Theoretical studies of iridium-mediated tautomerization of substituted pyridines

Room temperature reaction of [Ir(COD)2]BF4 (COD = 1,5-cyclooctadiene) and amide-tethered or simple 2,3′-bipyridyls gave iridium(I) complexes bearing chelating protic pyridylidenes. This protic pyridylidene tautomer is stabilized by both chelation effect and by hydrogen bonding. The mechanistic details of this tautomerization of N-heterocycles to N-heterocyclic carbenes (NHCs) were investigated using the density functional theory (DFT). DFT studies suggested that cyclometalation of 2,3′-bipyridyls took place to give an iridium(III) hydride, which subsequently undergoes formal 1,3-hydrogen shift from the iridium to the pyridyl nitrogen atom. Two possible mechanisms of this formal 1,3-hydrogen shift process have been examined: the β-insertion of the hydride into an olefin followed by proton abstraction and the water-assisted proton transfer via a cyclic transition state. The latter mechanism is strongly favored in the presence of a catalytic amount of water, and this mechanism is applicable to the tautomerization of both amide-tethered and amide-free 2,3′-bipyridyls.

The DFT studies showed that the water-assisted proton transfer mechanism is most likely for the tautomerization of 2,3′-bypyridyls with or without any adjacent amide group.Figure optionsDownload as PowerPoint slideHighlights
► Mechanistic details of iridium-mediated tautomerization of 2,3′-bipyridyls and analogues were investigated using the density functional theory.
► Two possible pathways have been examined: the β-insertion and the water-assisted proton relay pathways.
► The water-assisted proton relay mechanism is most likely followed in the tautomerization of various 2,3′-bypyridyls.