Carbocatalytic Acetylene Cyclotrimerization: A Key Role of Unpaired Electron Delocalization

Abstract

Development of sustainable catalysts for synthetic transformations is one of the most challenging and demanding goals. The high prices of precious metals and the unavoidable leaching of toxic metal species leading to environmental contamination make the transition metal-free catalytic systems especially important. Here we demonstrate that carbene active centers localized on carbon atoms at the zigzag edge of graphene represent an alternative platform for efficient catalytic carbon− carbon bond formation in the synthesis of benzene. The studied acetylene trimerization reaction is an efficient atom-economic route to build an aromatic ringa step ubiquitously important in organic synthesis and industrial applications. Computational modeling of the reaction mechanism reveals a principal role of the reversible spin density oscillations that govern the overall catalytic cycle, facilitate the product formation, and regenerate the catalytically active centers. Dynamic π-electron interactions in 2D carbon systems open new opportunities in the field of carbocatalysis, unachievable by means of transition metal-catalyzed transformations. The theoretical findings are confirmed experimentally by generating key moieties of the carbon catalyst and performing the acetylene conversion to benzene.

Synthesis of vinyl-substituted alcohols using acetylene as a C2 building block

“…This may be attributed to the unique properties of acetylene. 11 Moreover, the gaseous nature and flammable properties of acetylene make chemists stay away from it. 12 …”

Role of Acetylene in the Chemical Evolution of Carbon Complexity

“…467 Similarly, under certain conditions, PAHs can play a role in the synthesis of benzene from simple precursors. Recently, published DFT calculations and experimental data have confirmed the formation of benzene by trimerization of three acetylene molecules catalyzed by PAH carbene sites at ∼900 K. 468 The authors considered the carbene sites on the zigzag edge of PAHs to be catalytic sites. They considered PAHs of various sizes and configurations from C 6 H 6 to C 361 H 51 .…”

“…The initial form of the C 37 H 14 catalyst, location of the chemically reactive center, and designation of this C 37 H 14 structure in the catalytic cycle representations are shown in the magenta frame. Reproduced with permission from ref . Copyright 2020 American Chemical Society.…”

Nikolay Zelinsky (1861–1953): Mendeleev's Protege, a Brilliant Scientist, and the Top Soviet Chemist of the Stalin Era

“…They proposed an activated‐carbon‐catalyzed trimerization of acetylene into benzene, which could be applied to the synthesis of other aromatic hydrocarbons (Scheme 8). [36] In 1866, M. Berthelot (1827–1907) has established that heating acetylene to 550–600 °C leads to formation of benzene [37] . However, the benzene yields in this reaction were extremely low.…”