2045-75-2Relevant articles and documents
Effects of hindrance in N-pyridyl imidazolylidenes coordinated to iridium on structure and catalysis
Specht, Zephen G.,Grotjahn, Douglas B.,Moore, Curtis E.,Rheingold, Arnold L.
, p. 6400 - 6409 (2013)
The unhindered N-pyrid-2-yl imidazolidene NHC ligand has been shown to chelate to a CpIr fragment (1). With the goal of weakening the coordination of the pyridyl substituent and enabling its role as pendant base or hemilabile ligand, a tert-butyl group at C-6 next to the pyridyl N was installed. Attempted coordination of the newly synthesized carbene ligand avoided N-coordination entirely, but led to unexpected C-metalation at C-3 by the CpIr center. Successful formation of a weakly N-coordinated analogue was achieved by synthesizing a ligand with a second tert-butyl group at C-4. The complexes were studied using X-ray crystallography and NMR spectroscopy. The X-ray crystal structure of di-tert-butyl analogue 6 showed that in the solid the complex existed as a chloride-bridged dimer, with the pyridyl nitrogen uncoordinated. In solution, 15N chemical shift information revealed that 6 existed with the pyridyl substituent N-coordinated, presumably as a monomer, but that addition of an amine ligand readily opened the chelate. Finally, 6 was used as a catalyst for intramolecular hydroamination of primary and secondary alkenylamines. Comparing (heteroaryl)NHC species, each with a tert-butyl group next to the nitrogen, which enables hydroamination, the rate differences are very modest but increase in the order imidazolyl pyridyl pyrimidyl, which may be an effect of basicity but because of the similarity in rates is better ascribed to counterbalancing of more than one factor, including hemilability. The (4,6-di-tert-butyl)pyridyl species 6 was shown to be much more effective compared to parent compound 1 without the tert-butyl groups, in which the chelating group was more tightly bound.
