Catalyst shuttling enabled by a thermoresponsive polymeric ligand: facilitating efficient cross-couplings with continuously recyclable ppm levels of palladium.

A polymeric monophosphine ligand WePhos has been synthesized and complexed with palladium(ii) acetate [Pd(OAc)2] to generate a thermoresponsive pre-catalyst that can shuttle between water and organic phases, with the change being regulated by temperature. The structure of the polymeric ligand was confirmed with matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry and size-exclusion chromatography (SEC) analysis, as well as nuclear magnetic resonance (NMR) measurements. This polymeric metal complex enables highly efficient Pd-catalyzed cross-couplings and tandem reactions using 50 to 500 ppm palladium, and this can facilitate reactions that are tolerant to a broad spectrum of (hetero)aryl substrates and functional groups, as demonstrated with 73 examples with up to 99% isolated yields. Notably, 97% Pd remained in the aqueous phase after 10 runs of catalyst recycling experiments, as determined via inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements, indicating highly efficient catalyst transfer. Furthermore, a continuous catalyst recycling approach has been successfully developed based on flow chemistry in combination with the catalyst shuttling behavior, allowing Suzuki-Miyaura couplings to be conducted at gram-scales with as little as 10 ppm Pd loading. Given the significance of transition-metal catalyzed cross-coupling and increasing interest in sustainable chemistry, this work is an important step towards the development of a responsive catalyst, in addition to having high activity, by tuning the structures of the ligands using polymer science.