The regio- and stereoselective addition of C(1)-ammonium enolates – generated in situ from aryl esters and the isothiourea catalyst (R)-BTM – to pyridinium salts bearing an electron withdrawing substituent in...
A thermomorphic ionic-liquid-based
microemulsion system was successfully
applied for the Ru-catalyzed asymmetric transfer hydrogenation of
ketones. On the basis of the temperature-dependent multiphase behavior
of the targeted microemulsion, simple product separation as well as
catalyst recycling could be realized. The use of water-soluble ligands
improved the immobilization of the catalyst in the microemulsion phase
and significantly decreased the catalyst leaching into the organic
layer upon extraction of the product. Eventually, the optimized microemulsion
system could be applied to a wide range of aromatic ketones that were
reduced with good isolated yields (up to 98%) and enantioselectivities
(up to 97%), while aliphatic ketones were less successful.
α,β-Unsaturated acyl ammonium species are versatile intermediates that have been applied in a variety of transformations including Michael additions, domino reactions and cycloadditions.
We
report the design and synthesis of novel ion-tagged chiral ligands
for asymmetric transfer hydrogenation (ATH) in aqueous medium. Based
on (R,R)-1,2-diphenylethylene diamine
(DPEN) as structural motif, a straightforward three-step protocol
was developed that gave access to novel chiral ligands with carbamate-substructure
and pyridinium headgroup. The careful optimization of steric and electronic
properties in combination with the adaption of solubility via choice
of the anion gave a set of chiral and water-soluble ligands for use
in ruthenium-catalyzed asymmetric transfer hydrogenations in aqueous
medium. Eventually, a pool of aliphatic and aromatic ketones as well
as two imine substrates were reduced with excellent isolated yields
up to 95% and enantioselectivities >90% ee under environmentally
benign
conditions in the absence of additional surfactants.
A protocol for the enantioselective synthesis of substituted vinylcyclopentanes has been realised using cooperative palladium and isothiourea catalysis. Treatment of vinylcyclopropanes with Pd(PPh 3 ) 4 generates a zwitterionic π-allyl palladium intermediate that intercepts a catalytically generated α,β-unsaturated acyl ammonium species prepared from the corresponding α,β-unsaturated para-nitrophenyl ester and the isothiourea (R)-BTM. Intermolecular formal (3+2) cycloaddition between these reactive intermediates generates functionalised cyclopentanes in generally good yields and excellent diastereo-and enantiocontrol (up to > 95 : 5 dr, 97 : 3 er), with the use of LiCl as an additive proving essential for optimal stereocontrol. To the best of our knowledge a dual transition metal/organocatalytic process involving α,β-unsaturated acyl ammonium intermediates has not been demonstrated previously.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.