Phosphine-Based Z-Selective Ruthenium Olefin Metathesis Catalysts

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• 作者：Wietse Smit ; Vitali Koudriavtsev ; Giovanni Occhipinti ; Karl W. Törnroos ; Vidar R. Jensen
• 刊名：Organometallics
• 出版年：2016
• 出版时间：June 13, 2016
• 年：2016
• 卷：35
• 期：11
• 页码：1825-1837
• 全文大小：592K
• 年卷期：0
• ISSN：1520-6041

文摘

Whereas a number of highly Z-selective ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene ligands have been reported in recent years, Z-selectivity has so far been difficult to achieve for phosphine-based catalysts. Guided by predictive density functional theory (DFT) calculations, we have developed phosphine-based ruthenium olefin metathesis catalysts giving 70–95% of the Z-isomer product in homocoupling of terminal alkenes such as allylbenzene, 1-octene, allyl acetate, and 2-allyloxyethanol. Starting from a moderately selective catalyst, [P(Cy)₃](-S-2,4,6-Ph-C₆H₂)ClRu(═CH-o-OⁱPrC₆H₄) (4, Cy = cyclohexyl, ⁱPr = isopropyl), obtained by substituting a chloride of the Hoveyda–Grubbs first-generation catalyst with 2,4,6-triphenylbenzenethiolate, we moved on to replace Cl and PCy₃ by chelating, anionic phosphine ligands. Such ligands increase selectivity by limiting rotation around the P–Ru bond and by specifically directing the steric bulk of the phosphine substituents toward the selectivity-inducing thiolate ligand. In particular, DFT calculations predicted that o-(dialkylphosphino)phenolate ligands should improve selectivity and activity compared to 4. The most promising of these compounds (8b), based on the o-(di-tert-butylphosphino)phenolate ligand, directs the two P-bonded tert-butyl substituents toward the 2,4,6-triphenylbenzenethiolate and has little steric hindrance trans to the thiolate. This compound metathesizes terminal olefins such as allylbenzene and 1-octene with Z-selectivities above 80% and allylacetate above 90%. Although these phosphine-based ruthenium monothiolate catalysts in general achieve somewhat lower activities and Z-selectivities than their second-generation counterparts, they also offer examples giving less substrate and product isomerization and thus higher yields.