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A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). [1] Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of organic heterocycles .
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.
In such reactions "two or more unsaturated molecules unite directly to form a ring", [1] incorporating a metal bonded to one or more of the molecules. Cycloadditions involving metal centers are a staple of organic and organometallic chemistry, and are involved in many industrially-valuable synthetic processes.
[1] [2] Simple aromatic rings can be heterocyclic if they contain non-carbon ring atoms, for example, oxygen, nitrogen, or sulfur. They can be monocyclic as in benzene, bicyclic as in naphthalene, or polycyclic as in anthracene. Simple monocyclic aromatic rings are usually five-membered rings like pyrrole or six-membered rings like pyridine.
[6] [7] It was the German chemist Karl Gräbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a disubstituted aromatic ring (namely naphthalene). [8] In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene. [9]
The Nencki reaction (1881) is the ring acetylation of phenols with acids in the presence of zinc chloride. [24] In a green chemistry variation aluminium chloride is replaced by graphite in an alkylation of p-xylene with 2-bromobutane. This variation will not work with primary halides from which less carbocation involvement is inferred. [25]
Structure of (Me 4 N +) 2 [Fe(C 2 B 9 H 11) 2] +, showing only one Me 4 N +. [4] Closely related are the metal complexes containing H 3 C 3 B 2 R 2 ligands. [7] In addition to these, other sandwich complexes containing purely inorganic ligands are known, such as Fe(C 5 Me 5)(P 5) and [(P 5) 2 Ti] 2−. [8]
The driving force for ring-opening and polymerization is strain relief. The reaction is believed to be a living polymerization due to the lack of competing reactions. Because the two benzene rings are in close proximity this cyclophane type also serves as guinea pig for photochemical dimerization reactions as illustrated by this example: [21]