Aromatic alkylations

The rearrangement of carbonium ions that readily occurs according to the thermodynamic stabiUty of cations sometimes limits synthetic utility of aromatic alkylation. For instance, the alkylation of ben2ene with / -propyl bromide gives mostly isopropylben2ene (cumene) much less... 

Future Developments. The most recent advance in detergent alkylation is the development of a soHd catalyst system. UOP and Compania Espanola de Petroleos SA (CEPSA) have disclosed the joint development of a fixed-bed heterogeneous aromatic alkylation catalyst system for the production of LAB. Petresa, a subsidiary of CEPSA, has announced plans for the constmction of a 75,000 t/yr LAB plant in Quebec, Canada, that will use the UOP / -paraffin dehydrogenation process and the new fixed-bed alkylation process (85). 

Over the years, improvements in aromatic alkylation technology have come in the form of both improved catalysts and improved processes. This trend is expected to continue into the future. 

Catalysts. Nearly aU. of the industrially significant aromatic alkylation processes of the past have been carried out in the Hquid phase with unsupported acid catalysts. For example, AlCl HF have been used commercially for at least one of the benzene alkylation processes to produce ethylbenzene (104), cumene (105), and detergent alkylates (80). Exceptions to this historical trend have been the use of a supported boron trifluoride for the production of ethylbenzene and of a soHd phosphoric acid (SPA) catalyst for the production of cumene (59,106). 

In some cases, a mixture of natural petroleum feedstock is preblended with synthetic alkylated aromatics, such as detergent aromatic alkylate bottoms or with first-intent synthetic mono- or dialkylated aromatics, selected to provide a suitable molecular weight for cosulfonation and subsequent processing. The use of blended feedstocks may eliminate the need for conducting an oil extraction—concentrating step, particularly for a typical 40% Ca or Mg petroleum sulfonated product. 

The use of MSA as catalyst to prepare 2-alkylphenols and 2,6-dialk5lphenol has been described (407). MSA has also been used as an aromatic alkylation agent (408). 

Aromatic alkylations occur in numerous biological pathways, although there is of course no MCI3 present in living systems to catalyze the reaction. Instead, the carbocation electrophile is usually formed by dissociation of an organodiphosphate, as we saw in Section 11.6. The dissociation is typically assisted by complexation to a divalent metal cation such as Mg2+ to help neutralize charge. 

Upon calcination the template is removed and the zeolite s well-defined pores are available for adsorption and catalysis. Particularly challenging is the field of electrophilic aromatic substitution. Here often non-regenerable metal chlorides serve as the catalyst in present industrial practice. Zeolites are about to take over the job and in fact are doing so for aromatic alkylation. 

Zeoliltes seem particularly suited to take over the job and in fact are doing so already for aromatic alkylation. Thus in ethylbenzene manufacture (from benzene and ethene) modern processes apply zeolites (H-ZSM-5, H-Y) as the catalyst, substituting conventional processes based on AICI3 or BF3-on-alumina catalysis. Substantial waste reductions are achieved. 

Much of the early work into the rhodium(II)-catalysed formation of oxazoles from diazocarbonyl compounds was pioneered by the group of Helquist. They first reported, in 1986, the rhodium(II) acetate catalysed reaction of dimethyl diazomalonate with nitriles.<86TL5559, 93T5445, 960S(74)229> A range of nitriles was screened, including aromatic, alkyl and vinyl derivatives with unsaturated nitriles, cyclopropanation was found to be a competing reaction (Table 3). 

Lewis acid-catalyzed additions can be carried out in the presence of other chiral ligands that induce enantioselectivity.156 Titanium TADDOL induces enantioselectivity in alkylzinc additions to aldehydes. A variety of aromatic, alkyl, and a, (3-unsaturated aldehydes give good results with primary alkylzinc reagents.157... 

Scheme 12.22 provides some examples of the oxidation of aromatic alkyl substituents to carboxylic acid groups. Entries 1 to 3 are typical oxidations of aromatic methyl groups to carboxylic acids. Entries 4 and 5 bring the carbon adjacent to the aromatic ring to the carbonyl oxidation level. 

