Abstract
In the context of synthetic chemistry, Indium is one of the least explored elements of the notorious group 13 of the periodic table and has not attracted quite the same amount of attention as its fellow members, Aluminium and Boron, which have shown unprecedented synthetic applications for more than half a century. Nonetheless, Indium has emerged in recent years as a very valuable catalyst for multicomponent reactions. From the use of indium powder or easily accessible and cheap indium salts to more complex indium‐based metal‐organic frameworks or nanoparticles, a plethora of applications has been described throughout this last decade, showcasing not only the versatility of indium catalysis but also how much there is still to be explored. In the aftermath of the international year of the periodic table of the chemical elements in 2019, we navigated through the large inventory of multicomponent reactions (MCRs) to encounter the types of useful reactions leading to important target compounds (many of which are biologically active) catalyzed by this d‐block post‐transition metal.
Highlights
Indium was discovered by German chemists Ferdinand Reich and Theodor Richter in 1863 and successfully isolated one year later, and named indium due to the indigo colored line of its spectrum.[1]
With atomic number 49, placed in period 5 and group 13, this d-block post-transition metal caught the attention of researchers in the field of organic chemistry and catalysis in the early 1990s, due to the Lewis acidity of group 13 metal complexes.[2]
Despite the well-established application of these rare-earth metals in multicomponent reactions (MCRs), indium presents several advantages for catalytic purposes, as it is often cheaper, easier to handle, stable to moisture and air and it is suitable for a wide range of chemical transformations, mostly due to its low first ionization potential, which makes it a perfect candidate for single-electron transfer reactions, and its low heterophilicity, which makes indium catalysts ideal for C-C bond formation reactions and tolerant for multiple functional groups, very desirable for multicomponent approaches.[5]
Summary
Indium was discovered by German chemists Ferdinand Reich and Theodor Richter in 1863 and successfully isolated one year later, and named indium due to the indigo colored line of its spectrum.[1]. 3. Knoevenagel-initiated MCRs not being a multicomponent reaction per se, the Knoevenagel reaction is often the first mechanistic step of various cascade-based multicomponent approaches, especially combined with the Michael addition.[33] Catalytic versions of these chemical transformations are very popular, with several examples reporting the use of indium salts to promote these multicomponent synthetic processes, namely indium(III) chloride.
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