Gallium Catalysts

The gallium catalyst refers to a metal gallium, gallium salt or gallium complex having a catalytic function. Gallium is gray-blue or silver-white. It is an amphoteric metal located in the fourth cycle Group IIIA of the chemical periodic table. Gallium is chemically stable and does not react in dry air. Gallium oxidizes and tarnishes in humid air and ignites when heated to 500 °C. Gallium reacts slowly with water at room temperature, and reacts with boiling water to form gallium hydroxide, releasing hydrogen. Gallium has +1, +2, and +3 valences in chemical reactions. Its valence in most gallium catalysts is +3. The content of gallium in the earth's crust is very low. It is widely distributed but does not exist in a pure metal state. Instead, it exists in the form of CuGaS2.

Figure 1. Metal gallium catalyzed allylation

Applications:

Gallium catalysts are widely used in organic reactions due to their rich and special aqueous phase stability, strong Lewis acidity, high selectivity, easy recycling and environmental friendliness.

• Allylation and propargylation reaction: In organic synthesis, 3-Buten-1-ol is widely used as a very important intermediate. The metal gallium functions as a catalyst to catalyze the allylation reaction of aromatic and aliphatic aldehydes and ketones, and 3-Buten-1-ol compounds are obtained in high yield. When metal gallium catalyzes the reaction, the reaction conditions are mild and the operation is simple, and it is a simple and effective method for synthesizing acetylene alcohol.
• Cycloisomerization and Friedel-crafts reaction: The cycloisomerization reaction is an effective method for preparing structurally unique polycyclic compounds. The Friedel-crafts reaction is one of the main reactions in the construction of carbon-carbon bonds. Some gallium catalysts (such as GaCl₃ and Ga(OTf)₃) as a strong Lewis acid can efficiently catalyze this series of organic reactions. For example, GaCl₃ can catalyze the cycloisomerization of an enyne compound such as ω-phenyl-1-yne, resulting in the highthroughput production of dihydronaphthalene derivatives with high regioselectivity. In the presence of some electron-rich reagents (anisole, phenol and hydrazine derivatives, etc.), GaCl₃ can catalyze the Friedel-Crafts reaction of the cycloisomerization product, thereby synthesizing very valuable polycyclic compounds (such as four-hydrogen isoquinoline and tetrahydrobenzazepine compounds). Ga(OTf)₃ can catalyze propargyl bromide to form a propargyl organometallic reagent, which is then added to a 2,2'-diformyldiphenylacetylene compound to form a triacetylene intermediate. The triacetylene intermediate then undergoes a [2 + 2 + 2] cycloisomerization reaction to form a five-membered spiroene.vodopa in pharmaceutical waste water.
• Ring opening reaction：Many useful polyfunctional compounds are obtained by ring opening reactions. Gallium trichloride can catalyze the ring-opening reaction of substituted ethylene oxide with ammonium thiocyanate in the aqueous phase, with the corresponding β-hydroxy thiocyanate obtained with high regioselectivity and in quantity. Ga(OTf)₃ is also used for the ring opening reaction of an epoxy compound. For example, Ga(OTf)₃ can catalyze the addition ring opening reaction of arylthiophenol with an epoxy compound, with high regioselectivity and chemoselectivity to form the corresponding β-hydroxy sulfide. The gallium catalyst is used for the ring-opening reaction, and generally has the advantages of high reaction selectivity, short reaction time, easy recovery of the catalyst, and reusability.

Figure 2. Ga(OTf)₃ catalyzed ring opening reaction

• Other reactions: The gallium (III) catalyzed three-component [4+3] cycloaddition reaction can synthesize the corresponding cycloheptane hydrazine compound in one step at a high yield. Both Ga(OTf)₃ and GaBr₃ promote the reaction of hydrazine and aldehyde/ketone (ketal) with various diolefins. GaCl₃ as a catalyst can catalyze the three-component coupling reaction of naphthol, an alkyne and an aldehyde, thereby synthesizing a chromene compound in one step at a high yield. GaCl₃ catalyzes the insertion of an aryl isonitrile into the carbon-sulfur bond of the dithioacetal to form a thioimidate containing an α-alkylthio group. Ga(OTf)₃ can catalyze the direct substitution of various sulfur-containing nucleophiles with alcohols to obtain sulfur-containing intermediates. GaCl₃ can catalyze the intermolecular hydroamination of alkyne and arylamine, with Markovnikov hydroamination products obtained with high selectivity.

References

1. Agafonov, Yu. A.. (2018). "Propane Dehydrogenation on Chromium Oxide and Gallium Oxide Catalysts in the Presence of CO2." Kinetics and Catalysis 59(6), 744 -753.
2. Ino, Haruhiro. (2008), "Aromatization of waste polyolefin pyrolysate over H-ZSM-5 zeolite-supported gallium oxide catalysts." Journal of Material Cycles and Waste Management 10(2), 129-133.
3. Li, Yuejin. (1994), "Selective catalytic reduction of NO with methane on gallium catalysts." Journal of Catalysis 145(1), 1-9..