Chiral Recognition by CD-Sensitive Dimeric Zinc Porphyrin Host. 1. Chiroptical Protocol for Absolute Configurational Assignments of Monoalcohols and Primary Monoamines

Abstract

A general microscale protocol for the determination of absolute configurations of primary amino groups or secondary hydroxyl groups linked to a single stereogenic center is described. The chiral substrates are linked to the achiral trifunctional bidentate carrier molecule (3-aminopropylamino)acetic acid (1, H(2)NCH(2)CH(2)CH(2)NHCH(2)COOH) and the resultant conjugates are then complexed with dimeric zinc porphyrin host 2 giving rise to 1:1 host/guest sandwiched complexes. These complexes exhibit exciton-coupled bisignate CD spectra due to stereodifferentiation leading to preferred porphyrin helicity. Since the chiral sense of twist between the two porphyrins in the complex is dictated by the stereogenic center of the substrate, the sign of the couplet determines the absolute configuration at this center. The twist of the porphyrin tweezer in the complex can be predicted from the relative steric sizes of the groups flanking the stereogenic center, such that the bulkier group protrudes from the complex sandwich. In certain alpha-hydroxy esters and alpha-amino esters, electronic factors and hydrogen bonding govern the preferred conformation of the complex, and hence the CD spectra.

Supramolecular Chirality in Porphyrin Chemistry

“…The chiral memory on 103 was stable with just a small decrease of the CD intensity over one day. In the case of flexible bound porphyrins, the corresponding bis-porphyrin, 104 (Figure 28), required a bidentate enantiopure ligand to adopt the supramolecular chiral tweezer conformation [76,77]. Hence, the interaction of 104 with various chiral diamines resulted in the induction of a CD couplet in the region of porphyrin absorption.…”

Chiral Solvation Induced Supramolecular Chiral Assembly of Achiral Polymers

“…The complexation between CD-silent zinc porphyrins and chiral amines or alcohols is another important intermolecular interaction applied to prepare optically active complexes. Berova et al have designed an achiral zinc bis-porphyrin linked with a long pentamethylene diester (host molecule) as shown in Figure 1 [46][47][48]. By relatively intense coordination interactions between zinc porphyrin and nitrogen or oxygen, the host molecules successfully bound plenty of chiral diamines, amino alcohols and amino acids and showed clear ICD effects, due to the formation of CD-active 1:1 and 1:2 host-guest complexes.…”

Supramolecular Chirality: Solvent Chirality Transfer in Molecular Chemistry and Polymer Chemistry

“…Achiral CD-silent Zn bis-porphyrin linked with a flexible linker gave rise to CD-active 1:1 and 1:2 host-guest complexes with chiral amines and chiral alcohols, arising from relatively intense Zn/N and Zn/O coordination abilities. Berova, Nakanishi and coworkers [102][103][104] designed molecular chromophoric tweezers based on two Zn porphyrins linked with a long floppy pentamethylene diester to bind a number of chiral diamines, amino acid derivatives and amino alcohol derivatives in n-hexane, characterized by CD spectroscopy (Figures 7 and 8). Borovkov and Inoue [105][106][107][108][109] showed that a twisted zinc porphyrin rotamer linked with a shorter ethane spacer can efficiently switch between syn-and anti-forms with P(plus)-and M(minus) sense by step-wise coordination with a family of chiral secondary amines and chiral secondary alcohols, proven by variable temperature UV-Vis and CD spectra and ) of chiral fluid can induce certain chiral geometry in carefully designed non-chiral hosts in double-minimal-well or multi-minimal-well potentials dissolved in fluidic condition at ambient temperature.…”