Solvent‐ and Achiral‐Guest‐Triggered Chiral Inversion in a Planar Chiral pseudo[1]Catenane

Abstract

Chiral inversion systems have received a great deal of interest because they offer new applications in chemical sensing, enantioselective catalysis, and in the chirality switching of liquid crystals and photoluminescence. [1] However, chiral inversion has only been demonstrated in a few artificial molecules, such as helical oligomers and polymers [2,3] and axially chiral compounds. [4] In the present study, we present a new chiral inversion system, triggered by achiral guests and solvents, in a novel artificial molecule consisting of a planar chiral macrocyclic compound, pillar[5]arene ( Figure 1 a). [5-10, 12, 13] Pillar[5]arenes were first reported in 2008 [5] and were used as hosts to create novel supramolecular assemblies. [6] The planar chirality of pillar [5]arenes is caused by the substitution position of the alkoxy groups. [7] The units can rotate around the methylene bridges, meaning that pillar[5]arene has two equivalent most stable conformations (pS and pR), as shown in Figure 1 a. Interconversion between pS and pR occurs by rotation of these units at 25 8C in solution. [7a] Inhibiting this rotation is necessary to isolate the two enantiomers. One approach to hinder this rotation is to modify both rims with bulky substituents. [7b] The other approach is to form a rotaxane consisting of a pillar[5]arene wheel and a guest axle. [7c] Based on these approaches, we have successfully isolated planar chiral pS-and pR-pillar[5]arenes and [2]-and [3]-rotaxanes consisting of pS-and pR-pillar[5]arene wheels. [7b,c] However, the planar chirality of these molecules is static because the rotation of the units is completely inhibited. Herein, we report dynamic planar chiral inversions based on a new bicyclic structure based on pillar[5]arene, a pseudo[1]catenane 1 (Figure 1 c), in which the guest is an alkyl chain connected to one pillar[5]arene unit.Rotation of the pillar[5]arene unit connecting the alkyl chain causes inclusion and de-threading of the alkyl chain; thus 1 can be assigned as a pseudo[1]catenane. The inclusion and de-threading of the alkyl chain induces planar chiral inversion; thus 1 has four conformers: in-pS-1, in-pR-1, out-pS-1, and out-pR-1 (Figure 1 c). Guests and solvents can be included in the cavity of 1, and thus trigger the inclusion and de-threading of the alkyl chain. We show that achiral guests and solvents trigger the dynamic planar chiral inversions of 1.A pseudo[1]catenane 1 was prepared by a Sharpless click reaction between 1,12-diazidododecane and an A1/A2 dialkynylated pillar[5]arene 2 [8] (Figure 1 d). Purification of the crude product from the reaction mixture by silica gel chromatography afforded 1 in 17 % yield. The chemical structure of 1 was fully characterized by 1 H, 13 C, 2D 1 H-1 H COSY, HSQC, HMBC NMR spectroscopy, and high-resolution FAB mass spectrometry. Figure 2 shows the 1 H NMR spectrum of 1 and a model compound 3 in CDCl 3 at 25 8C. The signals from the alkyl chain of 1 (Figure 2 b, peaks a-e) were shifted upfield compared with that of 3 (Figure 2 a, peaks...

Sui Generis Helicene-Based Supramolecular Chirogenic System: Enantioselective Sensing, Solvent Control, and Application in Chiral Group Transfer Reaction

“…However, the distinctive behavior of fluorescence responses of 1 to 2 in these solvents is attributed to different solvent-effect parameters including the extent of favorable hydrogen bonding and solute–solvent π–π stacking interactions. 6 Comparatively, more polar halogenated solvents such as chloroform gave a rather weak recognition for R , R - 2 with an ef value of only 1.29, whereas in highly polar protic solvents such as methanol, no detectable chiral recognition was observed. By contrast, polar aprotic solvents, such as acetonitrile, showed a moderate sensing ability with an ef of 1.55 for R , R - 2 with emission shifted at a longer wavelength of 405 nm.…”

“…This solvent-dependent opposite trend in the CD spectrum can be attributed only to the conformational changes occurring in different solvents. 6 Thus, it is concluded that this solvent-dependent switching to a specific stereodynamic conformational feature is responsible for the inversion of chiral recognition observed in fluorescence as helicity also plays a key role in controlling the enantiomer recognition of 2 .…”

“…Indeed, there are several reports on the solvent control of supramolecular chirality recognition. 6 In turn, this can also be responsible for the stereoselectivity inversion of 2 enantiomers. To confirm the conformational switch in helicene chromophore, circular dichroism (CD) measurements of host 1 were taken in different solvents (see Supporting Information).…”

Resolution and Racemization of a Planar-Chiral A1/A2-Disubstituted Pillar[5]arene

“…The inhibition of the oxygen-through-annulus rotation will lead to a pair of planar-chiral enantiomers. Three approaches, including rotaxanation, the introduction of a side ring into one ring unit, as well as the chemical modification of bulky groups onto the rims, have been exploited for constructing chiral pillar [5]arenes [18][19][20]. Bulky groups, such as cyclohexylmethyl, phenyl, of bulky groups onto the rims, have been exploited for constructing chiral pillar [5]arenes [18][19][20].…”

“…Three approaches, including rotaxanation, the introduction of a side ring into one ring unit, as well as the chemical modification of bulky groups onto the rims, have been exploited for constructing chiral pillar [5]arenes [18][19][20]. Bulky groups, such as cyclohexylmethyl, phenyl, of bulky groups onto the rims, have been exploited for constructing chiral pillar [5]arenes [18][19][20]. Bulky groups, such as cyclohexylmethyl, phenyl, or bithienyl groups, have been chemically grafted onto one or more hydroquinone ether units, and the oxygen-through-annulus rotation was restrained or completely stopped [21,22].…”