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 (1)H 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 stereogenic center. The assignments, in turn, are correlated with the sign of the CD exciton couplet 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.
Kurtán
T
T
Department of Chemistry, Columbia University, New York, NY 10027, USA.
Nesnas
N
N
Koehn
F E
FE
Li
Y Q
YQ
Nakanishi
K
K
Berova
N
N
D000438:Alcohols / Q000737:chemistry*; D000588:Amines / Q000737:chemistry*; D002151:Calorimetry; D002942:Circular Dichroism; D009682:Magnetic Resonance Spectroscopy; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D012997:Solvents; D013237:Stereoisomerism; D015032:Zinc / Q000737:chemistry*
D000438:Alcohols; D000588:Amines; D008665:Metalloporphyrins; D012997:Solvents; C017802:zinc hematoporphyrin; D015032:Zinc