(560iw) Distinguishing Homogeneous and Heterogeneous Catalytic Pathway in Cuprous Oxide Nanoparticle-Catalyzed C-C Coupling Reactions

Carbon-carbon (C-C) coupling reactions are a diverse set of reactions that are used to produce a wide range of compounds including pharmaceuticals, aromatic-polymers, high performance materials and agrochemicals. The main drawback that limits the reactions though is the wide use of homogenous palladium catalyst. Homogenous palladium (Pd) catalyzed processes require numerous downstream operations to separate catalyst from the final product. Recently, metal and metal-oxide nanoparticle-based heterogeneous catalysts have emerged as attractive catalysts for these reactions. However, identification of the catalytic pathway remains a challenge in the field.

In this contribution, we show that by choosing appropriate reaction conditions, the reaction pathway can be controlled to occur via a truly homogeneous or heterogeneous pathway in Cu₂O-nanoparticle-catalyzed C-C coupling reactions. We utilized a combination of reactor study, flame atomic absorption spectroscopy, UV-Vis extinction spectroscopy, electrospray ionization mass spectrometry and transmission electron microscopy to characterize the role of homogenous Cu catalytic species under different reaction conditions. We also show that in-situ formed homogenous Cu complex can successfully catalyze (i) oxidative C-C homo-coupling of terminal alkynes, and (ii) C-C cross-coupling reactions between aryl halides and terminal alkyne. The molecular reaction mechanisms are proposed for both hetero and homo coupling activity and molecular structures for the intermediates are proposed. The mechanistic understanding from this work has the potential for the development of (a) an inexpensive copper based catalyst as a suitable replacement for expensive Pd based catalysts, and (b) a heterogeneous catalyst capable of exhibiting long term stability to develop energy efficient and environmentally friendly flow processes.