Dipyrrolyl Precursors to Bisalkoxide Molybdenum Olefin Metathesis Catalysts

摘要

Addition of two equivalents of lithium pyrrolide to Mo(NR)(CHCMe2R')(OTf)2(DME) (OTf = OSO2CF3; R = 2,6-i-Pr2C6H3, 1-adamantyl, or 2,6-Br2-4-MeC6H2; R' = Me or Ph) produces Mo(NR)(CHCMe2R')(NC4H4)2 complexes in good yield. All compounds can be recrystallized readily from toluene or mixtures of pentane and ether and are sensitive to air and moisture. An X-ray structure of a 2,6-diisopropylphenylimido species shows it to be an unsymmetric dimer, {Mo(NAr)(syn-CHCMe2Ph)(η⁵-NC4H₄)(η₁-NC₄H₄)}{Mo(NAr)(syn-CHCMe₂Ph)(η¹-NC₄H₄)₂}, in which the nitrogen in the η⁵-pyrrolyl bound to one Mo behaves as a donor to the other Mo. All complexes are fluxional on the NMR time scale at room temperature, with one symmetric species being observed on the NMR time scale at 50 °C in toluene-d₈. The dimers react with PMe₃ (at Mo) or B(C₆F₅)₃ (at a η⁵-NC₄H₄ nitrogen) to give monomeric products in high yield. They also react rapidly with two equivalents of monoalcohols (e.g., Me₃COH or (CF₃)₂MeCOH) or one equivalent of a biphenol or binaphthol to give two equivalents of pyrrole and bisalkoxide or diolate complexes in ~100% yield.