Medium-sized cyclophanes. XX. 13C nuclear magnetic resonance of cyclophanes: p-orbital compression shift and orbital deformation

Abstract

The 13C nmr spectra of [2.2]meta-, metapara-, and paracyclophanes were determined. By comparing the chemical shifts with those of acyclic models two types of anomalies were revealed.(1) Bridging methylene carbon resonances for [2.2]metacyclophanes having an alkyl group ortho to the bridge were shifted upfield as a result of van der Waals' compression. The upfield shift is much larger (5–12 ppm) for cyclophanes than for models (2–4 ppm) reflecting the rigid nature of the molecular geometry of the former.(2) Interestingly, inner aryl carbon resonances in [2.2]metacyclophanes and protonated aryl carbon resonances in [2.2]paracyclophanes were shifted downfield by 6–7 ppm. The complementary upfield shift of uncompressed atoms, hydrogen or carbon, connected to those carbons was also noticed. The downfield shift of compressed sp2 carbon results from interaction between two p-orbitals along the same orbital axis, p-orbital compression, and is attributable to decreased electron density.