CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

In the absence of a sterically-bulky substituent at the carbonyl or allyl bromide, the
allylation of an aldehyde with a γ-substituted allylic indium reagent occurs regioselectively
at the γ-position to afford the γ-homoallylic alcohol. Loh et al., however, have succeeded
in carrying out the In-mediated allylation via α-addition without the use of a stericallyhindered
substituent. 65g Interestingly, the solvent plays an important role in determining the
regioselectivity in these reactions. While water (10 M) and water/dichloromethane (10
M/10 M) exhibited excellent α-selectivity, the reactions in DMF, ethanol, THF and water
(0.5 M) followed exclusive γ-attack.
Compared to In, reports on the Ga-mediated allylation are limited. Till so far, the
Ga-mediated Barbier type reactions have been conducted in water and/ or organic solvents
using both metallic Ga/allyl halide 66 as well as preformed allyl-Ga dihalides. The Gamediated
allylation of aldehydes and ketones with allyl iodide in THF under
ultrasonication afforded the homoallylic alcohols. 66a Aldehydes were more reactive than
ketones, the reaction being more facile with allyl iodide. A similar reaction under heating
conditions is also reported in water. 66b Chemical additives, such as KI, LiCl (in THF) 66c or
HCl, NH 4 Cl (in H 2 O) 66b are required when the reaction is carried out with allyl bromide.
The reaction proceeded with excellent chemoselectivity with α,β-conjugated aldehydes/
ketones (exclusive 1,2-addition), and with esters, cyanides and acyl chlorides to furnish the
corresponding products. Regarding regioselectivity, only mono addition product was
formed with benzyl halides, while the γ-adducts were predominantly produced with the γ-
substituted allyl halides. Alternatively, the reaction could also be carried out under solventfree
conditions, but needed metal activation by ultrasound. 66d
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