78000-63-2Relevant articles and documents
Thermal defect engineering of precious group metal-organic frameworks: Impact on the catalytic cyclopropanation reaction
Heinz, Werner R.,Junk, Raphael,Agirrezabal-Telleria, Iker,Bueken, Bart,Bunzen, Hana,G?lz, Thorsten,Cokoja, Mirza,De Vos, Dirk,Fischer, Roland A.
, p. 8077 - 8085 (2020/12/31)
We report on the engineering of defects in precious group metal (PGM)-based HKUST-1 (Hong Kong University of Science and Technology) analogues (RhII,II, RuII,II, RuII,III) and the ramification on the catalytic activity by using the cyclopropanation of styrene with ethyl diazoacetate (EDA) as an analytical probe to investigate complex metal-organic framework (MOF) structures. We have characterized the active sites within the extended frameworks by their activity, product distribution and stereoselectivity. The role of the metal, its oxidation state and the availability of open metal sites is elucidated. With a set of 17 samples including reference to Cu-HKUST-1, metal nanoparticles and existing literature, conclusions on the tuneability of paddlewheel complexes within self-supported porous and crystalline frameworks are presented. In particular, additional axial ligands (OAc-/Cl-) accounting for charge compensation at the mixed-valent RuII,III nodes seem responsible for side-product formation during catalysis. Thermal defect-engineering allows for controlled and preferential removal of those axial ligands accompanied by reduction of the average metal oxidation state. This enhances the number of open metal sites (OMS) and the catalytic activity as well as improving the chemoselectivity towards cyclopropanes. The preference towards formation of trans-cyclopropane is assigned to the steric crowding of the paddlewheel moiety. This diastereoselectivity gradually diminishes with rising defectiveness of the PGM-HKUST-1 analogues featuring modified paddlewheel nodes.
"Syn-effect" in the conversion of (e)-α,β-unsaturated esters into the corresponding β,γ-unsaturated esters and aldehydes into silyl enol ethers
Guha, Samar Kumar,Shibayama, Atsushi,Abe, Daisuke,Sakaguchi, Maki,Ukaji, Yutaka,Inomata, Katsuhiko
, p. 2147 - 2157 (2007/10/03)
The stereochemistry in the conversion of (E)-α,β-unsaturated esters into the corresponding β,γ-unsaturated esters, and that in the conversion of aldehydes into the silyl enol ethers, were investigated. The Z/E ratios of the resulting β,γ-unsaturated ester
Carbon-carbon bond forming reactions of rhenium enolates with terminal alkynes. Evidence for an alkyne C-H oxidative addition mechanism and observation of highly stereoselective base-catalyzed proton transfer reactions in rhenium metallacycles
Stack, Jeffrey G.,Simpson, Robert D.,Hollander, Frederick J.,Bergman, Robert G.,Heathcock, Clayton H.
, p. 2716 - 2729 (2007/10/02)
The acetonitrile-substituted complexes fac-(MeCN)(OC)3(PPh3)ReCH(R2)CO(R1) (4, R1 = OEt R2 = H; 5, R1 = OEt R2 = Me) and the chelating amide complex 6 react with terminal alkynes (R3 C≡CH) to afford five-membered metallacycles 7-9 The metallacycles possess an exocyclic double bond which exists in the less thermodynamically stable Z stereochemistry, in which the alkyl group R3 is oriented proximate to the metal center. These complexes rearrange in the presence of a Lewis base to form the endocyclic isomers 10-12. The structure of the metallacycle 10g was determined by a single-crystal X-ray diffraction study. Labeling studies demonstrated that the 1,3-hydrogen shift involved in this rearrangement is intramolecular and stereospecific with respect to the rhenium center, even though it is mediated by an external reagent. Deuterium incorporation into the 7-position of the endo metallacycles can also be induced by base and once again occurs stereospecifically (anti to the phosphine ligand). Treatment of the metallacycles with acid results in removal of the metal and yields the unsaturated esters as a mixture of stereo- and regioisomers. Kinetic studies demonstrated that the reaction of 4 with terminal alkynes involves reversible dissociation of the coordinated acetonitrile to form an unsaturated intermediate 29 (R1 = CO2Et) This intermediate reacts faster with more electron-rich alkynes (HC≡CCMe3) than electron-deficient alkynes (HC≡CCO2Me) and displays a R3C≡CH/R3C≡CD kinetic isotope effect kH/kD of 2.0. On the basis of these results and information about reactions of related rhenium complexes, we suggest that formation of the metallacycles is best accommodated by the mechanism shown in Scheme VIII: insertion of 29 into the alkyne C-H bond leading to a 7-coordmate rhenium acetylide hydride 31, 1,3-migration of the hydride to the acetylide β-carbon, providing vinylidene complex 32, and then migration of the enolate ligand to the a-carbon of the vinylidene group, giving the exo metallacycle.
