Effects of Hindrance in N-Pyridyl Imidazolylidenes Coordinated to Iridium on Structure and Catalysis
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- 作者：Zephen G. Specht ; Douglas B. Grotjahn ; Curtis E. Moore ; Arnold L. Rheingold
- 刊名：Organometallics
- 出版年：2013
- 出版时间：November 11, 2013
- 年：2013
- 卷：32
- 期：21
- 页码：6400-6409
- 全文大小：432K
- 年卷期：v.32,no.21(November 11, 2013)
- ISSN：1520-6041
文摘
The unhindered N-pyrid-2-yl imidazolidene NHC ligand has been shown to chelate to a Cp*Ir fragment (<b>1b>). 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 Cp*Ir 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 <b>6b> 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 <b>6b> existed with the pyridyl substituent N-coordinated, presumably as a monomer, but that addition of an amine ligand readily opened the chelate. Finally, <b>6b> 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 <b>6b> was shown to be much more effective compared to parent compound <b>1b> without the tert-butyl groups, in which the chelating group was more tightly bound.