Orthometallated Pd(ii) C^N^S pincer complex catalyzed sustainable synthesis of bis(indolyl)methanes via acceptorless dehydrogenative coupling of alcohols

Abstract

Herein, we report the catalytic application of new orthometallated palladium(II) C^N^S pincer complexes (Pd1–Pd3) for the sustainable and cost-effective synthesis of pharmacologically important bis(indolyl)methanes (BIMs) using economically affordable and eco-friendly alcohols as the primary feedstock. Facile synthesis of the orthometallated complexes has been accomplished using [PdCl₂(PPh₃)₂] with thiourea ligand derivatives (L1–L3). FT-IR, NMR and HR-MS analyses have been employed to establish the formation of the new complexes. Single crystal XRD analysis for the representative complex Pd1 unveiled its three dimensional structure with tridentate coordination of the ligands assembling into a square planar geometry. Further, a variety of substituted BIMs (23 examples) were fabricated using new Pd(II) complexes via acceptorless dehydrogenative coupling of alcohols with indole. Noteworthily, only 0.5 mol% of Pd catalyst is well tolerant to a wide range of substrates yielding the desired bis(indolyl)methanes in excellent yields up to 95% in the absence of any oxidants or promoters. Further, to highlight the utility of the current protocol, a short synthesis of biologically important compounds such as turbomycin B, orphan nuclear receptors, and anti-inflammatory and anti-leukemic agents has been reported. HRMS and NMR studies confirm that the alkylation of indole proceeds through the formation of Pd-alkoxide and aldehyde intermediates, respectively.