Aromatic alkylation is illustrated in this paper for both individual a-olefin cuts (e.g. 1-decene and 1-dodecene) as well as typical samples of Ci0-Ci4 paraffin dehydrogenate that contain ca. 8.5% Go-Cm olefins, primarily internal olefins. Syntheses have, for the most part, been conducted in a continuous reactive distillation unit of the type illustrated in Figure 1, operated slightly above atmospheric pressure. Operational details for this unit may be found in the previous section. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Nitration versus alkylation. Upon the CT irradiation of an orange solution of the charge-transfer complex, the color bleaches rapidly, and either an aromatic nitration product (i.e. 3-nitro-4-methoxytoluene) or an aromatic alkylation product (i.e. 3-trinitromethyl-4-methoxytoluene) is obtained in high yield depending on the reaction conditions summarized in Scheme 22.4lc... 

A comparative analysis of the kinetics of the reactions of atoms and radicals with paraffinic (R1 ), olefinic (R2H), and aromatic alkyl-substituted (R3H) hydrocarbons within the framework of the parabolic model permitted a new important conclusion. It was found that the tt-C—C bond occupying the a-position relative to the attacked C—H bond increases the activation energy for thermally neutral reaction [11]. The corresponding results are presented in Table 6.9. 

Scheme 8.7. Nucleoside synthesis, glycoside synthesis, and aromatic alkylation with a dielectrophilic donor.

Following are several commercially available fluoromonomers (1-4) that contain functional groups and have been utilized to make polymeric membranes for ion separations6"9 or as catalysts for aromatic alkylation and acylation reactions.1011 They are also convenient starting precursors, allowing for further functionalization reactions. 

The Delft synthesis makes use of an acid-catalyzed ring closure - in fact an intramolecular aromatic alkylation - of a l-(3,5-dihydroxy-4-methoxybenzyl) isoquinoline derivative that is prepared starting from (natural) gallic acid. One of the hydroxyl groups is removed via a Pd/ C hydrogenation of the benzyl ether. Other catalytic steps play an important role some steps were improved recently [27]. The crucial step in the Rice synthesis makes use of a l-(2-bromo-5-hydroxy-4-methoxybenzyl)isoquinoline derivative that is also cyclized in an acid-catalyzed ring closure to the morphinan skeleton, followed by catalytic removal of the bromo substituent (Scheme 5.8). 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

Most of the commercial zeolite catalyzed processes occur either through acid catalysis fluid catalytic cracking (FCC), aromatic alkylation, methanol to olefins (MTO),... 

There are several means to maximize the yield in the desired monoalkylation product high aromatic/alkylation agent ratio, association of a transalkylation unit to the alkylation unit, and use of a shape selective zeolite as catalyst. 

Cumene was originally produced with SPA- [57], then FAU- or BEA-based catalysts, and most recently MWW. While most industrial processes use MWW-based catalysts [58], Dow and KeUog co-developed a dealuminated MOR based process called 3-DDM [59]. With each new process generation, conversion and selectivity to cumene has increased. These processes and the chemistry behind them are covered in Section 15.4. As the use of zeoHtes for alkylation reactions in industry increased, so did the study of the reaction and how the zeoHte topology affects the mechanism and selectivity to products, so that now many zeotypes are tested for aromatic alkylation as a way of figuring out a new structure s reaction pattern. Therefore, many zeotypes have been used to catalyze aromatic alkylation (Tables 12.9-12.11). 

Uopis, F., Sastre, G., and Corma, A. (2004) Xylene isomerization and aromatic alkylation in zeolites NU-87, SSZ-33, p, and ZSM-5 molecular dynamics and catalytic studies. 

Tellurium tetrachloride as reagent for the conversion of alcohols into alkyl chlorides and as a Lewis acid catalyst for aromatic alkylation... 

TeClf catalysed Friedel-Crafts aromatic alkylation (typical procedure. To a solution of 1-phenylethanol (3.7 g, 30 mmol) in toluene (30 mL) is added slowly TeCl4 (9.7 g, 36 mmol), keeping the temperature at 25°C (exothermic reaction). Small amounts of white precipitate appear immediately, and after a few minutes the colour of the mixture becomes dark brown. The mixture is stirred for 3 h and then quenched with HgO (20 mL). The organic layer is separated, washed with brine (2x20 mL) and dried (MgS04). Evaporation of the solvent leaves an oily residue which is distilled under vacuum, giving a mixture of 1-phenyl-1-tolylethanes (4.9 g (83.3%) b.p. 117-128°C/1 torr). GLC analysis (silicone OV-101, 0.24 mm X 30 m capillary column at 100-260°C, 4°C min i) reveals an ortholpara ratio of 12 88. 

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