Background and Synthesis of Rieke Indium

Indium was discovered in Germany in 1863. However, it was not until 1924 that gram quantities of the metal became available. The use of indium in organic chemistry has been very limited. Indium oxide does have extensive use in the area of photovoltaic devices. It serves as a transparent coating on glass which is electrically conducting. 

In 1974, we reported a general procedure for the preparation of highly reactive indium metal powder [1]. The procedure involves the reduction of anhydrous indium trichloride with alkali metals in hydrocarbon solvents under an argon atmosphere. A typical combination is potassium metal and xylene as the solvent. The process works well for most any combination with the requirement that the boiling point of the solvent should exceed the melting point of the alkali metal. 

Little work has been reported concerning the direct synthesis of alkylindium halides. Gynane and coworkers [2] reported the direct reaction of indium metal with alkyl halides. The reaction times were, however, on the order of a day or more, and the product was a mixture of R2lnX and RI11X2. We have found that our reactive indium will react with methyl and ethyl iodide in 2h at 80°C in xylene to give nearly quantitative yields of a single product, the dialkylindium iodide. The overall reaction scheme is summarized as follows  

Chemical Synthesis Using Highly Reactive Metals, First Edition. Reuben D. Rieke. 2017 John Wiley Sons, Inc. Published 2017 by John Wiley Sons, Inc. 

Under our reaction conditions, the indium monoiodide formed does not react further with additional alkyl iodide as has been reported [3, 4]. Additional reactions were attempted with indium monoiodide and alkyl iodides in the presence of alkali salts under our reaction conditions, and no reaction was observed. 

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