Para-benzyne
Arynes or benzynes are highly reactive species derived from an aromatic ring by removal of two ortho substituents. Arynes are usually best described as having a strained triple bond; however, they possess some biradical character as well. The term aryne is most closely associated with ortho-aryne (1,2-didehydrobenzene), however 1,3- and 1,4-didehydrobenzene intermediates have been described.
The discovery of benzyne led to rapid developments in synthetic methodologies to make this highly reactive intermediate useful for organic synthesis. A variety of natural products have been prepared using arynes as intermediates.
The alkyne representation of benzyne is the most widely encountered, however, the cumulene and diradical structures are significant resonance contributors.
Geometric constraints on the triple bond in ortho-benzyne result in reduced overlap of in-plane p-orbitals, and thus weaker triple bond. The vibrational frequency of the triple bond in benzyne was assigned by Rasziszhewski to be 1846 cm −1, indicating a much weaker triple bond than in unstrained alkyne with vibrational frequency of approximately 2150 cm−1. Nevertheless, strained alkyne is a better description of the triple bond nature of ortho-benzyne than biradical (larger singlet-triplet gap and alkyne-like reactivity). Geometric restrictions also result in significant lowering of the energy the LUMO of arynes (6.41 eV in 2-butyne to 1.33 eV in benzyne) while the energy of the HOMO remains essentially unchanged according to computations.
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.
The first hint of the existence of aryne intermediate came from the work of Stoermer and Kahlert in 1902. They observed that upon treatment of 3-bromobenzofuran with base in ethanol 2-ethoxybenzofuran is formed. Based on this observation they postulated an aryne intermediate.
Georg Wittig and coworkers proposed that the formation of biphenyl via reactions of fluorobenzene and phenyllithium proceeded via a zwitterionic intermediate. and experimentally confirmed by John D. Roberts in 1953.
In 1953 John D. Roberts performed the classic 14C labeling experiment, which provided strong support for benzyne. Roberts and his students performed the reaction of chlorobenzene-1-14C with potassium amide, and analyzed the 14C-label incorporation into the resulting aniline: equal amounts of aniline with 14C incorporation at C-1 and C-2 were observed. This result necessitated a symmetrical intermediate – now known as benzyne.
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