GaCl3 in Organic Synthesis

Abstract

AbstractGaCl3 and organogallium compounds generated from GaCl3 possess novel properties that can be used in organic synthesis. This microreview summarizes our studies on the use of GaCl3 as 1) a reagent to generate organogallium compounds, which can carbometalate unactivated unsaturated bonds, and 2) a Lewis acid which can activate organic molecules by interacting with the π‐, n‐, and σ‐electrons. Carbometalation of carbon–carbon triple bonds (carbogallation) is a characteristic reaction of organogallium compounds. Carbogallation of gallioacetylenes occurs at lower temperatures than with silylacetylenes, and addition to 1‐alkynes is slow. The regiochemistry and stereochemistry are in accordance with other carbometalation reactions, giving 1,1‐digallio or 1‐gallio‐1‐silyl intermediates in the cis‐addition mode. Carbogallation of enolates and acetylenes is effective for the α‐ethenylation of ketones, and a catalytic version of the reaction has been developed. When chloroacetylenes are used, α‐ethynylation of ketones proceeds by addition and β‐elimination. Aniline and phenols can be ethenylated or ethynylated at the o‐position of the heteroatom groups. GaCl3 has been used not only for the ethynylation of heteroatom compounds but also hydrocarbons such as 1,4‐enynes or 1,4‐diynes by C–H activation, and triethynylvinylmethanes and tetraethynylmethanes are obtained in one step via nucleophilic propargylgallium intermediates. As a Lewis acid, GaCl3 has the ability to interact with π‐acids such as silylacetylenes. The resulting gallium complexes are sufficiently electrophilic to react with aromatic hydrocarbons, even at –78 °C, to give an organogallium arenium intermediate. Arenium cations or vinyl cations appear to be stabilized in the presence of GaCl3. An unusual orientation has been observed in electrophilic aromatic substitution with GaCl3. GaCl3 activates even cycloalkane C–H bonds at σ‐electrons, and the reaction of cis‐perhydronaphthalene and naphthalene catalytically gives 2‐naphthylated trans‐perhydronaphthalene. Two C–H activation reactions of hydrocarbons with GaCl3 are notable as they generatenucleophilic and electrophilic species. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)