Crystal Structure and Conformation of a Dipyrrylmethane. The gem-Dimethyl Effect

Abstract

A crystal structure determination of the new dipyrrylmethane diethyl-2,3,5,5,7,8-hexamethyl-5,10-dihydrodipyrrin-1,9-dicarboxylate (1) is only the third reported for a dipyrrylmethane and the first with a gem-dimethyl group at the bridging carbon atom. Conformation determining torsion angles are compared to those from molecular mechanics calculations and to the corresponding data for an analogous dipyrrylmethane (2) with no gem-dimethyl moiety. The crystal structures of 1 and 2 differ significantly: 1 adopts the +ac,+ac or −ac,−ac conformation, whereas 2 exists in the −ac,+sc conformation in an intermolecularly hydrogen bonded dimer. There is no evidence for hydrogen bonding in crystals of 1, and its ac conformation is unlike that found about the central core of bilirubin (sc,sc). Taken collectively, the data indicate that the presence of a sterically demanding and potentially conformation distorting gem-dimethyl group located at the bridging carbon of a dipyrrylmethane (i) stabilizes a conformation that brings the pyrrole NH groups syn to the gem-dimethyls and (ii) would destabilize the ridge-title conformation of 10,10-dimethylbilirubin.