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 tweezer (host) and chiral monoalcohols and monoamines derivatized by a bidentate 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 preferred conformation is the one in which the L (larger) group protrudes from the porphyrin sandwich: this preferred helicity of the complex determines the CD of the complexes. It was found that the porphyrin ring-current induced 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) substituents attached to the stenogenic center. The assignments, in turn, are correlated with the sign of the CD exciton coupler that establishes the absolute configuration at the stereogenic center. Variable-temperature NMR experiments proved that the observed increase in CD amplitude at lower temperatures derives from conformational changes in the preferred offset geometry between two porphyrin rings.