Promoting effect

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

The symmetric coupling case has been examined by using Sethna s approximations for the kernel by Benderskii et al. [1990, 1991a]. For low-frequency bath oscillators the promoting effect appears in the second order of the expansion of the kernel in coj r, and for a single bath oscillator in the model Hamiltonian (4.40) the instanton action has been found to be... 

Small quaniiiies of acid or base may have major effects on both rate and product. Many reasons have been advanced to account for these promotional effects (65), and many examples have been cited 31,32,41,79,80). The effects of pH are very worthwhile exploring in reductions that somehow are less than satisfactory. Catalysts themselves may contain sufficient acid or base to alter the course of reduction (25). 

Rhodium (2J) and ruthenium are excellent catalysts for the reduction of aromatic rings. It is with these catalysts that the best chance resides for preservation of other reducible functions (2,10,13,18,41,42,52). Rhodium (41) and ruthenium (45) each reduced methylphenylcarbinol to methylcyclohexyl-carbinol in high yield. Palladium, on the other hand, gives ethylbenzene quantitatively. Water has a powerful promoting effect, which is unique in ruthenium catalysis (36). 

For aliphatic monoamine [43], it is shown that secondary amines R2NH always possess a higher promoting effect for the polymerization of AAM and even the primary amine PA will enhance the polymerization with Rr = 1.47 and Ea = 36.4 kJ/mol, while the tertiary aliphatic amine TPA will not provide the polymerization due to some steric hindrance (Table 6). All of the data of cyclic amines listed in Table 7 are effective, i.e., NMMP with Rr = 1.81 and Ea = 29.9 kJ/mol showing the absence of steric hindrance. 

Urea has no promoting effect on the A AM polymerization initiated with Ce(lV) ion. Recently, Qiu et al. [24] have studied the effect of N-acryloyl-N -4-tolylurea (ATU), N-methacryloyl-N -4-tolylurea (MTU), and N-acetyl-N -4-tolylurea (AcTU) on A AM polymerization initiated with Ce(IV) ion and found that these three urea compounds have a high promoting effect on the polymerization of AAM. The data are cited in Table 4. 

Hall and Hassell (50) continued these studies with the intention of proving that possible traces of oxide dissolved in the metal play no significant role in the poisoning or promoting effects arising from hydrogen which had been presorbed during the pretreatment procedure. The catalysts were prepared in essentially the same manner as before. The kinetics... 

Ei addition, clinical observations in EFN-a-treated hqiatitis C or melanoma patients have suggested a promoting effect of type I EFN in autoreactive skin disorders such as Lichen planus. This inflammatory skin disease was, however, not restricted to EFN-a treated patients but was found to be associated with expression by so far unknown mechanisms of the EFN-induced MxA... 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

It is worth noting that each Na atom appears to perturb the electron density of the Pt(lll) surface over large ( 12) atomic distances. This can explain nicely the observed long-range promotional effect of Na on Pt surfaces. It is strongly reminiscent of the IR spectroscopic work of Yates and coworkers who showed that a single adsorbed alkali atom can affect the IR spectra of up to 27 coadsorbed CO molecules.80... 

D.I. Kondarides, G.N. Papatheodorou, C.G. Vayenas, and X.E. Verykios, In situ High Temperature SERS study of Oxygen adsorbed on Ag Support and Electrochemical Promotion Effects, Ber. Buns. Phys. Chem. 97, 709-720 (1993). 

The observed pronounced electrochemical promotion effect is due to the weakening of the Rh = O bond and the strengthening of the bond... 

In this region of classical Ir02 promotion by Ti02 (0electrochemical promotion effect is negligible, as p(=r/ro) is below 1.5. 

Interestingly the electrochemical promotional effect was found only in the case of perchloric acid supporting electrolyte. No promotion effect was found in presence of strongly adsorbed anions (HS04 Cl ). 

Figure 11.13. Dependence of promotional effectiveness factor, r]p, on Thiele modulus 0>p and dimensionless current J.23...

The significance of Equation (11.31), in conjunction with Figure 11.13 and the definitions of 11, P and J (Table 11.1) is worth emphasizing. In order to obtain a pronounced electrochemical promotion effect, i.e. in order to maximize p (=r/ro), one needs large II and r p values. The latter requires large J and small 0P values (Fig. 11.13). Small k and L values satisfy both requirements (Table 11.1). This implies that the promoting species must not be too reactive and the catalyst film must be thin. 

In section 11.3 we saw how a classical reaction engineering approach45 can been used to model both electrochemical promotion and metal support interactions. The analysis shows that the magnitude of the effect depends on three dimensionless numbers, II, J and Op (Table 11.3) which dictate the actual value of the promotional effectiveness factor. 

I.V. Yentekakis, C.A. Pliangos, V.G. Papadakis, X.E. Verykios, and C.G. Vayenas, Support and NEMCA-induced Promotional Effects on the Activity of Automotive Exhaust Catalysts in A. Frennet and Journal-M. Bastin (eds.) Catalysis and Automotive Pollution Control HI, Stud. Surf. Sci. Catal. 96, 375-385 (1995). 

C. Pliangos, I.V. Yentekakis, V.G. Papadakis, C.G. Vayenas, and X.E. Verykios, Support-induced promotional effects on the activity of automotive exhaust catalysts 1. The case of oxidation of light hydrocarbons, Appl. Catal. B 14, 161-173 (1997). 

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