Catalyzed mechanism, Bronsted acid

Radioactive tracer experiments reported by Lombardo and Hall (4) showed that each butene isomer can be directly interconverted into the other two. These results are consistent with a common intermediate being in operation in this reaction. In Figure 3 the linear relationship between catalytic activity and percentage of Na+ replaced by H+ strongly favors a Bronsted acid catalyzed mechanism in which the common intermediate could be a secondary carbonium ion. This conclusion is also supported by the tracer experiments. 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

From a qualitative point of view, both acid-catalyzed mechanisms may account for the observed reactivity, and Bronsted acid catalysts seem to be more efficient than Lewis ones. However, limitations of catalyst modelling prevent us from drawing further conclusions. 

A mechanism frequently proposed for the Bronsted acid-catalyzed esterification reactions is based on fatty acid carbonyl activation by proton generated by the catalyst, followed by alcohol attacks in the carbonyhc carbon, generating a protonate intermediate who after water elimination results in the ester formation (Figure 5). 

Although SnCl2 being the only Lewis acid studied herein, and consequently it may display a different catalytic action mechanism, it was also included in this topic for comparison. Actually, it seems that the SnCl2 is also an efficient catalyst, taking in account that high yields in ethyl oleate were achieved, similarly to those obtained in the Bronsted acid-catalyzed reactions. 

Scheme 14 General mechanism for Bronsted acid-catalyzed vinyl polymerizations.

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. 

Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

This review aims at reporting on the synthesis of aliphatic polyesters by ROP of lactones. It is worth noting that lactones include cyclic mono- and diesters. Typical cyclic diesters are lactide and glycolide, whose polymerizations provide aliphatic polyesters widely used in the frame of biomedical applications. Nevertheless, this review will focus on the polymerization of cyclic monoesters. It will be shown that the ROP of lactones can take place by various mechanisms. The polymerization can be initiated by anions, organometallic species, cations, and nucleophiles. It can also be catalyzed by Bronsted acids, Lewis acids, enzymes, organic nucleophiles, and bases. The number of processes reported for the ROP of lactones is so huge that it is almost impossible to describe aU of them. In this review, we will focus on the more... 

Fig. 19 Monomer activation mechanism for the ROP of lactones catalyzed by Bronsted acids and initiated by nucleophilic alcohols...

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

In the presence of an electrophile, tautomerization of a substrate with a C=0 double bond to its enol only takes place when catalyzed by either a Bronsted- or a Lewis acid. The proton-catalyzed mechanism is shown for the ketone — enol conversion B — iso-B (Figure 12.4), the carboxylic acid —> enol conversion A — E (Figure 12.6), the carboxylic acid bromide — enol conversion E —> G (Figure 12.7) and the carboxylic acid ester — enol conversion diethyl-malonate —> E (Figure 12.9). Each of these enol formations is a two-step process consisting of the protonation to a carboxonium ion and the latter s deprotonation. The mechanism of a Lewis acid-catalyzed enolization is illustrated in Figure 12.5, exemplified by the ketone —> enol conversion A —> iso-A. Again, a protonation to a carboxonium ion and the latter s deprotonation are involved the Lewis acid-complexed ketone acts as a proton source (see below). 

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

---