Chiral Guest Binding as a Probe of Macrocycle Dynamics and Tautomerism in a Conjugated Tetrapyrrole

We report chiral guest binding as a probe of prototropic tautomerism and macrocyclic inversion in a highly conjugated tetrapyrrole studied using H-1 NMR spectroscopy in conjunction with mandelic acid as the chiral guest. Both tautomerism and macrocycle inversion can be influenced in a non-trivial way depending on temperature and the respective concentrations of tetrapyrrole host, chiral guest or water. Chirality of the interacting guest is the key feature since it permits separation and detailed observation of macrocyclic inversion and tautornerism. Based on this, a methodology was developed to identify and characterize the dynamic processes. Our observations suggest that yields of products (e.g., of asymmetric reactions) can be affected by reactivity of functional groups (in molecules undergoing tautomerism or inversion) by varying solution properties including reagent concentrations and impurities such as water. This work establishes a connection between the important chemical concepts of chirality, tautomerism, and macrocyclic dynamics.