Stereoisomerism, crystal structures, and dynamics of belt-shaped cyclonaphthylenes

Abstract

The chemistry of a belt-shaped cyclic array of aromatic panels, a socalled "nanohoop," has increasingly attracted much interest, partly because it serves as a segmental model of single-wall carbon nanotubes with curved sp 2 -carbon networks. Although the unique molecular structure of nanohoops is expected to deepen our understanding in curved π-systems, its structural chemistry is still in its infancy despite structural variants rapidly accumulated over the past several years. For instance, structural characteristics that endow the belt shapes with rigidity, an important structural feature relevant to carbon nanotubes, have not been clarified to date. We herein report the synthesis and structures of a series of belt-shaped cyclonaphthylenes. Random synthesis methods using three precursor units with different numbers of naphthylene panels allowed us to prepare 6 congeners consisting of 6 to 11 naphthylene panels, and relationships between the rigidity and the panel numbers, i.e., molecular structures, were investigated. Fundamental yet complicated stereoisomerism in the belt-shaped structures was disclosed by mathematical methods, and dynamics in the panel rotation was revealed by dynamic NMR studies with the aid of theoretical calculations.T he chemistry of hoop-shaped cycloarylenes (nanohoops) is being enriched by increasing variations in the molecular structures (1-4). The structural diversity in the constitutional arylene panels has gradually increased by starting from cyclopara-phenylenes (CPP; Fig. 1) with the most primordial panel being benzene (5-7), and the nanohoop chemistry is deepening our understanding of π-conjugated structures with belt-shaped, curved sp 2 -carbon networks that mimic single-wall carbon nanotubes (SWNTs) (8).Composed of arylene panels connected via multiple singlebond linkages, the belt-shaped nanohoop structures pose a fundamental and important question related to the isomerism and persistency of the curved sp 2 -carbon networks (8). Previously, we synthesized the first, to our knowledge, belt-persistent nanohoops to demonstrate that large arylene panels, such as chrysene and anthanthrene, endowed the nanohoop molecules with belt persistency. The belt-shaped atropisomers of [4]cyclochrysenylenes ([4]CC) (9, 10) and [4]cycloanthanthrenylenes ([4]CA) (11) were separated and identified as discrete molecular entities (Fig. 1), and the atropisomerism uniquely originated from restricted sp 2 -sp 2 rotations due to macrocyclic ring strain (12). However, the experimental relationship between the structure and dynamics of the arylene rotations in the nanohoops remains unclear (13-16) and will add novel insight to biaryl atropisomerism with structural and historical importance (17-20). We herein report our first attempt to reveal the structure-dynamics relationship in belt-shaped nanohoop molecules via the synthesis of a series of [n]cyclo-amphi-naphthylenes ([n]CaNAP, n = 6-11; Fig. 1) (21). Albeit simple at first glance, the molecular structures were rich in structural chemistry including ...

Stereoisomerism in Nanohoops with Heterogeneous Biaryl Linkages of E/Z- and R/S-Geometries

“…The high-temperature transition with the temperature peak at 50 °C could potentially be used for the kinetic analysis of the dynamics. 20 However, the population of the hidden, exchanging partners at this temperature was estimated as ca. 50% in comparison with the sharp peak at 40 °C and was too high for reliable analysis.…”

“…As was the case with [ n ]CaNAP, the diastereomers as well as the resonance splitting were not observed during the VT measurement, but the analysis of broadening-resharpening behaviors provided information about the dynamics. 20,33−35 Thus, a singlet proton resonance at the 4-position (H 4 ) appeared separately in the low-field region (∼9 ppm) and was suitable for the broadening-resharpening analysis. The temperature dependence of the full width at half-maximum of the H 4 resonance (H 4 -Δ 1/2 ) is summarized in Figure 7b together with the reference data of [ n ]CaNAP ( n = 8–10).…”

“…An enantiomer pair found in the crystal is shown. Naphthylene units identical to those of [8]CaNAP are shown in red and blue (see ref (20)). A minor structural deviation of one phenanthrenylene panel was found as a disordered structure, and the major structure with 83% occupancy was adopted as the representative structure for discussion.…”

“…In the case of [8]CaNAP, the coplanar belt shape resulted in 30 stereoisomers comprising 18 diastereomers and 12 enantiomer pairs, and the number of isomers was also obtained from mathematical equations. 20 However, the R/S linkages deviate the belt shape, and the non-belt shape containing axial chirality gives rise to a larger number of stereoisomers: there are 144 possible combinations of the E/Z and R/S linkages in [8]CPhen 3,9 structures, which results in 51 stereoisomers comprising 31 diastereomers and 20 enantiomer pairs (Table S1). We performed the density functional theory (DFT) calculations of all diasteroisomeric structures of [8]CPhen 3,9 , and the ( E , R , E , R , E , R , E , R )/( E , S , E , S , E , S , E , S )-enantiomers in crystal structures were found at the global minimum among 31 diastereomers.…”

“…In the belt-shaped nanohoops, the restricted biaryl rotations originated from macrocyclic ring strains to result in anomalous atropisomerism: 10−13,19,20 in the absence of steric repulsions, the biaryl linkages possessing 2,2′-binaphthyl structures in [6]CaNAP have been constrained at the coplanar, E/Z geometries. Conventionally, however, atropisomerism of biaryls such as 1,1′-binaphthyl was generated by steric repulsions between ortho -substituents, and the chirality emerged from the non-coplanar, R/S biaryl geometries.…”