An Analogue of Grubbs Third-Generation Catalyst with Fluorophilic Pyridine Ligands: Fluorous/Organic Phase-Transfer Activation of Ring-Closing Alkene Metathesis

The title catalyst (H₂IMes)[3,5-NC₅H₃(CH₂CH₂R_f8)₂]₂(Cl)₂Ru(=CHPh) [H₂IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, R_f8=(CF₂)₇CF₃] was prepared from the fluorous pyridine 3,5-NC₅H₃(CH₂CH₂R_f8)₂ (2.1 equiv.) and the pyridine complex (H₂IMes)(NC₅H₅)₂(Cl)₂Ru(=CHPh). 3,5-NC₅H₃(CH₂CH₂R_f8)₂ was synthesized by a Heck reaction of 3,5-dibromopyridine and the fluorous alkene H₂C=CHR_f8 [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 α,ω-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(CH₂CH₂R_f8)₂]₂(Cl)₂Ru(=CHPh), fluorophilic 3,5-NC₅H₃(CH₂CH₂R_f8)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(CH₂CH₂R_f8)₂]₂(Cl)₂Ru(=CHPh)], which decreases the fraction of unproductive events.