Thallium-based Catalysts

VANIA M. T. CARNEIRO, LUIZ S. LONGO JR AND LUIZ F. SILVA JR  

Avenida P.H. Rolfs, 36570-900 Vicosa, MG, Brazil Universidade Federal de Sao Paulo, Campus Diadema, Rua Prof. Artur Riedel, 275, 09972-270 Diadema, SP, Brazil Departamento de Quimica Fundamental, Institute de Quimica, Universidade de Sao Paulo, Av. Prof. Lineu Prestes, 748, 

Thallium compounds, especially in the form of thallium(i) and thallium(iii) salts, are ahle to mediate a great numbers of reactions, including oxidative rearrangements of ketones and alkenes, cyclisation reactions, a-oxidation of ketones, aromatic thallations, oxidative couplings, oxidations of phenols and nitrogen compounds and disulfide bond formation, thus, constituting a useful tool in synthetic organic chemistry. However, their application as catalysts has been less explored and there are only a few successful examples reported in the literature. 

Although used in several applications, thallium compounds are toxic. Because thallium(i) is more stable in the environment than thallium(iii), its toxicity has been most extensively studied. The toxic effect of thallium(i) is attributed to its ionic radius that is similar to potassium (Tl 1.54 and K 1.44 A). On the other hand, it is important to highlight that thallium is not accumulative and can be slowly excreted from the body by the Iddneys.  

Sustainable Catal)fsis With Non-endangered Metals, Part 2 

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The presence of thallium(0) led to an increase in activity and selectivity of metallic palladium catalysts supported on silica in aldose oxidation reactions. However, silica-supported thallium(0) had no activity by itself (entry 3). ° Similarly, the bimetallic catalyst platinum-thallium/ZSM-5, prepared by impregnation of thallium sulfate and chloroplatinic acid on Zeolite Socony Mobil-5 (ZSM-5), showed greater selectivity in propane aromatisation and almost the same catalytic activity as monometallic thallium/ZSM-5 (entry 4). Similar comparison of vanadium-caesium-copper and vanadium-caesium-copper-thallium catalysts supported on TiOa.SiC demonstrated that addition of thallium improved the catalytic activity in partial oxidation of p-tert-butyltoluene to p-tert-butyl-benzaldehyde (entry The application of solid-supported thallium-based catalysts in different processes includes (a) iron-thallium catalysts in carbon monoxide hydrogenations to form hydrocarbons and alcohols, and catalytic reforming of... 

Friedel-Crafts reactions involving electrophilic substitution of aromatic compounds have been reported on solid base catalysts such as thallium oxide and MgO. The rates of benzylation of toluene by benzyl chloride over MgO nanocrystals were found to be of the order CP-MgO > CM-MgO > AP-MgO.56 An important observation in the study was that x-ray diffraction of the spent catalyst... 

Karski and co-workers found that thallium acts also as a promoter and prevents the poisoning of the palladium particle with oxygen. Indeed, a bimetallic system which contained 5 wt% of Th led to 100% selectivity to gluconic acid at 95% conversion [117]. On their side, Boimeman et al. reported that charcoal-supported Pd-Pt catalysts prepared from a colloidal solution of Pd-Pt/NOct4Cl exhibit a superior activity and selectivity than industrial heterogeneous Pd-Pt-based catalysts [118]. 

It takes place at atmospheric pressure, between 450 and 550 C in the presence of a silver oxide based catalyst deposited on silica or of earth alkali metal oxides, thallium and lead, and with excess propylene. An inert (nitrogen, steam, etc.) is used as diluent, in order to absorb the heat generated daring the conversion, whose molar yield is 7Q per cent in relation to propylene. 

Heterogeneous catalysts containing thallium species have heen developed over the years to promote a number of different transformations. Table 20.1 shows examples for the use of thallium-based solid catalysts in different organic transformations. 

Table 20.1 Summary of some applications of thallium-based heterogeneous catalysts.

Phase-tranter Conditions. The Heck conditions described above are not useful, however, for a large number of alkenic substrates. A sometimes serious drawback is the high temperature (ca. 100 °C) often required. Upon addition of tetrabutylammonium chloride ( phase-transfer conditions or Jeffery conditions ), aromatic halides or enol triflates react under mild conditions with vinylic substrates or allylic alcohols. Variations of these conditions include the optional or additional presence of silver or thallium salts. The effect of using different salts, bases, catalysts, solvents, and protecting groups in the coupling of aminoacrylates with iodobenzene has been studied. ... 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

The selectivity (C-2 C-4 2,4-disubstitution) of Suzuki couplings of 2,4-dibromopyridine varies greatly with catalyst and conditions. The most selective C-2 mono-coupling is obtained using Pd(PPh3)4 with thallium hydroxide as base. (Caution Thallium hydroxide is very toxic.)... 

Method L is conducted with thallium hydroxide as the base and Pd(PPh3)4 as catalyst under very mild reaction conditions to afford a wide variety of biaryls, including highly encumbered, in fair yields [16]. The biaryl 189 was prepared from mesitylboronic acid (283) and iodobenzene in almost quantitative yield. Scheme 23. Whereas tri-ort/io-substituted biphenyls were obtained by this method in respective yields, synthesis of tetra-ort/io-substituted analogues failed [16], Among base-sensitive groups, only an ester function has survived thallium hydroxide-mediated reactions at room temperature. Concerning the extreme toxicity of thallium compounds, the method is applicable only in the small-scale labo-synthesis. 

To the solution of the ethoxylated thallium salt was added a solution of 2 mmol Grubbs catalyst (la) in 10 ml THF. The reaction mixture was stirred at room temperature for three hours. After evaporation of the solvent, the residu was dissolved in a minimal amount of benzene and cooled to 0°C. Thalliumchloride was removed via filtration. The desired complex was then washed with cold benzene (10 ml 3) and the filtrate was evaporated. The solid residue was recrystallized from pentane (- 70°C) to give the Shiff base modified Grubbs complex (2) as a brown solid. 

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