M. Peters, R. Breinbauer / Tetrahedron Letters 51 (2010) 6622–6625
6625
Acknowledgments
Financial support by the Volkswagenstiftung and the University
of Leipzig is gratefully acknowledged.
N
3d
N
N
N
N
Supplementary data
N
3j
Supplementary data associated with this article can be found, in
References and notes
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Diagram 1. Conversion-time-diagram for ligand 3j in comparison to ligand 3d.
0.75 mol % [Ir(OMe)(1.5-COD)]2 (7), 1.5 mol % ligand, 1.5 mol % KOtBu, 0.5 equiv
B2Pin2 (5), 1.0 equiv 2,6-dichloropyridine (11), THF, 65 °C.
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N
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N
N
N
N
N
N
3d
3h
N
N
N
3b
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Diagram 2. Conversion-time-diagram for the three most active NHCs in compar-
ison with the dtbpy ligand 6; 0.75 mol % [Ir(OMe)(1.5-COD)]2 (7), 1.5 mol % ligand,
1.5 mol % KOtBu (for 3b, 3d, 3h), 0.75 equiv B2Pin2 (5), 1.0 equiv 2,6-dichloropyr-
idine (11), MTBE, 65 °C.
14. Hydrogenation of ketones: (a) Cheng, Y.; Xu, H.-J.; Sun, J.-F.; Li, Y.-Z.; Chen, X.-
T.; Xue, Z.-L. Dalton Trans. 2009, 35, 7132–7140; (b) Cheng, Y.; Sun, J.-F.; Yang,
H.-L.; Xu, H.-J.; Li, Y.-Z.; Chen, X.-T.; Xue, Z.-L. Organometallics 2009, 28, 819–
823; Polymerization: (c) Xiao, X. Q.; Jin, G.-X. J. Organomet. Chem. 2008, 21–22,
3363–3368; (d) Chen, J. C. C.; Lin, I. J. B. Organometallics 2000, 19, 5113–5121.
Hydrosilylation: see 7c; cross-coupling: see 5..
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21. Detailed experimental procedures and full characterization of compounds can
be found in the Supplementary data.
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3. Conclusion
We were able to show that bidentate pyrido-NHC complexes
can easily be prepared by the deprotonation of pyridinyl-1H-imi-
dazolium iodides 2, which are readily accessible via Ullmann cou-
pling of 2-halopyridines 8 with 1H-imidazoles 10 and subsequent
alkylation at the imidazole nitrogen. In a test reaction we could
demonstrate that these ligands can be successfully applied in the
Ir-catalyzed borylation of arenes. While for this particular reaction
the mixed pyridyl-NHC-ligands 3 did not exceed the reactivity of
the best known literature ligand dtbpy (6), we do see considerable
potential that these precursors will find application in other orga-
nometallic reactions in which electron-rich bidentate ligands are
needed, as this ligand class is distinguished by a high tunability
regarding electronic and steric properties, made possible by the
flexible synthetic approach introduced in this report.