Cobalt–NHC promoted selective functionalization of alkynes via auxiliary-ligand modulation

Abstract

Herein, we described the synthesis and characterization of various pyridine appended ImNHC supported chelating Co-complexes (Co1–Co5) of compositions [(ImNHC)Co(acac)₂]⁺/[(ImNHC)Co(Cp*)I]⁺ with differing auxiliary ligands (acac, acetylacetonate or Cp*, pentamethyl cyclopentadienyl). These complexes were next applied as catalysts in alkyne functionalization and interestingly, this reaction can be directed either towards cyclotrimerization or hydroboration, by aptly selecting the auxiliary ligands (acac or Cp*) under solvent and base-free conditions. The presence of a rigid Cp* auxiliary ligand in Co4–Co5 facilitates the hydroboration reaction with β-selective trans-product formation, while the use of a labile acac ligand in Co1–Co3, that offers a flexible coordination environment around the Co-center, promotes cyclotrimerization under ambient conditions providing primarily the 1,2,4-substituted benzene derivatives (with selectivity of ∼90%) in good yields of 70–80% within 12 h. The present protocols are also compatible with various alkynes offering excellent functional group tolerance. Furthermore, detailed mechanistic probes via active-intermediate-capture, steric mapping, and various control experiments including deuterium labelling helped us to understand the underlying reaction mechanism leading to selective transformation.