Increased Efficiency in Cross-Metathesis Reactions of Sterically Hindered Olefins

Abstract

GENERAL INFORMATIONNMR spectra were recorded on an Oxford 300 MHz NMR spectrometer running Varian VNMR software. Chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane (TMS) with reference to internal solvent for 1 H NMR and 13 C NMR spectra. Chemical shifts are reported in parts per million (ppm) downfield from H 3 PO 4 for 31 P NMR spectra. Multiplicities are abbreviated as follows: singlet (s),, and broad (br). Analytical thin-layer chromatography (TLC) was performed using silica gel 60 F254 precoated plates (0.25 mm thickness) with a fluorescent indicator. Visualization was performed with standard potassium permanganate stains or UV light. Flash column chromatography of organic compounds was performed using silica gel 60 (230-400 mesh). All glassware was either oven dried or flame dried, and reactions were done under an atmosphere of argon unless otherwise noted. All organic solvents were dried by passage through solvent purification columns containing activated alumina. All commercial chemicals were used as obtained. GENERAL CROSS METATHESIS PROCEDUREA 10 mL 2-neck round bottom flask was charged with a stir bar and outfitted with a reflux condenser and a septum. The apparatus was flame-dried under active vacuum, and then allowed to cool to ambient temperature under a positive pressure of argon. The flask was then charged the catalyst (0.025 mmol) as a solid. Dry, degassed CH 2 Cl 2 (2.0 mL) was then added via syringe. The substrates (0.5 mmol of limiting olefin reagent)were then added simultaneously via syringe to the stirring solution. The septum was replaced with a glass stopper, and the solution was heated to reflux for the indicated amount of time. After cooling to ambient temperature, ethyl vinyl ether (0.5 mL) was

Copper‐Catalyzed Asymmetric Allylic Alkylation of Halocrotonates: Efficient Synthesis of Versatile Chiral Multifunctional Building Blocks

“…General Procedure for the Cross Metathesis (CM) of b,g-Unsaturated Esters [53] In a dried Schlenk tube equipped with septum and stirring bar under a N 2 atmosphere the corresponding catalyst (5.0 mol%) and dichlorobenzoquinone (10 mol%) were dissolved in anhydrous CH 2 Cl 2 (4.0 mL mmol À1 substrate). After stirring for 2 min a mixture of both the substrate (1.0 equiv.)…”

Relevance of cis- and trans-dichloride Ru intermediates in Grubbs-II olefin metathesis catalysis (H2IMesCl2RuCHR)

“…12 This has led to several reports considering the side-bound pathway as highly significant, if not the most likely operative pathway. 3,6 We consider that this lack of a clear understanding of the catalytic pathway (side vs. bottom) is hindering the development of selective metathesis catalysts. To provide a basis for assessing the side-and bottom-bound metathesis pathways, we applied DFT methods to investigate the relative energies and the expected E : Z olefin product ratio of the cis-and trans-dichloro Ru pathways for the metathesis of E-and Z-2-butene with the Grubbs-II benzylidene catalyst (Fig.…”

Improved Chiral Olefin Metathesis Catalysts: Increasing the Thermal and Solution Stability via Modification of a C1‐Symmetrical N‐Heterocyclic Carbene Ligand

“…It was initially expected that catalysts 16-Bn and 17-Bn might decompose through insertion into the benzylidene unit as previously reported by Blechert [13] and Grubbs. [14] However, 16-Bn and 17-Bn are stable over the same period of 4-5 months. Thus, even a relatively small alkyl group can increase the steric bulk of the NHC ligand to such a point as to increase the catalysts thermal and solution stability.…”

“…This observation is in agreement with those of Grubbs and co-workers, who have observed large increases in reactivity in Ru-based catalysts bearing NHC ligands that are less sterically demanding. [14,20] The new catalysts have been evaluated in desymmetrization reactions of meso-trienes and have shown surprising results. The change of the Nalkyl group of the NHC ligands provided lower enantioselectivities in almost all cyclization reactions.…”