m-Terphenyls (also known as meta-terphenyls, meta-diphenylbenzenes, or meta-triphenyls) are organic molecules composed of two phenyl groups bonded to a benzene ring in the one and three positions. The simplest formula is C18H14, but many different substituents can be added to create a diverse class of molecules. Due to the extensive pi-conjugated system, the molecule it has a range of optical properties and because of its size, it is used to control the sterics in reactions with metals and main group elements. This is because of the disubstituted phenyl rings, which create a pocket for molecules and elements to bond without being connected to anything else. It is a popular choice in ligand, and the most chosen amongst the terphenyls because of its benefits in regards to sterics. Although many commercial methods exist to create m-terphenyl compounds, they can also be found naturally in plants such as mulberry trees.
History
Discovery The earliest known synthesis of meta-terphenyl was completed in 1866 by Pierre Eugène Marcellin Berthelot by heating benzene to high temperatures leading to a mixture of hydrocarbons including a mixture of meta-terphenyl and para-terphenyl. Meta-terphenyl was isolated in 1874 by Gustav Schultz (1851–1928) by taking the mixture of the compounds, mixing them in a solvent and allowing meta-terphenyl to melt off as it has a lower melting and boiling point than para-terphenyl. Meta-terphenyl, and other aromatic compounds, remained of much interest to scientists in throughout the end of the 20th century, with many of the physical properties being measured and compared during this time. This also led to the first alternate form of meta-terphenyl synthesis, which involved passing gaseous benzene and toluene through a hot glass tube.
Reactivity By the 1930s, focus had shifted to experimenting with the reactivity of meta-terphenyl and its potential use as a ligand. The first verified modified version of meta-terphenyl was created in 1932 by Arthur Wardner and Alexander Lowy and led to the creation of nitro-substituted meta-terphenyls as well as amino-meta-terphenyls from the oxidation of the nitro-substituted compounds. Walter and Kathryn Cook halogenated meta-terphenyl with chlorine and bromine with further applications such as use in as Grignard reaction, the first such suggestion for meta-terphenyl as a ligand for a main group element. They later confirmed their results using the Stepanow Method. This trend continued with C.K. Breadsher and I. Swerlick publishing a review of all known reactions that meta-terphenyl could undergo. G. R. Ames also wrote an article detailing not only reactions of meta-terphenyls, but also covering all the different experimental methods to obtain meta-terphenyl known at the time.
Early synthetic methods During this time, the method of producing meta-terphenyl had remained the same. While people did experiment with other ways to obtain the compound, for the most part the method of heating benzene in a glass tube remained the primary method. In 1948, however, G. Woods and Irwin Tucker put forth an alternative method. Instead of heating benzene, they found that a combination of dihydroresocinol and two equivalents of phenyllithium would create unsymmetrical meta-terphenyl molecules. This was significant as the previous method required the separation of meta-terphenyl from other compounds and this novel synthesis allowed meta-terphenyls to be the major product and much more easily isolatable. This method would remain the most popular form of making meta-terphenyls until the end of the 20th century.
It was during this time that it was discovered that meta-terphenyls occurred in nature. In 1975, Karl-Werner Glombitza, Hans-Willi Rauwald, and Gert Eckhardt isolated two meta-terphenyls from the algae Fugus vesiculosus. More naturally occurring meta-terphenyls have been isolated since then and have shown promising applications in the field of biochemistry.
Synthesis
Hart Method As the demand for meta-terphenyl and its derivatives grew through the latter half of the 20th century, it became necessary to increase the yield of reactions producing meta-terphenyls as well as have the ability to uniquely create symmetric and unsymmetric meta-terphenyls to investigate their reactivity as well utilize their increased steric control. Such a method was discovered by Akbar Saednya and Harold Hart in 1986. Using an excess of Grignard reagent that had a phenyl group attached, meta-terphenyl was able to be made quickly, in one step, with a relatively high yield. This method also allowed for a variety of meta-terphenyl compounds to be made as so long as it could successfully be made into a Grignard reagent.
This method continues to remain very popular in terms of the creation of symmetrical meta-terphenyl compounds, but that has not stopped attempts to quicken synthesis, increase yield, and create more sterically bulky m-terphenyls. The method developed by Saednya and Hart has provided the basis for improved synthesis of meta-terphenyls and has often been used as a comparison when it comes to resulting structures and methods. Saednya and Hart continued their work and provided two alternate paths to create meta-terphenyls in 1996. One involved using a halogenated benzene and three equivalents of the phenyl group attached to the benzene. The second involved a dichloro-substituted benzene and butyl lithium followed by two equivalents of the Grignard reagents mentioned above. This led to increased yields of larger terphenyl compounds, however as the size of the substituents has been hypothesized to have a limit due to the increased steric hindrance of the molecule.
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