Gallium reactions with alkynes

An interesting feature of gallium(III) compounds as classified Lewis acids is their ability to interact with unsaturated bonds particularly with alkynes. GaCl3 formed a tt complex with alkynes, which strongly activated the triple bond toward nucleophilic attack. Carbometalation (carbogallation) with alkynes is another characteristic reaction of organogallium(III) compounds, and it is likely that the initial step for this reaction is the interaction of the tt electrons of alkynes with the Lewis acid gallium (III) center. 

Gallium The hydroamination products ArCH(NArR )CH3 have now been obtained from the reaction of alkynes ArC=CH with aromatic amines ArNHR, catalysed by GaCl3, followed by reduction of the imine intermediates ArC(NArR )=CH2 with LiAlH4. DFT calculations suggest that the key step proceeds as a Markovnikov-type iyn-addition of GaCl3 and the amine across the C=C bond. ... 

Hydrogen fluoride (HF) is often used as a catalyst for alkenes, since there is less tar (fewer decomposition and polymeric by-products) and the volatile catalyst is easily removed. Both ferric chloride (FeCl3) and BF3 are common catalysts for alkene-aromatic coupling. Typical alkyne catalysts are aluminum chloride, gallium trichloride (GaCl3), BF3, and sulfuric acid. Reaction of a mixture of xylenes with a trace amount of BF3 in liquid HF gave the thermodynamic mixture 18% ortho, 60% meta, and 22% para but treatment with excess BF3 gave virtually 100% m-xylene. 07b,117... 

---