Lewis acid catalysts variations

Another variation on this theme is the use of a scandium salt of a hydrophobic polystyrene-supported sulfonic acid (PS-S03H) as an effective heterogeneous Lewis acid catalyst in aqueous media [149]. 

Conjugate addition and conjugate-addition-initiated ring-closure (CAIRC) reactions ofp-dicarbonyl compounds with Michael acceptors have been studied using several Lewis acid catalysts under solvent-free conditions. Bi(N03)3 was found to be an extraordinarily effective catalyst in some cases. Its catalytic behavior depended on the structural variations of P-dicarbonyl compounds (Equation 6) [28]. 

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

It has to be taken into account that these catalysts are not yet fully developed. Introduction of electron-withdrawing groups is suspected to increase Lewis acidity, which influences activity, and the optimum spacer length may enable formation of chiral polymers, but such variations are synthetically challenging. 

The variations of acidic properties in the surface layers and in the bulk solid catalysts after calcination, reduction, or coking were examined by pyridine Nls XPS [4,7] and by the pyridine infrared adsorption techniques, respectively. This provides a means to compare the changes in the characteristic BrBnsted and Lewis acidity functions after those treatment conditions. First of all, TPD of ammonia revealed that both coked and regenerated samples exhibited much decreased acidity as compared with either calcined or reduced samples before the reaction of n-heptane conversion in either N2 or H2 stream [7]. The adsorption of pyridine may cause further perturbation to the Pt4+ or Pt 2+ species in the zeolite as indicated by the increase in binding energies of Pt3d5/2 electrons, as shown in Table 3 and Figure 4,... 

Several catalysts with similar characteristics have been reported. They are cationic and dicationic half-sandwich Lewis acid complexes of the elements Rh, Ir, and Ru and incorporate non-Ci-symmetric bidentate phosphorus and nitrogen ligands. The best results obtained with these Lewis acids in the reaction of methacrolein with cyclopentadiene are shown in Sch. 48. Ligand variation afforded less active and/or less selective catalysts. 

The evidence would certainly support activity residing in Bronsted and/or Lewis-acid sites. However, subtle variation in activity can occur, effected by the strength of associated basic sites in dehydration catalysts. 

A variation of the above method uses a Lewis acid in place of the protic acid. Here the advantage is that the acid coordinates to the oxygen of the product, thus retarding further degradation. Aids is an effective catalyst to afford mainly ortho and para substitution. 

Five-coordinate W alkylidene complexes, where L can be alkoxide or halide or SO3CF3, are highly effective olefine metathesis catalysts in the presence of Lewis acids. The vast number of compounds synthesized can be understood by the variation and combination of different ligands. 

Fig. 7 shows the variation of the steady conversion as a function of the integrated areas of the NH/ deformation band. The linear relationship verifies that the Bronsted acidity is directly ruling the isomerization activity of the catalysts. Although no reliable values could be established for the Lewis acidity, the bands in the N-H stretching region suggest a relative decrease of the Lewis/Bronsted acid ratio with increasing sulfate content. 

The addition of potassium to AI2O3- [5] or Ti02-supported [17-19] vanadia catalysts increases the selectivity to olefins during the ODH of n-butane and propane, respectively. This variation of the selectivity to oxydehydrogenation products has been related to a decrease in the number of Lewis acid sites [5] or to a decrease in the heat of the propylene adsorption [17-19]. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

In all of the examples considered so far, the chiral element has been employed in stoichiometric quantities. Ultimately, it would be desirable to require only a small investment from the chirality pool. This is only possible if the chiral species responsible for enantioselectivity is catalytic. It is worth stating explicitly that, in order to achieve asymmetric induction with a chiral catalyst, the catalyzed reaction must proceed faster than the uncatalyzed reaction. One example of an asymmetric aldol addition that has been studied is variations of the Mukaiyama aldol reaction [110] whereby silyl enol ethers react with aldehydes with the aid of a chiral Lewis acid. These reactions proceed via open transition structures such as those shown in Figure... 

Problem 8.22 The problem of reproducibility of rates and the discrepancy between results obtained by various authors is usually attributed to variations in impurity levels, the most important being the traces of water. To determine the minimum concentration levels at which water can still be effective, consider a polymerization where PVI] = 0.1 mol/L, ki = 3 L/mol-s, and the DP of the polymer formed is 10 (these values are reasonably close to a styrene polymerization) and assume that a rate of 1 % conversion per 24 h can be determined with sufficient accuracy. Determine the concentration of water in the system containing a Lewis acid (L) catalyst in a relatively large concentration. 

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