1H NMR Study of a Chiral Argentivorous Molecule/Ag+ Complex: Assignment of Proton Signals of Four Aromatic Rings with Slightly Different Environments.

The proton signals at the 2'- and 6'-positions of the aromatic side arms of a silver(I) complex with a chiral tetra-armed cyclen ((S)-L2) are assigned by comparison with 1H NMR spectra and X-ray structure of Ag+ complexes with three analogues of (S)-L2: (S)-L23D having one benzyl group and three deuterium-substituted benzyl groups, (S)-L22D having two benzyl groups and two deuterium-substituted benzyl groups, and (S)-L2F having three benzyl groups and one 4'-fluorobenzyl group. An interaction factor is defined by using the X-ray structures. The chemical shift values of the proton signals in the aromatic side arms can be explained by using the interaction factors that are defined by the dihedral angles, the angles between two mean planes, and the CH···π distances from the X-ray data of the (S)-L2/Ag+ complex.

Argentivorous Molecules Exhibiting Highly Selective Silver(I) Chiral Enhancement.

We report the synthesis of chiral tetra-armed cyclens, having an asymmetric center at one side-arm, that show selective chiral enhancement with silver(I). When the chiral ligand forms a silver(I) complex, the side-arms cover the silver(I) incorporated into the cyclen. The asymmetric center controls the conformation of the side-arms in one direction, increasing the molar ellipticity of the CD spectrum. Chiral cyclens substituted with biphenyl groups exhibit large molar ellipticity values with strong exciton coupling effects.

Inclusion of alkyl nitriles by tetra-armed cyclens with styrylmethyl groups.

Synthesis of tetra-armed cyclens (2a-2e), with substituted styrylmethyl groups as side-arms, and their Ag+ complexes is reported. The Ag+ complex with a tetra styrylmethyl-armed cyclen (2a) incorporates alkyl nitriles in a pseudo-cavity formed by the four styrylmethyl side-arms. This prompted us to apply this system to the determination of the absolute configurations of chiral nitriles with low [α]D and low circular dichroism (CD) intensity. In the CD spectra of the 2a/Ag+ complex, (S)- and (R)-G1 did not show a specific Cotton effect, while when chiral nitriles were added to the 2a/Ag+ complex, drastic spectral changes were observed. The (S)-G1@2a/Ag+ system exhibited first a negative and then a positive Cotton effect, whereas the (R)-G1@2a/Ag+ system showed the mirror image of the Cotton effect of (S)-G1@2a/Ag+. We have, therefore, demonstrated a new technique for determining the absolute configurations of weak optical rotation molecules using the Ag+ complex with 2a.