Well-Defined Palladium N‑Heterocyclic Carbene Complexes: Direct C–H Bond Arylation of Heteroarenes

A series of palladium N-heterocyclic
carbene (NHC) complexes of
type trans-{(NHC)­PdCl2L} (L = C5H5N, 3-ClC5H4N, and PPh3) (3–5) have been developed as efficient
precatalysts for direct C–H bond arylation of various heteroarenes.
In particular, an in situ generated new NHC ligand derived from {1,3-di-(2,6-diethylphenyl)­acenaphtho­[1,2-d] imidazolium} chloride (2) is used for the
stabilization of the palladium metal center. Among the screened palladium
precatalysts (3–5), the most active
PEPPSI themed complex (3) was successfully employed
toward direct C–H bond arylation of various heteroarenes and
aryl bromides. A range of functional groups on aryl bromides as well
as on heteroarenes sustained throughout the standard reaction conditions
for easy access of various arylated heterocyclic compounds. Significantly,
the utility of the protocol was demonstrated by the effective synthesis
of a precursor of raloxifene, a selective estrogen receptor modulator.