Reusable reagent system

Another reagent system that has been recently employed in the Paal synthesis of thiophenes is the combination of bis(trialkyltin)- or bis(triaryltin) sulfides with boron trichloride. Known as the Steliou reagent,it has been utilized in the transformation of 1,4-diketone 11 to thiophene 12. Higher yields are obtained in shorter reaction times in contrast to the use of Lawesson s reagent. Additionally, others have noted the relative ease of the work-up procedure using the Steliou conditions, and the fact that the tributyltinchloride byproduct of the reaction is reusable. Similarly, the combination of the bis(trimethylsilyl)sulfide has been used in conjunction with trimethylsilyltriflate for the preparation of thiophenes in an analogous manner. ... 

Study of the reusability of the catalyst shows a 10% decrease in the activity of the tetramethylammonium hydroxide/RuCl2(PPh3)3 reagent system in the oxidation of... 

The related system 31 has been prepared by thionation of the diketone 32 with P4S10 or Lawesson s reagent. Furthermore, dithieno[2,3-6 2 ,3 -c( thiophene 31 was also submitted to electrochemical polymerisation <04TL3405>. Lawesson s reagent has also been used to effect conversion of several 1,4-diketones to thiophenes employing a new reusable catalytic system consisting of Bi(OTf)3 and the ionic liquid [bmim]BF4 (l-butyl-3-methylimidazolium tetrafluoroborate) <04TL5873>. 

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... 

Silica-supported reagents have been exploited as nontoxic, inexpensive, reusable, and environmentally acceptable catalysts for developing stoichiometric reaction methods in organic chemistry and specifically in carbohydrate chemistry. Apart from being easy to handle and to store, these systems allow facile workup, the catalyst being removed by simple filtration, and the reaction products isolated by chromatographic purification, if necessary. 

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