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An alkyne trimerisation is a [2+2+2] cycloaddition reaction in which three alkyne units (C≡C) react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. [1] Being a cycloaddition reaction, it has high atom economy.
Triazenes are prepared from the N-coupling reaction between diazonium salts and primary or secondary amines. [4] The coupling reactions are typically mild, using a base such as sodium acetate, [5] sodium carbonate, [5] or sodium bicarbonate. [4] The diazonium reagents are themselves available starting from amines.
In organic chemistry, alkynylation is an addition reaction in which a terminal alkyne (−C≡CH) is added to a carbonyl group (C=O) to form an α-alkynyl alcohol (R 2 C(−OH)−C≡C−R). [1] [2] When the acetylide is formed from acetylene (HC≡CH), the reaction gives an α-ethynyl alcohol. This process is often referred to as ethynylation.
Upon complexation the C-C bond elongates and the alkynyl carbon bends away from 180º. For example, in the phenylpropyne complex Pt(PPh 3) 2 (MeC 2 Ph), the C-C distance is 1.277(25) vs 1.20 Å for a typical alkyne. The C-C-C angle distorts 40° from linearity upon complexation. [4]
The reaction is assisted by the copper, which, when coordinated with the acetylide lowers the pKa of the alkyne C-H by up to 9.8 units. Thus under certain conditions, the reaction may be carried out even in the absence of a base. In the uncatalysed reaction the alkyne remains a poor electrophile. Thus high energy barriers lead to slow reaction ...
The 1,2,4-triazines can react with electron-rich dienophiles in an inverse electron demand Diels-Alder reaction. This forms a bicyclic intermediate which normally then extrudes a molecule of nitrogen gas to form an aromatic ring again. In this way the 1,2,4-triazines can be reacted with alkynes to form pyridine rings.
Reactions of alkenyl- and alkynylaluminium compounds involve the transfer of a nucleophilic alkenyl or alkynyl group attached to aluminium to an electrophilic atom. Stereospecific hydroalumination, carboalumination, and terminal alkyne metalation are useful methods for generation of the necessary alkenyl- and alkynylalanes.
The acidity of the terminal alkynyl proton allows the alkynylzinc compound to be generated in situ from the appropriate alkyne with an alkylzinc reagent or zinc triflate, Zn(OTf) 2. [1] The first example of catalytic asymmetric addition of alkynylzinc compounds to aldehydes was reported by KensÅ Soai and co-workers in 1990.