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. 2019 Jun 5;141(22):9072-9078.
doi: 10.1021/jacs.9b03611. Epub 2019 May 23.

1,2-Azaborine's Distinct Electronic Structure Unlocks Two New Regioisomeric Building Blocks via Resolution Chemistry

Cameron R McConnell  1 , Fredrik Haeffner  1 , Andrew W Baggett  1 , Shih-Yuan Liu  1
Affiliations

1,2-Azaborine's Distinct Electronic Structure Unlocks Two New Regioisomeric Building Blocks via Resolution Chemistry

Cameron R McConnell et al. J Am Chem Soc. .

Abstract

Two new 1,2-azaborine building blocks that enable the broad diversification of previously not readily accessible C4 and C5 ring positions of the 1,2-azaborine heterocycle are developed. 1,2-Azaborine's distinct electronic structure allowed the resolution of a mixture of C4- and C5-borylated 1,2-azaborines. The connection between the electronic structure of C4 and C5 positions of 1,2-azaborine and their distinct reactivity patterns is revealed by a combination of reactivity studies and kinetic measurements that are supported by DFT calculations. Specifically, we show that oxidation by N-methylmorpholine N-oxide (NMO) is selective for the C4-borylated 1,2-azaborine, and the Ir-catalyzed deborylation is selective for the C5-borylated 1,2-azaborine via kinetically controlled processes. On the other hand, ligand exchange with diethanolamine takes place selectively with the C4-borylated isomer via a thermodynamically controlled process. These results represent the first examples for chemically distinguishing a mixture of two aryl mono-Bpin-substituted isomers.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Electronic structure calculations (M062X/6–31G(d,p), computed using polarizable continuum model (PCM) simulating CH2Cl2 solvent): select Löwdin charges in parentheses (au), LUMO energies, and orbital illustrations.
Figure 2
Figure 2
Calculated Gibbs free energy reaction coordinate profile of the NMO oxidation of 4Bpin and 5Bpin, computed at 298 K and 1 atm using the M062X/6–31G(d,p) method. The polarizable continuum model (PCM) was used to model the effects on the reaction by the CH2Cl2 solvent. Energies are in kcal/mol. All H atoms as well as the methyl on the pinacol, mesityl, and t-butyl substituents have been omitted for clarity.
Figure 3
Figure 3
Electronic structure calculations (M062X/6–31G(d,p), computed using polarizable continuum model (PCM) simulating CH2Cl2 solvent): select Löwdin charges in parentheses (au), HOMO energies, and orbital illustrations.
Scheme 1
Scheme 1
Late-Stage Building-Block Functionalization Approach
Scheme 2
Scheme 2
New 1,2-Azaborine Building Blocks via Resolution Chemistry
Scheme 3
Scheme 3
Resolution of 5Bpin and 4Bpin via Selective Oxidation of 4Bpin
Scheme 4
Scheme 4
Kinetic Resolution via Ir-Catalyzed Protodeborylationa aIndependent rate measurements performed at 40 °C.
Scheme 5
Scheme 5
Reaction of Borylated Isomers with Diethanolamine
Scheme 6
Scheme 6
Kinetic and Thermodynamic Studies of Transesterification Reaction
Scheme 7
Scheme 7
Functionalizations of 4Bpin and 5Bpin
See this image and copyright information in PMC

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