Enantioselective synthesis of cyclic enol ethers and all-carbon quaternary stereogenic centers through catalytic asymmetric ring-closing metathesis

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that offer a plausible rationale for the identity of major enantiomers as well as the observed levels of enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%).     

Dipyrrolyl precursors to bisalkoxide molybdenum olefin metathesis catalysts

Addition of 2 equiv 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)(eta5-NC4H4)(eta1-NC4H4)}{Mo(NAr)(syn-CHCMe2Ph)(eta1-NC4H4)2}, in which the nitrogen in the eta5-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 degrees C in toluene-d8. The dimers react with PMe3 (at Mo) or B(C6F5)3 (at a eta5-NC4H4 nitrogen) to give monomeric products in high yield. They also react rapidly with 2 equiv of monoalcohols (e.g., Me3COH or (CF3)2MeCOH) or 1 equiv of a biphenol or binaphthol to give 2 equiv of pyrrole and bisalkoxide or diolate complexes in approximately 100% yield.