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Introduction to arynes:
Arynes are intermediates based on aromatic molecules with a “triple” bond in the ring. Conceptually, imagine taking a benzene ring, and removing homolytically two neighbouring hydrogen atoms (i.e. two neighbouring •H). What would remain is the same ring but with an electron in each of the two neighbouring sp2 orbitals previously used to hold onto the hydrogens we took away. These two singly occupied sp2 orbitals can overlap (weakly) and form a new bond in the ring. And that is benzyne, the simplest aryne. Other arynes are similar, but have different functional groups somewhere else on the ring.
Figure 3: Generation of an aryne and its capture.
The versatility of arynes
The extra bond makes arynes strained, so they are quite reactive and short-lived. However, it is possible to generate arynes under mild conditions from various precursors and then convert their extra bond to a huge variety of functional groups. Moreover, existing functional groups present on the aryne already can be used to steer the regioselectivity of these reactions.
This approach allows the simultaneous introduction of two different functional groups. The sheer variety of functional groups that are compatible with this process is enormous. Arynes can form new C-H, C-C, C-B, C-N, C-O, C-S, C-Se, C-Te, C-P, C-halogen, and even C-transition metal bonds (including to Ni, Pd, Pt, Cu, Ag and Au). They also participate in reactions through various different mechanisms. These properties make arynes ideal as the basis of very versatile building blocks for organic synthesis.
Our work on arynes:
We have developed a new approach to way to derivatise aryne precursors via Ir-catalysed C-H borylation. This brings a whole new dimension to both aryne and organoboronate chemistry. The resulting boryl aryne precursors are incredibly versatile. The boronate group and the aryne can be activated seperately, in whichever order is preferred, and used to add an extremely large number of functional groups. Thus, releasing and manipulating the aryne’s triple bond can enable the synthesis of a range of previously impossible-to-make aryl boronates. This is important because aryl boronates are very useful in many different reactions (e.g. Suzuki coupling, Chan-Lam aminations, fluorination reactions, and far beyond). On the other hand, the boronate can be reacted to make previously impossible aryne precursors, enabling the use of aryne reactivity in new chemical environments.