CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

Given that the Ga-mediated Barbier-type allylation reactions require metal
activation, other methods for the generation of allylgallium species via transmetallation of
(i) allylmagnesium bromide with GaCl 3 , and (ii) allylindium sesquihalides with Ga metal
have been developed. The allylic-Ga reagents, prepared from the first route, could execute
allylation of carbonyl compounds in excellent yields in aqueous as well as organic
media. 66e Some related reactions like propargylation of aldehydes, 66f,g
allylation of
epoxides, 66h aromatic hydrazones and N-oxides, 66i as well as addition of propargyl
bromide to terminal alkynes 66j and alkylation of haloesters 66k have also been accomplished
using metallic Ga.
In an innovative approach, Ga has been used for the allyl transfer from a bulky
substrate to an aldehyde to get the homoallylic alcohols. 67a-c In these approaches, treatment
of the bulky homoallylic alcohols such as 57 with a Grignard reagent and GaCl 3 led to
retro-allylation furnishing the σ-allylgallium reagents, which on reaction with a suitable
aldehyde/ketone furnished the new homoallylic alcohols 58. Notably, depending on the
stereochemistry (threo or erythro) of 57, the retro-allylation reaction selectively generated
the E- or Z-allylgallium reagents from the γ-substituted allyl halides. This is advantageous
for better diastereocontrol with these allyl halides (Scheme II.2.5).
OH
Pr
Pr
57
i,ii GaL n
iii
R 1 OH
58
(i) MeMgI, dioxane; (ii) GaCl 3 ; (iii) RCHO.
Scheme II.2.5
35