Structures of alkyl-substituted Tröger's base derivatives illustrate the importance of<i>Z</i>′ for packing in the absence of strong crystal synthons

Abstract

Crystal structures of Tröger's base (5,11-methano-2,8-dimethyl-5,6,11,12-tetrahydrodibenzo[b,f][1,5]diazocine) analogues with the methyl groups replaced by ethyl, iso-propyl and tert-butyl groups were studied. The incidence of Z' > 1 structures increases to rather conspicuous levels. The reasons behind this trend are expanded upon, and a possible explanation is given in the flexibility of the alkyl substituents and van der Waals stabilization. In combination these effects allow for an additional stabilization of the packing by small changes in the molecular conformations, thus expanding the size of the asymmetric unit.