Triphenylphosphine-grafted, RAFT-synthesised, porous monoliths as catalysts for Michael addition in flow synthesis

We describe the preparation and application of triphenylphosphine functional polystyrene-based porous monolithic catalysts for use as flow reactors. RAFT (reversible addition-fragmentation chain transfer)-crosslinking polymerisation of styrene and divinylbenzene provided monoliths which were then functionalized by RAFT "grafting from" polymerisation of 4-styryldiphenylphosphine mediated by the retained thiocarbonylthio functionality. Under the chosen conditions, the retention of the RAFT functionality was proved by chain extension experiments in which the monoliths were five times re-subjected to the grafting conditions. Importantly, the bound triphenylphosphine-functionality was demonstrated to be catalytically active when the monoliths were used as flow reactors in performing Michael addition of 1-hexanethiol to tert-butyl acrylate and of acetyl acetone to diethyl azodicarboxylate. Conversions from reagent to product of up to 77% were achieved. The monoliths were susceptible to oxidation of the phosphine groups most likely caused by adventitious oxygen in the non-degassed reaction medium, but were successfully regenerated by trichlorosilane reduction and reused.