An Analogue of Grubbs Third-Generation Catalyst with Fluorophilic Pyridine Ligands: Fluorous/Organic Phase-Transfer Activation of Ring-Closing Alkene Metathesis
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2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. The title catalyst (H2IMes)[3,5-NC5H3(CH2CH2Rf8)2]2(Cl)2Ru(=CHPh) [H2IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, Rf8=(CF2)7CF3] was prepared from the fluorous pyridine 3,5-NC5H3(CH2CH2Rf8)2 (2.1 equiv.) and the pyridine complex (H2IMes)(NC5H5)2(Cl)2Ru(=CHPh). 3,5-NC5H3(CH2CH2Rf8)2 was synthesized by a Heck reaction of 3,5-dibromopyridine and the fluorous alkene H2C=CHRf8 [2.4 equiv.; Pd(OAc)2 (cat.), n-Bu4N+ Br-/NaOAc (2.0 equiv.)], followed by hydrogenation. The catalyst shows dramatic rate accelerations in the ring-closing metatheses of ,-dienes under fluorous/organic liquid/liquid biphasic conditions [e.g., perfluoro(methyldecalin)/CD2Cl2] relative to rates under monophasic organic conditions (e.g., CD2Cl2). These catalysts require initial dissociation of the pyridine ligands to generate the active species, which can either combine with an alkene (productive) or recombine with a pyridine (unproductive). In the case of (H2IMes)[3,5-NC5H2(CH2CH2Rf8)2]2(Cl)2Ru(=CHPh), fluorophilic 3,5-NC5H3(CH2CH2Rf8)2 transfers to the fluorous phase, in accord with its CF3C6F11/toluene partition coefficient [93.9:6.1 vs. 39.8:60.2 for (H2IMes)[3,5-NC5H3(CH2CH2Rf8)2]2(Cl)2Ru(=CHPh)], which decreases the fraction of unproductive events.
