Investigation on the Synthesis, Application and Structural Features of Heteroaryl 1,2-Diketones

A set of unsymmetrical heteroaryl 1,2-diketones were
synthesized
by a heteroarylation/oxidation sequence with up to 65% isolated yields.
Palladium catalyst XPhos Pd G4 and SeO2 were the key reagents
used in this methodology, and microwave irradiation was utilized to
facilitate an efficient and ecofriendly process. The application of
heteroaryl 1,2-diketones is demonstrated through the synthesis of
an unsymmetrical 2-phenyl-3-(pyridin-3-yl)­quinoxaline (5a) from 1-phenyl-2-(pyridin-3-yl)­ethane-1,2-dione (4a). The lowest energy conformations of 4a and 5a were located using Density Functional Theory (DFT) at the M06-2X/def2-TZVP
level of theory. Two lowest energy conformations of 4a differ with respect to the position of the N atom in the pyridyl
ring and 0.27 kcal/mol energy difference between them corresponds
to 60.4 and 39.6% at 50 °C in toluene. Four lowest energy conformations
for 5a have the energy differences of 0.01, 0.03 and
0.07 kcal/mol that corresponds to 26.0, 25.7, 24.9 and 23.4%, respectively.
A comparison of 4a and 5a to the less hindered
analogs (oxalyl chloride and oxalic acid) is used to investigate the
structural features and bonding using Natural Bond Orbital (NBO) analysis.