PLANAR BORON HETEROCYCLES WITH NUCLEIC ACID-LIKE HYDROGEN-BONDING MOTIFS

To promote the development of boron-containing purine analogues that e xist in planar, non-zwitterionic dominant structural form in aqueous s olution, rigorous solution and solid state structural analyses of 1-hy droxy-1H-2,3,1-benzoxazaborine (1), 1,2-dihydro-1-hydroxy-2,3,1-benzod iazaborine (2), and related 2,3,1-benzodiheteraborines were undertaken . With the aid of isotope-enriched compounds, a multisolvent H-1, C-13 , B-11, and N-15 NMR spectroscopic analysis of 1 and 2 was conducted, providing structurally-diagnostic chemical shift data for all non-oxyg en atoms that constitute their heterocyclic peripheries. In addition, single-crystal X-ray diffraction analyses of 1 and 2 were performed. I n stark contrast to their 2,4,1 isomeric counterparts, the 2,3,1-benzo xaza- and benzodiazaborines exist in planar structural form in protic solution and in the solid state and display proton dissociative and as sociative tendencies reflective of the predominant Bronsted, yet still Lewis acidic-capable character of the B-OH group together with the ba sic one at the C4-N3 imine group. In the solid state, 1 and 2 display intermolecular hydrogen-bonding patterns not too dissimilar from the m otifs of certain natural nucleic acid bases. Diazaborine 2 was shown b y VT-NMR to undergo a triple hydrogen-bonding solution association wit h a 2',3',5'-tri-O-protected cytidine in a demonstration of one biomim etic potential held by a 1-hydroxy-2,3,1-diheteraborine periphery. In general, 1, 2, and related 1-hydroxy-2,3,1-benzodiheteraborine heteroc ycles were found to be characterized by an environment-dependent O1--> N3 Bronsted prototropy and B-OH group Bronsted/Lewis acid ambidency so sensitive and subtle that certain past difficulties encountered in at tempts to delineate their physicochemical properties now become readil y appreciated.