Transalkylation mechanism

The transalkylation of fm-alkylbenzenes follows a different route, since they have no abstractable benzylic hydrogen. They were shown to transalkylate by a dealkylation-transalkylation mechanism with the involvement of free ferf-alkyl cations. The exceptional ability of fcrt-alkyl groups to undergo transalkylation led to the extensive utilization of these groups, especially the fm-butyl group, as positional protective groups in organic synthesis pioneered by Tashiro.223... 

Further isomerization of m-xylene as well as transalkylation of trimethylbenzene and toluene to form m-xylene can occur. Evidence for the bimolecular transalkylation mechanism was provided by observation of a peak at m/e 109 in the mass spectra for CD3 substitution of toluene. These data rule out unimolecular 1,2-methyl shifts as the sole means of formation of xylenes. The higher the A1 content of the ultrastable faujasite the greater the extent of bimolecular transalkylation. These observations have significant implications for unimolecular kinetic models that have been proposed as well as reported activation energies and turnover frequencies. 

Bolton et al. (8) proposed that isomerization of diethylbenzenes at 170°C over a modified Y-type faujasite occurs via a transalkylation mechanism involving diphenylethane type intermediates. A similar bi-molecular mechanism v as proposed for xylene isomerization (32). Hovv -ever, as Csicsery (13) observed, this reaction cannot proceed v hen easily-extractable a-hydrogen atoms are unavailable—i.e., when the alkyl group is tert-hutyl. In his studies of the isomerization of I-methyl-2-ethyl-benzene over Ca-NH4-Y-t) e faujasite, Csicsery has shown further that intermolecular isomerization predominates at temperatures below 200°C, while at temperatures above 300°C, isomerization proceeds via 1,2-shifts. 

While it was found by means of isotopic studies than on amorphous silica-alumina the reaction proceed by an intramolecular mechanism (194), in zeolite Y, the distribution of isomers in the trimethylbenzene fraction indicates that some of the isomers could be obtained by a bimolecular mechanism (172,175). In a very recent work (196,197) it has been demonstrated by means of isotopic studies, that on some 12 MR zeolites such as Y, and mordenite, xylenes are isomerized by both uni and bimolecular transalkylation mechanism. The ratio of the uni to bimolecular increases when increasing the Si/Al ratio, and decreases when increasing the reaction temperature, the partial pressure of the feed, and the contact time. Another 12 MR, Beta zeolite, while being able to disproportionate xylene, does not isomerize via the bimolecular mechanism. This was explained by space constraints to accommodate a xylene and a trimethylbenzene as a bimolecular intermediate in the channels of the zeolite. A medium pore zeolite (ZSM-5) does isomerize only through a unimolecular 1,2 methyl-shift mechanism. 

In this section, the reactivities of organosilicon compounds for the Friedel-Crafts alkylation of aromatic compounds in the presence of aluminum chloride catalyst and the mechanism of the alkylation reactions will be discus.sed, along with the orientation and isomer distribution in the products and associated problems such as the decomposition of chloroalkylsilanes to chlorosilanes.. Side reactions such as transalkylation and reorientation of alkylated products will also be mentioned, and the insertion reaction of allylsilylation and other related reactions will be explained. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

The data are summarized in Table II. They have been normalized to kx x s i for each zeolite catalyst. In general it is seen that the7transfer of an ethyl group (E,E E,X) occurs faster than that of a methyl group (X,E X,X). This is in agreement with the indicated mechanism for transalkylation (Figure 4) which involves a benzylic carbenium ion intermediate. In the case of methyl transfer, this is a primary cation,... 

The mechanism of the transalkylation reaction is complex because of the many products involved and the possibility of internal rearrangement within the complex. The first step in the reaction may be the alkylation of I to III (Equation 6) or elimination of benzyl bromide to form II or IV. Since the transalkylation of II and III with p-nitrobenzvl bromide is slower than the total transalkylation of I with the same agent, I —> IV seems to be the slow step in the reaction. 

Numerous studies have dealt with the mechanism of metathesis.6,7 14 20-31 The first important question to answer in an understanding of the mechanism is whether the carbon-carbon single bonds or the carbon-carbon double bonds are cleaved during the reaction. The corresponding transformations taking place are transalkylation [Eq. (12.9)] or transalkylidenation [Eq. (12.10)], respectively ... 

Aliphatic amine transalkylation has been observed on alumina by Derrien and Jungers (10) and by Gatry and Jungers (11) at a higher temperature. Under the experimental conditions used here, an acid-catalyzed mechanism seems to be acceptable. An explanation that would lead to... 

Fig.9.3 Mechanisms of xylene isomerization- a Intramolecular mechanism (1,2 methyl shifts) b Intermolecular mechanism via disproportionation and transalkylation steps...

Mechanism of transalkylation reaction to give structural unit B. (From... 

The transalkylation and isomerization reactions can be satisfactorily explained by the Streitwieser mechanism( ). This mechanism proposes a 1,1-diphenylethane-type intermediate. For example, para-diethylbenzene. (Figure 3) Such an intermolecular mechanism is consistent with the experimental data and does not require the assumption of a sequence of intramolecular 1,2 shifts. The decay of the polyethylbenzenes towards equilibrium is consecutive and not concurrent. The catalyst seems to be associated with the most basic center and when it reaches steady-state, the catalyst transfers to the next most basic one. There is also a concurrent intramolecular isomerization such as 1,2,4 triethylbenzene going to 1,3,5 triethylbenzene. There is hence a movement towards isomer equilibrium as well as product equilibrium. 

The HDN of aliphatic amines (equation 25) is relevant to the HDN of indoles (equation 26), pyridine (equation 27), and quinoline (equation 28) because these heterocycles are first hydrogenated to the aliphatic amines. A general mechanism proposed for the HDN of aliphatic amines is based on metal cluster catalysis of the transalkylation reaction in equation (33) and on metal complex catalyzed exchange of deuterimn for hydrogen in tertiary aliphatic amines (equation 34). There are other examples of amine activation in metal complexes. 

The kinetics and mechanism of the transalkylation reaction between 2-alkoxy-1-methyl-benzimidazoles and thiophenol have been studied. The results demonstrate a rapid protonation of the azole followed by an 5n2 reaction which forms the benzimidazolinone. The basicity of the benzimidazole and the pH both affect the reaction rate (Scheme 126). The corresponding sulfur analogue, l-methyl-2-methylthiobenzimidazole, does not react with thiophenol, probably because of the poor electron-donating ability of the sulfur atom of thiophenate (71 JCS(B)2299). 

The reaction mechanism for a very strong one-electron donor centre in the dehydrogenation of alkylaromatic hydrocarbons is similar to that proposed by Krause for ethylbenzene dehydrogenation [reactions (5) and (6)]. The mechanism for n-propylbenzene transalkylation and cyclization on the radical pathway has been suggested. ... 

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