Unusual crowded organic architectures
Molecules with unusual steric crowding are always interesting in chemistry. They give the opportunity to explore the limits of stable molecular structures and the synthesis of unnatural products. They also provide points of calibration for modern computational methods. This dissertation describes the design, synthesis and characterization of two types of crowded complex aromatic compounds. The goal of the first project was to synthesize in-keto cyclophanes, that is, molecules with ketone oxygens pressed toward the centers of benzene rings. Several likely precursors were synthesized and fully characterized, but attempts to make the in-keto cyclophanes themselves were unsuccessful. The nonbonded interaction between the ketone oxygen and basal benzene ring may be so close in the target structures as to prevent the formation of an in-keto cyclophane. The second project describes the design, synthesis and characterization of several macrobicyclic, bis(triarylelement)-containing cyclophanes with various bridgehead heteroatoms. Computational studies accurately predicted that when the bridgehead substituents are small (lone pairs or protons), an in,in bridgehead stereochemistry is strongly favored, but larger bridgehead substituents favor the formation of in,out stereoisomers. The NMR spectra of several of these compounds show unusual through-space spin-spin coupling between atoms along the central axis. Most importantly, one of these compounds, an in,in-bis(hydrosilane), possesses a hydrogen-hydrogen nonbonded contact distance of approximately 1.56 Å, a new “world record” for such a contact in any crystallographically characterized compound.