BINOL-Containing Chiral Porous Polymers as Platforms for Enantiorecognition

The enantioselective discrimination of racemic compounds can be achieved through the design and preparation of a new family of chiral conjugated BINOL–porous polymers (CBPPs) from enantiopure (R)- or (S)-BINOL derivatives and 1,3,5-tris(4-phenylboronic acid)benzene or 1,3,5-tris(4-ethynylphenyl)benzene, 1,3,5-triethynyl-2,4,6-trifluorobenzene, and tetra(4-ethynylphenyl)methane as comonomers following Suzuki–Miyaura and Sonogashira–Hagihara carbon–carbon coupling approaches. The obtained CBPPs show high thermal stability, a good specific surface area, and a robust framework and can be applied successfully in the fluorescence recognition of enantiomers of terpenes (limonene and α-pinene) and 1-phenylethylamine. Fluorescence titration of CBPPs-OH in acetonitrile shows that all Sonogashira hosts exhibit a preference for the (R)-enantiomer over the (S)-enantiomer of 1-phenylethylamine, the selectivity being much higher than that of the corresponding BINOL-based soluble system used as a reference. However, the Suzuki host reveals a preference toward (S)-phenylethylamine. Regarding the sensing of terpenes, only Sonogashira hosts show enantiodifferentiation with an almost total preference for the (S)-enantiomer of limonene and α-pinene. Based on the computational simulations and the experimental data, with 1-phenylethylamine as the analyte, chiral recognition is due to the distinctive binding affinities resulting from N···H–O hydrogen bonds and the π–π interaction between the host and the guest. However, for limonene, the geometry of the adsorption complex is mostly governed by the interaction between the hydroxyl group of the BINOL unit and the C=C bond of the iso-propenyl fragment. The synthetic strategy used to prepare CBPPs opens many possibilities to place chiral centers such as BINOL in porous polymers for different chiral applications such as enantiomer recognition.


1.-Materials and Characterization Methods
All of the reagents were obtained from commercial sources and used without further purification unless otherwise indicated.Solvents were dried by standard methods or by elution using a PureSolv Innovative Technology drying system.
Mass spectra were acquired on a linear MALDI TOF/TOF (ULTRAFLEX III BRUKER).The analysis was done using DCTB as a matrix with Positive Reflector model and 355 nm laser NdYAG.The HR-MS analysis was carried out by using an Agilent 1200 Series LC system (equipped with a binary pump, an autosampler, and a column oven) coupled to a 6520 quadrupoletime of flight (QTOF) mass spectrometer.Acetonitrile: water (75:25, v:v) was used as mobile phase at 0.2 mL min -1 .The ionization source was an ESI interface working in the positive-ion mode.
Nuclear magnetic resonance (NMR) spectra were recorded with a BRUKER AVANCE III HD (Larmor frequencies of 400 and 101 MHz for 1 H and, 13 C respectively) for liquids and a Bruker AV400 WB spectrometer (Larmor frequencies of 400 and 100 MHz, using 4 mm MAS probes spinning at 10 kHz rate for 13 C solid-state MAS-NMR measurements.The 13 C CP-MAS spectra were obtained using 3. Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer equipped with 1.0 cm quartz cells.The widths of both the excitation slit and the emission slit were set to 5.0 nm.Fluorescence experiments were recorded in acetonitrile solution (for soluble (R)-2Ad-BINOL) and in solid-state and acetonitrile suspension (for CBPPs) at room temperature.

2.-Preparation of BINOL monomers
The synthesis of (R)-BINOL precursor (P1) with adamantine groups in the 6,6' position has been already published by our group and the synthetic route shown in scheme 1.

Preparation of CBPPs-OH
General deprotection procedure: An excess of BBr 3 in DCM (10 mL per 100 mg of polymer) was added at -78 ºC to a suspension of the CBPP and stirred for 2 hours; then the mixture was heated to room temperature and stirred two days at said temperature.To quench the reaction, a saturated aqueous solution of NaHCO 3 (5 mL) was added and the mixture was stirred for 2 hours, the resulting polymer was filtered and exhaustively washed with water, subsequently the polymer was stirred in warm methanol for two additional hours.Finally, the solid was filtered and washed with methanol, acetone and diethyl ether.

Procedures for quenching measurements
All materials were soaked in deoxygenated acetonitrile to remove any solvent remaining in the porous.Then, the solids were dried under vacuum and mechanically grinded with an agate mortar and pestle.Exactly 1.0 mg of the corresponding CBPPs-OH was placed in a quartz cuvette with 4 mL of acetonitrile leading a cloudy dark suspension.Moreover, 0.5 M enantiopure solutions of quenchers were prepared.

Stern-Volmer plot measurements
Fluorimeter instrumental parameters were set as follows: slit and bandwidths = 5 nm; rate = 600 nm/min.The voltage was adjusted to achieve a good signal to noise without saturation.For (S)-CBPPSu: excitation λ = 278 nm, emission λ = 424 nm; for (R)-CBPPSo1 excitation λ = 270 nm, emission λ = 372 nm and for (R)-CBPPSo2 excitation λ = 280 nm and λ em = 337 nm.A careful reading was performed before the addition of quencher solutions (to obtain I 0 under identical conditions for normalization) and again after each addition of the quencher to the corresponding suspension of the CBPPs-OH in acetonitrile.Samples were allowed to stir for three minutes after addition of each quencher to give enough time for the diffusion of quencher through the CBPPs-OH porous framework.An average intensity was then taken from 5 measurements, after each addition, and used as the intensity for that given quencher concentration. The

Figure S7 -FigureFigure S18 .Figure S19 .Figure S20 .
Figure S7-S10.Fluorescence quenching upon titration with limonene S12-S13 Figure S11-S13.Fluorescence quenching upon titration with α-Pinene S14 Figure S14-S17.Fluorescence quenching upon titration with 1-phenylethylamine S15-S16 8.-Computational Simulations S17-S19 Figure S18.Binding sites of 1-phenylethylamine in CBPPs S17 Figure S19.Binding sites of limonene in CBPPs S18 Figure S20.Binding sites of limonene in CBPPs (II) S19 5 ms contact time and 4 s relaxation time.The number of scans was 1024 of13  C CP-MAS spectra.ATR-FTIR spectra were recorded (cm −1 ) on a PerkinElmer Spectrum Two spectrometer with a Fourier equipped with a diamond internal element.Specific rotation of the optically active samples was determined on a JASCO P-2000 Polarimeter using sodium lamp (589 nm).Circular Dichroism was performed on a J-815 equipment of Jasco provided with a peltier set at 25ºC with 1.0 cm quartz cells.The microwave used was Discover SP® from CEM Corporation 3100 and/or Monowave 300 from Anton PAAR.Acquisition Gas Chromatographic (GC) was done using KONIK HRGC 5000B; a CP-CHIRASIL-DEX CB varian capillary column (25 m, 0.25 mm, 0.25 µm) and KAP-120212 capillary column (15 m, 0.25 mm, 0.25 µm).Nitrogen adsorption isotherms where measured at 77 K using a Micromeritics ASAP 2020 M and Quantachrome surface and porosity analyzer.Prior to measurement, the samples were degassed for 12 h at 100°C.Specific surface areas were determined by N 2 adsorption-desorption at 77K and the pore distribution by DFT methods.