[7440-74-6] · In · Indium · (MW 114.82)
(effective mediator for the allylation of carbonyl compounds)
Physical Data: mp 156 °C; d 7.300 g cm-3.
Form Supplied in: available in the form of bars, foils, wire, granules, shot, and powders of various sizes. For synthetic purposes, indium powder of about 60 mesh is the most convenient form.
Handling, Storage, and Precautions: no special precautions are required.
Indium is a very effective mediator for the addition of allyl groups to carbonyl compounds under very mild reaction conditions. Typically, the organoindium reagent is prepared in situ by the addition of an allyl iodide to a suspension of indium powder in a suitable solvent. The preparation of highly activated indium powder (Rieke indium) via reduction of indium(III) chloride has been reported,2 but in the vast majority of cases commercial indium powder is sufficient. Allyl bromides are equally well suited for this reaction, but allyl chlorides are not sufficiently reactive, resulting in diminished yields. Studies by Butsugan and co-workers1 suggest that the active organoindium species is the allylic indium sesquihalide (eq 1), which is in accord with results of Gynane and Worrall.3
Studies of the variation of the yields of allylation as a function of the ratio of indium:allyl iodide:ketone have shown that two-thirds of the allyl groups are transferred to carbonyl compounds, and thus the stoichiometry of the reaction is 2:3:2 (indium:allyl halide:ketone).1 In cases where quantitative utilization of the allyl halide is important, this can be achieved by substituting indium(I) iodide for metallic indium.4
Allylindium sesquihalides react readily with a variety of carbonyl compounds, including ketones (eqs 2 and 3),1 aldehydes (eqs 4 and 5),1 acid anhydrides,5 quinones,6 and aldimines.7 In general, the stereoselectivity obtained in these reactions is low to moderate. For example, indium-mediated allylation of 4-t-butylcyclohexanone (eq 3) gives an 83:17 mixture of axial:equatorial products. In the case of substituted allyl halides (eqs 4 and 5), regioselectivity is extremely high as only the products of g-coupling are observed. a,b-Unsaturated carbonyl compounds give exclusively 1,2-addition products (eq 6).1
One significant advantage of allylindium sesquihalides over alternative organometallic allylating reagents (allylmagnesium halides, allyllithium compounds) is their low basicity. Yields remain high even in the presence of an active hydrogen, such as in the case of salicylaldehyde (eq 7).1 This property has been exploited by Chan and Li, who have reported indium-mediated allylations of carbonyl compounds carried out in water.8 These workers report that in contrast to similar reactions with zinc and tin, where acid catalysts, heat, or sonication are often required, the indium-mediated reaction proceeds readily without the need of a promoter.
The mildness of the reaction conditions employed for indium-mediated allylation can also be advantageous when allylating labile carbonyl compounds. For example, attempts to allylate the tricarbonyliron butadiene complex (1) with allylmagnesium bromide fail completely, yielding only unidentifiable decomposition products. In contrast, the indium-mediated allylation of (1) (eq 8) proceeds smoothly to give the corresponding homoallyl alcohols in 91% yield.9
Timothy B. Lowinger
The Ohio State University, Columbus, OH, USA