Gallium trichloride

[13450-90-3]  · GaCl3  · (MW 179.03)

(used as a Lewis acid or a precursor of organogallium reagents)

Physical Data: mp 77.9 °C; bp 201.3 °C; d 2.47 g cm-3.

Solubility: soluble in hexane, benzene, diethyl ether, ethanol, and other organic solvents as well as water.

Form Supplied in: colorless solid.

Purification: high-purity (99.999%) gallium trichloride is available.

Handling, Storage, and Precautions: moisture sensitive. A convenient way of handling this compound is to prepare a methylcyclohexane stock solution. The solvent is liquid over a wide range of temperature from -126 °C to 101 °C. Toxicity of gallium nitrate (i.v.) rat LD50: 46 mg kg-1.

Lewis Acid

Gallium trichloride has been used in Friedel-Crafts alkylation and acylation reactions as a Lewis acid.1 Although its acidity is generally considered to be lower than that of aluminum trichloride, another group 13 Lewis acid, its high solubility in organic solvents makes it useful for kinetic studies. Recent studies, however, have revealed novel aspects of this compound in electrophilic aromatic substitution reactions. Spectroscopy revealed that gallium trichloride interacts with p-acids such as silylethyne2 or silylallene.3 The complexes are strongly electrophilic compared with conventional alkenylating reagents, and react with aromatic hydrocarbons even at -78 °C to give b-silylalkenylated arenes. Ipso-substitution takes place with 1,2,3-trimethoxybenzene at the 2-position, and treatment of the arenium cation thus formed with organomagnesium compounds gives the 2,5-dihydrobenzene alkylated at the 5-position. The silylethyne-gallium trichloride complex, in the absence of an aromatic hydrocarbon, spontaneously trimerizes to a conjugated trienyl cation which, on treatment with organolithium or magnesium compounds, gives alkylated trienes.4 In the presence of gallium trichloride, cationic species appear to gain a lifetime sufficient to allow attack by organometallic reagents. Some unusual orientations have also been observed in aromatic substitutions using gallium trichloride. The reaction of toluene and bis-silylated buta-1,3-diyne gives an ortho-substituted product exclusively,5 and even isopropylbenzene reacts at the ortho-position predominantly. The tendency of the reaction to occur at the vicinity of the alkyl substituent, however, is restricted to diyne-based electrophiles. Other closely related electrophiles derived from silylethyne, silylallene, or bis-silylated 1,3,5,7-octatetrayne and gallium trichloride exhibit normal orientation. Gallium trichloride is also used in catalytic aromatic acylation reactions.6 Aliphatic and aromatic acid anhydrides react with anisole derivatives in the presence of 10 mol % gallium trichloride and 10-20 mol % silver perchlorate to give the para-acylated products.

The interaction of gallium trichloride with a p-acid results in regioselective reduction of an aldehyde group located in the vicinity of an ethynyl group.7

Gallium trichloride shows strong affinity toward halogens.8 ortho-Acylation of anilines with nitriles has been known to take place in the presence of boron trichloride. When gallium trichloride is added, the reaction is accelerated; this is ascribed to the abstraction of chlorine from the organoboron intermediate with concomitant formation of the stable gallium tetrachloride anion. It turns out that gallium trichloride is more effective than aluminum trichloride in this transformation.

The soft nature of gallium is effectively utilized in the activation of dithioacetals. In the presence of gallium trichloride and water, thioacetals are hydrolyzed to aldehydes and ketones.9 Allylstannanes react with thioacetals to give allylated products (1).10

Asymmetric Catalysis

Gallium-sodium-bis(binaphthoxide) (GaSB) prepared from gallium trichloride, sodium tert-butoxide (four equiv) and BINOL (two equiv) is an excellent catalyst for the asymmetric Michael addition of malonate to cyclopentenone and cyclohexenone.11 Use of 10 mol % of the catalyst gives the adducts in high yields and high enantiomeric excesses up to 98% ee (2). The reaction can be accelerated by the presence of one more equivalent of sodium tert-butoxide, indicating the rapid complex formation between sodium malonate and GaSB. Gallium-lithium-bis(binaphthoxide) (GaLB) catalyst is prepared from gallium trichloride and lithiated binaphthol, and is used in the asymmetric ring-opening reaction of meso-epoxides with 1,1-dimethylethanethiol.12 The reaction, being accelerated by the presence of MS 4A, is conducted with 10 mol % of the complex giving the thioalcohol of 98% ee from cyclohexene oxide. The asymmetric ring-opening of the same epoxide oxide with para-methoxyphenol is catalyzed by 20 mol % of the GaLB catalyst to give the alkoxycyclohexanol in 93% ee.13


Carbometalation (carbogallation) with a carbon-carbon triple bond has become an important reaction of organogallium compounds. Carbogallation was first found in the dimerization of alkynylgallium reagents;14 reaction of silylated 1-alkynes with gallium trichloride gave enynes. Alkynyldichlorogalliums generated by transmetalation turn out to be unstable in hydrocarbon solvents and spontaneously dimerize to give bis-gallated enynes. Such carbogallation reactions also take place between allylgallium and 1-alkynes to give gallated 1,3-dienes.15 The alkyne should either be 1-gallated or 1-silylated for the carbogallation to occur. Gallium enolate and ethynylgallium, which are generated from silyl enol ethers and silylethyne by treatment with gallium trichloride undergo carbogallation (3).16 After acid work-up, a-ethenylated ketones are obtained. This is a novel and convenient method to ethenylate enolate derivatives. Equatorial preferences are observed in the ethenylation of cyclohexanone enolates.17 These results are in contrast to the stereochemistry observed in enolate alkylation, which takes place from the axial surface of the enolate plane. In general, isomerization to the thermodynamically stable conjugated enone is not observed. The ethenylation also occurs with silylated 1,3-dicarbonyl compounds,18 and ethenylmalonate possessing an acidic proton can be synthesized by this method. The ethenylmalonate is relatively insensitive to acid, while it rapidly isomerizes to the conjugated product in the presence of triethylamine. Analogously, gallium phenoxide reacts with silylethyne to give ortho-(b-silylethenyl)phenols.19 These studies revealed that organogallium compounds undergo carbometalation reactions similar to organotin compounds. The enolate and phenoxide of both organometallic reagents carbometalate with carbon-carbon triple bonds. The above serves as an interesting example of the fact that organometallic reagents of elements arranged diagonally in the periodic table exhibit similar reactivities.

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Masahiko Yamaguchi

Tohoku University, Aoba, Sendai, Japan

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