Escape from Flatland: Pericyclic Reactions and Rearrangements of Diazofluorene with Biarylcyclooctynes for the Generation of Helicene‐Like Phenanthro‐Pyrazoles

We report the voyage of two flat, highly unsaturated systems into the third dimension. Specifically, we investigated the 1,3-dipolar cycloadditions of 9-diazofluorene with a rapidly growing class of strained dipolarophile—biarylcyclooctynes. We found that the initial spirocyclic 3H-pyrazole cycloadducts are isolable, and can be subjected to controllable [1,5]-sigmatropic shifts. These van Alphen–Hüttel rearrangements generate either the metastable 4H-pyrazoles—another spirocyclic fused-ring system with a highly contorted structure—or the thermodynamically favored 1H-pyrazoles. The 1H-pyrazoles display extended conjugation and can be classified as [5]-helicenes containing a phenanthro-pyrazole fused to a dihydro-cyclooctatetraene. Overall, these studies provide a modular framework to control heteroatom placement, ring size, and ring fusion in the syntheses of carbon-rich materials.