Ultrasound catalytic processes

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

One of the important application area of ultrasound (US) is catalytic reactions with the participation of low molecular mass compounds and heterogeneous catalysts. The effect of ultrasound on catalytic reactions in the presence of platinum and rhodium catalysts of various dispersities was investigated in Ref [1]. It was demonstrated that ultrasound can provide for the occmrence of chemical processes that cannot be performed even in the presence of catalysts. It is assumed that the main mechanism of its action on catalytic processes consists in the dispersion of catalyst particles however, as was shown in Ref [1], the adhesion of particles can occur during the action of the so-called Bjerknes forces, that is, forces that promote the attraction of particles (primarily small particles) to a deformed bubble followed by their sticking together. As a consequence, the diffusion of reagents to the surface of a particle becomes more pronoimced and the rate of the process inereases. 

Sommelet process org chem The preparation of thiophene aldehydes by treatment of thiophene with hexamethylenetetramine.. so-mal ya, pra-s3s ) Sonnenschein s reagent analychem A solution of phosphomolybdicadd that forms a yellow precipitate with alkaloid sulfates. z6n-3n,shTnz r5,a-j3nt) sonocatalysis chem 1. Initiation of a catalytic reaction by irradiation with sound or ultrasound. 2. Use of sound to impart catalytic activity to a chemical compound. san-3-k3 tal-3-s3s)... 

A number of novel process methods have been described. For example, ultrasound [1142] and phase-transfer catalytic techniques [1085] have been employed to Increase reaction rates in the synthesis of polycarbonates. 

The ultrasound-induced changes on samarium metal were not studied, but its reaction with iodine is strongly accelerated by preliminary sonication in THF. The yellow Smia is formed as a solvate which releases the synthetically useful Sml2 by adding catalytic amounts of mercury under sonication. The overall process requires less than 30 min. ... 

As a last example of an ultrasound application to catalytic reactions using solid catalysts, we refer to unpublished results. The hydrolysis of inuline (Eq. 12) is catalyzed by acid substances, i.e., inorganic or organic acids in aqueous solution, or acid solids or enzymes. The products of acid hydrolysis are fructose and glucose. Because of the use of the reaction in the food industry, an acid catalyst should not pollute the products at the end of the process. Therefore, solid acids, much more easily separable from the reaction products than liquid acids, must be preferred. In the present work, the employed catalyst was Amberlite IR-120-H (Carlo Erba), that is to say, a solid catalyst with a particle size from 15 to 45 mesh. The reaction was studied in a batch and in a continuous sonicated reactor. 

Reaction with Alkynes. Acetals of dihaloacetophenones are prepared from phenylethynes with 7V-halosuccinimide (NIS or NBS) and catalytic amounts of HTIB in methanol (eq 33). When an equimolar quantity of NIS is used in the presence of a catalytic amount of HTIB, a-diiodoacetophenone acetal Is formed in an excellent 3deld. If the ratio of NIS to phenyleth3me Is increased with concomitant increase in the amount of HTIB, the 3deld of the acetal decreases to 7 0% and methyl phenylglyoxylate is formed as a minor product. In this process, there is no evidence of formation of methyl phenylacetate, which is typically formed in the reaction of equimolar HTIB with phenylethjme in hot methanol. The acetal of methyl phenylglyoxylate is also prepared as the sole product by treatment of 2-lodo-l-phenylethyne with NIS and a catalytic amount of HTIB (eq 34). Terminal alkynes are also reported to react with HTIB in an ultrasound enhanced system to furnish arylsulfonates in a quick and simple one-step process (eq 35). ... 

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