Aryne

Arynes or benzynes are highly reactive species derived from an aromatic ring by removal of two substituents. The most common arynes are ortho but meta- and para-arynes are also known. o-Arynes are examples of strained alkynes. Arynes or benzynes are highly reactive species derived from an aromatic ring by removal of two substituents. The most common arynes are ortho but meta- and para-arynes are also known. o-Arynes are examples of strained alkynes. The alkyne representation of benzyne is the most widely encountered. o-Arynes, or 1,2-didehydroarenes, are usually described as having a strained triple bond. Geometric constraints on the triple bond in ortho-benzyne result in diminished overlap of in-plane p-orbitals, and thus weaker triple bond. The vibrational frequency of the triple bond in benzyne was assigned by Radziszewski to be 1846 cm −1, indicating a weaker triple bond than in unstrained alkyne with vibrational frequency of approximately 2150 cm−1. Nevertheless, ortho-benzyne is more like a strained alkyne than a biradical, as seen from the large singlet–triplet gap and alkyne-like reactivity. LUMO orbital of aryne lies much lower than LUMO of unstrained alkynes, which makes it a better energy match for HOMO of nucleophiles. Hence, benzyne possesses electrophilic character and undergoes reactions with nucleophiles. A detailed MO analysis of benzyne was presented in 1968. Due to their extreme reactivity, arynes must be generated in situ. Typical of other reactive intermediates, benzyne must be trapped, otherwise it dimerises to biphenylene. Early routes to benzyne involved dehydrohalogenation of aryl halides: Such reactions require strong base and high temperatures. Ortho-disubstituted arenes serve as precursors to benzynes under milder conditions. Benzyne is generated by the dehalogenation of magnesium and 2-bromofluorobenzene.Anthranilic acid can be converted to 2-diazoniobenzene-1-carboxylate by diazotization and neutralization. Although explosive, this zwitterionic species is a convenient and inexpensive precursor to benzyne. Another method is based on trimethylsilylaryl triflates. Fluoride displacement of the trimethylsilyl group induces elimination of triflate and release of benzyne: A hexadehydro Diels-Alder reaction (HDDA) involves cycloaddition of 1,3-diyne and alkyne.