‘GaI’: A new reagent for chemo- and diastereoselective C–C bond forming reactions

Abstract

The potential of ‘GaI’ as a reductant in organic transformations, in particular C–C bond forming reactions involving α-functionalised ketones, has been investigated. Of most interest are the diastereoselective aldol coupling reactions of the α-alkoxy ketones, PhC(O)C(H)(OR)Ph, R = Me, Et or ⁱPr, which give the novel γ-alkoxy, β-hydroxy ketones, PhC(O)C(H)(Ph)C(OH)(Ph)C(H)(OR)Ph, containing three contiguous stereogenic centres. The diastereostereoselectivity of these reactions was established from NMR spectroscopic and X-ray crystallographic studies of the products. When R = Me, the 2R,3S,4R/2S,3R,4S-enantiomeric pair is formed, whereas when R = Et or ⁱPr, 2R, 3R, 4S/2S, 3S, 4R-diastereoisomers result. These differences are rationalised in terms of the likely transition states of the reactions. The same products are not formed when higher oxidation salts of gallium, or InI, are employed as the inorganic reagent. The reactivity of “GaI” towards α-halo ketones, a 1,2-diketone and α,β-unsaturated ketones has also been explored. Again, the outcomes of these reactions have been compared to those involving Ga(II) and Ga(III) reagents.