Steric Effects in Benzopyran Imine‐Hemiaminal Ring–Chain Tautomerism: Tautomeric Free Energies as a Measure of Substituent Size

We have synthesized and characterized a series of tetrahydrobenzo[c]pyranamines that exist in an imine/hemiaminal ring–chain tautomerism. As was the case for a series of homologous isobenzofuranamines, their tautomeric free energies were found to vary depending on the steric bulk of common substituents. The results indicate that even for systems of differing ring size, tautomeric free energies give consistent values of the steric bulk of substituents. Six single‐crystal X‐ray diffraction structures have been determined, two on tetrahydrobenzo[c]pyranamines and one on an open‐chain tautomer. Two further on dihydrobenzo[c]furanamines and one open‐chain tautomer of this system. H‐atoms have been placed close to the neutron positions using refinement with aspherical atomic scattering factors and a detailed analysis of H‐bonding was undertaken.