The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14‐hexafluoropentacene (1) and the related 1,2,9,10,11‐pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X‐ray absorption spectroscopy data indicate that the single‐molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F⋅⋅⋅H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid‐state properties such as the exciton binding energy and the sublimation enthalpy.
A cyclometalated ruthenium complex with exclusively metal-centered
chirality catalyzes the conversion of diazoketones to chiral flavanones
with up to 99% yield and with up to 96% ee. A competing oxygen attack
pathway involving the formation and [1,2]-shift (Stevens rearrangement)
of an oxonium ylide intermediate was successfully suppressed in favor
of a catalytic enantioselective ring-closing C(sp3)–H
carbene insertion. Density functional theory calculations provide
a rationale for the observed C–H insertion over the undesirable
C–O formation pathway. The method provides access to a variety
of chiral flavanones which are considered privileged scaffolds with
diverse biological activities.
A non-C 2 -symmetric and sterically demanding chiral-at-rhodium catalyst is demonstrated to efficiently catalyze the highly enantioselective α-fluorination [12 examples, up to >99% enantiomeric excess (ee)] and α-chlorination (12 examples, up to 98% ee) of N-acyl pyrazoles in high yields. Based on two sterically distinct cyclometalating ligands, the nonracemic rhodium(III) catalyst can conveniently be accessed in an enantiomerically pure fashion (>99% ee) via an established auxiliary-mediated approach. Comparison of the catalytic performance with the related C 2 -symmetric rhodium catalysts revealed the explicit superiority of the non-C 2 -symmetric design for the presented α-halogenation reactions, which are generally featured by a very simple synthetic protocol.
A method for the synthesis of a bis-cyclometalated
rhodium complex
containing two different cyclometalating ligands is reported and applied
to asymmetric catalysis. The preparation of this previously inaccessible
class of tris-heteroleptic bis-cyclometalated rhodium(III) complexes
was achieved by a stepwise protocol that relies on the formation of
an isolable mono-cyclometalated rhodium(III) species in the first
step, providing the opportunity to introduce a different second ligand
in a subsequent additional cyclometalation step. The obtained racemic
complex was resolved into its single enantiomers using an established
chiral auxiliary ligand approach. The final Λ- and Δ-configured
chiral-at-metal rhodium complexes contain a cyclometalated 5-tert-butyl-1-methyl-2-phenylbenzimidazole, a cyclometalated
5-tert-butyl-2-phenylbenzothiazole, and two acetonitrile
ligands, complemented by a hexafluorophosphate counterion, and proved
to be highly efficient for asymmetric [2 + 2] photocycloadditions.
The synthesis of enantiomerically pure bis-cyclometalated rhodium(III) complexes using chiral bis(oxazoline) ligands as C 2 -symmetric chiral auxiliaries is described. Bis(oxazolines) are versatile chiral ligands for asymmetric catalysis but have not been applied to the resolution of racemic mixtures of transition-metal complexes. Due to their C 2 symmetry, chiral bis(oxazolines) are particularly useful for the synthesis of nonracemic transitionmetal complexes with lower symmetry, and this is demonstrated with the synthesis of an enantiomerically pure rhodium(III) complex containing two different cyclometalated ligands.
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