Pyridine-Stabilized Fast-Initiating Ruthenium Monothiolate Catalysts for Z-Selective Olefin Metathesis

Bulky Cyclometalated Ruthenium Nitrates for Challenging Z-Selective Metathesis: Efficient One-Step Access to α-Oxygenated Z-Olefins from Acrylates and Allyl Alcohols

α-Oxygenated Z-olefins are ubiquitous in biologically active molecules and serve as versatile handles for organic synthesis, but their syntheses are often tedious and less selective. Here we report the efficient Z-selective metathesis of various terminal acrylates and allyl alcohols, which enables facile and selective construction of high value-added α-oxygenated Z-olefins from readily available feedstock chemicals. These challenging metathesis transformations are enabled by novel cyclometalated Ru-carbene-nitrate complexes bearing bulky-yet-flexible side arms, whose assembly was unlocked by new organometallic syntheses.

Toward E-selective Olefin Metathesis: Computational Design and Experimental Realization of Ruthenium Thio-Indolate Catalysts

The selective transformation of 1-alkenes into E-olefins is a long-standing challenge in olefin metathesis. Density functional theory (DFT) calculations predict high E-selectivity for catalysts incorporating a bidentate, dianionic thio-indolate ligand within a RuXX’(NHC)(py)(= CHR) platform (NHC = N-heterocyclic carbene; py = pyridine). Such complexes are predicted to yield E-olefins by favoring anti-disposed substituents in the transition state expected to be rate-determining: specifically, that for cycloreversion of the metallacyclobutane intermediate. Three pyridine-stabilized catalysts Ru21a-c were synthesized, in which the thio-indolate ligand bears a H, Me, or Ph substituent at the C2 position, and the NHC ligand is the unsaturated imidazoline-2-ylidene Me2IMes (which bears N-mesityl groups and methyl groups on the C4,5 backbone). Single-crystal X-ray diffraction analysis of Ru21c confirms the ligand orientation required for E-selective metathesis, with the thio-indolate sulfur atom binding cis to the NHC, and the indolate nitrogen atom trans to the NHC. However, whereas the new complexes mediated metathetic exchange of their 2-thienylmethylidene ligand in the presence of the common metathesis substrates styrene and allylbenzene, no corresponding self-metathesis products were obtained. Only small amounts of 2-butene (73% (Z)-2-butene) were obtained in self-metathesis of propene using Ru21a. Detailed DFT analysis of this process revealed that product release is surprisingly slow, limiting the reaction rate and explaining the low metathesis activity. With the barrier to dissociation of (Z)-2-butene being lower than that of (E)-2-butene, the calculations also account for the observed Z-selectivity of Ru21a. These findings provide guidelines for catalyst redesign in pursuit of the ambitious goal of E-selective 1-alkene metathesis.

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Enantioselective anti- and syn-(Borylmethyl)allylation of Aldehydes via Brønsted Acid Catalysis

The enantioselective anti- and syn-(borylmethyl)allylation of aldehydes via phosphoric acid catalysis is reported. Both (E)- and (Z)-γ-borylmethyl allylboronate reagents were prepared via the Cu-catalyzed highly stereoselective protoboration of 1,3-dienylboronate. Chiral phosphoric acid-catalyzed aldehyde allylation with either the (E)- or (Z)-allylboron reagent provided 1,2-anti- or 1,2-syn-adducts in good yields with high enantioselectivities. The application to the synthesis of morinol D was accomplished.

Supported Ru olefin metathesis catalysts via a thiolate tether

Ruthenium alkylidene complexes can be successfully immobilized on hybrid mesostructured silica via thiolate tethers to give heterogeneous, thiolate-coordinated olefin metathesis catalysts.