Functional group tolerant iron-catalyzed cyclotrimerization of terminal and internal alkynes.

Arenes play a crucial role in various fields of chemistry. Since Kekulé proposed the molecular structure of benzene in 1865, chemists have been working to develop methods for preparing functionalized arenes. Most of these methods focus on introducing functional groups through substitution reactions (electrophilic aromatic substitution, cross-coupling, C-H activation) using aromatic starting materials. In contrast, the formal [2 + 2 + 2] cycloaddition of three different alkyne moieties allows for the introduction of functional groups before synthesizing the aromatic core. Although straightforward, this strategy faces challenges, including the inherent reactivity of heteroatom-substituted alkynes, restrictions on the substitution patterns of the alkynes, and the use of noble metal-based catalysts. Herein, we report an iron-catalyzed cyclotrimerization of internal and terminal alkynes to provide densely functionalized aromatic products. The method accommodates alkoxy-, amido-, and even boryl alkynes, yielding the corresponding aryl ethers, amides, or boronic acids in up to quantitative yields at moderate temperatures within minutes.