Efficient Synthesis of Cu(II)‐N‐Heterocyclic Carbene Complexes in Water and Their Activity Towards Aerobic Alcohol Oxidation

Abstract

The majority of the copper N-heterocyclic carbene (NHC) complexes contain Cu in a + 1 oxidation state. The Cu(II)-NHC complexes reported so far usually require strict inert atmosphere conditions. In this work, we report a very facile and quantitative preparation of a series of new Cu(II)-NHC complexes ( 2a-c) starting from the inexpensive and readily available Cu(II) salts like Cu(OAc) 2 and CuCl 2 • 2H 2 O with imidazolium salts (1 a-c), wherein 2 b and 2 c were exclusively synthesized in water under aerobic condition. It should be emphasized that this is the first example where Cu(II)-NHC complexes have been prepared in the aqueous medium. All the complexes have manifested with a heterobidentate C, O chelate type of coordination to Cu(II) centers forming trans-[Cu(IRpyO) 2 ] (where R = Me, Mes, Dipp) type of complexes where 2 b forms a perfect square planar Cu(II)-NHC complex. Complexes 2 a-c were evaluated as catalysts for the aerobic oxidation of alcohols.

Selective aerobic oxidation of biomass model compound veratryl alcohol catalyzed by air-stable copper(ii) complexes in water

“…Many catalytic oxidation reactions of primary alcohols were performed in hazardous organic solvent (instead of water). 77,78,137 Therefore, it may be concluded that our optimized catalytic protocol is greener, less energy-intensive and more sustainable as compared to many past reports.…”

Three water-soluble copper(ii) N-heterocyclic carbene complexes: toward copper-catalyzed ketone reduction under sustainable conditions

“…20 Copper( ii )-NHC complexes containing N -hydroxypyridine functionalized ligands have been used as catalysts for the aerobic oxidation of a variety of activated alcohols. 27 Copper( ii )-NHC complexes using imidazolium carboxylate precursors shows good reactivity for homogeneous catalysis. 28 Tetradentate copper( ii )-NHC complexes have been used as catalyst for generating C–N bonds.…”