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

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.