Planar Boron Heterocycles with Nucleic Acid-Like Hydrogen-Bonding Motifs^†

To promote the development of boron-containing purine analogues that exist in planar, non-zwitterionic dominant structural form in aqueous solution, rigorous solution and solid state structural analyses of 1-hydroxy-1H-2,3,1-benzoxazaborine (1), 1,2-dihydro-1-hydroxy-2,3,1-benzodiazaborine (2), and related 2,3,1-benzodiheteraborines were undertaken. With the aid of isotope-enriched compounds, a multisolvent 1H, 13C, 11B, and 15N NMR spectroscopic analysis of 1 and 2 was conducted, providing structurally-diagnostic chemical shift data for all non-oxygen atoms that constitute their heterocyclic peripheries. In addition, single-crystal X-ray diffraction analyses of 1 and 2 were performed. In stark contrast to their 2,4,1 isomeric counterparts, the 2,3,1-benzoxaza- and benzodiazaborines exist in planar structural form in protic solution and in the solid state and display proton dissociative and associative tendencies reflective of the predominant Brønsted, yet still Lewis acidic-capable character of the B-OH group together with the basic one at the C4−N3 imine group. In the solid state, 1 and 2 display intermolecular hydrogen-bonding patterns not too dissimilar from the motifs of certain natural nucleic acid bases. Diazaborine 2 was shown by VT-NMR to undergo a triple hydrogen-bonding solution association with a 2‘,3‘,5‘-tri-O-protected cytidine in a demonstration of one biomimetic potential held by a 1-hydroxy-2,3,1-diheteraborine periphery. In general, 1, 2, and related 1-hydroxy-2,3,1-benzodiheteraborine heterocycles were found to be characterized by an environment-dependent O1→N3 Brønsted prototropy and B−OH group Brønsted/Lewis acid ambidency so sensitive and subtle that certain past difficulties encountered in attempts to delineate their physicochemical properties now become readily appreciated.