Chiral Atropisomeric Indenocorannulene Bowls: Critique of the Cahn–Ingold–Prelog Conception of Molecular Chirality

Abstract

Chiral corannulenes abound, but suffer generally from configurational lability associated with bowl-to-bowl inversion, [1] thus obviating questions of stereogenicity and stereoelement construction. [2] In contrast, peri-annulated corannulenes show greatly increased barriers for bowl-to-bowl inversion;s pecifically indenocorannulenes invert on at ime scale too slow to observe by normal NMR methods and raise the possibility of creating chiral atropisomeric bowl-shaped aromatics. [3] Twom ethods for preparing indenocorannulene from simple 2-haloarylcorannulenes-silyl cation C-F activation, [4] and Pd-mediated C-Cl activation [5] -enable the synthesis of an arrayo fs uch chiral atropisomeric indenocorannulenes. [6] Resolution of the enantiomers by high-performance liquid chromatography over chiral support phases motivates the study of chiroptical properties,the assignment of absolute "Cartesian" configuration, and the assessment of configurational stability. [7] These studies bring into question any systematic assignment of nontrivial stereoelements (i.e.not the molecule in its entirety) and refute any assertion of congruence between "Cahn-Ingold-Prelog elements" and the physical or "Cartesian" basis of chirality. Theminimum-energystaticbowlformofindenocorannulenemanifests bilateral (C s )symmetry.All of the hydrogen atoms are chirotopic (local symmetry C 1 )and therefore replacement of any single hydrogen atom by anon-hydrogen atom lowers the symmetry of the molecule to C 1 .T his study focuses on chiral molecules resulting from substitutions to the indeno six-membered ring.Iodocorannulene couples efficiently with av ariety of 2haloarylboronic acids to provide the immediate synthetic precursors to indenocorannulenes 1a-1f(Scheme 1). Fluoro precursors were subjected to silyl cation C-F activation/ coupling,whereas chloro precursors underwent Pd-catalyzed C-Cl activation/coupling.A lthough both methods cleanly provide product, the yields for Pd-catalyzed C-Cl activation/ coupling are in general higher (80 %v s. 40 %, see the Supporting Information) and the reaction is less sensitive to moisture and oxygen.Indenocorannulenes in general embody useful photophysical and electrochemical properties.C ompared to corannulene with af irst reduction potential of À2.49 V, the parent monoindenocorannulene has af irst reduction potential of À2.06 Va nd azaindenocorannulene 1f has af irst reduction potential of À2.00 V. [8] Clearly the effect of introducing an indeno annulation (ca. 0.5 V) outweighs the modulating influence of simple substituents (< 0.1 V). Across the series,the optical spectra display absorption peaks around 270 nm and 300-350 nm, and one broad emission peak at about 580 nm (ca. 100 nm width at half-height). Quantum efficiencies are routinely observed to be less than 1%.Indenocorannulenes are predicted to have high barriers and low rates for bowl inversion. [3] As such, one expects the products of the reactions described above,m onosubstituted derivatives 1a-1f, to be nonfluxional racemic mixtures. HPLC over ac hi...

On-Surface Stereochemical Characterization of a Highly Curved Chiral Nanographene by Noncontact Atomic Force Microscopy and Scanning Tunneling Microscopy

“…The nanographene synthesis involves two simple synthetic steps (Scheme 1). Initially, iodocorannulene 39,40 was subjected to a palladium-catalyzed Heck reaction with 1,3,5-trivinylbenzene to give the precursor 1 in an isolated yield of 63%. A subsequent oxidative photocyclization led to the formation of nanographene 2 in an isolated yield of 45%.…”