Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 1. Chiroptical protocol for absolute configurational assignments of monoalcohols and primary monoamines

A general microscale protocol for the determination of absolute configurati ons of primary amino groups or secondary hydroxyl groups linked to a single stereogenic center is described, The chiral substrates are linked to the a chiral trifunctional bidentate carrier molecule (3-aminopropylamino)acetic acid (1, H2NCH2CH2CH2NHCH2COOH) and the resultant conjugates are then compl exed with dimeric zinc porphyrin host 2 giving rise to 1:1 host/guest sandw iched complexes. These complexes exhibit exciton-coupled bisignate CD spect ra due to stereodifferentiation leading to preferred porphyrin helicity. Si nce 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 couple t determines the absolute configuration at this center. The twist of the po rphyrin tweezer in the complex can be predicted from the relative steric si zes of the groups flanking the stereogenic center. such that the bulkier gr oup protrudes from the complex sandwich. In certain alpha -hydroxy esters a nd alpha -amino esters, electronic factors and hydrogen bonding govern the preferred conformation of the complex, and hence the CD spectra.