Stable‐Ion NMR Spectroscopy and GIAO‐DFT Study of Carbocations Derived from Multibridged [3n]Cyclophanes

Abstract

AbstractMono‐ and diprotonation of the tri‐, tetra‐, and pentabridged cyclophanes [33](1,3,5)cyclophane (4), [34](1,2,3,5)cyclophane (5), and [35](1,2,3,4,5)cyclophane (6) were realized, and NMR spectroscopic studies of the resulting carbocations are reported. Unlike [2.2]paracyclophane and its fluorinated analogs that are protonated at a position ipso to the ethano‐bridge, the monocations derived from 4, 5, and 6 are protonated at the unsubstituted ring positions. Protonation regioselectivity in the dications is pseudo‐meta for 4 and 5 at unsubstituted ring positions, but for more‐crowded 6 the second protonation occurs at a position that is ipso to the trimethylene bridge. Transannular π–π interactions in the monocations are manifested in the observed proton deshielding in the unprotonated π‐deck. DFT and GIAO‐DFT were employed to study the mono‐ and dications for comparison with the solution studies in superacids. GIAO‐derived ΔNICS(1)zz data for the [3n]cyclophane monocations imply decreased aromaticity in the cofacial unprotonated arene, consistent with transannular donor–acceptor interactions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)