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 5а′ with two intramolecular H-bonds ОН⋯ОС 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 ОН⋯ОС. 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.

► Vorländer adducts of dimedone and phenalen-1,3-dione exist in the dienol form. ► Adduct of dimedone with 2-pyridinealdehyde show reversible ring-chain interconversion. ► Adduct of phenalenedione with 4-pyridinealdehyde forms zwitterion in the crystal. ► Phenalenedione adducts do not undergo ring closure due to extended conjugation system.