An experimental and theoretical study of intramolecular regioselective oxidations of 6-substituted 2,3-dimethylquinoxaline derivatives

An experimental and theoretical study of the regioselective Riley oxidation was conducted on a series of 2,3-dimethyl-6-substituted-quinoxalines bearing EWG (NO 2 , CN, CF 3 , Cl, Br, F, COOH, COOMe, COPh) and EDG (2,3-dimethylquinoxaline, OMe, OH, NH 2 ) substituents. The nitrogen lone pair of electrons of the symmetric benzopyrazine moiety initiates the oxidation and promotes nucleophilic competition between the two active sites to give carbaldehyde regioisomers a and b . The mesomeric effect provides the dominant contribution to the regioselectivity. The compounds were characterized by NMR, measuring the 1 H, 13 C, pfg-HSQC, pfg-HMBC, and 15 N, 1 H correlation signals established by pfg-HMQC. The nucleophilic reactivity of nitrogen was evaluated by 1 H NMR titration and analyzed using Perrin linearization to determine the reactivity ratio, Δ K , of the N4 and N1 nitrogen atoms. The structures were optimized using density functional theory at the ωB97XD/6-311G++(d,p) level of theory. The highest occupied molecular orbitals modeled using the HF/6-311G++(d,p) functionals revealed an asymmetric electron density that confirmed the asymmetric nucleophilicity of the nitrogen centers. These values agreed with the experimentally measured Δ K ratios. The PM6 theoretical calculations of the heats of formation of the mesomeric forms and intermediates of (2,3-dimethyl-6-substituted-quinoxalines)-SeO 2 allowed us to identify the reaction routes that minimized energy expenditures. The regioselectivities were explained in terms of the energetic diagrams of the regioisomers. All compounds evaluated indicated a preference toward forming regioisomer b , except for the derivative bearing the EDG substituent (2,3-dimethylquinoxaline) which displayed a preference for regioisomer a . The long-range substituent effects on the Riley reaction mechanism were determined by NMR and DFT calculations.