Catalysts frameworks

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

Our calculations suggest that the stereoselectivity of the hydrosilylation is determined by the thermodynamic stability of the ri3-allylic complex that forms after styrene insertion. This opens up the possibility of improving the enantioselectivity by modifying the catalyst framework to alter the stability of the exo versus the endo T 3-allylic intermediate. 

Another advantage of the catalyst 29 is that it enables rational fine-tuning for substrate-specific optimization simply by changing the substitution pattern at the catalyst framework [33, 34]. A current drawback for large scale applications might be access to these impressive catalysts, which are prepared starting from binaph-thol in a six-step synthesis. 

As indicated from computational studies, the catalyst-activated iminium ion MM3-2 was expected to form with only the (E)-conformation to avoid nonbonding interactions between the substrate double bond and the gem-dimethyl substituents on the catalyst framework. In addition, the benzyl group of the imidazolidinone moiety should effectively shield the iminium-ion Si-face, leaving the Re-face exposed for enantioselective bond formation. The efficiency of chiral amine 1 in iminium catalysis was demonstrated by its successful application in several transformations such as enantioselective Diels-Alder reactions [6], nitrone additions [12], and Friedel-Crafts alkylations of pyrrole nucleophiles [13]. However, diminished reactivity was observed when indole and furan heteroaromatics where used for similar conjugate additions, causing the MacMillan group to embark upon studies to identify a more reactive and versatile amine catalyst. This led ultimately to the discovery of the second-generation imidazolidinone catalyst 3 (Fig. 3.1, bottom) [14],... 

Although electron-rich aromatics typically undergo 1,2-carbonyl addition, the iminium ions derived from 4/f-imidazol-4-one 456a are inert to the 1,2-pathway due to steric constraints imposed by the catalyst framework. The heteroaromatic nucleophiles react via the less sterically demanding 1,4-addition pathway (Equation 109). With TEA as the cocatalyst, ee values of 89-97% were obtained with a range of substituted pyrroles (R = Me, Bn, allyl). In addition, ee values of 87-93% were obtained with alkyl, aromatic, or electron-withdrawing substituents on the ot, 3-unsaturated aldehydes (R = Me, Pr , Bn, Ph, MeC02). 

As discussed in the literature [10,11], it is likely that the occurrence of the SCR reaction requires the participation of the catalysts framework oxygen, resulting in catalyst reduction, according to the stoichiometry ... 

For the higher activity Ziegler-Natta catalysts (Table II) based on reaction products of specific magnesium, titanium, and aluminum compounds, the similarity in size, coordination preference, electronic structure, and electronegativity of Ti(IV), Mg(II), and Al(III) ions is reflected in structural parameters and chemical properties (38) (Table III). The similarity in size between Mg(II) and Ti(IV) probably permits an easy substitution between ions in a catalyst framework. 

Triflic acid-functionalized Zr-TMS (zirconium oxide with a mesostructured framework TMS, transition metal oxide mesoporous molecular sieves) have been extensively studied by Chidambaram et alP by the use of a variety of spectroscopic methods including DD/MAS NMR. The observed chemical shift of ca. 119 ppm and a Jcf coupling of ca 310 Hz showed that the triflic acid remained intact on the catalyst framework. 

In 2000, MacMillan and his co-workers presented the first enantioselective organocatalytic 1,3-dipolar cycloaddition of nitrones 187 and a,p-unsaturated aldehydes 28 (dipolarphiles) to afford the e do-(45)-isoxazolidine adducts 188, Scheme 3.59 [75]. With the LUMO-lowering activation of a,p-unsaturated aldehydes 28 and enforced formation of (fi)-iminium isomer, the HClO -salt of catalyst 30 effectively promote cycloaddition of the dipolarphile. In addition, e do-cycloaddition effectively alleviated nonbonding interaction between the nitrone phenyl group and the neopentyl methyl substituent on the catalyst framework. Later, in 2002, Karlsson and Hbgberg reported the organocatalytic enantioselective 1,3-dipolar cycloaddition of... 

The flexibility of the catalyst framework helps to accommodate the different steric requirements of reactant, transition and product states. The steric match of the pretransition state and the transition state should not be so tight that it prevents the entropic movement of substrate. The product that forms must be unfavorably bound so that it will desorb from the active site once it forms. 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

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