Retained: An N-heterocyclic carbene with eight cyclohexyl groups (see figure) provides increased electron density for a highly active olefin metathesis catalyst as well as sufficient steric bulk to allow the efficient separation of such a complex from the organic products in the solvent-resistant nanofiltration.
A mass-tagged N-mesityl imidazolinium salt with four additional -CH2NCy2 substituents was synthesized, leading to a molecular mass of nearly 1100 g mol−1 in the corresponding carbene ligand. This mass-tagged ligand was used to generate the respective Grubbs II and Grubbs–Hoveyda type complexes. The catalytic activity of the latter complex was tested in several olefin metathesis reactions and found to be slightly superior to that of the related N-mesityl based complex. In batchwise solvent resistant nanofiltration experiments the ruthenium complex dissolved in toluene and following a metathesis reactions was efficiently retained (>99.8 %) by a single nanofiltration; the permeate contained less than 4 ppm of Ru. Equally efficient catalyst retention was observed in a membrane reactor utilized for the continuous synthesis of a RCM product.
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