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 1H 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 tautomerism. 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.