A straightforward route to organonitriles, versatile intermediates in organic synthesis, would be cross metathesis (CM) with acrylonitrile. Unfortunately, acrylonitrile is definitely not the best substrate for metathesis reactions. Electron deficient olefins are generally difficult substrates for metathesis reactions, but α,β-unsaturated esters, aldehydes and ketones can be crossed with terminal olefins in good yields using second-gen Grubbs catalyst. Acrylonitrile is less reactive under these conditions, probably due to the strong ability of the nitrile group to coordinate to the metal and decompose the catalyst.
3Although ruthenium initiators currently available for alkene metathesisare endowed with many beneficial properties, there is still room forimprovement and many research groups are actively pursuing the quest forthe next generation of alkene metathesis catalysts.
Olefin Metathesis, Grubbs Reaction - Organic chemistry
A novel series of Schiff-base-substituted ruthenium carbene complexes have been prepared by the treatment of RuCl2(CHPh)(PCy3)2 with a variety of Schiff-base ligands as the thallium salts. Modification of the Schiff-base electronic and/or structural substituents allowed for the tuning of the complexes activities in olefin metathesis reactions. The structures of the complexes were unambiguously characterized by NMR studies and X-ray analysis. These newly prepared complexes are highly stable to air, moisture, and temperature and even exhibit catalytic activity in polar protic solvents.
Because of these new catalysts, the metathesis reaction has ..
Initial catalyst investigations were conducted with 8-nonenol (9a) and trans-1,4-hexadiene (). Reactions were conducted in open vessels to facilitate efficient ethylene removal. Ruthenium complex 5 provided cross product 12a in good yield and Z-selectivity (entry 1). Catalyst 6, bearing the bulkier 2,6-diisopropylphenyl group, has recently been shown to exhibit superior reactivity and Z-selectivity. Indeed, complex 6 provided diene 12a in similar yields with essentially complete stereoselectivity (entry 2). Production of diene 12a was observed with catalyst loadings as low as 0.1 mol %, though the conversion was diminished (entries 3 and 4). Unchelated dichloride catalyst 1 provided no observable amount of the desired cross product (entry 5), producing primarily oligomeric products of unselective cross metathesis of 1,4-hexadiene. Less reactive catalyst 2, provided some 12a, however the yield and stereoselectivity were low (entry 6). For future investigations, we chose 1 mol % of catalyst 6 in a 0.5 M solution of substrate in 1:1 THF:hexadiene as optimal conditions to achieve sufficient reaction rate with most substrates.
precatalysts for alkene and enyne metathesis for ..
Ahmed, T.S.; Grubbs, R. H. “Fast-Initiating, Ruthenium-based Catalysts for Improved Activity in Highly E-Selective Cross Metathesis” J. Am. Chem. Soc., 2017, 139, 1532-1537. [10.1021/jacs.6b11330]A recent publication from the Grubbs’ group …
Synthesis and application of a new polystyrene …
Chelated ruthenium catalyst 6 is sensitive to the steric environment of the reacting olefin. As a result, monosubstituted terminal olefins are also preferentially reacted over disubstituted alkenes (). When diene 14 was reacted with 8-nonenol (9a), as either the limiting reagent or in excess, metathesis only occured at the less substituted olefin to produce diene 15 as the major product.