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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 cor-annulenes show greatly increased barriers for bowl-to-bowl inversion; specifically indenocorannulenes invert on a time scale too slow to observe by normal NMR methods and raise the possibility of creating chiral atropisomeric bowl-shaped aromatics. [3] Two methods 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 array of such chiral atropisomeric indeno-corannulenes. [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-energystaticbowlformofindenocorannulene manifests bilateral (C s) symmetry. All of the hydrogen atoms are chirotopic (local symmetry C 1) and therefore replacement of any single hydrogen atom by a non-hydrogen atom lowers the symmetry of the molecule to C 1. This study focuses on chiral molecules resulting from substitutions to the indeno six-membered ring. Iodocorannulene couples efficiently with a variety of 2-haloarylboronic acids to provide the immediate synthetic precursors to indenocorannulenes 1 a-1 f (Scheme 1). Fluoro precursors were subjected to silyl cation C-F activation/ coupling, whereas chloro precursors underwent Pd-catalyzed C-Cl activation/coupling. Although both methods cleanly provide product, the yields for Pd-catalyzed C-Cl activation/ coupling are in general higher (80 % vs. 40 %, see the Supporting Information) and the reaction is less sensitive to moisture and oxygen. Indenocorannulenes in general embody useful photo-physical and electrochemical properties. Compared to cor-annulene with a first reduction potential of À2.49 V, the parent monoindenocorannulene has a first reduction potential of À2.06 V and azaindenocorannulene 1 f has a first 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, monosubstituted derivatives 1 a-1 f, to be nonfluxional racemic mixtures. HPLC over a chiral stationary phase effected the resolution of 1 a-1 f, specifically using (S,S)-WHELK-O1, Chirapak ID, (S,S)-WHELK-O1, Chirapak IE, Chirapak IG, and Chirapak IC, respectively (see the Supporting Information). Kinetic studies on the first-order decay of optical activity allowed determination of activation free energies for race-mization by bowl-to-bowl inversion (Table 1). The kinetics of enantiomerization were measured in ethanol at 78 8 8C. Rate constants of enantiomerization were determined assuming first-order decay of the optical activity during the early stages of the reaction. Half-lives of racemization were determined using the first-order rate constants. Scheme 1. Chiral indenocorannulenes prepared by C-F or C-Cl activation. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, DMA = dimethylaceta-mide, MW = microwave irradiation. Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under:
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Yujia Wang, Oliver Allemann, Teodor Silviu Balaban, Nicolas Vanthuyne, Anthony Linden, et al.. Chiral Atropisomeric Indenocorannulene Bowls: Critique of the Cahn-Ingold-Prelog Conception of Molecular Chirality. Angewandte Chemie International Edition, 2018, 57 (22), pp.6470-6474. ⟨10.1002/anie.201801325⟩. ⟨hal-02093243⟩



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