Unusual behavior of 2,2'-(Quinolin-8-ylmethylene)bis(1H-indene-1,3(2H)-dione): Gear-like rotation, merry-go-round tautomerization, reversible fragmentation and intramolecular cyclization

1,3-Indandione reacts with 8-quinolinecarbaldehyde to give, depending on the conditions, 2,2'-(quinolin-8-ylmethylene)bis(1H-indene-1,3(2H)-dione) 3 or 2-(quinolin-8-ylmethylene)-1H-indene-1,3(2H)-dione 4. The 2:1 adduct 3 exhibits dynamic behavior distinct from that of the 2-pyridyl analogue. On cooling, the singlets of the central CH and 2-CH protons resolve into a triplet and doublet, respectively. On heating, all aromatic protons of the two indandione moieties in 3 are averaged due to reversible gear-like rotation of the quinoline fragment driven by the keto-enol tautomerization. In strongly basic DMSO, adduct 3 is reversibly fragmented into 1,3-indandione and 1:1 adduct 4, the first example of 1:1 adduct of indanones with electron-deficient hetaryl aldehydes. Protonation with trifluoroacetic acid at low temperature reveals the presence of several protonated forms differing by the configuration of the central carbon. Theoretical DFT calculations of adduct 3, its tautomers, rotamers, transition states and protonated forms were performed to support the experimental results.