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Synthesis of New Chiral 2,2-Bipyridyl-Type Ligands

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 Conclusions

In conclusion, we have synthesized a series of bipyridine-type chiral ligands 5-12 via a de novo construction of the pyridine nucleus that had been annulated
to a chiral synthon originating from the monoterpene realm. Complexes of these ligands with Mo(0),
Pd(II), and Cu(II) have been prepared and several of
them characterized by single-crystal X-ray crystallography (38-41); interestingly, 38 exhibited polymorphism. The C2-symmetrical Cu(II) complex 41 is characterized by unique geometry of the metal coordination
(Figure 11). The Mo and Pd complexes showed modest
asymmetric induction in allylic substitution. The Cu(I)
catalyst, derived from ligand (+)-10 (PINDY), exhibited
promising enantioselectivity (∼50-75% ee) and reaction
rate (e30 min at room temperature) in allylic oxidation
(Scheme 8). Further encouraging results (e72% ee) were
obtained with the Cu(I) complex of ligand (-)-11
(MINDY) for asymmetric cyclopropanation (Scheme 9).
These experiments have demonstrated the effectiveness
of this class of chiral bipyridine ligands. Their chiral
cavity can, in principle, be further tuned by varying the
sterics through employing different chiral building
blocks. Furthermore, the de novo synthesis of the
pyridine rings should allow controlling the electronics
of the ligating nitrogen atoms through substitution in
4- and 4′-positions. The number of reactions known to
be catalyzed by nonchiral bipyridine complexes suggests
that these ligands can be expected to enjoy a broad scope
in asymmetric catalysis.




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