A Nonempirical Approach for Direct Determination of the Absolute Configuration of 1,2-Diols and Amino Alcohols Using Mg(II)bisporphyrin

Abstract

We report here a simple, facile, and direct nonempirical protocol for determining the absolute stereochemistry of a variety of chiral 1,2-diols and amino alcohols at room temperature with no chemical derivatization using Mg(II)bisporphyrin as a host. Addition of excess substrates resulted in the formation of a 1:2 host–guest complex in which two substrates bind in an unusual endo-endo fashion because of interligand H-bonding within the bisporphyrin cavity leading to the formation of a unidirectional screw in the bisporphyrin moiety that allowed us an accurate absolute stereochemical determination of the chiral substrate via exciton-coupled circular dichroism (ECCD). The sign of the CD couplet has also been found to be inverted when the stereogenic center is moved by one C atom simply from the bound to an unbound functionality and thus able to discriminate between them successfully. Strong complexation of the alcoholic oxygen with Mg(II)bisporphyrin rigidifies the host–guest complex, which eventually enhances its ability to stereochemically differentiate the asymmetric center. The ECCD sign of a large number of substrates has followed consistent and predictable trends; thus, the system is widely applicable. Moreover, computational calculations clearly support the experimental observations along with the absolute stereochemistry of the chiral substrate.