Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal "Defect" and Helicene Strain for
Zheng Zhou1, Rahul Kisan Kawade2, Zheng Wei1
1Department of Chemistry, University at Albany, State University of New York, Albany, NY, 12222, USA.
Abstract:
Incorporation of a five-membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red-shift in the absorbance spectrum and injects a charge into a helical conjugated π-system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co-crystallized with {K+ (18-crown-6)(THF)} and {Cs+ 2 (18-crown-6)3 }. UV/Vis-monitoring of these systems shows a time-dependent formation of mono- and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.
More Related Videos
09:45Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene
Published on: March 20, 2017
11:44Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds
Published on: October 18, 2018
Related Concept Videos
Aromatic Hydrocarbon Cations: Structural Overview
Removing one hydrogen from the intervening CH2 group...
Aromatic Hydrocarbon Anions: Structural Overview
Due to the absence of continuous...
Criteria for Aromaticity and the Hückel 4n + 2 Rule
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n +...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
Thermal Electrocyclic Reactions: Stereochemistry
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Basicity of Heterocyclic Aromatic Amines
