Increased catalytic productivity for nanofiltration-coupled Heck reactions using highly stable catalyst systems

Abstract

The recycle of homogeneous Heck catalysts from post-reaction mixtures using solvent resistant nanofiltration (SRNF) membranes is known to increase catalytic productivity. However, the technique is sensitive to catalyst stability when applied to conventional catalysts: catalyst deactivation will cause declining reaction rates and higher reactor occupancy with an increasing number of catalyst recycles. In this study, a conventional Heck catalyst {bis(acetato)bis(triphenylphosphine)palladium(II)}, was recycled six times, before the reaction rate dropped below 20% of the original value. A cumulative turnover number (TON) of 690 in 120 h was obtained, whilst providing a product substantially lowered in organometallic impurities. Significant improvements in system performance were realised by employing, at identical Pd loading, state-of-the-art catalysts with greater chemical stability. An imidazolylidene catalyst, bis(1,3-dibenzylimidazoline-2-ylidene)diiodopalladium(II), yielded an equal TON for six recycles in 40 h with substantially less reaction rate decline. Palladium(II) acetate stabilised by the quaternary phosphonium salt (quat) tetraphenylphosphonium bromide even gave six recycles in under 30 h after careful solvent selection. In all cases, the membrane showed good selectivity against the catalyst (up to 96% Pd rejection), while allowing the reaction product to permeate completely. Through the sustainable high reaction rates, reactor occupancy can be minimised, and waste generated by downstream processing reduced. Indeed, the imidazolylidene and the quat-stabilised catalyst systems surpassed 10 catalyst recycles with TONs >1000 within 60 and 45 h, respectively.