Alkynylation with Hypervalent Iodine Reagents

Alkynes are among the most versatile functional groups in organic synthesis. They are also frequently used in chemical biology and materials science. Whereas alkynes are traditionally added as nucleophiles into organic molecules, hypervalent iodine reagents offer a unique opportunity for the development of electrophilic alkyne synthons. Since 1985, alkynyliodonium salts have been intensively used for the alkynylation of nucleophiles, in particular soft carbon nucleophiles and heteroatoms. They have made an especially strong impact in the synthesis of highly useful ynamides. Nevertheless, their use has been limited by their instability. Since 2009, more stable ethynylbenziodoxol(on)e (EBX) reagents have been identified as superior electrophilic alkyne synthons in many transformations. They can be used for the alkynylation of acidic C–H bonds with bases or aromatic C–H bonds using transition metal catalysts. They were also highly successful for the functionalization of radicals or transition metal-catalyzed domino processes. Finally, they allowed the alkynylation of a further range of heteroatom nucleophiles, especially thiols, under exceptionally mild conditions. With these recent developments, hypervalent iodine reagents have definitively demonstrated their utility for the efficient synthesis of alkynes based on non-classical disconnections.