Hydrogen-Bonded Double Strands: Crystal Structure and Specroscopic Properties of a 2,2'-Dipyrryl Ketone

摘要

The synthesis, crystal sructure determination, conformational analysis, and spectroscopic properties of 3,3'-diethyl-4,4'-dimethyl-2,2'-dipyrryl ketone (1) are reported. The dipyrryl ketone is a model for the dipyrrole core of 10-oxobilirubin, a presumed metabolite in alternate paathways of excretion of the yellow pigment of jaundice, bilirubin. In the crystal, 1 adopts a helical conformation, wiht a molecule of one helicity being hydrogen-bonded to two molecules of the opposite helicity . Thus, 1 self-assembles via hydrogen bonding into supramolecular double-stranded arrays, where moloecules of the same helicity comprise one strand and are paired through hydrogen bonding to molecules of opposite helicity in the second strand. In the obsered molecular conformation each pyrrole ring and adjacent carbonyl group are rotated into an sc conformation (torsion angle approx 29 deg), with each N-H pointing in the same direction as the C=O. Molecular mechanics/dynamics calculations calculations predict the sc, sc conformation, absent hydrogen bonding, to be the most stable, but only by a few tenths of kj/mol. In CHCl_3, 1 is monomeric according to vapor pressure osmonetry studies (MW_obs= 251 (+-) 10 vs. MW_calc =244). ~1H NMR NH chemical shifts in CDCl_3 suggests a predominantl yanti orientation of the C=O and pyrrole NHs, which is opposite to the orientation observed in the crystal.