access discrete Co(III)–C bonded complexes for alternative
reactivity, but knowledge of how ligand and/or substrate structures
affect catalytic steps is pivotal to reaction design and catalyst
optimization. In this vein, experimental studies that can determine
the exact nature of elementary organometallic steps remain limited,
especially for single-electron oxidative addition pathways. Herein,
we utilize cyclic voltammetry combined with simulations to obtain
kinetic and thermodynamic properties of the two-step, halogen-atom
abstraction mechanism, validated by analyzing kinetic isotope and
substituent effects. Complex Hammett relationships could be disentangled
to allow understanding of individual effects on activation energy
barriers and equilibrium constants, and DFT-derived parameters used
to build predictive statistical models for rates of new ligand/substrate
combinations.
Cobalt complexes have shown great promise as electrocatalysts in applications ranging from hydrogen evolution to C−H functionalization. However, the use of such complexes often requires polydentate, bulky ligands to stabilize the catalytically active Co(I) oxidation state from deleterious disproportionation reactions to enable the desired reactivity. Herein, we describe the use of bidentate electronically asymmetric ligands as an alternative approach to stabilizing transient Co(I) species. Using disproportionation rates of electrochemically generated Co(I) complexes as a model for stability, we measured the relative stability of complexes prepared with a series of N,N-bidentate ligands. While the stability of Co(I)Cl complexes demonstrates a correlation with experimentally measured thermodynamic properties, consistent with an outer-sphere electron transfer process, the set of ligated Co(I)Br complexes evaluated was found to be preferentially stabilized by electronically asymmetric ligands, demonstrating an alternative disproportionation mechanism. These results allow a greater understanding of the fundamental processes involved in the disproportionation of organometallic complexes and have allowed the identification of cobalt complexes that show promise for the development of novel electrocatalytic reactions.
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