Arylazo under Extreme Conditions: [2 + 2] Cycloaddition and Azo Metathesis

Abstract

The four-membered nitrogen ring (N4-ring) is predicted to be a high-energy density moiety and has been the target of chemical synthesis for quite a long time. Here, by compressing the 1:1 co-crystal of trans-azobenzene and trans-perfluoroazobenzene up to ∼40 GPa, the azo groups were restrained closely in parallel in the crystal and underwent two competitive addition reactions. One is [4 + 2] cycloaddition with the azo group as a part of diene and phenyl as dienophile. The other is [2 + 2] cycloaddition between two azo groups, which produced an unprecedented N4-ring structure as evidenced by the metathesis product. The content of the N4-ring structure significantly increases under higher pressure, and we found that it was the external pressure that decreased the kinetic barrier and realized such a high-tensile moiety. Our work shows that high pressure is an alternative synthetic strategy for these high-tensile structures, which can be very effective under the cooperation of crystal engineering.

Hetero-ene Metathesis

“…Given the wide range of applications of the photoresponsive NN functionalitymolecular motors, azo dyes, actuating materials, and radical initiators to name a fewcatalytic diazene metathesis would be an attractive transformation to enable their synthesis. Diazene metathesis could theoretically be accomplished through any of the mechanism types discussed above, as we propose in Figure ; however, although uncatalyzed diazene metathesis has been observed at pressures of 20–40 GPa, catalytic diazene metathesis has not been realized to date.…”

“…Yet, for all of the power and versatility of olefin metathesis, it is inherently restricted to carbon–carbon double and triple bonds, which may not map onto valuable heteroatom-rich materials, including functional polymers, − biomacromolecules, dyes, and natural products (Figure B). , Synthesis of such materials could be dramatically streamlined and precisely controlled through metathesis of the corresponding heteroatom-containing double bonds (hetero-enes). Although some of these double bonds have been metathesized (e.g., carbonyls, − imines, − and carbodiimides − ), the catalysis and double bond scope (Figure C) have considerable room to improve. − From our perspective, these challenges are exciting opportunities to advance the young field of hetero-ene metathesis, in terms of both fundamental science and its applications.…”

“…Several examples of classes of heteroatom-rich compounds. C. Reported double bond metatheses. − If both catalyzed and uncatalyzed methodologies exist for a given functionality combination, then only the catalytic methodology is indicated.…”

Computational high‐pressure chemistry: Ab initio simulations of atoms, molecules, and extended materials in the gigapascal regime

“…Nevertheless, studies on Diels–Alder reactions under pressure that intimately link experimental and computational techniques are still scarce. In a recent work, 287 PBC‐based metadynamics simulations of an azobenzene co‐crystal have helped shed light on the reaction mechanisms of competing Diels–Alder ([4 + 2]) and [2 + 2] cycloaddition reactions. Moreover, Zholdassov and co‐workers recently used a multiscale modeling approach to distinguish between uniaxial stress and hydrostatic compression in a sophisticated nanoreactor setup that induced Diels–Alder reactions between a surface‐immobilized anthracene and different dienophiles 288 .…”

Six-Membered Iridacycles with Five Nitrogen Atoms in the Ring

“…Currently, diazene homometathesis has only been observed under extreme pressures (20−40 GPa) within an engineered crystalline lattice, as reported by Zheng, Kang, and co-workers. 6 Catalysis may enable this metathesis to occur under more accessible conditions. En route to this goal, several examples of stoichiometric metathesis of diazenes with carbenes have been reported.…”