Post‐Assembly Reactivity of N‐Aryl Iminoboronates: Reversible Radical Coupling and Unusual B−N Dynamic Covalent Chemistry

Abstract

Post‐assembly reaction of a dynamic covalent iminoboronate system following addition of Cp2Co resulted in the formation of a series of new reductively coupled dianionic dimers via C−C bond formation. The dimers formed as a mixture of BN‐containing isomeric products: diastereomers rac 5 and meso 5, with coupled five‐membered rings, and enantiomeric rac 6, with a fused six‐membered ring bicyclic system from C−C bond formation and rearrangement of the B−N bonds. Each isomer was identified using 1H NMR spectroscopy in combination with single crystal X‐ray structure determination. Interestingly, interconversion between the coupled five‐membered rings (rac 5) and fused bicyclic systems (rac 6) was found to occur through an unprecedented breaking and reforming of the B−N covalent bond. Further, the coupled products could be converted quantitatively back to their iminoboronate precursors with addition of the electron abstractor Ph3C+.

The Dynamic Covalent Chemistry of Amidoboronates: Tuning the rac5/rac6 Ratio via the B-N and B-O Dynamic Covalent Bonds

“…We have previously reported the solid-state and solution structures of [meso5-3b](Cp2Co)2, [rac6-3b-d](Cp2Co)2 and [rac5-4c](Cp2Co)2 as well as the solid-state structure of [rac5-4c](Cp*2Co)2. [26] Decamethylcobaltocene was chosen as an alternative reductant in these studies to investigate: i) whether different amidoboronate isomers could be crystallised with decamethylcobaltocenium rather than cobaltocenium countercations; ii) the influence of the countercation on the rac5/rac6 rearrangement.…”

“…Comparisons can be made on the amidoboronate products obtained by crystallisation from the series of reductive couplings varying the aniline para substituent, catechol and countercation in this and the previous study. [26] Within the pyrocatechol series (3a-3e), the rac6 isomer crystallised from the reaction mixtures as the cobaltocenium salt with the exception of [rac5-3e](Cp2Co)2 with NMe2 substituents (Figures S57-S62). Furthermore, crystals of [meso5-3a](Cp2Co)2 (Figure 1a, S27-S31) and [meso5-3b](Cp2Co)2 [26] with the electron-withdrawing Cl and F substituents, respectively, were obtained on separate occasions to the rac6 isomer.…”

“…[26] Within the pyrocatechol series (3a-3e), the rac6 isomer crystallised from the reaction mixtures as the cobaltocenium salt with the exception of [rac5-3e](Cp2Co)2 with NMe2 substituents (Figures S57-S62). Furthermore, crystals of [meso5-3a](Cp2Co)2 (Figure 1a, S27-S31) and [meso5-3b](Cp2Co)2 [26] with the electron-withdrawing Cl and F substituents, respectively, were obtained on separate occasions to the rac6 isomer. In contrast, the rac5 isomer crystallised for the tetrachlorocatechol series with cobaltocenium (4c-4e) and decamethylcobaltocenium counterions ( 4a, 4c-4e).…”

“…The increased interconversion is attributed to the greater electron density on the nitrogen and boron centres, facilitating B-N bond cleavage. [26] It was not possible to investigate the influence of the countercation on the interconversion as crystals were not obtained from the reductive couplings with decamethylcobaltocene.…”

“…[21c] We also recently exploited their reactivity in a reductive coupling with cobaltocene to access up to three amidoboronate products following C-C bond formation: the diastereomeric meso5 and rac5 products and the rac6 product with a fused six-membered heterocyclic ring system formed from the rac5 isomer via rearrangement of the covalent B-N bonds (Scheme 1). [26] We report the expanded scope of the reductive coupling in acetonitrile varying both the para substituent of the aniline and catechol to obtain single isomers of the rac5 or rac6 isomers via crystallisation. The rac5/rac6 interconversion and resulting isomeric ratio of the redissolved crystals was tuned by electronic effects via the B-N and B-O dynamic covalent bonds (Scheme 1).…”