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

A novel dioxa[6]helicene-based supramolecular chirogenic system (1) as a specific chiral recognition host for enantiopure trans-1,2-cyclohexanediamine (2) is reported. Host 1 with an inherent free phenolic group and a (1S)-camphanate chiral handle on the opposite terminal rings of the helicene chromophore acted as an efficient turn on fluorescent sensor for S,S-2 with an excellent enantioselective factor, α = KSS/KRR = 6.3 in benzene. This specific host–guest interaction phenomenon is found to be solvent-dependent, which leads to an enantioselective chiral (camphanate) group transfer to the diamine guest molecule. In the case of R,R-2, the de value is up to 68% even at room temperature. Intriguingly, the induced helicity in dioxa[6]helicene diol 6, upon supramolecular hydrogen-bonding interactions, is of opposite sense with positive helicity for S,S-2 and negative helicity for R,R-2, as shown by circular dichroism spectroscopy and in combination with theoretical calculations. This chiral supramolecular system is found to be an excellent host–guest pair for enantiomeric recognition of 2, based on their electronic and steric factors.


Job's plot analysis of supramolecular complexes using UV-vis spectroscopy (Continuous variation method)
A series of solutions containing host 1 and guest 2 (CHD) were prepared such that the sum of the total host and guest concentration remained constant (6.9 x 10 -5 M) in benzene. The mole fraction (X) of guest 2 (CHD) was varied from 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 to 1.0. The corrected absorption (at 347 nm x mole fraction (X) of guest 2 (CHD)) was plotted against the molar fraction (X) of the guest 2 (CHD), which confirmed 1:1 stoichiometry.

S9
Fluorescence sensing studies: Following guest molecules were evaluated for chiral molecular recognition studies. The stock solution of the host (6.9 x 10 -5 M) was made by dissolving 0.9 mg of helicene-1-(S)-camphanate (1) in 25 ml of solvent. From this stock solution, 1 ml was pipetted out and transferred to various 10 ml standard flasks containing 0.0 (blank), 0.2, 0.4, 0.6, 0.8, 1.0 equivalents of (1R,2R) and (1S,2S)-cyclohexanediamine (CHD) in two separate set of experiments. The flasks were kept at room temperature for four hours prior to record their fluorescence responses. S10 Figure S9: a) UV and b) fluorescence spectra of host 1 in benzene solvent (6.9 x 10 -5 M) (Slit width 7.5, 7.5) c) Comparative UV and d) fluorescence spectra of host 1 and 6 in toluene solvent The following sensing experiments were repeated thrice to obtain consistence and reproducible results. The graphs were plotted using Orgin-06 software.

I 0 /(I-I 0 )=[b/(a-b)] [ 1/(K[M]) + 1]
Where I 0 is the fluorescence intensity of the sensor in the absence of substrates [guest]; I is the fluorescence intensity of the sensor in the presence of substrates; [M] is the concentration of the substrates; and K is the association constant between the sensor and the substrates.
Terms a and b are constants.
The [b/(a-b)] can be found out by plotting the I 0 /(I-I 0 ) against the inverse of the concentration term, M -1 .
The Benesi-Hildebrand equation is similar to equation for straight line, y = mx + C. The intercept of the graph gives the [b/(a-b)]; the I 0 and I are found out experimentally and hence K can then be calculated.

S16
The Gibbs free energy changes (∆G 0 ) related to the stability of the diastereomeric supramolecular complexes were calculated at 298K as shown in the following table.

S21
The Gibbs free energy changes (∆G 0 ) related to the stability of the diastereomeric supramolecular complexes were calculated at 298K as shown in the following table.

S31
Following table shows the experiments performed towards the preliminary study of host 1 to behave as reagent for kinetic resolution for racemic CHD. Similar NMR studies for desymmetrization of meso-1,2-DPED was without much fruitful results.
The following 1-3 solutions were made by dissolving the weighed quantities of host and guest in 0.5 ml of CDCl 3 in NMR test tubes. After overnight incubation at RT, the 1 H NMR of the solutions was recorded as it is without any further purification.

Reagent
in CDCl 3 at 300 MHz Note. Starting material (SM)/ host 1 has disymmetric NMR pattern, which on reaction converts to parenet helicene diol 6 (product 1/ P1) having symmetric NMR pattern. Thus the 1 H integration is going to be doubled after reaction but actual concentration will remain the same. Thus correction factors needs to be taken in account to calculate exact concentration of the species.
Similarly, the reverse situation is for cyclohexanediamine, initially the C 2 -symmetric CHD on chiral camphanate group transfer reaction gives dissymmetric CHD camphanate amide product. As the sum of total concentration corresponding species will remains the same, thus corrected concentration is calculated for kinetic measurements.         Table S11: Induced CD studies between stereodynamic parent helicene diol 6 and CHD enantiomers in chloroform Job's plot analysis of supramolecular complexes between host 6 and 2 using Uv-Visible spectroscopy A series of solutions containing host 6 and guest 2 (CHD) were prepared such that the sum of the total host and guest concentration remained constant (1 x 10 -4 M) in chloroform. The mole fraction (X) of guest 2 (CHD) was varied from 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 to 1.0. The corrected emission (at 345 nm x mole fraction (X) of guest 2 (CHD)) was plotted against the molar fraction (X) of the guest 2 (CHD), which confirmed 1:1 stoichiometry.  Figure S33. Job's plot analysis of supramolecular complexes between host 6 and 2 using Uv-Visible spectroscopy