Friedel-Crafts benzylation

It is worth noting here that the results of some other studies of aromatie substitutions, sueh as the Friedel-Crafts benzylation and iso-propylation of alkylbenzenes, and the bromination of alkylbenzenes with bromine, eatalysed by ferrie ehloride, are under suspicion as depending upon slow mixing. As regards halogenation eatalysed by Lewis aeids, positive evidenee to support this eritieism has been obtained. ... 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

In the early 1970s, Olah et al. investigated the Friedel-Crafts benzylation of benzene and toluene with benzyl chloride derivatives in the presence of various... 

Table 41.2 shows the results of the Friedel-Crafts benzylation of benzene at 60°C over several TUD-1 based materials, and compared with bulk Fe203. Comparison between different metal-containing Si-TUD-1 catalysts with Si/M ratio 50 indicates that Fe is the most active metal, as conversion reaches 100% within 180 minutes. 

Another recent patent (22) and related patent application (31) cover incorporation and use of many active metals into Si-TUD-1. Some active materials were incorporated simultaneously (e.g., NiW, NiMo, and Ga/Zn/Sn). The various catalysts have been used for many organic reactions [TUD-1 variants are shown in brackets] Alkylation of naphthalene with 1-hexadecene [Al-Si] Friedel-Crafts benzylation of benzene [Fe-Si, Ga-Si, Sn-Si and Ti-Si, see apphcation 2 above] oligomerization of 1-decene [Al-Si] selective oxidation of ethylbenzene to acetophenone [Cr-Si, Mo-Si] and selective oxidation of cyclohexanol to cyclohexanone [Mo-Si], A dehydrogenation process (32) has been described using an immobilized pincer catalyst on a TUD-1 substrate. Previously these catalysts were homogeneous, which often caused problems in separation and recycle. Several other reactions were described, including acylation, hydrogenation, and ammoxidation. 

Zinc chloride was used as a catalyst in the Friedel Crafts benzylation of benzenes in the presence of polar solvents, such as primary alcohols, ketones, and water.639 Friedel-Crafts catalysis has also been carried out using a supported zinc chloride reagent. Mesoporous silicas with zinc chloride incorporated have been synthesized with a high level of available catalyst. Variation in reaction conditions and relation of catalytic activity to pore size and volume were studied.640 Other supported catalytic systems include a zinc bromide catalyst that is fast, efficient, selective, and reusable in the /wa-bromination of aromatic substrates.641... 

Olah and co-workers219 have applied Nafion-H in the benzylation of benzene with benzyl alcohols [Eq. (5.83)] and also reported the reaction of benzyl alcohol with substituted aromatics (toluene, xylenes, mesitylene) to yield diphenylmethanes. The reaction is performed under mild conditions and produces the corresponding dibenzyl ethers as byproducts (2-22%). The substrate and positional selectivity in competitive benzylation of benzene and toluene (1 1 molar ratio) was found to be almost the same as observed in solution-phase Friedel-Crafts benzylation with benzyl chloride (AICI3-CH3NO2). Cyclic products 56 and 57 resulting from cyclialkylation were isolated when Nafion-H-catalyzed benzylation was applied to 2-(hydroxymethyl) diphenylmethane and 3,4-dimethoxybenzyl alcohol, respectively. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

A Mechanistic Study Substrate and Positional Selectivity in a Series of Analogous Friedel-Crafts Benzylations... 

More recently, Olah et al. (1970) have varied the reactivity of the electrophile in a systematic way and observed a number of convincing examples of reactivity- selectivity relationships. The Friedel-Crafts benzylation reaction on benzene and toluene was conducted for a large number of substituted benzyl chlorides. The data are in Table 18 and indicate that the selectivity of electrophilic attack decreases as the substituent becomes more electron withdrawing. 

Shrigadi, N. B., A. B. Shinde, and S. D. Samant. 2003. Study of catalytic activity of free and KlO-supported iron oxyhydroxides and oxides in the Friedel-Crafts benzylation reaction using benzyl chloride/alcohol to understand their role in the catalysis by the Fe-exchanged/impregnated K10 catalysts. Appl. Catal. A 252 23-25. 

Silica-supported polytrifluoromethanesulfosiloxane (Si02-Si-SCF3) has been prepared by Jiang et al., and has been used to catalyze the Friedel-Crafts benzylation of arene with benzyl alcohol [124]. The selectivity of the reaction was influenced by catalyst concentration, reaction time and arene/benzyl alcohol molar ratios. In the range 30 1 to 10 1 of toluene/benzyl alcohol molar ratios, the yield of alkylated product was quantitative, but with a molar ratio of 5 1, 37% the intramolecular condensed product, dibenzyl ether, was formed (Scheme 5.23). 

Scheme 5.23 Effect of the molar ratio of reactants on selectivity in the Friedel-Crafts benzylation of toluene with benzyl alcohol.

Friedel-Crafts benzylation. The title salt is a stable precursor of benzylcarbenium ion. On thermolysis (at 80°) of its mixture with an electron-rich arene the cation is generated and trapped. 

Inorganic supported reagents such as clayzic (add-treated clay supported zinc chloride)42,93-95 have a limited but valuable range of applications, notably in Friedel-Crafts benzylations (Figure 4.8), where a remarkable synergism between the individually weakly active components results in a very active catalyst. 

Arata, K., Yabe, K., and Toyoshima, I. 1976. Friedel-Crafts reaction in the heterogeneous system—V. Friedel-Crafts benzylation and benzoylation of toluene catalyzed by calcined iron sulfates. /. Catal. 44 385-391. 

Friedel-Crafts benzylation. The catalyst is CuCh supported on AI2OJ. Chlorination. CuCh is a mild chlorinating agent for sodiomalonate esters. The AI2O.1 supported reagent has also been used in a-chlorination of conjugated... 

Friedel-Crafts benzylation. The benzylation of arenes proceeds in good yields with benzyl chloride as reagent when ZnCh is activated by the addition of a ketone, a primary alcohol, or water. ... 

One example (36) describes Co-TUD-1 for liquid-phase oxidation of cyclohexane. Another example (37) describes the synthesis, characterization, and catalytic performance of Fe-TUD-1 for Friedel-Crafts benzylation of benzene. Other reactions were described ... 

Friedel-Crafts benzylation and phenethylation reactions of several aromatic compounds with benzyl and phenethyl benzothiazol-2-yl carbonate derivatives also proceeded in the presence of Sc(OTf)3 [48]. The use of reactive aromatic compounds and electrophiles are key for this process. Secondary alcohol methanesulfonates and 2-pyridylsulfonamide also acted as Friedel-Crafts alkylating reagents to react with aromatic compounds in the presence of Sc(OTf)3 [49, 50]. 

While the Lewis acidity of HS-AlFj is too strong to be used for reactions such as Friedel-Crafts benzylation, acylation, and alkylation due to the formation of unwanted side reactions, the created Lewis acidity in Fe /MgFj, Ga /MgFj, and In /MgFj catalysts has been advantageously used for the above syntheses [67-70]. 

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