Unusual spontaneous oxidation of 2,2′‐(quinolin‐2‐ylmethylene)bis(1H‐indene‐1,3(2H)‐dione): An experimental and theoretical study of the mechanism

Abstract

Abstract2‐Quinolinecarbaldehyde reacts with 1,3‐indandione to give 2‐[(3‐hydroxy‐1‐oxo‐1H‐inden‐2‐yl)(quinolin‐2‐yl)methylene]‐1H‐indene‐1,3(2H)‐dione, which is the first example of self‐oxidation of the Michael adducts of cycloalkanones with aldehydes. The structure of the product of oxidation is also proved by NMR and elemental analysis of its bromination product 2‐[(2‐bromo‐1,3‐dioxo‐2,3‐dihydro‐1H‐inden‐2‐yl)(1,3‐dioxo‐1,3‐dihydro‐2H‐inden‐2‐ylidene)‐methyl]quinolinium tribromide salt. The mechanism of this unusual reaction is studied experimentally and theoretically in comparison with that of a similar reaction with 2‐pyridinecarbaldehyde, which does not lead to self‐oxidation. The three‐step mechanism includes the formation of a pre‐reaction complex, which suffers proton transfer to the pyridinic nitrogen atom, and the hydride ion transfer to the protonated aldehyde. The transition states for the two reactions differ by the position on the reaction coordinate and the heights of the free energy barriers (~25 and ~4 kcal/mol), which allows to explain the observed different reactivity of the two similar hetaryl aldehydes.