Acid/base cooperativity

The first event in this process is the formation of a RNA-DNA hetero-duplex between the complementary 19-mer sequences of 16 and 17. This allows the catalytic moiety to be correctly placed near the selective scission site. The cleavage proved to be most efficient at pH 8, where the ethylenediamine exists mostly as a monocation (pfCa values are 9.2 and 6.5). This observation is consistent with a mechanism involving an intramolecular acid-base cooperation between the neutral amine and the ammonium ion of the DETA moiety, as depicted in Scheme 13.5. The basic amino moiety pulls a proton from the 2 -hydroxyl group of the ribose to promote its attack toward the phosphorus atom, while the protonated ammonium residue provides Lewis acid assistance to the reaction [7d, 19]. This... 

These, and several other studies indicate that this reaction is not simple. Results suggest that, whilst basic sites are essential for side-chain alkylation the reaction proceeds more effectively via acid-base cooperative catalysis [81]. Experimented evidence strongly suggests that formaldehyde is an intermediate and that crystal stmcture [82], or at any rate microporosity [83], is also important in selectivity. This last point suggests that selectivity is improved by a preferred local geometry and this is suggested as a feature in the higher yields with KX rather than K.M catalysts. The low activity... 

In the breakdown of the intermediate, the water bound to Ce(IV) functions as acid catalyst. With this catalysis, the alkoxide ion of 5 -OH can be promptly removed from the phosphorus atom. Otherwise, the leaving group is poor and hard to be removed. Consistently, the DNA hydrolysis by Ce(IV) is accompanied by a notable D2O solvent isotope effect ( 20/ 20 = 2.2-2.4). The proton transfer is, at least partially, rate limiting. The large coordination-number of the Ce(IV) ions in the bimetallic cluster is favorable for this acid catalysis, exactly as described above for the first step of DNA hydrolysis (formation of the pentacoordinated intermediate). This mechanism is proposed on the basis of the kinetic analysis in acidic solutions. When DNA hydrolysis is carried out at around pH 7, similar acid-base cooperation by Ce(IV)-bound water molecule and the corresponding hydroxide should occur in higher aggregates of [Ce 2(OH)4] +. 

Vasihev and Gahnsky [224] and Palomares et al. [225] studied X-type zeolites exchanged with K, Rb, and Cs and found that the Cs-exchanged X zeolite was the most active for side chain alkylation, producing nearly no xylene isomers. On the other hand, toluene alkylation over Na-X mainly led to the formation of xylenes. An IR-spectroscopic study in [225] revealed that on Na-X, methanol was by far the most abimdant species on the catalyst smface, while on the more basic Cs,Na-X it was toluene. Miyamoto et aL [226] visualized the acid-base cooperative catalysis in the side chain alkylation of toluene by computer graphics and also used it to explore geometrical factors. 

Acid-base cooperative effect. Figure 3.38 shows the relationship between the ratio (Sa/Sk) of acidic surface area (Sa) to the basic surface area (Sk) and the... 

It is clearly seen from Fig. 3.38 that the ratio (Sa/Sr) of sm-face coverage of acid and base also shows two peaks along with change of Fe +/Fe +, which is well consistent with the hump-type activity curve (Fig. 3.28). The activity increases first followed by decrease and then increase again with the increasing Sa/Sr value, reaches the maximum when Sa/Sr is in the range of 1.1 1.2, and is very low when Sa/Sr < 0.9. Therefore, the high activity of Fei xO based catalyst is related with their surface acid-base cooperative effect. 

Fei xO. The ratio of 1.1 also reflects the adsorpion of N2 being more important than desorption of NH3. For this reason, it is considered that the acid-base cooperative effect of catalyst surface is one of the important reasons for Fei xO based catalysts to have the higher activity. It is quite a common phenomenon in catalysts such as acid, base and oxide ones, but it is not noticed as fully as possible in the case of metal catalyst such as iron catalyst. At the same time, it is found that the phenomena of acid and base coverage and their ratio relates with the properties and compositions of precursor iron oxide are derived from the severe enrichments as well as the excess coverage of alkali on the surface and resulting in the decreasing of the activity. 

Acid-Base Cooperative Catalysis for Organic Reactions by Designed Solid Surfaces with Organofunctional Groups... 

Acid-Base Cooperative Catalysis for Organic Reactions... 

Because of the weak interaction between the H" " site and immobilized tertiary amine group, both the acid site and the amine group can act as catalytically active species. On the other hand, a nonimmobilized tertiary amine, such as triethylamine, strongly adsorbed on the acid site and deactivated each other. Therefore, SA-NEtj showed much higher activity for the above-mentioned Michael reaction and cyano-ethoxycarbonylation. These reactions hardly occurred with only siHca-alumina or the tertiary amine. The catalytic reaction pathway including acid-base cooperative activation is shown in Scheme 1.10. Similar to the case of silica-alumina, Al-MCM-41 was also reported as a support, which can enhance the amine-catalyzed nitroaldol reaction [14]. 

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