Supported Lewis acids metal chlorides

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

The use of inorganic supports for Bronsted and Lewis acids25 was described in Chap. 5. These included the metal chlorides, as well as protonic acids, on inorganic supports that are sold by Contract Chemicals as Envirocats 26. They are air-stable, nontoxic powders that can be reused several times. 

The preparation and use of indium trichloride, gallium trichloride, and zinc chloride supported on MCM-41 as Lewis acids in the Friedel-Crafts acylation of aromatics with acyl chlorides was investigated. The support itself shows no catalytic activity in the benzoylation of benzene with BC, whereas the highest activity is showed by the supported indium trichloride. The order for acylation activity of the supported metal chloride (indium trichloride > gallium trichloride zinc chloride) is quite similar to that of the redox potential of the metals [E , +/, (-0.34 V) > E°Ga /Ga ( 0.53 V) > E 2n +/zn ( 0.74 V)] and confirms a possible relationship between the redox potential and the catalytic activity of the supported metal chloride. The reaction can be efficiently applied to a variety of aromatic compounds, including toluene, para-xylene, mesitylene, anisole, and 2-MN (70%-90% yield), confirming the moisture insensitivity of the catalyst. ... 

Reaction conditions were 200 °C or less and at least 200 atm of carbon monoxide. Benzene, chlorobenzene, cyclohexane, or 1,1,2-trichloro-1,2,2-trifluoroethane were used as solvents. The noble metal could be supported, but even unsupported or in the form of salt, oxide, or complex. Among the Lewis acid cocatalysts used successfully were FeCU, FeCU, FeBrs, AICI3, AlBrs, SnCl4, CuCL, and anhydrous HCl, but the highest yields were obtained with supported metals and with FeCL as cocatalyst. The presence of FeCL allowed for the use of milder experimental conditions with respect to the monometallic system, also affording higher conversions and better selectivities [24-26]. For example, nitrobenzene (24.6 g), Rh/C (5 % on Rh, 5.0 g), anhydrous ferric chloride (0.4 g), under 500 atm of carbon monoxide and at 190 °C for 5.5 h in benzene (100 ml), gave 100 % conversion with formation of PhNCO (35 % after vacuum fractional distillation). Diphenylurea, PhNHC(0)NHPh, and 1,3,5-... 

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