Intramolecular interactions in dimedone and phenalen-1,3-dione adducts of 2(4)-pyridinecarboxaldehyde: Enol-enol and ring-chain tautomerism, strong hydrogen bonding, zwitterions

The 2:1 adducts of dimedone and phenalen-1,3-dione with 2- and 4-pyridine carboxaldehyde, in spite of similar chemical behavior of their diketone precursors, have quite different tautomeric structure both in solid state and in solution. 2,2′-(Pyridin-2-ylmethanediyl)-bis(5,5-dimethyl-cyclohexane-1, 3-dione) 5 exists as an equilibrium mixture of its dienol tautomer 5′ with two intramolecular H-bonds ⋯ and OH⋯N and the epimeric products of its reversible cyclization, that is, 4a-hydroxy-9-(pyridin-2-yl)-2,3,4,4a,6, 7,9,9a-octahydro-5-H-xanthene-1,8-diones 5b (major) and 5c (minor), the latter appears only in polar media like DMSO. 2,2′-(Pyridin-4-ylmethanediyl) bis(5,5-dimethylcyclohexane-1,3-dione) 4, like other 2:1 dimedone-aldehyde adducts, both in solution and in solid state exists as dienol with two intramolecular H-bonds ⋯. 4-[Bis(1H-phenalen-1,3(2H)-dione)methyl] pyridine 6 in nonpolar media like chloroform exists as dienol, but crystallizes from this solvent as zwitter-ion 6b with one very strong ionic hydrogen bond OH⋯O and protonated pyridine nitrogen. The same zwitterion is formed in polar media (DMSO). For 2-[bis(1H-phenalen-1,3(2H)-dione)-methyl]-pyridine 7, fast exchange between its dienol tautomer 7a and zwitter-ion 7b occurs even in CD2Cl2, whereas in DMSO the equilibrium shifts towards zwitter-ion 7b.