Base promoter effect

By the same token, in base-promoted E2 dehydrohalogenations, the rate of elimination of the halogen as a halide ion is expected to be I > Br > Cl F This element effect has indeed been documented in many instances, a few examples of which are listed in Table I... 

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). 

As an example of enolate-ion reactivity, aldehydes and ketones undergo base-promoted o halogenation. Even relatively weak bases such as hydroxide ion are effective for halogenation because it s not necessary to convert the ketone completely into its enolate ion. As soon as a small amount of enolate is generated, it reacts immediately with the halogen, removing it from the reaction and driving the equilibrium for further enolate ion formation. 

Base-promoted deformylation (KOH/A) to afford the same secondary phos-phine-boranes is less effective and an acceptable compromise between yield and selectivity is difficult to reach. When the reaction proceeds smoothly, it is accompanied by significant racemization [93]. 

Based on the characterization studies and the behavior of the model systems, it appears that the promotion effect of oxides of Ti, V,... 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Now, regarding the SCR with hydrocarbons in 02 excess, numerous investigations have shown a low activity below 200°C. However, it was found that H2 can promote the reduction of NO below 200°C on molybdenum and sodium-modified Pt/Si02 and Pt/Al203 catalysts [103]. Such a promotional effect also observed on silver-based catalysts originates extensive investigations in this field and offers new perspectives in the developments of non-noble metal-based catalysts. However, further developments of that variety of catalysts seem to be questionable due to their low sulphur tolerance. 

Morales, F., and Weckhuysen, B. M. 2006. Promotion effects in Co-based Fischer-Tropsch catalysis. Catalysis 19 1 10. 

Based on these observations and several other experimental results with cofeeding of ethene and 1-alkene,9 the selectivity of branched hydrocarbons,11 and the different promoter effects of Li-, Na-, K-, and Cs-carbonate/oxide,1213 a novel mechanism has been proposed that is consistent with these various experimental results.14 The formulation of this mechanism follows the knowledge of analogous reactions in homogeneous catalysis and gives a detailed insight in the crucial step of C-C linkage formation. The aim of this work is to discuss in detail these experiments and their relationship to the proposed mechanism. 

The effect of crown ethers on the geometrical orientation in base-promoted E2 eliminations has been studied by several groups. Bartsch et al. (1973) have investigated the effect of several parameters on the potassium alkoxide-promoted eliminations of HBr from 2-bromobutane (Table 45). In the absence of crown ethers the relative amount of 1-butene formed increases, while the trans/cis ratio of the 2-butenes decreases, with decreasing solvent polarity. Furthermore, the proportion of 1-butene increases and the trans/cis ratio decreases on increasing the base concentration. These effects were explained in terms of steric interactions between the base and a- and /7-alkyl groups in the... 

Both acid- and base-promoted reactions may be affected by acidic surfaces and, hence, by the factors which influence the surface acidity. Kinetic evidence for increased Br nsted acidity at clay surfaces has been presented by McAuliffe and Coleman (80) who studied the hydrolysis of ethylacetate and the inversion of sucrose. They noted that potentionmetrie pH measurements did not explain the catalytically effective H+-concentration at the clay surface. 

Rate data for hydrolysis reactions in homogeneous aqueous solutions have been reviewed (79), but application of these data to environmental conditions involving mineral surfaces remains difficult due to the unknown effects sorption may have. Several studies have demonstrated that acid-catalyzed reactions are promoted if the substrate is sorbed at clay surfaces (70-74 and other works reviewed by Theng, 8), but inhibition may also occur if substrate hydrolysis is base-promoted (74). 

Phosphonium and uronium salt-based reagents effect couplings by first reacting with the anion of the starting acid (Figure 2.22, path A). The benzotriazolyl ester is then one of the two possible precursors of the peptide. Operating in a research climate in which HOBt was commonly used as additive, Hudson rationalized that if the ester is the precursor of the peptide, additional HOBt in the form of the anion would be beneficial because it would by mass action promote formation of the ester. A favorable effect from adding HOBt was reported, so one variant of the use of... 

Although the isolation and reactivity of acyl complexes strongly support the hydride mechanism, the other mechanism cannot be excluded. For example H20, the acid or molecular hydrogen, which can act as a hydride source, can promote the Pd - C splitting of the Pd-alkylcarboalkoxy intermediate in the alkoxy cycle as well. More convincing for the hydride route is the fact that the acid, which does not promote the formation of a Pd-OCH3+ species, has a promoting effect on the catalysis and can activate a Pd(0) complex, otherwise inactive, whilst a base, which not only promotes the formation of this species, but also deprotonates a Pd - H+ species to Pd(0), suppresses the catalysis. 

The acid-catalysed hydrolysis of the acylal, 1-phenoxyethyl propionate (13), has been studied using the PM3 method in the gas phase. The kinetics and mechanism of the hydrolysis of tetrahydro-2-furyl and tetrahydropyran-2-yl alkanoates (14) in water and water-20% ethanol have been reported. In acidic and neutral media, kinetics, activation parameters, isotope-exchange studies, substituent effects, solvent effects and the lack of buffer catalysis pointed clearly to an Aai-1 mechanism with formation of the tetrahydro-2-furyl or tetrahydropyran-2-yl carbocation as the rate-limiting step (Scheme 1). There is no evidence of a base-promoted Bac2 mechanism up to pH 12. ... 

TT-Stacking interactions and solvation effects within the highly preorganized cleft of a bifunctional C-shaped host are believed to benefit the base-promoted conversion of 5-nitrobenzisoxazole to 2-cyano-5-nitrophenolate relative to the acetate-promoted reaction structural variation of the host has been explored. ... 

Another a, i-unsaturated aldehyde analyzed is cinnamaldehyde. Its liquid-phase hydrogenation has been studied in our research group [20, 51, 94], using Pt, Ni and Cu-based tin-modified hi- and organobimetaUic catalysts (in all cases with Si02 as support). The catalytic results obtained showed that in aU cases there was a marked promoting effect of Sn on the selectivity to cinnamic alcohol (UOL). The specific modification of the monometallic systems due to Sn addition from the application of SOMC/M markedly increases the selectivity to UOL, especially in the case of Ni, where it goes from zero selectivity for the monometallic to 25% for the NiSn catalyst. Pt-based systems modified by Sn yield the best Suol values. 

Heteronuclear carbonyls containing Fe-Mn and Fe-Ru frames are available, with different metal ratios, and these are useful to study the promoter effects of both Mn and Ru on Fe-based catalysts. 

Two examples using i-BuLi/(—)-sparteine mixture as base have been reported to promote effective allylic alcohols formation in moderate ee for the spiro epoxide 82 and cyclooctadiene oxide 83 ° (Scheme 33). In the last case, such regioselectivity could be explained by the relatively flat conformation of the medium-sized oxirane which favors the syn base-oxirane complex (see Section n.A). 

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