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

摘要

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 octafluoroderivatives 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.