Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α‑Hydroxyimine Palladium Complexes

In
the present work, a series of α-hydroxyimine palladium
complexes with bulky substituents (i.e., {[Ar-NC­(R)–C­(R)2–OH]­PdCl2} (C1, R = Me, Ar
= 2-diphenylmethyl-4,6-dimethylphenyl; C2, R = Me, Ar
= 2,6-bis­(diphenylmethyl)-4-methylphenyl; C3, R = Me,
Ar = 2,6-bis­(diphenylmethyl)-4-methyoxylphenyl; C4, R
= Me, Ar = 2,6-bis­(diphenylmethyl)-4-chlorophenyl; C5, R = Ph, Ar = 2,6-dimethylphenyl; C6, R = Ph, Ar =
2,6-diisopropylphenyl)) were synthesized and characterized. The structures
of palladium complexes C1 and C2 were determined
by X-ray diffraction. These bidentate N,O-palladium complexes were
applied for direct arylation under aerobic conditions. The effects
of the reaction conditions and ligand substitution on the catalytic
activity were evaluated. Upon a low palladium loading of 0.5 mol %,
the bulky palladium complex C6 was successfully used
to catalyze the cross-coupling of a variety of five-membered heteroarenes
and their benzo-condensed derivatives with (hetero)­aryl bromides.
The mechanistic investigation on the direct arylation supported the
involvement of a Pd(0)/Pd­(II) CMD process.