Coupling reactions in water, indium-promoted

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water (Eq. 8.49).108 When the allylation was mediated by indium in water, the reaction went smoothly at room temperature without any promoter, whereas the use of zinc and tin usually requires acid catalysis, heat, or sonication. The mildness of the reaction conditions makes it possible to use the indium method to allylate a methyl ketone in the presence of an acid-sensitive acetal functional group (Eq. 8.50). Furthermore, the coupling of ethyl 2-(bromomethyl)acrylate with carbonyl compounds proceeds equally well under the same reaction conditions, giving ready access to various hydroxyl acids including, for example, sialic acids. 

Lanthanide trillates, which are water-tolerant Lewis acids, were shown to greatly improve the yields, and therefore the scope of the aqueous Mukaiyama reaction (Kobayashi, 1991 Kobayashi and Hachiya, 1992). This is detailed elsewhere in this book. In keeping with the same concept, indium chloride in catalytic amounts promoted efficient cross-coupling (Loh et al, 1996b), as did the use of tris(pentafluorophenyl)boron (Ishihara et al., 1993). 

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