Arynes, which are formally derived from aromatic rings by abstraction of two hydrogen atoms, have been a focus of organic chemistry for 100 years. In contrast to ortho-benzyne, which is mentioned in almost every introductory textbook on organic reaction mechanisms as a reactive intermediate of nucleophilic aromatic substitution, the meta and para isomers were regarded as rather exotic until recently. This situation has changed dramatically with the discovery of the enediyne antibiotics, a promising new class of antitumor drugs, and has aroused the interest of research groups from all branches of chemistry. Nowadays, arynes and related compounds are among the most intensively studied systems in chemistry. However, many aspects of the chemistry of these reactive intermediates are not well understood yet. In this review we outline the historical developement with an emphasis on recent progress in this challenging field of research.

We have studied the matrix photolysis of 2,4,6-triazido-1,3,5-triazine (cyanuric triazide, 1). Stepwise generation of the corresponding mononitrene, dinitrene, and trinitrene was observed by matrix IR and electron paramagnetic resonance (EPR) spectroscopy. The generated species were identified by comparison of their matrix IR spectra with density functional theory (DFT) computational results. The generation of 2,4,6-trinitreno-1,3,5-triazine with a septet ground state was confirmed for the first time by matrix EPR spectroscopy. The trinitrene readily decomposed into three NCN molecules upon further photoirradiation. This process was also confirmed by matrix EPR spectroscopy.

The fluorinated m-didehydrobenzenes 5-H-, 5-deuterio-, 5-(trifluoromethyl)-, 5-(trimethylsilyl)-, and 5-iodo-2,4,6-trifluoro-1,3-didehydrobenzene 6b−6f were generated by UV photolysis of the corresponding 1,3-diiodobenzene derivatives 4b−4f in solid neon at 3 K. The photolysis of 1 proceeds stepwise via the phenyl radical intermediates 5b−5f. The radicals 5 and m-benzynes 6 were identified by comparison of their experimentally measured IR spectra with those produced by DFT calculations. The formation of mono-and diradicals 5 and 6, respectively, is reversible, and annealing of [a]