Search results for "Catalytic cycle"
showing 10 items of 31 documents
Can Green Dimethyl Carbonate Synthesis be More Effective? A Catalyst Recycling Study Benefiting from Experimental Kinetics and DFT Modeling
2014
Dibutyldimethoxystannanes are known to catalyze the reaction between carbon dioxide and methanol leading to dimethyl carbonate. Despite similarities between din-butyl- and ditert-butyldimethoxystannane, the recycled complexes have different structural features. In the din-butyl series, a decatin(IV) complex has been characterized and is less active than the stannane precursor. Kinetic experiments likely indicate that all the tin centers are not active, which is confirmed in comparing with the related dinuclear 1,3-dimethoxytetran-butyldistannoxane complex. In the ditert-butyl series, the tritin(IV) complex isolated upon recycling features the steric effect of bulky tBu ancillary ligands. In…
ChemInform Abstract: Bioinspired Functional Analogs of the Active Site of Molybdenum Enzymes: Intermediates and Mechanisms
2015
Abstract Molybdenum(VI) complexes of the general type MoVI(L)2(E)O (L = 2-(aryl N CH)-pyrrolylato, E = O, NtBu) and their relevance as bioinspired functional analogs for molybdenum-containing oxotransferases are reviewed. All complexes are capable of transfering oxygen atoms to PR3 (forward oxygen atom transfer) giving OPR3 and phosphane molybdenum(IV) complexes MoIV(L)2(E)(PR3) (with a second equivalent PR3) via the transient phosphoryl complex MoIV(L)2(E)(OPR3) and the five-coordinate intermediate MoIV(L)2(E). Reactivity of MoIV(L)2(E) and the favored stereochemistry of products from excess PR3 MoIV(L)2(E)(PR3) depend on the steric demand of the chelate ligands (L)−, the π donor ligand E …
Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms
2015
Abstract Molybdenum(VI) complexes of the general type MoVI(L)2(E)O (L = 2-(aryl N CH)-pyrrolylato, E = O, NtBu) and their relevance as bioinspired functional analogs for molybdenum-containing oxotransferases are reviewed. All complexes are capable of transfering oxygen atoms to PR3 (forward oxygen atom transfer) giving OPR3 and phosphane molybdenum(IV) complexes MoIV(L)2(E)(PR3) (with a second equivalent PR3) via the transient phosphoryl complex MoIV(L)2(E)(OPR3) and the five-coordinate intermediate MoIV(L)2(E). Reactivity of MoIV(L)2(E) and the favored stereochemistry of products from excess PR3 MoIV(L)2(E)(PR3) depend on the steric demand of the chelate ligands (L)−, the π donor ligand E …
Stereocontrol in Diphenylprolinol Silyl Ether Catalyzed Michael Additions : Steric Shielding or Curtin-Hammett Scenario?
2017
The enantioselectivity of amine-catalyzed reactions of aldehydes with electrophiles is often explained by simple steric arguments emphasizing the role of the bulky group of the catalyst that prevents the approach of the electrophile from the more hindered side. This standard steric shielding model has recently been challenged by the discovery of stable downstream intermediates, which appear to be involved in the rate-determining step of the catalytic cycle. The alternative model, referred to as Curtin-Hammett scenario of stereocontrol, assumes that the enantioselectivity is related to the stability and reactivity of downstream intermediates. In our present computational study, we examine th…
The catalytic reduction of nitrobenzene at the [MoVIO2(O2CC(S)(C6H5)2)2]2? complex intercalated in a Zn(II)-Al(III) layered double hydroxide host: A …
2001
The heterogeneous reduction of nitrobenzene by thiophenol catalyzed by the dianionic bis(2-sulfanyl-2,2-diphenylethanoxycarbonyl) dioxomolybdate(VI) complex, [MoVIO2(O2CC(S)(C6H5)2)2]2−, intercalated into a Zn(II)–Al(III) layered double hydroxide host [Zn3−xAlx(OH)6]x+, has been investigated under anaerobic conditions. Aniline was found to be the only product formed through a reaction consuming six moles of thiophenol for each mol of aniline produced. The kinetics of the system have been analyzed in detail. In excess of thiophenol, all reactions follow first-order kinetics (ln([PhNO2]/[PhNO2]0) = −kappt) with the apparent rate constant kapp being a complex function of both initial nitrobenz…
High resolution crystal structures of triosephosphate isomerase complexed with its suicide inhibitors: The conformational flexibility of the catalyti…
2011
The key residue of the active site of triosephosphate isomerase (TIM) is the catalytic glutamate, which is proposed to be important (i) as a catalytic base, for initiating the reaction, as well as (ii) for the subsequent proton shuttling steps. The structural properties of this glutamate in the liganded complex have been investigated by studying the high resolution crystal structures of typanosomal TIM, complexed with three suicide inhibitors: (S)-glycidol phosphate ((S)-GOP, at 0.99 A resolution), (R)-glycidol phosphate, ((R)-GOP, at 1.08 A resolution), and bromohydroxyacetone phosphate (BHAP, at 1.97 A resolution). The structures show that in the (S)-GOP active site this catalytic glutama…
Heterogeneously catalyzed Suzuki-Miyaura conversion of broad scope
2012
The reaction tolerates a broad range of functional groups in the coupling partners and is usually performed in solution under homogeneous conditions at T ¢ 60 uC using 2–3 mol% catalytic amounts. The catalyst is often a Pd(0) complex with triarylphosphane ligands. 2 The catalytic cycle (Scheme 2) begins with the oxidative addition of an aryl halide to a Pd(0) species formed in situ to form an arylpalladium(II) halide intermediate. 3 Chloroarenes, especially nonactivated aryl chlorides, are notoriously less reactive due to the stability of the C–Cl bond (the relative reactivity of Ar–X is correlated to the respective bond dissociation energy: Ph–Cl: 96 kcal mol 21 ,P h–Br: 81 kcal mol 21 ,P …
Enantioselective Protonation of the Lithium Transient Enolate of2-Methyltetralone with 2-Sulfinyl Alcohols
2005
A new catalytic cycle for the enantioselective protonation of cyclic ketone enolates with sulfinyl alcohols has been developed. An enol trifluoroacetate that can be easily obtained from the corresponding ketone is used for the first time as an enolate precursor of a cyclic ketone enolate. In this method, the achiral alcohol plays two roles: it is involved, as is usual in catalytic asymmetric protonation reactions, in the turnover of the chiral proton source and also in the generation of a transient enolate through the reaction of its corresponding alkoxide with the enol trifluoroacetate precursor. Stereoselectivity is highly dependent on the structure of the achiral alcohol. High levels of …
Imidazolium-2-carboxylate as an afficient, expeditious and eco-friendly organocatalyst for glycerol carbonate synthesis
2009
An improved and greener approach towards the synthesis of glycerol carbonate, via transesterification, using 1-n-butyl-3-methylimidazolium-2-carboxylate as catalyst is described. The catalyst loading as low as 1% was sufficient to yield quantitative conversions. A plausible mechanism is proposed for the catalytic cycle leading to product formation.
“Release and catch” catalytic systems
2013
In this perspective article the “release and catch” catalytic system concept is discussed. A “release and catch” catalytic system is prepared by non-covalent immobilization of the catalytic moiety on a suitable support, but differently from the usual non-covalently supported catalyst, the catalytic moiety is released in solution over the course of the reaction and it is recaptured at the end of the reaction. Such a “catalyst- sponge like” or “boomerang” system allows one to combine the benefits of homogeneous and heterogeneous catalysis and can be applied to organometallic-based catalysts, organocatalysts and metal-based catalysts.