Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3‐rich small molecules. The ability of boron‐containing functional groups to modify the reactivity of α‐radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p‐orbitals have a significant stabilizing effect on α‐radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N‐methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α‐radicals via σB‐Nhyperconjugation in a manner that allows site‐selective C−H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α‐radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p‐orbitals, but that hyperconjugation of tetrahedral boron‐containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α‐carbon.
- Award ID(s):
- 1828117
- NSF-PAR ID:
- 10301423
- Date Published:
- Journal Name:
- Synlett
- Volume:
- 32
- Issue:
- 07
- ISSN:
- 0936-5214
- Page Range / eLocation ID:
- 655 to 662
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3‐rich small molecules. The ability of boron‐containing functional groups to modify the reactivity of α‐radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p‐orbitals have a significant stabilizing effect on α‐radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N‐methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α‐radicals via σB‐Nhyperconjugation in a manner that allows site‐selective C−H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α‐radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p‐orbitals, but that hyperconjugation of tetrahedral boron‐containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α‐carbon.
-
more » « less
Despite their desirable attributes, boronic acids have had a minimal impact in biological contexts. A significant problem has been their oxidative instability. At physiological pH, phenylboronic acid and its boronate esters are oxidized by reactive oxygen species at rates comparable to those of thiols. After considering the mechanism and kinetics of the oxidation reaction, we reasoned that diminishing electron density on boron could enhance oxidative stability. We found that a boralactone, in which a carboxyl group serves as an intramolecular ligand for the boron, increases stability by 104-fold. Computational analyses revealed that the resistance to oxidation arises from diminished stabilization of the p orbital of boron that develops in the rate-limiting transition state of the oxidation reaction. Like simple boronic acids and boronate esters, a boralactone binds covalently and reversibly to 1,2-diols such as those in saccharides. The kinetic stability of its complexes is, however, at least 20-fold greater. A boralactone also binds covalently to a serine side chain in a protein. These attributes confer unprecedented utility upon boralactones in the realms of chemical biology and medicinal chemistry.
-
more » « less
Abstract Synthesis of the C−C bonds of ketones relies upon one high‐availability reagent (carboxylic acids) and one low‐availability reagent (organometallic reagents or alkyl iodides). We demonstrate here a ketone synthesis that couples two different carboxylic acid esters,
N ‐hydroxyphthalimide esters andS ‐2‐pyridyl thioesters, to form aryl alkyl and dialkyl ketones in high yields. The keys to this approach are the use of a nickel catalyst with an electron‐poor bipyridine or terpyridine ligand, a THF/DMA mixed solvent system, and ZnCl2to enhance the reactivity of the NHP ester. The resulting reaction can be used to form ketones that have previously been difficult to access, such as hindered tertiary/tertiary ketones with strained rings and ketones with α‐heteroatoms. The conditions can be employed in the coupling of complex fragments, including a 20‐mer peptide fragment analog of Exendin(9–39) on solid support. -
more » « less
Abstract Synthesis of the C−C bonds of ketones relies upon one high‐availability reagent (carboxylic acids) and one low‐availability reagent (organometallic reagents or alkyl iodides). We demonstrate here a ketone synthesis that couples two different carboxylic acid esters,
N ‐hydroxyphthalimide esters andS ‐2‐pyridyl thioesters, to form aryl alkyl and dialkyl ketones in high yields. The keys to this approach are the use of a nickel catalyst with an electron‐poor bipyridine or terpyridine ligand, a THF/DMA mixed solvent system, and ZnCl2to enhance the reactivity of the NHP ester. The resulting reaction can be used to form ketones that have previously been difficult to access, such as hindered tertiary/tertiary ketones with strained rings and ketones with α‐heteroatoms. The conditions can be employed in the coupling of complex fragments, including a 20‐mer peptide fragment analog of Exendin(9–39) on solid support.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1055/s-0040-1707297
