Synthesis of New Chiral 2,2‘-Bipyridyl-Type Ligands, Their Coordination to Molybdenum(0), Copper(II), and Palladium(II), and Application in Asymmetric Allylic Substitution, Allylic Oxidation, and Cyclopropanation

Abstract

A series of chiral bipyridine-type ligands 5−12 has been synthesized via a de novo construction of the pyridine nucleus. The chiral moieties of the ligands originate from the monoterpene realm, namely, pinocarvone (13 → 6, 7, and 9), myrtenal (18 → 5), nopinone (21 → 8 and 10), and menthone (28 → 11 and 12); the first three precursors can be obtained in one step from β- and α-pinene, respectively. Complexes of these ligands with molybdenum(0) (38−40) and copper(II) (41) have been characterized by single-crystal X-ray crystallography. While complex 38 exhibits polymorphism (monoclinic and tetragonal forms crystallize from the same batch), 41 is characterized by a tetrahedrally distorted geometry of the metal coordination. The Mo and Pd complexes exhibit modest asymmetric induction in allylic substitution (43 → 44), and the Cu(I) counterpart of 41, derived from 10 (PINDY) and Cu(OTf)2, shows promising enantioselectivity (49−75% ee) and reaction rate (≥30 min at room temperature) in allylic oxidation of cyclic olefins (47 → 48). The Cu(I) complex of 11 (MINDY) proved effective in cyclopropanation (49 → 50) with up to 72% ee.