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(1 H -indene-1,3(2 H)-dione) 3 or 2-(quinolin-8-ylmethylene)-1 H -indene-1,3(2 H)-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. [Display omitted] [ABSTRACT FROM AUTHOR]