Chiral Dibenzopentalene‐Based Conjugated Nanohoops through Stereoselective Synthesis

Abstract

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π‐systems. They can possess unexpected optoelectronic properties and their radially oriented π‐system makes them attractive for host–guest chemistry. Bending the π‐subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel “corner unit” in the synthesis of the hoops. The [6]DBP[4]Ph‐hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO–LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Enantiopure nanohoops through racemic resolution of diketo[n]CPPs by chiral derivatization as precursors to DBP[n]CPPs

“…56,57 Addition of its Grignard reagent furnishes diastereomers, which can be separated by column chromatography, as we previously demonstrated for the diketone unit by itself. 28 Performing the racemic resolution at the late nanohoop stage in compound 3 has several advantages compared to an early-stage racemic resolution of the diketone precursor S1 (dibromide precursor to boronic ester 5 in Scheme 1, for structure see ESI): In an early-stage resolution (1) due to the material loss in the synthesis of diketo [6]CPP 3 from S1 (14% yield over four steps) significantly higher amounts of the chiral auxiliary would be required and (2) in order to obtain both enantiomers (S,S)-and (R,R)-3, their syntheses from S1 would have to be performed in two separate batches. Furthermore, in order to determine the enantiopurity of the chiral hoops via HPLC, a racemic sample of 3 is required in any case.…”

“…We synthesized chiral sulfoximine (R)-11 from thioanisol in three steps including racemic resolution of the intermediate sulfoximine before N-methylation. 28,[58][59][60] Its Grignard reagent diastereoselectively adds to the carbonyl groups in rac-3 from their convex side 28 and furnished four products 12-15 as a result of a single or double addition in an overall yield of 76%. The double adducts 12 and 15 as well as the single adducts 13 and 14 are diastereomers.…”

“…[17][18][19][20][21][22] Exceptions are the use of a chiral catalyst to obtain an enantiomeric excess of a cyclophenylene 26 and a rhodium-catalyzed diastereo-and enantioselective synthesis of CPPs, 27 both recently reported by Tanaka, and the stereoselective synthesis of chiral nanohoops using enantiomerically pure bent precursors reported by us. 28 Preparative chiral HPLC is costly and time-intensive, since multiple runs or a recycling mode using expensive chiral columns are required, and hence only allows to isolate small quantities of enantiopure hoops. Racemic resolution by preparative synthetic chemistry, on the other hand, can easily be upscaled.…”

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero‐ or Polycyclic Aromatics

“…Isobe employed an oxanorbornadiene derivative as a precursor to a 9,10‐connected anthracene corner unit in 2017(Scheme 1 e), [39] highlighted in the same section. Lastly, our group used a bent and chiral diketone precursor to incorporate dibenzo[ a , e ]pentalenes into nanohoops (Scheme 1 f), as will be discussed in Section 6.1 [40, 41] …”

Asymmetric synthesis, structures, and chiroptical properties of helical cycloparaphenylenes

“…On the other hand, our research group synthesized CP c 12 with 98% ee by the rhodium-catalyzed intramolecular alkyne cyclotrimerization, 14 which demonstrated the high utility of the alkyne cyclotrimerization for the enantioselective synthesis of chiral CNRs and CNBs. 15,16 In this article, we report the perfectly diastereo-and enantiocontrolled catalytic synthesis (>99% ee) and good chiroptical responses of multiply-helical cycloparaphenylene (CPP) (Sp,Sp)-(M,M,M,M)-1a with four helical and two planar chiralities, which is designed by cutting out configurationally stable helical units from the armchair-type CNT (Fig. 1D).…”