Ultrasonic Acceleration of Organic Reactions

Initial investigations showed that use of a phase transfer catalyst was unnecessary. However, it later transpired that the reaction was auto-catalysed although addition of an independent catalyst was necessary in some cases [182], 

Long chain a-bromo-to-carboxylic acids were converted to their 128-iodo-derivatives in quantitative yield simply by sonolysis of the acid with 128-iodine in ethyl acetate solution for 20 min [183]. Furthermore, investigations showed that the system would tolerate up to 7.5 % volume water. As yet there are no examples of this simple procedure being used in place of the commonly used Finkelstein method [184] for preparation of alkyl iodides from the equivalent bromide. 

A further example of the use of ultrasound to simplify a standard organic procedure is the discovery that a variety of sugar acetals can be prepared 

A Japanese patent describes the preparation of iV-formyl aspartic anhydride by direct reaction of L-aspartic acid with formic acid in the presence of acetic anhydride and an optional catalyst. Reaction is complete after 12 h of sonolysis at 45 °C and gives a 93 % isolated yield of the anhydride [198], 

The stereochemical outcome of the reaction is also identical to that of the thermal reaction. 

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The ultrasonic field considerably accelerates processes and increases yields of Al-organic complexes with low-active alkyl halides, complexes of sodium, naphthaline, benzoquinoline, and other aromatic compounds [742], Use of US treatment in Ulmann condensation [742] in the presence of copper powder (3.285) allows acceleration of the reaction rate 50 times and a yield of 81% in comparison with <1.5% without US ... 

Some catalytic organic reactions, for example, hydrogenations, dechlorinations, were reviewed recently.Boudjouk et al. found that ultrasonic irradiation accelerated the hydrosilylation of alkenes in the presence of PtJC with good to excellent yields at under atmospheric pressure, while higher temperature above (lOCC) and pressure (over... 

Application of ultrasounds is among the new alternative techniques which can accelerate heterogeneous reactions by increasing the surface (dispersity) of the reagents, while in homogeneous systems ultrasounds assist the even heat distribution in a reactor. Thus, the apphcation of ultrasounds or of MW irradiation may substantially shorten the reaction time in oxidations of alcohols with PCC, from hours to minutes.It is beheved that the ultrasound produces erosion on the PCC surface and therefore accelerates its interaction with the organic substrates. The involvement of ultrasonic irradiation in a drastic reduction on the amount of PCC used was also indicated. 

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