Zinc porphyrin tweezer in host-guest complexation: determination of absolute configurations of primary monoamines by circular dichroism

A nonempirical exciton chirality circular dichroic (CD) method for determining the absolute configurations of primary monoamines with amino group directly linked to the stereogenic center is described. Conventional exciton chirality CD method cannot be applied to these compounds since they lack the two sites for attaching the interacting chromophores. This was solved by covalently linking the monoamine to a trifunctional bidentate carrier moiety 1. Treatment of the carrier/monoamine conjugate with the porphyrin tweezer 4 consisting of two pentanediol-linked zinc porphyrins gives rise to 1:1 host-guest macrocyclic complexes that exhibit exciton-coupled CD spectra. The sign of the CD couplet can then be correlated with the absolute configuration of the monoamine as follows: a clockwise arrangement of the L, M, and S (large, medium, small) groups in the Newman projection of the monoamine with the amino group in the rear gives rise to a positive CD couplet, and vice versa; the assignments of L, M, S groups are based on conformational energies (A values). This method is applicable to cyclic and acyclic aliphatic amines, aromatic amines, amino esters, amides, and cyclic amino alcohols, and can be performed at the several microgram level.