Transannular π–π interactions in janusenes and in related rigid systems with cofacial aromatic rings; gauging aromaticity in the hydrocarbons and in model carbocations; a DFT study

Abstract

The utility of NICS (nuclear independent chemical shift) as a probe for detecting/sensing variation in aromaticity due to transannular π–π interactions in janusene 2, a [3.3]orthocyclophane having two cofacial benzene rings within van der Waals distance, its tetrafluoro- and octafluoro-derivatives 4 and 5, and in tropiliojanusene 6 was studied by DFT at the B3LYP/6-31G(d) level. The related hydrocarbons 8 and 8a with a buried double-bond and their carbocations were also included in this study. Whereas NICS(0) and NICS(1) are rather insensitive to transannular interactions, computed NICS(1)_zz, values are larger and more negative for both π-decks in the interannular space and this is consistent with increased transannular π–π interactions in the cofacial rings, previously shown in these systems via spectroscopic studies (UV and NMR), and by electrophilic chemistry. Transannular effects in 2, 4, and 6 were also probed by examining the forms of HOMO-LUMOs. Attempts to measure donor-accepter interactions between electron rich/electron poor cofacial decks via NICS (1)_zz through substituent effects proved unsuccessful, resulting in only very small changes. Protonation of the double-bond buried in between the two π-decks in 8 and 8a results in internally π-stabilized carbocations that exhibit more negative NICS(1) and NICS(1)_zz values in the interannular space. GIAO NMR data were computed for the neutral hydrocarbons and their derived carbocations, as a guiding tool for planned experimental studies.