RESUMEN
Optically active triple helicenes (TH-1) were prepared via a palladium-catalyzed enantioselective cross-cyclotrimerization of two helicenyl arynes 5, which are generated in situ from 3, with dialkyl acetylenedicarboxylate 4. Enantiomeric ratios of up to 98:2 were obtained when using 4a and (S)-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereochemistry of TH-1a was revealed to be (M,P,M) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantiomerically pure TH-1a at elevated temperatures were conducted based on a high-performance liquid chromatography analysis. The activation energy for the racemization was found to be 29.1 kcal mol-1. Density functional theory calculations revealed that the palladium-catalyzed enantioselective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered palladacycle 6a with two [5]helicenes. Several initially formed stereoisomers of 6a eventually isomerize into the most thermodynamically stable palladacycle intermediate (M,P,M)-6a by inversion of the [5]helicenyl moiety. Then, the insertion of 4 into 6a to form (M,P,M)-12a, followed by a reductive elimination, leads to the formation of (M,P,M)-TH-1a in a stereoselective manner. The optical properties of TH-1a were studied by circular dichroism and circularly polarized luminescence.
RESUMEN
The reaction mechanisms of the plausible reaction process for the synthesis of hexapole helicene via a palladium-catalyzed [2 + 2 + 2] cyclotrimerization of [5]helicenyl aryne were examined using a theoretical approach. In a previous experimental study, this reaction selectively produced C2-symmetrical hexapole helicene, even though the D3-symmetrical structure is thermodynamically more stable. To clarify the mechanism underlying this reaction, density functional theory (DFT) and transition-state-theory calculations were used to evaluate the reaction profile and kinetic rate constants of the primary reactions. The thus obtained results suggest that the first step of the [2 + 2 + 2] cyclotrimerization is not a Diels-Alder reaction but an insertion of the helicenyl aryne into a metallacyclopentadiene. Subsequently, we clarified that the formation of the D3-symmetrical product is precluded by the high free-energy barrier of this reaction, while the C2-symmetrical product can be obtained at 300 K. Simulations of the time evolution of the molar fractions of the isomers were carried out based on the evaluated kinetic rate constants. The experimental result that the C2-symmetrical product is formed predominantly at 300 K was successfully reproduced in the simulations, while the isomerization into the more stable D3 hexapole helicene structure is predicted to occur at 400 K.
RESUMEN
Hexapole helicenes 1, which contain six [5]helicene substructures, were synthesized by Pd-catalyzed [2+2+2]cycloadditions of aryne precursor 6. Among the possible 20 stereoisomers, which include ten pairs of enantiomers, HH-1 was obtained selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated by chiral HPLC. Single-crystal X-ray diffraction analyses revealed a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for (P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion, the structure of HH-1 is significantly distorted and exhibits the largest twisting angle reported so far (up to 35.7° per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO-LUMO gap on account of the extended π-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on 1H NMR spectroscopy, HPLC analysis, and DFT calculations.