Allylation of carbonyl compounds mediated by indium

Compared to other metals, indium has proved attractive, owing to three major properties (i) indium is unaffected by boiling water (ii) it does not form oxides readily in the air and (iii) its first ionization potential is low (5.79 eV). Li and Chan (1991b) took advantage of these properties to undertake the first allylation of aldehydes and ketones mediated by indium in water, without the help of an additive, such as a salt or a cosolvent, as well as the help of ultrasound. 

The interest of indium, compared to other metals, has been emphasized by Chan et al. (1994) in the allylation of carbonyl compounds containing acid-labile groups. Only indium as the metal gave the desired product in good yield  

As the zero-valent indium species is regenerated by either zinc or aluminum, a catalytic amount of InCls, together with either zinc or aluminum, could be utilized in the allylation of carbonyl compounds in tetrahydrofuran-water mixture. The presence of water was found to be effective in suppressing undesired side-reactions (Araki et al, 1992). 

Likewise, indium trichloride as the catalyst in the presence of the stoichiometric amount of tin could replace indium as the promotor in the aqueous allylation of trifluoroacetaldehyde (Loh and Li, 1996). 

The problem of regeneration of the indium powder after its use in an allylation process, which is crucial from economical and ecological points of view, was resolved by simple electrochemical deposition of the metal on an aluminum cathode in a conventional electrolysis unit (Prenner et al, 1994). 

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Regio- and Stereoselectivity. For the allylation of carbonyl compounds mediated by indium and other compounds in aqueous media, usually the carbon-carbon bond forms at the more substituted carbon of the allyl halide, irrespective of the position of halogen in the starting material. However, the carbon-carbon bond forms at the less-substituted carbon when the y-substituents of allyl halides are large enough (e.g., trimethylsilyl or tert-butyl) as shown by Chan et al.139 (Scheme 8.10). The following conclusions can be drawn ... 

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