Granular Indium Barbier Allylation of Carbonyl Compounds: A More Economical Protocol

Abstract

A new protocol, amenable to be used in large-scale preparations, using a cheaper form of indium metal and mild warming is reported for the Barbier allylation of aldehydes and ketones with allyl bromide in N,N-dimethylformamide.

indium - carbonyl complexes - allylation - homoallylic alcohols - addition reactions

References and Notes

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Purchased from Alfa Aesar Chemical Co. A somewhat more expensive indium wire, 99.99% pure, purchased from Indium Corporation of America, also gave identical results.

<A NAME="RS15206ST-5">5</A>

Indium is so soft that an ingot of metal can be easily flattened by pressing with a spatula (or even strong fingernails) between two pieces of clean filter paper, and then cut into small pieces of a few square millimeters with scissors.

<A NAME="RS15206ST-6">6</A> It was previously reported that when a THF suspension of In powder and allyl bromide was stirred for 90 min, and then allowed to react with a ketone, a yellow-green color developed: Capps SM. Clarke TP. Charmant JPH. Höppe HAF. Lloyd-Jones GC. Murray M. Peakman TM. Stentiford RA. Walsh KE. Worthington PA. Eur. J. Org. Chem.  2000,  963 

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If the reaction is run in water instead of DMF, only a slightly greenish yellow color is observed in the reaction suspension that turns to an orange-ocher color when the reaction is complete. In this case, a better indication of reaction progress is to observe the metal surface, dark at the beginning, and shiny at the end.

<A NAME="RS15206ST-8">8</A> Das S. Hung CH. Goswani S. Inorg. Chem.  2003,  42:  5153 ; and references cited therein

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Radical reactions initiated by In:

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For a study on the direct binding of carbonyl compounds to the In surface, see:

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<A NAME="RS15206ST-15">15</A> For a similar NMR analysis of Ga and Al allyl derivatives, see: Takai K. Ikawa Y. Org. Lett.  2002,  4:  1727 

It is also possible that no actual organometallic compounds form under Barbier conditions, but rather a series of single electron-transfer processes occur at the metal surface, as previously proposed for other metals:

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