Lewis acid centers

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

It was an interesting idea to create the giant tube-like structures 46 capped with a Lewis acid center by the homologation reaction of 42 with dimethylsulfoxonium methylide and their deboronation to the three-armed star polymethylene polymers 47 incorporating a m, r-l,3,5-trisubstituted cyclohexane core (Scheme 12) <2003JA12179, 20010L3063>. 

The LLB catalyst system needs a rather long reaction time and the presence of excess ketone to get a reasonable yield. Yamada and Shibasaki63 found that another complex, BaBM (91), was a far superior catalyst. Complex 91 also contains a Lewis acidic center to activate and control the orientation of the aldehyde, but it has stronger Bronsted basic properties than LLB. The preparation of BaBM is shown in Scheme 3-35. 

With the Ti4+ ions acting as Lewis acid centers, a strong interaction with ammonia and water with these centers is expected. There is in fact abundant spectroscopic evidence for the coordination of NH3 and H20 molecules to tetrahedral Ti4+ centers and for the corresponding expansion of their coordination spheres. 

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

In 1997 Howarth [180] reported the preparation of ionic liquids 65 and 66. They reported that imidazolium cations can be used as Lewis acid centers in catalytic amount rather than as solvent (Scheme 66). The bromide salts 65a and 66 were prepared by a literature procedure [181] from TMS protected imidazole 64 and ethyl bromide or (5)-l-bromo-2-methylbutane in refluxing toluene in 46 and 21% yield, respectively. Salt 65a was converted into salt 65b with AgCF3COO in 89% yield. 

Ichikawa ( ) also observed the lowering of the CO adsorption frequency on Rh-Mn/SiO catalysts and ascribed it to this type of interaction. Recently, Blyholder et al. (40), by theoretical calculations, showed that such interaction of the oxygen atom with Lewis acid centers would have a stabilizing effect on the formyl species. This stabilization could cause the high selectivity to oxygenates by a reaction pathway like the following ( ). ... 

Prohibit degrative attack to Lewis acidic center... 

In most cases the receptors for anions are positively charged ions. However, they can also be neutral molecules which bind ions exclusively by hydrogen bonding, ions-dipole interaction or coordinate anions as Lewis acid centers ofan organometallic ligand. 

In analogy to the carboxylate binding by zinc-containing cyclodextrin 10 (see Sect. 2), Lewis acidic centers such as a copper(II) histamine unit may also serve for the chelation of the (deprotonated) 2-aminoacetate substructure of a-amino acids [51], Rizzarelli, Marchelli et al. used a respective j8-cyclodextrin derivative for the formation of the ternary complexes 36 and 37 with racemic... 

Impellizzeri G, Maccarrone G, Rizzarelli E, Vecchio G, Corradini R, Marchelli R (1991) Angew Chem Int Ed Engl 30 1348. The capacity of the aminoacetate subunit of a-amino acids to chelate Lewis-acidic centers has also been used in U-tube experiments with lipophilic Cu(II) complexes (a) and phenylboronic acid (b), respectively, as transport mediators a) Scrimin P, Tonellato U, Zanta N (1988) Tetrahedron Lett 29 4967 b) Mohler LK, Czarnik AW (1993) J Am Chem Soc 115 7037... 

The reaction of water sensitive compounds with alumina surfaces is complicated not only by the multiplicity of the surface hydroxyl groups but also by the Lewis acid centers which can give a strong physical adsorption of the compound without chemical reaction. 

There is evidence of a promoting action of chromium on nickel catalysts for the reaction of hydrogenation of valeronitrile in our conditions. Introduction of chromium increased the initial specific activity and the selectivity. The promoting effect of chromium on activity could be correlated to the increase of the metallic surface. Another explanation could be that the Cr+ segregated at the surface of the catalyst may play the role of a Lewis acid center and may be responsible for a better chemisorption of valeronitrile on the catalysts, through nitrogen lone pair electrons or the n orbital of the CN bond. However, further examination of the results obtained (see Fig. 3)... 

In addition to interacting with the Lewis acid center of the C02 molecule, these same low-valent metal complexes may also interact with the carbon-oxygen 7t-bonds in C02, in much the same way as olefins interact with electron-rich complexes. Finally, the oxygen atoms in C02 may be expected to show weak electron donating ability, possibly coordinating to a very electron-poor metal, although this mode of coordination of C02 is not presently known. 

Since the metal-alkene association preceding the peroxymetalation reaction in mechanism (B) is a pure Lewis acid/Lewis base interaction, it would be expected that compounds having alkylperoxy groups bonded to a Lewis acid center should be active for the epoxidation of alkenes. This is indeed found for boron compounds, which are active as catalysts for the epoxidation of alkenes by alkyl hydroperoxides.246,247 Isolated boron tris(alkyl peroxides), B(OOR)3, have been shown to epoxidize alkenes stoichiometrically, presumably through alkylperoxyboration of the double bond (equation 76).248... 

Lewis acid centers, which were thought to be the primary catalytic sites. Boreskova et al. (51) studied the poisoning effect of quinoline on the cracking of cumene over Na, H—Y zeolite and observed a linear decrease in activity with the amount of quinoline added until a constant level of activity was reached. The catalytic activity was attributed to trivalent aluminum centers (Lewis acids), which were poisoned by coordinately bound quinoline. In a similar study of cumene cracking, Turkevich et al. (50) also concluded on the basis of magnetic resonance experiments that Lewis centers were the active sites. 

Infrared has also been used to assess the type of acidity present on a catalyst. The method involves measuring the spectrum of adsorbed pyridine on the catalyst certain characteristic absorption frequencies are assigned to Lewis acid centers (coordinately bound pyridine) and others to Brqnsted sites (pyridine adsorbed as pyridinium ion). 

The silicon atom in Wiberg s silenes is a Lewis acidic center and can be coordinated with donors. These donors may be halide ions [like, e.g., in 104 LiX(T2-C-4)831, ethers or nitrogen bases84. Coordination with the donor stabilizes the silenes and, when the basicity suffices (LbOcBr, THF, NMe3, F ), the adducts serve as stock compounds from which the silenes may be liberated (equation 32). 92 decomposes at —100 °C, but its trimethylamine adduct is stable at room temperature85. Noticeable amounts of 97 are available from 97-LiBr already at — 78 °C, whereas from 97 LiF the temperature has to be raised to +30 °C. Structures of such silene adducts will be discussed in Section I.C.l. The silene adduct 124-TIIF has been discussed in Section I.A.2. 

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