Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 2. Structural studies of host-guest complexes with chiral alcohol and monoamine conjugates

A structural study of complexes formed between a dimeric zinc porphyrin twe ezer (host) and chiral monoalcohols and monoamines derivatized by a bidenta te carrier molecule (guest) confirmed that their CD couplets arise from the preferred porphyrin helicity of 1:1 host-guest complexes, NMR experiments and molecular modeling of selected tweezer complexes revealed that the pref erred conformation is the one in which the L (larger) group protrudes from the porphyrin sandwich: this preferred helicity of the complex determines t he CD of the complexes. It was found that the porphyrin ring-current induce d H-1 chemical shifts and molecular modeling studies of the complex lead to the assignments of relative steric size of the L (large)/M (medium) substi tuents attached to the stenogenic center. The assignments, in turn, are cor related with the sign of the CD exciton coupler that establishes the absolu te configuration at the stereogenic center. Variable-temperature NMR experi ments proved that the observed increase in CD amplitude at lower temperatur es derives from conformational changes in the preferred offset geometry bet ween two porphyrin rings.