Gallium enolates

Triethylgallium has been used as a non-nucleophilic base to generate enolates from ketones, both cyclic and acyclic, without forming carbonyl addition products.290 The gallium enolates can then be C-benzoylated, and can participate in aldol reactions. Unsymmetrical ketones preferentially enolized at the methylene, under kinetic control. 

Treatment of cr-bromo ketone with gallium triiodide or methylgallium iodide provides the corresponding gallium enolate, which reacts with carbonyl compounds and imines to give /3-hydroxy ketones and /3-amino ketones, respectively, in moderate yields (Scheme 131).417... 

Treatment of silyl enolates with methyllithium followed by an addition of gallium trichloride affords the corresponding gallium enolates. The reaction of the resulting gallium enolates with a-halo carbonyl compounds in the presence of triethylborane provides 1,4-dicarbonyl compounds in good yields (Scheme 142).433... 

Gallium enolate and ethynylgallium, generated, respectively, from silyl enol ether and silylethyne by treatment with gallium trichloride, undergo carbogalla-tion, giving a-ethenylated ketones after work-up with aqueous acid (Scheme 7.28)... 

Similar to aluminum, the synthetic application of metallic gallium or its salts in the aldol reaction produced gallium enolate and its Lewis acidic salt (161). The first example of the gallium enolate aldol reaction was reported by Han and Huang in 1998 (162). The gallium enolates could be formed easily from a-bromoketones and unmodified ketones with gallium triiodide and triethylgal-lium, respectively (163). 

Tris(trimethylsilyl)silane [20,21], thiols [22], germanes [23-25] and gallium hydride [26] can be added easily to terminal alkynes in the presence of Et3B/02. This process was extended to internal alkenes (Scheme 8, Eq. 8a) as well as silyl enol ethers (Eq. 8b) by using tri-2-furylgermane. In this last case, basic or acidic treatment of the main syn /J-siloxygcrmanc furnishes the corresponding E- or Z-alkene, respectively [24],... 

Allylgallium and propargylgallium compounds can be prepared by the direct reaction of the corresponding bromides or iodides with gallium metal.402,403Gallium enolates are also prepared from a-halo carbonyl compounds with gallium.404... 

This chapter is primarily concerned with keto-enol equilibrium and the chemistry of dissociated enols (enoxides) and lithium enolates. The acid-base aspects of the chemistry of other metal enolates (e.g. silyl enol ethers "", boron enol ethers" " " , aluminium , tin " , gallium , bismuth , zinc " ", rhodium , palladium " , manganese ", copper , nickel , magnesium " , titanium " , molybdenum , zirconium" " and ammonium" " enolates) have been reported elsewhere. 

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