Synthetic applications of N–N linked heterocycles. Part 17. The crystal and molecular structures of N-(4-oxopyridin-1-yl)pyridinium tetrafluoroborate, and its 2,6-dimethyl analogue: evidence for steric hindrance to approach by nucleophiles at the pyridinium α positions in the latter, and a note of protonation effects on chemical shifts

Abstract

The crystal structures of the title compounds (1) and (2) have been determined by X-ray diffraction; both are monoclinic with Z= 4, and have space groups and unit cell dimensions, respectively, P2₁/c, a= 10.876(2), b= 8.913(2), c= 12.805(4)Å, β= 112.16(2)°, and C2/m, a= 22.390(5), b= 6.944(2), c= 9.330(2)Å, β= 98.96(1)°. Both structures were determined by the Patterson method; refinements were to R 0.057 for 637 independent reflections in (1) and to R 0.079 for 1 065 in the dimethyl analogue (2). The N–N bond lengths, and the torsion angles between the ring planes, are, respectively, 1.417(7)Å and 74(3)°, and 1.411(5)Å and 90°; the positions of the methyl groups in (2) effectively shield the pyridinium α-positions from attack by bulky groups, accounting for the observed regiospecific addition of nucleophiles at the γ-position. U.v., and ¹H and ¹³C n.m.r. spectra in both aqueous and acidic media, for compounds (1), (2), and the 2-methyl analogue (3), while providing no evidence for steric interactions between the methyl groups and the pyridinium ring, demonstrate the electronegativity of both rings. In acid solution protonation occurs at the carbonyl oxygen atoms; protonation effects on ¹H, ¹³C, and ¹⁵N chemical shifts are discussed.