MIDA- and TIDA-Boronates Stabilize α-Radicals Through B-N Hyperconjugation

Area of Science:

• Organic Chemistry
• Organoboron Chemistry
• Radical Chemistry

Background:

• Organoboron compounds are crucial for synthesizing complex Csp3-rich molecules.
• Boron functional groups modify alpha-radical reactivity, enabling diverse chemical reactions.
• Boronic esters with vacant p-orbitals stabilize alpha-radicals via spin delocalization.

Purpose of the Study:

• To investigate the radical stabilizing effect of coordinatively saturated boronates, such as MIDA and TIDA boronates.
• To elucidate the mechanism by which these saturated boronates stabilize secondary alkyl alpha-radicals.
• To demonstrate their application in site-selective C-H bromination reactions.

Main Methods:

• Density Functional Theory (DFT) calculations for radical stabilization energies and spin density.
• Kinetic analysis using LED-NMR to study photochemical bromination rates.
• Synthesis and characterization of N-methyliminodiacetic acid (MIDA) and TIDA boronates.

Main Results:

• Coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl alpha-radicals via sigma B-N hyperconjugation.
• This stabilization facilitates site-selective C-H bromination.
• DFT and kinetic data support the hyperconjugation mechanism, distinct from vacant p-orbital delocalization.

Conclusions:

• The alpha-radical stabilizing effect of boronic esters involves both vacant p-orbital delocalization and hyperconjugation from tetrahedral boron groups.
• Boron ligands offer a tunable approach to control reactivity at the alpha-carbon.
• This work expands the understanding of organoboron chemistry in radical reactions.